diff --git a/apps/web/package.json b/apps/web/package.json
index 5dff12d..3bde1b4 100644
--- a/apps/web/package.json
+++ b/apps/web/package.json
@@ -14,7 +14,8 @@
   "dependencies": {
     "next": "^15.0.0",
     "react": "^19.0.0",
-    "react-dom": "^19.0.0"
+    "react-dom": "^19.0.0",
+    "three": "^0.185.1"
   },
   "devDependencies": {
     "@testing-library/dom": "^10.4.0",
@@ -23,12 +24,13 @@
     "@types/node": "^20.0.0",
     "@types/react": "^19.0.0",
     "@types/react-dom": "^19.0.0",
+    "@types/three": "^0.185.4",
     "@vitejs/plugin-react": "^4.4.0",
+    "autoprefixer": "^10.0.0",
     "jsdom": "^26.0.0",
-    "typescript": "^5.7.0",
-    "tailwindcss": "^3.4.0",
     "postcss": "^8.0.0",
-    "autoprefixer": "^10.0.0",
+    "tailwindcss": "^3.4.0",
+    "typescript": "^5.7.0",
     "vitest": "^3.0.0"
   }
 }
diff --git a/apps/web/src/app/visualization/VisualizationDemo.tsx b/apps/web/src/app/visualization/VisualizationDemo.tsx
index b9c3756..696419e 100644
--- a/apps/web/src/app/visualization/VisualizationDemo.tsx
+++ b/apps/web/src/app/visualization/VisualizationDemo.tsx
@@ -10,10 +10,9 @@ import {
 /**
  * Client-side demonstration of the Phase 6.1 visualization foundation.
  *
- * Shows the data lifecycle (loading → success / empty / error) flowing
- * through the reusable `VisualizationContainer`. The module catalog maps the
- * full Phase 6.2–6.11 platform, each entry resolving to the viewer(s) shown
- * above on this page.
+ * Shows the module catalog flowing through the reusable
+ * `VisualizationContainer`. The catalog maps the full Phase 6.2–6.12
+ * platform, each entry resolving to the viewer(s) shown above on this page.
  */
 export function VisualizationDemo() {
   const { status, data, error, refetch } = useVisualizationData(
@@ -22,7 +21,7 @@ export function VisualizationDemo() {
   )
 
   return (
-    <div className="grid w-full grid-cols-1 gap-6 lg:grid-cols-2">
+    <div className="flex w-full flex-col gap-6">
       <VisualizationContainer
         title="Delivered visualization modules"
         description="Foundation demo loading the visualization module catalog."
@@ -45,19 +44,6 @@ export function VisualizationDemo() {
           ))}
         </ul>
       </VisualizationContainer>
-
-      <div className="flex w-full flex-col gap-6">
-        <VisualizationContainer
-          title="Empty state"
-          status="empty"
-          emptyMessage="No samples match the current selection."
-        />
-        <VisualizationContainer
-          title="Error state"
-          status="error"
-          error={{ message: 'The visualization service could not be reached.' }}
-        />
-      </div>
     </div>
   )
 }
diff --git a/apps/web/src/app/visualization/molecular-structure/MolecularStructureDemo.tsx b/apps/web/src/app/visualization/molecular-structure/MolecularStructureDemo.tsx
new file mode 100644
index 0000000..9cbdb85
--- /dev/null
+++ b/apps/web/src/app/visualization/molecular-structure/MolecularStructureDemo.tsx
@@ -0,0 +1,19 @@
+'use client'
+
+import { MolecularStructureViewer } from '@/components/molecular/MolecularStructureViewer'
+import { P53_HELIX_STRUCTURE_FIXTURE } from '@/lib/molecular/molecular.fixtures'
+import { useMolecularStructureViewer } from '@/lib/molecular/useMolecularStructureViewer'
+
+/**
+ * Phase 6.12 demo: Molecular Structure Viewer over the development fixture.
+ *
+ * The backend does not yet expose a molecular structure endpoint, so this
+ * demo feeds the synthetic typed fixture through the same normalizer the
+ * production adapter uses. See `docs/visualization/molecular-structure.md`.
+ */
+export function MolecularStructureDemo() {
+  const result = useMolecularStructureViewer({
+    loader: async () => P53_HELIX_STRUCTURE_FIXTURE,
+  })
+  return <MolecularStructureViewer result={result} />
+}
diff --git a/apps/web/src/app/visualization/molecular-structure/page.tsx b/apps/web/src/app/visualization/molecular-structure/page.tsx
new file mode 100644
index 0000000..e21cff6
--- /dev/null
+++ b/apps/web/src/app/visualization/molecular-structure/page.tsx
@@ -0,0 +1,36 @@
+import type { Metadata } from 'next'
+import Link from 'next/link'
+
+import { MolecularStructureDemo } from './MolecularStructureDemo'
+
+export const metadata: Metadata = {
+  title: 'Molecular Structure Viewer — GenomeAI',
+  description:
+    'Interactive 3D molecular structure viewer (Phase 6.12): cartoon, ball-and-stick, and space-filling representations over a synthetic development fixture.',
+}
+
+export default function MolecularStructurePage() {
+  return (
+    <main className="mx-auto flex min-h-screen w-full max-w-6xl flex-col gap-6 p-4 sm:p-8">
+      <div className="flex w-full flex-col gap-2">
+        <h1 className="text-2xl font-bold text-gray-900">Molecular Structure Viewer</h1>
+        <p className="text-sm text-gray-600">
+          Phase 6.12 interactive 3D molecular structure rendering: orbit, zoom, and pan the camera;
+          switch between cartoon / ribbon, ball-and-stick, and space-filling representations; and
+          reset or fit the view. The demo renders a synthetic development fixture through the same
+          typed normalizer the production structure adapter will use — no structure endpoint exists
+          yet (see the molecular structure documentation).
+        </p>
+      </div>
+      <nav aria-label="Visualization pages" className="flex w-full flex-wrap gap-3">
+        <Link
+          href="/visualization"
+          className="rounded-md bg-gray-900 px-4 py-2 text-sm font-medium text-white hover:bg-gray-700"
+        >
+          Back to Visualization
+        </Link>
+      </nav>
+      <MolecularStructureDemo />
+    </main>
+  )
+}
diff --git a/apps/web/src/app/visualization/page.tsx b/apps/web/src/app/visualization/page.tsx
index 9b88dd3..64cf558 100644
--- a/apps/web/src/app/visualization/page.tsx
+++ b/apps/web/src/app/visualization/page.tsx
@@ -38,6 +38,12 @@ export default function VisualizationPage() {
         >
           Open Integrated Research Workspace
         </Link>
+        <Link
+          href="/visualization/molecular-structure"
+          className="rounded-md border border-gray-300 px-4 py-2 text-sm font-medium text-gray-700 hover:bg-gray-50"
+        >
+          Open Molecular Structure Viewer
+        </Link>
       </nav>
       <GenomeBrowserDemo />
       <GeneTranscriptDemo />
diff --git a/apps/web/src/components/molecular/MolecularStructureViewer.test.tsx b/apps/web/src/components/molecular/MolecularStructureViewer.test.tsx
new file mode 100644
index 0000000..31f2da0
--- /dev/null
+++ b/apps/web/src/components/molecular/MolecularStructureViewer.test.tsx
@@ -0,0 +1,183 @@
+import { cleanup, fireEvent, render, screen, waitFor } from '@testing-library/react'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+
+import { P53_HELIX_STRUCTURE_FIXTURE } from '@/lib/molecular/molecular.fixtures'
+import type { MolecularViewer } from '@/lib/molecular/render/types'
+import type { MolecularStructureViewerResult } from '@/lib/molecular/useMolecularStructureViewer'
+
+import { MolecularStructureViewer } from './MolecularStructureViewer'
+
+function result(
+  overrides: Partial<MolecularStructureViewerResult> = {},
+): MolecularStructureViewerResult {
+  const base: MolecularStructureViewerResult = {
+    status: 'success',
+    error: undefined,
+    refetch: vi.fn(),
+    structure: P53_HELIX_STRUCTURE_FIXTURE,
+    summary: {
+      name: 'p53 N-terminal helix (synthetic fixture)',
+      chains: 1,
+      residues: 15,
+      atoms: 60,
+      bonds: 75,
+    },
+    representation: 'cartoon',
+    setRepresentation: vi.fn(),
+    visible: true,
+    setVisible: vi.fn(),
+    focus: { target: { x: 0, y: 0, z: 0 }, radius: 5, version: 0 },
+    resetView: vi.fn(),
+    fitToView: vi.fn(),
+  }
+  return { ...base, ...overrides }
+}
+
+function fakeViewer() {
+  const viewer: MolecularViewer = {
+    setStructure: vi.fn(),
+    setRepresentation: vi.fn(),
+    setVisible: vi.fn(),
+    focusCamera: vi.fn(),
+    resize: vi.fn(),
+    dispose: vi.fn(),
+  }
+  const createViewer = vi.fn(() => viewer)
+  return { viewer, createViewer }
+}
+
+afterEach(() => {
+  cleanup()
+})
+
+describe('MolecularStructureViewer', () => {
+  it('renders the canvas with a labelled image and a live summary', () => {
+    const { createViewer } = fakeViewer()
+    render(<MolecularStructureViewer result={result()} createViewer={createViewer} />)
+    const canvas = screen.getByRole('img')
+    expect(canvas).toBeInTheDocument()
+    expect(canvas.getAttribute('aria-label')).toContain('15 residues')
+    expect(screen.getByTestId('structure-status').textContent).toContain('60 atoms')
+  })
+
+  it('renders the loading state without creating a viewer', () => {
+    const { createViewer } = fakeViewer()
+    render(
+      <MolecularStructureViewer
+        result={result({ status: 'loading', structure: undefined })}
+        createViewer={createViewer}
+      />,
+    )
+    expect(screen.getByText('Loading molecular structure...')).toBeInTheDocument()
+    expect(createViewer).not.toHaveBeenCalled()
+  })
+
+  it('renders the empty state message', () => {
+    const { createViewer } = fakeViewer()
+    render(
+      <MolecularStructureViewer
+        result={result({ status: 'empty', structure: undefined })}
+        createViewer={createViewer}
+      />,
+    )
+    expect(screen.getByText('No molecular structure to display.')).toBeInTheDocument()
+    expect(createViewer).not.toHaveBeenCalled()
+  })
+
+  it('renders the error state and retries', () => {
+    const { createViewer } = fakeViewer()
+    const refetch = vi.fn()
+    render(
+      <MolecularStructureViewer
+        result={result({
+          status: 'error',
+          error: { message: 'down' },
+          structure: undefined,
+          refetch,
+        })}
+        createViewer={createViewer}
+      />,
+    )
+    expect(screen.getByText('Failed to load molecular structure')).toBeInTheDocument()
+    fireEvent.click(screen.getByRole('button', { name: /retry/i }))
+    expect(refetch).toHaveBeenCalledTimes(1)
+    expect(createViewer).not.toHaveBeenCalled()
+  })
+
+  it('creates the viewer once and feeds it the structure and focus', () => {
+    const { viewer, createViewer } = fakeViewer()
+    render(<MolecularStructureViewer result={result()} createViewer={createViewer} />)
+    expect(createViewer).toHaveBeenCalledTimes(1)
+    expect(viewer.setStructure).toHaveBeenCalledWith(P53_HELIX_STRUCTURE_FIXTURE, 'cartoon')
+    expect(viewer.focusCamera).toHaveBeenCalledWith({ x: 0, y: 0, z: 0 }, 5)
+  })
+
+  it('switches representation through the labelled select', () => {
+    const { viewer, createViewer } = fakeViewer()
+    const setRepresentation = vi.fn()
+    render(
+      <MolecularStructureViewer
+        result={result({ setRepresentation })}
+        createViewer={createViewer}
+      />,
+    )
+    const select = screen.getByRole('combobox', { name: 'Structure representation' })
+    fireEvent.change(select, { target: { value: 'space-filling' } })
+    expect(setRepresentation).toHaveBeenCalledWith('space-filling')
+    expect(viewer.setStructure).toHaveBeenCalledWith(P53_HELIX_STRUCTURE_FIXTURE, 'cartoon')
+  })
+
+  it('toggles visibility with an accessible pressed state', () => {
+    const { viewer, createViewer } = fakeViewer()
+    const setVisible = vi.fn()
+    const { unmount } = render(
+      <MolecularStructureViewer result={result({ setVisible })} createViewer={createViewer} />,
+    )
+    const toggle = screen.getByRole('button', { name: 'Hide structure' })
+    expect(toggle.getAttribute('aria-pressed')).toBe('true')
+
+    fireEvent.click(toggle)
+    expect(setVisible).toHaveBeenCalledWith(false)
+    expect(viewer.setVisible).toHaveBeenCalledWith(true)
+
+    unmount()
+  })
+
+  it('re-frames the camera when the focus version changes', async () => {
+    const { viewer, createViewer } = fakeViewer()
+    const first = result()
+    const { rerender } = render(
+      <MolecularStructureViewer result={first} createViewer={createViewer} />,
+    )
+    expect(viewer.focusCamera).toHaveBeenCalledTimes(1)
+
+    const refocused = result({ focus: { target: { x: 1, y: 1, z: 1 }, radius: 6, version: 1 } })
+    rerender(<MolecularStructureViewer result={refocused} createViewer={createViewer} />)
+    await waitFor(() => expect(viewer.focusCamera).toHaveBeenCalledTimes(2))
+  })
+
+  it('exposes reset and fit controls', () => {
+    const { createViewer } = fakeViewer()
+    const resetView = vi.fn()
+    const fitToView = vi.fn()
+    render(
+      <MolecularStructureViewer
+        result={result({ resetView, fitToView })}
+        createViewer={createViewer}
+      />,
+    )
+    fireEvent.click(screen.getByRole('button', { name: 'Reset view' }))
+    fireEvent.click(screen.getByRole('button', { name: 'Fit to structure' }))
+    expect(resetView).toHaveBeenCalledTimes(1)
+    expect(fitToView).toHaveBeenCalledTimes(1)
+  })
+
+  it('disposes the viewer on unmount', () => {
+    const { viewer, createViewer } = fakeViewer()
+    const { unmount } = render(
+      <MolecularStructureViewer result={result()} createViewer={createViewer} />,
+    )
+    unmount()
+    expect(viewer.dispose).toHaveBeenCalledTimes(1)
+  })
+})
diff --git a/apps/web/src/components/molecular/MolecularStructureViewer.tsx b/apps/web/src/components/molecular/MolecularStructureViewer.tsx
new file mode 100644
index 0000000..180b6f9
--- /dev/null
+++ b/apps/web/src/components/molecular/MolecularStructureViewer.tsx
@@ -0,0 +1,182 @@
+'use client'
+
+import { type ChangeEvent, useEffect, useRef } from 'react'
+
+import { VisualizationContainer } from '@/components/visualization/VisualizationContainer'
+import { createThreeViewer } from '@/lib/molecular/render/threeViewer'
+import type { CreateMolecularViewer, MolecularViewer } from '@/lib/molecular/render/types'
+import { REPRESENTATIONS, type RepresentationId } from '@/lib/molecular/representations'
+import type { MolecularStructureViewerResult } from '@/lib/molecular/useMolecularStructureViewer'
+
+/**
+ * 3D molecular structure viewer (Phase 6.12).
+ *
+ * Renders a `MolecularStructure` through a Three.js viewer behind the shared
+ * `VisualizationContainer` lifecycle. The viewer instance is created once per
+ * mount and updated in place (structure, representation, visibility, camera
+ * framing) so the renderer is never recreated unnecessarily, and it is fully
+ * disposed on unmount.
+ *
+ * The WebGL canvas is a supplementary visual: the labelled controls and the
+ * textual structure summary carry the keyboard/assistive interaction, and the
+ * canvas itself is exposed as a labelled image (see the accessibility section
+ * of `docs/visualization/molecular-structure.md`).
+ */
+export interface MolecularStructureViewerProps {
+  /** View model produced by `useMolecularStructureViewer`. */
+  result: MolecularStructureViewerResult
+  /** Container heading. */
+  title?: string
+  /**
+   * Viewer factory (defaults to the Three.js implementation). Inject a fake
+   * in tests to drive the lifecycle without a GPU.
+   */
+  createViewer?: CreateMolecularViewer
+}
+
+function structureCanvasLabel(result: MolecularStructureViewerResult): string {
+  const summary = result.summary
+  if (summary === undefined) return '3D molecular structure'
+  return [
+    `3D structure of ${summary.name}`,
+    `${summary.chains} chain${summary.chains === 1 ? '' : 's'}`,
+    `${summary.residues} residues`,
+    `${summary.atoms} atoms`,
+    `${summary.bonds} bonds`,
+  ].join(', ')
+}
+
+export function MolecularStructureViewer({
+  result,
+  title = 'Molecular Structure Viewer',
+  createViewer = createThreeViewer,
+}: MolecularStructureViewerProps) {
+  const containerRef = useRef<HTMLDivElement | null>(null)
+  const viewerRef = useRef<MolecularViewer | null>(null)
+  const createViewerRef = useRef<CreateMolecularViewer>(createViewer)
+  createViewerRef.current = createViewer
+
+  // Create the viewer once the success-state container exists; dispose it on
+  // unmount (or when the component leaves the success state, e.g. refetch).
+  const viewerCreatedRef = useRef(false)
+  useEffect(() => {
+    const container = containerRef.current
+    if (container === null || viewerCreatedRef.current || result.status !== 'success') return
+    viewerCreatedRef.current = true
+    viewerRef.current = createViewerRef.current(container)
+    return () => {
+      viewerRef.current?.dispose()
+      viewerRef.current = null
+      viewerCreatedRef.current = false
+    }
+  }, [result.status])
+
+  useEffect(() => {
+    const viewer = viewerRef.current
+    if (viewer === null || result.structure === undefined || result.status !== 'success') return
+    viewer.setStructure(result.structure, result.representation)
+  }, [result.status, result.structure, result.representation])
+
+  useEffect(() => {
+    const viewer = viewerRef.current
+    if (viewer === null || result.focus === undefined) return
+    viewer.focusCamera(result.focus.target, result.focus.radius)
+  }, [result.focus])
+
+  useEffect(() => {
+    viewerRef.current?.setVisible(result.visible)
+  }, [result.visible])
+
+  function handleRepresentationChange(event: ChangeEvent<HTMLSelectElement>) {
+    result.setRepresentation(event.target.value as RepresentationId)
+  }
+
+  return (
+    <VisualizationContainer
+      title={title}
+      description={
+        result.summary
+          ? `${result.summary.name} · ${result.summary.chains} chain${result.summary.chains === 1 ? '' : 's'} · ${result.summary.residues} residues · ${result.summary.atoms} atoms · ${result.summary.bonds} bonds`
+          : undefined
+      }
+      status={result.status}
+      error={result.error}
+      loadingLabel="Loading molecular structure..."
+      emptyMessage="No molecular structure to display."
+      errorTitle="Failed to load molecular structure"
+      onRetry={result.refetch}
+    >
+      {result.status === 'success' && result.structure ? (
+        <div className="flex w-full flex-col gap-3">
+          <div
+            ref={containerRef}
+            className="h-96 w-full rounded-md border border-gray-200 bg-white"
+            role="img"
+            aria-label={structureCanvasLabel(result)}
+          />
+          <output
+            className="text-xs text-gray-500"
+            aria-live="polite"
+            data-testid="structure-status"
+          >
+            {structureCanvasLabel(result)}
+          </output>
+
+          <div className="flex w-full flex-wrap items-center gap-3">
+            <fieldset
+              className="flex items-center gap-1 border-0 p-0"
+              aria-label="3D viewport controls"
+            >
+              <legend className="sr-only">3D viewport controls</legend>
+              <button
+                type="button"
+                onClick={result.resetView}
+                aria-label="Reset view"
+                title="Reset view (also fit the structure to the viewport)"
+                className="rounded-md border border-gray-300 px-2 py-1 text-sm text-gray-700 hover:bg-gray-50"
+              >
+                Reset view
+              </button>
+              <button
+                type="button"
+                onClick={result.fitToView}
+                aria-label="Fit to structure"
+                title="Fit the structure to the viewport"
+                className="rounded-md border border-gray-300 px-2 py-1 text-sm text-gray-700 hover:bg-gray-50"
+              >
+                Fit to structure
+              </button>
+            </fieldset>
+
+            <label htmlFor="molecular-representation" className="text-sm text-gray-600">
+              Representation
+            </label>
+            <select
+              id="molecular-representation"
+              value={result.representation}
+              onChange={handleRepresentationChange}
+              aria-label="Structure representation"
+              className="rounded-md border border-gray-300 px-2 py-1 text-sm text-gray-700"
+            >
+              {REPRESENTATIONS.map((option) => (
+                <option key={option.id} value={option.id}>
+                  {option.label}
+                </option>
+              ))}
+            </select>
+
+            <button
+              type="button"
+              onClick={() => result.setVisible(!result.visible)}
+              aria-pressed={result.visible}
+              aria-label={result.visible ? 'Hide structure' : 'Show structure'}
+              className="rounded-md border border-gray-300 px-2 py-1 text-sm text-gray-700 hover:bg-gray-50"
+            >
+              {result.visible ? 'Hide structure' : 'Show structure'}
+            </button>
+          </div>
+        </div>
+      ) : null}
+    </VisualizationContainer>
+  )
+}
diff --git a/apps/web/src/lib/molecular/api.test.ts b/apps/web/src/lib/molecular/api.test.ts
new file mode 100644
index 0000000..888c398
--- /dev/null
+++ b/apps/web/src/lib/molecular/api.test.ts
@@ -0,0 +1,152 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+
+import { GenomeApiError } from '@/lib/genome/api'
+
+import { fetchMolecularStructure, toStructure } from './api'
+
+const rawFetch = globalThis.fetch
+
+afterEach(() => {
+  globalThis.fetch = rawFetch
+  vi.restoreAllMocks()
+})
+
+function jsonResponse(payload: unknown, status = 200) {
+  return {
+    ok: status >= 200 && status < 300,
+    status,
+    json: () => Promise.resolve(payload),
+  } as Response
+}
+
+describe('toStructure', () => {
+  it('normalizes a raw structure record', () => {
+    const structure = toStructure({
+      id: 'fixture-mini-p53-helix',
+      name: 'p53 N-terminal helix',
+      kind: 'protein',
+      organism: 'Homo sapiens',
+      description: 'Synthetic.',
+      chains: [{ id: 'A', residues: [{ index: 1, name: 'MET', atom_indices: [1, 2, 3] }] }],
+      atoms: [
+        {
+          index: 1,
+          element: 'N',
+          atom_name: 'N',
+          residue_name: 'MET',
+          residue_index: 1,
+          chain_id: 'A',
+          x: 0,
+          y: 0,
+          z: 0,
+        },
+        {
+          serial: 2,
+          element_symbol: 'C',
+          position_x: 1,
+          position_y: 1,
+          position_z: 1,
+          residue: 1,
+          chain: 'A',
+        },
+        { index: 3, element: 'O', x: 2, y: 2, z: 2, residue_index: 1, chain_id: 'A' },
+      ],
+      bonds: [
+        { atom_a: 1, atom_b: 2, order: 1 },
+        { atom1: 2, atom2: 3 },
+      ],
+      metadata: { source: 'fixture', numeric: 3 },
+    })
+
+    expect(structure.id).toBe('fixture-mini-p53-helix')
+    expect(structure.name).toBe('p53 N-terminal helix')
+    expect(structure.kind).toBe('protein')
+    expect(structure.organism).toBe('Homo sapiens')
+    expect(structure.chains[0].residues[0].name).toBe('MET')
+    expect(structure.atoms).toHaveLength(3)
+    expect(structure.atoms[1].element).toBe('C')
+    expect(structure.atoms[1].x).toBe(1)
+    expect(structure.atoms[1].residueIndex).toBe(1)
+    expect(structure.atoms[1].chainId).toBe('A')
+    expect(structure.bonds).toEqual([
+      { atomA: 1, atomB: 2, order: 1 },
+      { atomA: 2, atomB: 3 },
+    ])
+    expect(structure.metadata).toEqual({ source: 'fixture', numeric: 3 })
+  })
+
+  it('drops malformed atoms and bonds while keeping well-formed ones', () => {
+    const structure = toStructure({
+      id: 'partial',
+      atoms: [
+        { index: 1, element: 'C', x: 0, y: 0, z: 0, residue_index: 1 },
+        { index: 2, element: 'N' },
+      ],
+      bonds: [{ atom_a: 1, atom_b: 2 }, { atom_b: 1 }],
+    })
+    expect(structure.atoms).toHaveLength(1)
+    expect(structure.bonds).toHaveLength(1)
+  })
+
+  it('falls back to molecule_type for the kind and omits unknown kinds', () => {
+    expect(toStructure({ id: 'x', molecule_type: 'nucleic-acid' }).kind).toBe('nucleic-acid')
+    expect(toStructure({ id: 'x', kind: 'virus' }).kind).toBeUndefined()
+  })
+
+  it('filters non-scalar metadata values', () => {
+    const structure = toStructure({ id: 'x', metadata: { source: 'fixture', nested: { a: 1 } } })
+    expect(structure.metadata).toEqual({ source: 'fixture' })
+  })
+
+  it('defaults the id to an empty string and optional fields to undefined', () => {
+    const structure = toStructure({})
+    expect(structure.id).toBe('')
+    expect(structure.name).toBeUndefined()
+    expect(structure.chains).toEqual([])
+    expect(structure.atoms).toEqual([])
+    expect(structure.bonds).toEqual([])
+  })
+})
+
+describe('fetchMolecularStructure', () => {
+  it('fetches, normalizes, and validates a structure', async () => {
+    const fetchMock = vi.fn(async () =>
+      jsonResponse({
+        id: 'a1b2c3',
+        name: 'Minimal',
+        chains: [{ id: 'A', residues: [{ index: 1, atom_indices: [1] }] }],
+        atoms: [{ index: 1, element: 'C', x: 0, y: 0, z: 0, residue_index: 1, chain_id: 'A' }],
+        bonds: [],
+      }),
+    )
+    globalThis.fetch = fetchMock as unknown as typeof fetch
+
+    const structure = await fetchMolecularStructure('a1b2c3', new AbortController().signal)
+    expect(structure.id).toBe('a1b2c3')
+    expect(structure.atoms).toHaveLength(1)
+    expect(fetchMock).toHaveBeenCalledWith(
+      'http://localhost:8000/structures/a1b2c3',
+      expect.objectContaining({ signal: expect.any(AbortSignal) }),
+    )
+  })
+
+  it('throws a GenomeApiError for a non-2xx response', async () => {
+    globalThis.fetch = vi.fn(async () => jsonResponse({}, 404)) as unknown as typeof fetch
+    await expect(fetchMolecularStructure('missing')).rejects.toMatchObject({
+      name: 'GenomeApiError',
+      status: 404,
+    })
+  })
+
+  it('throws a GenomeApiError for an invalid payload', async () => {
+    globalThis.fetch = vi.fn(async () => jsonResponse(null)) as unknown as typeof fetch
+    await expect(fetchMolecularStructure('a1b2c3')).rejects.toThrow(GenomeApiError)
+  })
+
+  it('throws a GenomeApiError when the payload does not describe a valid structure', async () => {
+    globalThis.fetch = vi.fn(async () =>
+      jsonResponse({ id: 'empty', atoms: [] }),
+    ) as unknown as typeof fetch
+    await expect(fetchMolecularStructure('a1b2c3')).rejects.toThrow(GenomeApiError)
+  })
+})
diff --git a/apps/web/src/lib/molecular/api.ts b/apps/web/src/lib/molecular/api.ts
new file mode 100644
index 0000000..516f8fb
--- /dev/null
+++ b/apps/web/src/lib/molecular/api.ts
@@ -0,0 +1,205 @@
+/**
+ * Molecular structure data adapter (Phase 6.12).
+ *
+ * Documents the future GenomeAI structure contract and normalizes raw wire
+ * records into the canonical `MolecularStructure` model:
+ *
+ *     GET /structures/{structure_id}
+ *
+ * The backend does **not yet expose any molecular structure endpoint**. This
+ * adapter defines the normalized shape a future source is expected to return
+ * and provides `toStructure` so a real endpoint can be wired in without
+ * touching the viewer. Until then the demo uses the clearly isolated
+ * development fixture in `lib/molecular/molecular.fixtures.ts`, routed through
+ * the same normalizer as production. See `docs/visualization/molecular-structure.md`.
+ */
+
+import { API_BASE_URL, GenomeApiError, asNumber, asString } from '@/lib/genome/api'
+import type {
+  MolecularStructure,
+  StructureAtom,
+  StructureBond,
+  StructureChain,
+  StructureKind,
+  StructureResidue,
+} from './types'
+import { firstStructureError } from './validate'
+
+/** Raw record shape a future `GET /structures/{id}` endpoint would return. */
+export interface RawStructureRecord {
+  id?: unknown
+  name?: unknown
+  molecule_name?: unknown
+  kind?: unknown
+  molecule_type?: unknown
+  organism?: unknown
+  description?: unknown
+  chains?: unknown
+  atoms?: unknown
+  bonds?: unknown
+  metadata?: unknown
+  [key: string]: unknown
+}
+
+function isObject(value: unknown): value is Record<string, unknown> {
+  return typeof value === 'object' && value !== null && !Array.isArray(value)
+}
+
+function asKind(value: unknown): StructureKind | undefined {
+  return value === 'protein' || value === 'nucleic-acid' || value === 'other' ? value : undefined
+}
+
+function toAtom(value: unknown): StructureAtom | undefined {
+  if (!isObject(value)) return undefined
+  const index = asNumber(value.index) ?? asNumber(value.serial)
+  const x = asNumber(value.x) ?? asNumber(value.position_x)
+  const y = asNumber(value.y) ?? asNumber(value.position_y)
+  const z = asNumber(value.z) ?? asNumber(value.position_z)
+  const residueIndex =
+    asNumber(value.residue_index) ?? asNumber(value.residue_seq_number) ?? asNumber(value.residue)
+  const chainId = asString(value.chain_id) ?? asString(value.chain)
+  const element = (asString(value.element) ?? asString(value.element_symbol) ?? '').toUpperCase()
+  if (index === undefined || x === undefined || y === undefined || z === undefined) {
+    return undefined
+  }
+  return {
+    index,
+    element,
+    x,
+    y,
+    z,
+    residueIndex: residueIndex ?? 0,
+    chainId: chainId ?? '',
+    ...(asString(value.residue_name) !== undefined
+      ? { residueName: asString(value.residue_name) }
+      : {}),
+    ...(asString(value.atom_name) !== undefined ? { atomName: asString(value.atom_name) } : {}),
+  }
+}
+
+function toBond(value: unknown): StructureBond | undefined {
+  if (!isObject(value)) return undefined
+  const atomA = asNumber(value.atom_a) ?? asNumber(value.atom1)
+  const atomB = asNumber(value.atom_b) ?? asNumber(value.atom2)
+  const order = asNumber(value.order)
+  if (atomA === undefined || atomB === undefined) return undefined
+  return { atomA, atomB, ...(order !== undefined ? { order } : {}) }
+}
+
+function toResidue(value: unknown): StructureResidue | undefined {
+  if (!isObject(value)) return undefined
+  const index = asNumber(value.index) ?? asNumber(value.residue_number)
+  const atomIndices = Array.isArray(value.atom_indices)
+    ? value.atom_indices
+        .map((entry: unknown) => (typeof entry === 'number' ? entry : asNumber(entry)))
+        .filter((entry: number | undefined): entry is number => entry !== undefined)
+    : []
+  if (index === undefined) return undefined
+  return {
+    index,
+    ...(asString(value.name) !== undefined ? { name: asString(value.name) } : {}),
+    atomIndices,
+  }
+}
+
+function toChain(value: unknown): StructureChain | undefined {
+  if (!isObject(value)) return undefined
+  const id = asString(value.id) ?? asString(value.chain_id)
+  const residues = Array.isArray(value.residues)
+    ? value.residues
+        .map(toResidue)
+        .filter((residue): residue is StructureResidue => residue !== undefined)
+    : []
+  if (id === undefined) return undefined
+  return { id, residues }
+}
+
+function toMetadata(value: unknown): Record<string, string | number | boolean> | undefined {
+  if (!isObject(value)) return undefined
+  const entries = Object.entries(value).filter(
+    (entry): entry is [string, string | number | boolean] => {
+      const [, field] = entry
+      return typeof field === 'string' || typeof field === 'number' || typeof field === 'boolean'
+    },
+  )
+  return entries.length > 0 ? Object.fromEntries(entries) : undefined
+}
+
+/**
+ * Normalizes a raw structure record into the canonical `MolecularStructure`.
+ * Records that are not objects (or lack coordinates) are dropped; the result
+ * should be validated with `validateStructure` before rendering.
+ */
+export function toStructure(item: RawStructureRecord): MolecularStructure {
+  const id = asString(item.id) ?? asString(item.structure_id) ?? ''
+  const atoms = Array.isArray(item.atoms)
+    ? item.atoms.map(toAtom).filter((atom): atom is StructureAtom => atom !== undefined)
+    : []
+  const bonds = Array.isArray(item.bonds)
+    ? item.bonds.map(toBond).filter((bond): bond is StructureBond => bond !== undefined)
+    : []
+  const chains = Array.isArray(item.chains)
+    ? item.chains.map(toChain).filter((chain): chain is StructureChain => chain !== undefined)
+    : []
+
+  const name = asString(item.name) ?? asString(item.molecule_name)
+  const kind = asKind(item.kind) ?? asKind(item.molecule_type)
+  const organism = asString(item.organism)
+  const description = asString(item.description)
+  const metadata = toMetadata(item.metadata)
+
+  return {
+    id,
+    ...(name !== undefined ? { name } : {}),
+    ...(kind !== undefined ? { kind } : {}),
+    ...(organism !== undefined ? { organism } : {}),
+    ...(description !== undefined ? { description } : {}),
+    chains,
+    atoms,
+    bonds,
+    ...(metadata !== undefined ? { metadata } : {}),
+  }
+}
+
+/** True when a raw record could plausibly describe a structure. */
+export function isStructureRecord(value: unknown): value is RawStructureRecord {
+  return isObject(value)
+}
+
+/**
+ * Fetches a single structure by id and normalizes it, reusing the caller's
+ * `AbortSignal`. Only usable once the backend exposes `GET /structures/{id}`;
+ * it throws a descriptive error otherwise.
+ */
+export async function fetchMolecularStructure(
+  structureId: string,
+  signal?: AbortSignal,
+): Promise<MolecularStructure> {
+  const response = await fetch(`${API_BASE_URL}/structures/${encodeURIComponent(structureId)}`, {
+    headers: { 'Content-Type': 'application/json' },
+    signal,
+  })
+
+  if (!response.ok) {
+    throw new GenomeApiError(
+      `GenomeAI API returned ${response.status} for structure ${structureId}`,
+      response.status,
+    )
+  }
+
+  const payload = (await response.json()) as RawStructureRecord | null
+  if (!isStructureRecord(payload)) {
+    throw new GenomeApiError(
+      `GenomeAI API returned an invalid payload for structure ${structureId}`,
+    )
+  }
+
+  const structure = toStructure(payload)
+  const error = firstStructureError(structure)
+  if (error !== null) {
+    throw new GenomeApiError(
+      `GenomeAI API returned an invalid structure for ${structureId}: ${error}`,
+    )
+  }
+  return structure
+}
diff --git a/apps/web/src/lib/molecular/geometry.test.ts b/apps/web/src/lib/molecular/geometry.test.ts
new file mode 100644
index 0000000..af3b3a1
--- /dev/null
+++ b/apps/web/src/lib/molecular/geometry.test.ts
@@ -0,0 +1,147 @@
+import { describe, expect, it } from 'vitest'
+
+import {
+  backboneTrace,
+  cameraFocusForStructure,
+  elementColor,
+  elementCounts,
+  elementPresentation,
+  structureBounds,
+  structureCentroid,
+  structureRadius,
+  structureSummary,
+} from './geometry'
+import type { MolecularStructure } from './types'
+
+function structure(overrides: Partial<MolecularStructure> = {}): MolecularStructure {
+  return {
+    id: 'geom',
+    name: 'Test helix',
+    chains: [
+      {
+        id: 'A',
+        residues: [
+          { index: 1, name: 'ALA', atomIndices: [1, 2] },
+          { index: 2, name: 'GLY', atomIndices: [3] },
+        ],
+      },
+      { id: 'B', residues: [{ index: 1, atomIndices: [4] }] },
+    ],
+    atoms: [
+      { index: 1, element: 'C', x: 0, y: 0, z: 0, residueIndex: 1, chainId: 'A', atomName: 'CA' },
+      { index: 2, element: 'N', x: 2, y: 0, z: 0, residueIndex: 1, chainId: 'A', atomName: 'N' },
+      { index: 3, element: 'O', x: 0, y: 4, z: 0, residueIndex: 2, chainId: 'A', atomName: 'CA' },
+      { index: 4, element: 'S', x: 0, y: 0, z: 6, residueIndex: 1, chainId: 'B', atomName: 'CA' },
+    ],
+    bonds: [
+      { atomA: 1, atomB: 2 },
+      { atomA: 3, atomB: 4 },
+    ],
+    ...overrides,
+  }
+}
+
+describe('elementPresentation', () => {
+  it('returns CPK defaults for a known element', () => {
+    expect(elementPresentation('C').color).toBe('909090')
+    expect(elementPresentation('o').color).toBe('ff0d0d')
+  })
+
+  it('falls back to a default presentation for unknown elements', () => {
+    expect(elementPresentation('Xe').color).toBe('ff7f7f')
+  })
+
+  it('exposes elementColor', () => {
+    expect(elementColor('FE')).toBe('e06633')
+  })
+
+  it('keeps van der Waals radii for space filling larger than ball radii', () => {
+    for (const symbol of ['C', 'N', 'O', 'S', 'P', 'ZN']) {
+      const presentation = elementPresentation(symbol)
+      expect(presentation.vanDerWaalsRadius).toBeGreaterThan(presentation.ballRadius)
+    }
+  })
+})
+
+describe('structureBounds', () => {
+  it('returns undefined for no atoms', () => {
+    expect(structureBounds([])).toBeUndefined()
+  })
+
+  it('computes the bounding box', () => {
+    const bounds = structureBounds(structure().atoms)
+    expect(bounds).toEqual({
+      min: { x: 0, y: 0, z: 0 },
+      max: { x: 2, y: 4, z: 6 },
+    })
+  })
+})
+
+describe('structureCentroid and structureRadius', () => {
+  it('returns the mean position', () => {
+    expect(structureCentroid(structure().atoms)).toEqual({ x: 0.5, y: 1, z: 1.5 })
+  })
+
+  it('returns the origin for an empty structure', () => {
+    expect(structureCentroid([])).toEqual({ x: 0, y: 0, z: 0 })
+  })
+
+  it('computes the largest distance from the centroid', () => {
+    const centroid = structureCentroid(structure().atoms)
+    const radius = structureRadius(structure().atoms, centroid)
+    expect(radius).toBeCloseTo(Math.sqrt(21.5))
+  })
+})
+
+describe('cameraFocusForStructure', () => {
+  it('frames around the centroid with a minimum radius', () => {
+    const focus = cameraFocusForStructure(structure())
+    expect(focus.target).toEqual({ x: 0.5, y: 1, z: 1.5 })
+    expect(focus.radius).toBeGreaterThanOrEqual(1)
+  })
+})
+
+describe('backboneTrace', () => {
+  it('traces one point per residue, preferring the CA atom', () => {
+    const trace = backboneTrace(structure())
+    expect(trace.get('A')).toEqual([
+      { x: 0, y: 0, z: 0 },
+      { x: 0, y: 4, z: 0 },
+    ])
+    expect(trace.get('B')).toEqual([{ x: 0, y: 0, z: 6 }])
+  })
+
+  it('falls back to the first atom of a residue without a CA', () => {
+    const modified = structure()
+    modified.chains[0].residues[1].atomIndices = [5]
+    modified.atoms = [
+      ...modified.atoms,
+      { index: 5, element: 'C', x: 9, y: 9, z: 9, residueIndex: 2, chainId: 'A', atomName: 'N' },
+    ]
+    const trace = backboneTrace(modified)
+    expect(trace.get('A')?.[1]).toEqual({ x: 9, y: 9, z: 9 })
+  })
+
+  it('omits chains with no residues', () => {
+    const modified = structure()
+    modified.chains = [{ id: 'C', residues: [] }]
+    expect(backboneTrace(modified).size).toBe(0)
+  })
+})
+
+describe('elementCounts and structureSummary', () => {
+  it('tallies atoms per element case-insensitively', () => {
+    expect(elementCounts(structure().atoms)).toEqual({ C: 1, N: 1, O: 1, S: 1 })
+  })
+
+  it('summarizes chains, residues, atoms, and bonds', () => {
+    const summary = structureSummary(structure())
+    expect(summary.name).toBe('Test helix')
+    expect(summary.chains).toBe(2)
+    expect(summary.residues).toBe(3)
+    expect(summary.atoms).toBe(4)
+    expect(summary.bonds).toBe(2)
+    expect(summary.elements.C).toBe(1)
+    expect(summary.radius).toBeGreaterThan(0)
+  })
+})
diff --git a/apps/web/src/lib/molecular/geometry.ts b/apps/web/src/lib/molecular/geometry.ts
new file mode 100644
index 0000000..3f0a50a
--- /dev/null
+++ b/apps/web/src/lib/molecular/geometry.ts
@@ -0,0 +1,187 @@
+/**
+ * Pure structure geometry (Phase 6.12).
+ *
+ * Coordinate math, element presentation, and camera framing for the
+ * Molecular Structure Viewer. Everything here is plain data — no Three.js,
+ * no DOM, no WebGL — so it is fully unit-testable and shared by the viewer
+ * and its tests.
+ */
+
+import type { MolecularStructure, StructureAtom } from './types'
+
+/** A point in 3D space (angstroms). */
+export interface Point3 {
+  x: number
+  y: number
+  z: number
+}
+
+/** Presentation defaults for a chemical element. */
+export interface ElementPresentation {
+  /** CPK-style display colour as a hex string (no leading `#`). */
+  color: string
+  /** Van der Waals radius in angstroms (space-filling spheres). */
+  vanDerWaalsRadius: number
+  /** Ball-and-stick sphere radius in angstroms. */
+  ballRadius: number
+}
+
+const ELEMENTS: Record<string, ElementPresentation> = {
+  C: { color: '909090', vanDerWaalsRadius: 1.7, ballRadius: 0.35 },
+  N: { color: '3050f8', vanDerWaalsRadius: 1.55, ballRadius: 0.35 },
+  O: { color: 'ff0d0d', vanDerWaalsRadius: 1.52, ballRadius: 0.35 },
+  S: { color: 'ffff30', vanDerWaalsRadius: 1.8, ballRadius: 0.4 },
+  H: { color: 'ffffff', vanDerWaalsRadius: 1.2, ballRadius: 0.2 },
+  P: { color: 'ff8000', vanDerWaalsRadius: 1.8, ballRadius: 0.4 },
+  FE: { color: 'e06633', vanDerWaalsRadius: 1.8, ballRadius: 0.45 },
+  ZN: { color: '7f80cc', vanDerWaalsRadius: 1.39, ballRadius: 0.45 },
+  CL: { color: '1ff01f', vanDerWaalsRadius: 1.75, ballRadius: 0.4 },
+  BR: { color: 'a62929', vanDerWaalsRadius: 1.85, ballRadius: 0.45 },
+  CA: { color: '3dff3d', vanDerWaalsRadius: 1.8, ballRadius: 0.45 },
+}
+
+/** Default presentation used for any element without an entry. */
+const DEFAULT_ELEMENT: ElementPresentation = {
+  color: 'ff7f7f',
+  vanDerWaalsRadius: 1.6,
+  ballRadius: 0.35,
+}
+
+/** Atom serial → atom lookup for the geometry helpers. */
+export type AtomIndex = Map<number, StructureAtom>
+
+export function atomIndex(atoms: StructureAtom[]): AtomIndex {
+  return new Map(atoms.map((atom) => [atom.index, atom]))
+}
+
+/** Presentation defaults for an element symbol (uppercase, unknown-safe). */
+export function elementPresentation(element: string): ElementPresentation {
+  return ELEMENTS[element.toUpperCase()] ?? DEFAULT_ELEMENT
+}
+
+/** Display colour (hex, no `#`) for an element symbol. */
+export function elementColor(element: string): string {
+  return elementPresentation(element).color
+}
+
+/** Bounding-box span of a set of atoms, or `undefined` when empty. */
+export function structureBounds(atoms: StructureAtom[]): { min: Point3; max: Point3 } | undefined {
+  if (atoms.length === 0) return undefined
+  const min = {
+    x: Number.POSITIVE_INFINITY,
+    y: Number.POSITIVE_INFINITY,
+    z: Number.POSITIVE_INFINITY,
+  }
+  const max = {
+    x: Number.NEGATIVE_INFINITY,
+    y: Number.NEGATIVE_INFINITY,
+    z: Number.NEGATIVE_INFINITY,
+  }
+  for (const atom of atoms) {
+    min.x = Math.min(min.x, atom.x)
+    min.y = Math.min(min.y, atom.y)
+    min.z = Math.min(min.z, atom.z)
+    max.x = Math.max(max.x, atom.x)
+    max.y = Math.max(max.y, atom.y)
+    max.z = Math.max(max.z, atom.z)
+  }
+  return { min, max }
+}
+
+/** Centroid (mean) of all atoms, or `{0,0,0}` when empty. */
+export function structureCentroid(atoms: StructureAtom[]): Point3 {
+  if (atoms.length === 0) return { x: 0, y: 0, z: 0 }
+  let x = 0
+  let y = 0
+  let z = 0
+  for (const atom of atoms) {
+    x += atom.x
+    y += atom.y
+    z += atom.z
+  }
+  const count = atoms.length
+  return { x: x / count, y: y / count, z: z / count }
+}
+
+/** Largest distance from the centroid to any atom (angstroms). */
+export function structureRadius(atoms: StructureAtom[], centroid: Point3): number {
+  let radius = 0
+  for (const atom of atoms) {
+    const dx = atom.x - centroid.x
+    const dy = atom.y - centroid.y
+    const dz = atom.z - centroid.z
+    radius = Math.max(radius, Math.sqrt(dx * dx + dy * dy + dz * dz))
+  }
+  return radius
+}
+
+/** Camera framing for a structure: the point to look at and its radius. */
+export function cameraFocusForStructure(structure: MolecularStructure): {
+  target: Point3
+  radius: number
+} {
+  const centroid = structureCentroid(structure.atoms)
+  return { target: centroid, radius: Math.max(structureRadius(structure.atoms, centroid), 1) }
+}
+
+/**
+ * Backbone trace points per chain, in residue order. Uses the C-alpha atom of
+ * each residue when present, otherwise the first atom of the residue — the
+ * simplest meaningful spline backbone for the cartoon representation.
+ */
+export function backboneTrace(
+  structure: MolecularStructure,
+  atoms: StructureAtom[] = structure.atoms,
+): Map<string, Point3[]> {
+  const byChain = new Map<string, Point3[]>()
+  const byIndex = atomIndex(atoms)
+  for (const chain of structure.chains) {
+    const points: Point3[] = []
+    for (const residue of chain.residues) {
+      const residueAtoms = residue.atomIndices
+        .map((serial) => byIndex.get(serial))
+        .filter((atom): atom is StructureAtom => atom !== undefined)
+      const backbone =
+        residueAtoms.find((atom) => (atom.atomName ?? '').toUpperCase() === 'CA') ?? residueAtoms[0]
+      if (backbone !== undefined) {
+        points.push({ x: backbone.x, y: backbone.y, z: backbone.z })
+      }
+    }
+    if (points.length > 0) byChain.set(chain.id, points)
+  }
+  return byChain
+}
+
+/** Tally of atoms per element symbol, keyed by uppercase symbol. */
+export function elementCounts(atoms: StructureAtom[]): Record<string, number> {
+  const counts: Record<string, number> = {}
+  for (const atom of atoms) {
+    const element = atom.element.toUpperCase()
+    counts[element] = (counts[element] ?? 0) + 1
+  }
+  return counts
+}
+
+/** Human-readable summary of a structure for status lines and tests. */
+export function structureSummary(structure: MolecularStructure): {
+  name: string
+  chains: number
+  residues: number
+  atoms: number
+  bonds: number
+  elements: Record<string, number>
+  radius: number
+  centroid: Point3
+} {
+  const residues = structure.chains.reduce((total, chain) => total + chain.residues.length, 0)
+  return {
+    name: structure.name ?? structure.id,
+    chains: structure.chains.length,
+    residues,
+    atoms: structure.atoms.length,
+    bonds: structure.bonds.length,
+    elements: elementCounts(structure.atoms),
+    radius: structureRadius(structure.atoms, structureCentroid(structure.atoms)),
+    centroid: structureCentroid(structure.atoms),
+  }
+}
diff --git a/apps/web/src/lib/molecular/molecular.fixtures.test.ts b/apps/web/src/lib/molecular/molecular.fixtures.test.ts
new file mode 100644
index 0000000..a83c76b
--- /dev/null
+++ b/apps/web/src/lib/molecular/molecular.fixtures.test.ts
@@ -0,0 +1,33 @@
+import { describe, expect, it } from 'vitest'
+
+import { P53_HELIX_STRUCTURE_FIXTURE } from './molecular.fixtures'
+import { isValidStructure, validateStructure } from './validate'
+
+describe('P53_HELIX_STRUCTURE_FIXTURE', () => {
+  it('is a structurally valid, usable fixture', () => {
+    expect(validateStructure(P53_HELIX_STRUCTURE_FIXTURE)).toEqual([])
+    expect(isValidStructure(P53_HELIX_STRUCTURE_FIXTURE)).toBe(true)
+  })
+
+  it('is stable across repeated access (deterministic generation)', () => {
+    expect(JSON.stringify(P53_HELIX_STRUCTURE_FIXTURE.atoms)).toBe(
+      JSON.stringify(P53_HELIX_STRUCTURE_FIXTURE.atoms),
+    )
+    const bonds = P53_HELIX_STRUCTURE_FIXTURE.bonds
+    expect(bonds.length).toBeGreaterThan(0)
+    expect(P53_HELIX_STRUCTURE_FIXTURE.chains).toHaveLength(1)
+    expect(P53_HELIX_STRUCTURE_FIXTURE.chains[0].id).toBe('A')
+    expect(P53_HELIX_STRUCTURE_FIXTURE.kind).toBe('protein')
+  })
+
+  it('describes a small polymer: one chain, many residues, more atoms than residues', () => {
+    const residues = P53_HELIX_STRUCTURE_FIXTURE.chains[0].residues.length
+    expect(residues).toBeGreaterThanOrEqual(10)
+    expect(P53_HELIX_STRUCTURE_FIXTURE.atoms.length).toBeGreaterThan(residues)
+    expect(P53_HELIX_STRUCTURE_FIXTURE.bonds.length).toBeGreaterThan(residues)
+  })
+
+  it('uses the synthetic marker metadata', () => {
+    expect(P53_HELIX_STRUCTURE_FIXTURE.metadata?.source).toBe('fixture')
+  })
+})
diff --git a/apps/web/src/lib/molecular/molecular.fixtures.ts b/apps/web/src/lib/molecular/molecular.fixtures.ts
new file mode 100644
index 0000000..dc79ddd
--- /dev/null
+++ b/apps/web/src/lib/molecular/molecular.fixtures.ts
@@ -0,0 +1,152 @@
+/**
+ * Development fixture for the Molecular Structure Viewer (Phase 6.12).
+ *
+ * The GenomeAI backend does **not yet expose any molecular structure
+ * endpoint**. This module provides a small, clearly isolated, synthetic
+ * structure that mimics what a future structure source would return, so the
+ * viewer, its geometry math, and its tests can be developed now.
+ *
+ * ## Boundary
+ *
+ * This is a **development fixture, not a real API or a real molecule**. It is
+ * generated deterministically (a straight alpha-helix-like trace with
+ * backbone atoms N/CA/C/O per residue and a few side-chain atoms) purely so
+ * the 3D rendering pipeline has realistic input. It flows through the same
+ * `toStructure` normalizer the production adapter uses, so the seam is
+ * exercised exactly as production would. See
+ * `docs/visualization/molecular-structure.md`.
+ */
+
+import { type RawStructureRecord, toStructure } from './api'
+import type { MolecularStructure } from './types'
+
+const RESIDUE_NAMES = [
+  'MET',
+  'GLU',
+  'GLU',
+  'PRO',
+  'GLN',
+  'SER',
+  'ASP',
+  'PRO',
+  'SER',
+  'VAL',
+  'GLU',
+  'PRO',
+  'PRO',
+  'LEU',
+  'SER',
+]
+
+const RESIDUES_COUNT = RESIDUE_NAMES.length
+const HELIX_RADIUS = 4.5
+const RISE_PER_RESIDUE = 1.5
+const RESIDUES_PER_TURN = 3.6
+
+/**
+ * Synthetic raw structure record: a short alpha-helix-like trace over the
+ * N-terminal residues of TP53. Coordinates are generated deterministically
+ * (no random values), so the fixture is stable for tests and rendering.
+ */
+function buildRawFixture(): RawStructureRecord {
+  const atoms: unknown[] = []
+  const bonds: unknown[] = []
+  const residues: unknown[] = []
+  const chainId = 'A'
+
+  let serial = 0
+  const previousCarbon: number[] = []
+  const cAlphaByResidue: number[] = []
+
+  for (let residueIndex = 0; residueIndex < RESIDUES_COUNT; residueIndex++) {
+    const angle = (residueIndex / RESIDUES_PER_TURN) * 2 * Math.PI
+    const z = residueIndex * RISE_PER_RESIDUE
+    const caX = HELIX_RADIUS * Math.cos(angle)
+    const caY = HELIX_RADIUS * Math.sin(angle)
+
+    const backbone = [
+      { name: 'N', dx: 0.5, dy: 0.4, dz: -0.6 },
+      { name: 'CA', dx: 0, dy: 0, dz: 0 },
+      { name: 'C', dx: -0.4, dy: -0.5, dz: 0.6 },
+      { name: 'O', dx: -0.9, dy: -0.6, dz: 0.6 },
+    ]
+
+    const residueAtoms: number[] = []
+    for (const position of backbone) {
+      serial += 1
+      residueAtoms.push(serial)
+      const isCarbon = position.name === 'CA' || position.name === 'C'
+      atoms.push({
+        index: serial,
+        element: position.name === 'O' ? 'O' : isCarbon ? 'C' : 'N',
+        atom_name: position.name,
+        residue_name: RESIDUE_NAMES[residueIndex],
+        residue_index: residueIndex + 1,
+        chain_id: chainId,
+        x: caX + position.dx,
+        y: caY + position.dy,
+        z: z + position.dz,
+      })
+      if (position.name === 'CA') cAlphaByResidue.push(serial)
+    }
+
+    // Peptide bond from the previous residue's carbonyl carbon to this N.
+    if (previousCarbon.length > 0) {
+      bonds.push({ atom_a: previousCarbon[0], atom_b: residueAtoms[0], order: 1 })
+    }
+    bonds.push({ atom_a: residueAtoms[0], atom_b: residueAtoms[1], order: 1 })
+    bonds.push({ atom_a: residueAtoms[1], atom_b: residueAtoms[2], order: 1 })
+    bonds.push({ atom_a: residueAtoms[2], atom_b: residueAtoms[3], order: 1 })
+    previousCarbon[0] = residueAtoms[2]
+
+    // A few side-chain atoms (CA-CB stub) for visual variety.
+    if (residueIndex % 3 === 0) {
+      serial += 1
+      atoms.push({
+        index: serial,
+        element: 'C',
+        atom_name: 'CB',
+        residue_name: RESIDUE_NAMES[residueIndex],
+        residue_index: residueIndex + 1,
+        chain_id: chainId,
+        x: caX + 1.1,
+        y: caY - 0.4,
+        z: z + 0.3,
+      })
+      bonds.push({ atom_a: cAlphaByResidue[residueIndex], atom_b: serial, order: 1 })
+      residueAtoms.push(serial)
+    }
+
+    residues.push({
+      index: residueIndex + 1,
+      name: RESIDUE_NAMES[residueIndex],
+      atom_indices: residueAtoms,
+    })
+  }
+
+  return {
+    id: 'fixture-mini-p53-helix',
+    name: 'p53 N-terminal helix (synthetic fixture)',
+    kind: 'protein',
+    organism: 'Homo sapiens',
+    description:
+      'Synthetic alpha-helix-like trace over the N-terminal residues of TP53. Development fixture, not a real molecule.',
+    chains: [{ id: chainId, residues }],
+    atoms,
+    bonds,
+    metadata: { source: 'fixture', format: 'synthetic-helix' },
+  }
+}
+
+/** The raw fixture record, routed through the production normalizer. */
+export const P53_HELIX_STRUCTURE_FIXTURE: MolecularStructure = toStructure(buildRawFixture())
+
+/** A deliberately empty structure fixture for the empty-state tests/demo. */
+export const EMPTY_STRUCTURE_FIXTURE: MolecularStructure = toStructure({
+  id: 'fixture-empty',
+  name: 'Empty structure',
+  kind: 'protein',
+  chains: [],
+  atoms: [],
+  bonds: [],
+})
diff --git a/apps/web/src/lib/molecular/render/representationBuilder.test.ts b/apps/web/src/lib/molecular/render/representationBuilder.test.ts
new file mode 100644
index 0000000..ec2b14d
--- /dev/null
+++ b/apps/web/src/lib/molecular/render/representationBuilder.test.ts
@@ -0,0 +1,107 @@
+import type * as THREE from 'three'
+import { describe, expect, it, vi } from 'vitest'
+
+import type { MolecularStructure } from '../types'
+import { buildStructureGroup, disposeGroup } from './representationBuilder'
+
+function structure(): MolecularStructure {
+  return {
+    id: 'builder',
+    chains: [
+      {
+        id: 'A',
+        residues: [
+          { index: 1, atomIndices: [1, 2] },
+          { index: 2, atomIndices: [3, 4] },
+        ],
+      },
+    ],
+    atoms: [
+      { index: 1, element: 'C', x: 0, y: 0, z: 0, residueIndex: 1, chainId: 'A', atomName: 'CA' },
+      {
+        index: 2,
+        element: 'N',
+        x: 0.5,
+        y: 0.4,
+        z: -0.6,
+        residueIndex: 1,
+        chainId: 'A',
+        atomName: 'N',
+      },
+      { index: 3, element: 'C', x: 4, y: 0, z: 0, residueIndex: 2, chainId: 'A', atomName: 'CA' },
+      {
+        index: 4,
+        element: 'O',
+        x: 4.4,
+        y: -0.6,
+        z: 0.5,
+        residueIndex: 2,
+        chainId: 'A',
+        atomName: 'O',
+      },
+    ],
+    bonds: [
+      { atomA: 1, atomB: 2 },
+      { atomA: 2, atomB: 3 },
+      { atomA: 3, atomB: 4 },
+    ],
+  }
+}
+
+function countMeshes(group: THREE.Group): number {
+  let count = 0
+  group.traverse((object) => {
+    if ((object as THREE.Mesh).isMesh) count += 1
+  })
+  return count
+}
+
+describe('buildStructureGroup', () => {
+  it('builds one sphere per atom for ball-and-stick', () => {
+    const group = buildStructureGroup(structure(), 'ball-and-stick')
+    expect(countMeshes(group)).toBe(4 + 3)
+  })
+
+  it('builds spheres only for space-filling (no bonds)', () => {
+    const group = buildStructureGroup(structure(), 'space-filling')
+    expect(countMeshes(group)).toBe(4)
+  })
+
+  it('builds a cartoon ribbon tube per chain backbone', () => {
+    const group = buildStructureGroup(structure(), 'cartoon')
+    const ribbons = group.children.filter(
+      (child) => (child as THREE.Mesh).isMesh && child.name.startsWith('cartoon-'),
+    )
+    expect(ribbons).toHaveLength(1)
+  })
+
+  it('returns an empty group for a structure without atoms', () => {
+    const empty: MolecularStructure = { id: 'empty', chains: [], atoms: [], bonds: [] }
+    expect(buildStructureGroup(empty, 'ball-and-stick').children).toHaveLength(0)
+    expect(buildStructureGroup(empty, 'cartoon').children).toHaveLength(0)
+  })
+})
+
+describe('disposeGroup', () => {
+  it('disposes every geometry and material owned by the group', () => {
+    const group = buildStructureGroup(structure(), 'ball-and-stick')
+    const disposeCalls: string[] = []
+    group.traverse((object) => {
+      const mesh = object as THREE.Mesh
+      if (mesh.isMesh) {
+        const geometry = mesh.geometry
+        const material = mesh.material as THREE.Material
+        vi.spyOn(geometry, 'dispose').mockImplementation(() => {
+          disposeCalls.push('geometry')
+        })
+        vi.spyOn(material, 'dispose').mockImplementation(() => {
+          disposeCalls.push('material')
+        })
+      }
+    })
+
+    disposeGroup(group)
+    expect(disposeCalls.filter((entry) => entry === 'geometry')).toHaveLength(7)
+    expect(disposeCalls.filter((entry) => entry === 'material')).toHaveLength(7)
+  })
+})
diff --git a/apps/web/src/lib/molecular/render/representationBuilder.ts b/apps/web/src/lib/molecular/render/representationBuilder.ts
new file mode 100644
index 0000000..c7c5eeb
--- /dev/null
+++ b/apps/web/src/lib/molecular/render/representationBuilder.ts
@@ -0,0 +1,135 @@
+/**
+ * Structure representation builder (Phase 6.12).
+ *
+ * Builds the Three.js object graph for one representation of a structure.
+ * This module only creates Three.js **objects** (geometries, materials,
+ * meshes, curves) — it never touches WebGL — so it is fully unit-testable in
+ * jsdom. Rendering and lifecycle live in `threeViewer.ts`.
+ *
+ * Representation extensions: add a branch in `buildStructureGroup` and a
+ * catalog entry in `lib/molecular/representations.ts`.
+ */
+
+import * as THREE from 'three'
+
+import { backboneTrace, elementPresentation } from '@/lib/molecular/geometry'
+import type { Point3 } from '@/lib/molecular/geometry'
+import type { RepresentationId } from '@/lib/molecular/representations'
+import type { MolecularStructure, StructureAtom } from '@/lib/molecular/types'
+
+/** Colour used by the cartoon ribbon. */
+const CARTOON_COLOR = 0x4f7cc0
+/** Radius of the cartoon ribbon tube, in angstroms. */
+const CARTOON_TUBE_RADIUS = 0.6
+/** Cylinder radius for ball-and-stick bonds, in angstroms. */
+const BOND_RADIUS = 0.14
+
+function atomMesh(atom: StructureAtom, radius: number): THREE.Mesh {
+  const presentation = elementPresentation(atom.element)
+  const geometry = new THREE.SphereGeometry(radius, 16, 12)
+  const material = new THREE.MeshStandardMaterial({ color: `#${presentation.color}` })
+  const mesh = new THREE.Mesh(geometry, material)
+  mesh.position.set(atom.x, atom.y, atom.z)
+  return mesh
+}
+
+/**
+ * A cylinder between two points, oriented along the bond axis. Returns a
+ * mesh whose disposal must be tracked alongside its geometry/material.
+ */
+function bondCylinder(a: Point3, b: Point3, radius: number): THREE.Mesh {
+  const start = new THREE.Vector3(a.x, a.y, a.z)
+  const end = new THREE.Vector3(b.x, b.y, b.z)
+  const direction = new THREE.Vector3().subVectors(end, start)
+  const length = direction.length()
+  const geometry = new THREE.CylinderGeometry(radius, radius, length, 8, 1)
+  const material = new THREE.MeshStandardMaterial({ color: 0xb0b0b0 })
+  const mesh = new THREE.Mesh(geometry, material)
+  const midpoint = new THREE.Vector3().addVectors(start, end).multiplyScalar(0.5)
+  mesh.position.copy(midpoint)
+  if (length > 0) {
+    mesh.quaternion.setFromUnitVectors(new THREE.Vector3(0, 1, 0), direction.clone().normalize())
+  }
+  return mesh
+}
+
+function buildBallAndStick(structure: MolecularStructure): THREE.Group {
+  const group = new THREE.Group()
+  const byIndex = new Map(structure.atoms.map((atom) => [atom.index, atom]))
+
+  for (const atom of structure.atoms) {
+    group.add(atomMesh(atom, elementPresentation(atom.element).ballRadius))
+  }
+  for (const bond of structure.bonds) {
+    const atomA = byIndex.get(bond.atomA)
+    const atomB = byIndex.get(bond.atomB)
+    if (atomA === undefined || atomB === undefined) continue
+    group.add(bondCylinder(atomA, atomB, BOND_RADIUS))
+  }
+  return group
+}
+
+function buildSpaceFilling(structure: MolecularStructure): THREE.Group {
+  const group = new THREE.Group()
+  for (const atom of structure.atoms) {
+    group.add(atomMesh(atom, elementPresentation(atom.element).vanDerWaalsRadius))
+  }
+  return group
+}
+
+function buildCartoon(structure: MolecularStructure): THREE.Group {
+  const group = new THREE.Group()
+  const traces = backboneTrace(structure)
+  for (const [chainId, points] of traces) {
+    if (points.length < 2) continue
+    const curve = new THREE.CatmullRomCurve3(
+      points.map((point) => new THREE.Vector3(point.x, point.y, point.z)),
+    )
+    const geometry = new THREE.TubeGeometry(
+      curve,
+      Math.max(points.length * 4, 16),
+      CARTOON_TUBE_RADIUS,
+      8,
+      false,
+    )
+    const material = new THREE.MeshStandardMaterial({ color: CARTOON_COLOR })
+    const mesh = new THREE.Mesh(geometry, material)
+    mesh.name = `cartoon-${chainId}`
+    group.add(mesh)
+  }
+  return group
+}
+
+/**
+ * Builds the object graph for a representation. Callers own the returned
+ * group and must dispose its geometries/materials (see `disposeGroup`).
+ */
+export function buildStructureGroup(
+  structure: MolecularStructure,
+  representation: RepresentationId,
+): THREE.Group {
+  switch (representation) {
+    case 'ball-and-stick':
+      return buildBallAndStick(structure)
+    case 'space-filling':
+      return buildSpaceFilling(structure)
+    case 'cartoon':
+      return buildCartoon(structure)
+  }
+}
+
+/** Disposes every geometry and material owned by a group (recursively). */
+export function disposeGroup(group: THREE.Object3D): void {
+  group.traverse((object) => {
+    const mesh = object as THREE.Mesh
+    if (mesh.isMesh) {
+      mesh.geometry?.dispose()
+      const material = mesh.material
+      if (Array.isArray(material)) {
+        for (const entry of material) entry.dispose()
+      } else {
+        material?.dispose()
+      }
+    }
+  })
+}
diff --git a/apps/web/src/lib/molecular/render/threeViewer.test.ts b/apps/web/src/lib/molecular/render/threeViewer.test.ts
new file mode 100644
index 0000000..ff28055
--- /dev/null
+++ b/apps/web/src/lib/molecular/render/threeViewer.test.ts
@@ -0,0 +1,176 @@
+import * as THREE from 'three'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+
+import type { MolecularStructure } from '../types'
+import { createThreeViewer } from './threeViewer'
+import type { ThreeRenderer } from './types'
+
+interface FakeRendererHarness {
+  renderer: ThreeRenderer & {
+    setPixelRatio: ReturnType<typeof vi.fn>
+    setSize: ReturnType<typeof vi.fn>
+    setClearColor: ReturnType<typeof vi.fn>
+    dispose: ReturnType<typeof vi.fn>
+  }
+  loop: ((time: number) => void) | null
+  lastScene: unknown
+  lastCamera: unknown
+  setAnimationLoopCalls: Array<((time: number) => void) | null>
+}
+
+function createFakeRenderer(): FakeRendererHarness {
+  const harness: FakeRendererHarness = {
+    renderer: {
+      domElement: document.createElement('canvas'),
+      setPixelRatio: vi.fn(),
+      setSize: vi.fn(),
+      setClearColor: vi.fn(),
+      setAnimationLoop: vi.fn(),
+      render: vi.fn(),
+      dispose: vi.fn(),
+    },
+    loop: null,
+    lastScene: undefined,
+    lastCamera: undefined,
+    setAnimationLoopCalls: [],
+  }
+  const renderer = harness.renderer
+  renderer.setAnimationLoop = vi.fn((callback: ((time: number) => void) | null) => {
+    harness.setAnimationLoopCalls.push(callback)
+    harness.loop = callback
+  })
+  renderer.render = vi.fn((scene: unknown, camera: unknown) => {
+    harness.lastScene = scene
+    harness.lastCamera = camera
+  })
+  return harness
+}
+
+class MockResizeObserver implements ResizeObserver {
+  callback: ResizeObserverCallback
+  observed = new Set<Element>()
+  constructor(callback: ResizeObserverCallback) {
+    this.callback = callback
+  }
+  observe(target: Element) {
+    this.observed.add(target)
+  }
+  unobserve(target: Element) {
+    this.observed.delete(target)
+  }
+  disconnect() {
+    this.observed.clear()
+  }
+}
+
+function structure(): MolecularStructure {
+  return {
+    id: 'viewer',
+    chains: [
+      {
+        id: 'A',
+        residues: [
+          { index: 1, atomIndices: [1] },
+          { index: 2, atomIndices: [2] },
+        ],
+      },
+    ],
+    atoms: [
+      { index: 1, element: 'C', x: 0, y: 0, z: 0, residueIndex: 1, chainId: 'A', atomName: 'CA' },
+      { index: 2, element: 'N', x: 4, y: 0, z: 0, residueIndex: 2, chainId: 'A', atomName: 'CA' },
+    ],
+    bonds: [],
+  }
+}
+
+let previousResizeObserver: typeof ResizeObserver | undefined
+
+afterEach(() => {
+  if (previousResizeObserver === undefined) {
+    ;(globalThis as Record<string, unknown>).ResizeObserver = undefined
+  } else {
+    globalThis.ResizeObserver = previousResizeObserver
+  }
+  previousResizeObserver = undefined
+  vi.restoreAllMocks()
+})
+
+describe('createThreeViewer', () => {
+  it('appends the canvas and renders frames through the animation loop', () => {
+    const fake = createFakeRenderer()
+    const container = document.createElement('div')
+    const viewer = createThreeViewer(container, { createRenderer: () => fake.renderer })
+
+    expect(container.contains(fake.renderer.domElement)).toBe(true)
+    viewer.setStructure(structure(), 'cartoon')
+    fake.loop?.(0)
+
+    expect(fake.renderer.render).toHaveBeenCalledTimes(1)
+    expect(fake.lastCamera).toBeInstanceOf(THREE.PerspectiveCamera)
+    expect(fake.lastScene).toBeInstanceOf(THREE.Scene)
+  })
+
+  it('frames the camera around a target with a distance proportional to the radius', () => {
+    const fake = createFakeRenderer()
+    const container = document.createElement('div')
+    const viewer = createThreeViewer(container, { createRenderer: () => fake.renderer })
+
+    fake.loop?.(0)
+    const camera = fake.lastCamera as THREE.PerspectiveCamera
+
+    viewer.focusCamera({ x: 0, y: 0, z: 0 }, 5)
+    expect(camera.position.length()).toBeCloseTo(15)
+    expect(camera.near).toBeCloseTo(0.05)
+    expect(camera.far).toBe(250)
+
+    viewer.focusCamera({ x: 10, y: 0, z: 0 }, 1)
+    const position = camera.position
+    const distance = new THREE.Vector3(position.x - 10, position.y, position.z).length()
+    expect(distance).toBeCloseTo(Math.max(1 * 3, 2))
+  })
+
+  it('handles an explicit resize', () => {
+    const fake = createFakeRenderer()
+    const container = document.createElement('div')
+    const viewer = createThreeViewer(container, { createRenderer: () => fake.renderer })
+    viewer.resize(400, 300)
+    expect(fake.renderer.setSize).toHaveBeenCalledWith(400, 300)
+  })
+
+  it('observes the container size when ResizeObserver is available', () => {
+    previousResizeObserver = globalThis.ResizeObserver
+    globalThis.ResizeObserver = MockResizeObserver
+    const fake = createFakeRenderer()
+    const container = document.createElement('div')
+    const viewer = createThreeViewer(container, { createRenderer: () => fake.renderer })
+    viewer.setStructure(structure(), 'ball-and-stick')
+    expect(globalThis.ResizeObserver).toBe(MockResizeObserver)
+    viewer.dispose()
+  })
+
+  it('disposes the renderer, stops the loop, and detaches the canvas', () => {
+    const fake = createFakeRenderer()
+    const container = document.createElement('div')
+    const viewer = createThreeViewer(container, { createRenderer: () => fake.renderer })
+    viewer.setStructure(structure(), 'space-filling')
+
+    viewer.dispose()
+    expect(fake.renderer.dispose).toHaveBeenCalledTimes(1)
+    expect(fake.setAnimationLoopCalls.at(-1)).toBeNull()
+    expect(fake.renderer.domElement.parentNode).toBeNull()
+  })
+
+  it('keeps the render loop healthy across representation and visibility changes', () => {
+    const fake = createFakeRenderer()
+    const container = document.createElement('div')
+    const viewer = createThreeViewer(container, { createRenderer: () => fake.renderer })
+    viewer.setStructure(structure(), 'cartoon')
+    viewer.setRepresentation('ball-and-stick')
+    viewer.setVisible(false)
+    viewer.setVisible(true)
+
+    fake.loop?.(0)
+    fake.loop?.(0)
+    expect(fake.renderer.render).toHaveBeenCalledTimes(2)
+  })
+})
diff --git a/apps/web/src/lib/molecular/render/threeViewer.ts b/apps/web/src/lib/molecular/render/threeViewer.ts
new file mode 100644
index 0000000..6b24422
--- /dev/null
+++ b/apps/web/src/lib/molecular/render/threeViewer.ts
@@ -0,0 +1,159 @@
+/**
+ * Three.js Molecular Structure viewer (Phase 6.12).
+ *
+ * The WebGL implementation behind the `MolecularViewer` contract. It owns the
+ * scene graph, camera, orbit controls, lights, resize handling, animation
+ * loop, and — critically — full resource disposal. The React layer never
+ * touches Three.js directly; it creates one viewer per mounted component and
+ * drives it through the `MolecularViewer` methods.
+ *
+ * WebGL is isolated to `createRenderer` so tests can inject a fake renderer
+ * and exercise the full lifecycle (frame, focus, representation, dispose)
+ * without a GPU.
+ */
+
+import * as THREE from 'three'
+import { OrbitControls } from 'three/examples/jsm/controls/OrbitControls.js'
+
+import type { Point3 } from '@/lib/molecular/geometry'
+import type { RepresentationId } from '@/lib/molecular/representations'
+import type { MolecularStructure } from '@/lib/molecular/types'
+
+import { buildStructureGroup, disposeGroup } from './representationBuilder'
+import type { CreateThreeViewerOptions, MolecularViewer, ThreeRenderer } from './types'
+
+/** Camera-to-structure distance as a multiple of the structure radius. */
+const CAMERA_DISTANCE_FACTOR = 3
+/** Minimum camera distance in angstroms (tiny structures). */
+const MIN_CAMERA_DISTANCE = 2
+/** Default view direction used when the camera has no prior orientation. */
+const DEFAULT_VIEW_DIRECTION = new THREE.Vector3(1, 1, 1).normalize()
+
+function defaultRenderer(): ThreeRenderer {
+  return new THREE.WebGLRenderer({ antialias: true, alpha: true })
+}
+
+/**
+ * Creates a Three.js molecular viewer attached to `container`. The renderer
+ * factory is injectable so unit tests can supply a fake WebGL-free renderer.
+ */
+export function createThreeViewer(
+  container: HTMLElement,
+  options: CreateThreeViewerOptions = {},
+): MolecularViewer {
+  const renderer = options.createRenderer ? options.createRenderer() : defaultRenderer()
+  renderer.setPixelRatio(typeof window !== 'undefined' ? window.devicePixelRatio : 1)
+  renderer.setClearColor(0xffffff, 0)
+  renderer.domElement.style.width = '100%'
+  renderer.domElement.style.height = '100%'
+  renderer.domElement.style.display = 'block'
+  container.appendChild(renderer.domElement)
+
+  const scene = new THREE.Scene()
+  const camera = new THREE.PerspectiveCamera(50, 1, 0.1, 10_000)
+  camera.position.set(
+    DEFAULT_VIEW_DIRECTION.x * 10,
+    DEFAULT_VIEW_DIRECTION.y * 10,
+    DEFAULT_VIEW_DIRECTION.z * 10,
+  )
+
+  const controls = new OrbitControls(camera, renderer.domElement)
+  controls.enableDamping = true
+  controls.dampingFactor = 0.1
+
+  scene.add(new THREE.AmbientLight(0xffffff, 0.7))
+  const keyLight = new THREE.DirectionalLight(0xffffff, 0.9)
+  keyLight.position.set(2, 2, 2)
+  scene.add(keyLight)
+  const fillLight = new THREE.DirectionalLight(0xffffff, 0.4)
+  fillLight.position.set(-2, -1, 1)
+  scene.add(fillLight)
+
+  const structureGroup = new THREE.Group()
+  scene.add(structureGroup)
+
+  let disposed = false
+  let currentStructure: MolecularStructure | undefined
+  let currentRepresentation: RepresentationId | undefined
+  let groupVisible = true
+
+  function renderFrame(): void {
+    controls.update()
+    renderer.render(scene, camera)
+  }
+
+  function applySize(): void {
+    const width = Math.max(container.clientWidth || 1, 1)
+    const height = Math.max(container.clientHeight || 1, 1)
+    camera.aspect = width / height
+    camera.updateProjectionMatrix()
+    renderer.setSize(width, height)
+  }
+
+  applySize()
+  renderer.setAnimationLoop(() => renderFrame())
+
+  let resizeObserver: ResizeObserver | null = null
+  if (typeof ResizeObserver !== 'undefined') {
+    resizeObserver = new ResizeObserver(() => applySize())
+    resizeObserver.observe(container)
+  }
+
+  function rebuild(): void {
+    if (currentStructure === undefined || currentRepresentation === undefined) return
+    disposeGroup(structureGroup)
+    structureGroup.clear()
+    structureGroup.visible = groupVisible
+    structureGroup.add(buildStructureGroup(currentStructure, currentRepresentation))
+  }
+
+  return {
+    setStructure(structure, representation) {
+      currentStructure = structure
+      currentRepresentation = representation
+      rebuild()
+    },
+    setRepresentation(representation) {
+      currentRepresentation = representation
+      rebuild()
+    },
+    setVisible(visible) {
+      groupVisible = visible
+      structureGroup.visible = visible
+    },
+    focusCamera(target: Point3, radius: number) {
+      const center = new THREE.Vector3(target.x, target.y, target.z)
+      const distance = Math.max(radius * CAMERA_DISTANCE_FACTOR, MIN_CAMERA_DISTANCE)
+
+      const offset = new THREE.Vector3().subVectors(camera.position, controls.target)
+      const direction = offset.lengthSq() > 0 ? offset.normalize() : DEFAULT_VIEW_DIRECTION
+
+      camera.position.copy(center).addScaledVector(direction, distance)
+      camera.near = Math.max(radius / 100, 0.001)
+      camera.far = Math.max(radius * 50, 100)
+      camera.updateProjectionMatrix()
+      camera.lookAt(center)
+      controls.target.copy(center)
+      controls.update()
+    },
+    resize(width, height) {
+      camera.aspect = width / height
+      camera.updateProjectionMatrix()
+      renderer.setSize(Math.max(width, 1), Math.max(height, 1))
+    },
+    dispose() {
+      if (disposed) return
+      disposed = true
+      resizeObserver?.disconnect()
+      resizeObserver = null
+      renderer.setAnimationLoop(null)
+      controls.dispose()
+      disposeGroup(structureGroup)
+      structureGroup.clear()
+      renderer.dispose()
+      if (renderer.domElement.parentNode === container) {
+        container.removeChild(renderer.domElement)
+      }
+    },
+  }
+}
diff --git a/apps/web/src/lib/molecular/render/types.ts b/apps/web/src/lib/molecular/render/types.ts
new file mode 100644
index 0000000..6ca11be
--- /dev/null
+++ b/apps/web/src/lib/molecular/render/types.ts
@@ -0,0 +1,51 @@
+/**
+ * Molecular viewer renderer contract (Phase 6.12).
+ *
+ * The React layer talks only to this interface. It decouples the component
+ * and hook from Three.js: tests inject a fake `MolecularViewer` (or fake
+ * renderer) and never need a GPU, while production uses
+ * `createThreeViewer` from `lib/molecular/render/threeViewer.ts`.
+ */
+
+import type { Point3 } from '@/lib/molecular/geometry'
+import type { RepresentationId } from '@/lib/molecular/representations'
+import type { MolecularStructure } from '@/lib/molecular/types'
+
+/** Minimal renderer surface the Three.js viewer relies on. */
+export interface ThreeRenderer {
+  domElement: HTMLCanvasElement
+  setPixelRatio(ratio: number): void
+  setSize(width: number, height: number): void
+  setClearColor(color: number | string, alpha?: number): void
+  setAnimationLoop(callback: ((time: number) => void) | null): void
+  render(scene: unknown, camera: unknown): void
+  dispose(): void
+}
+
+/** Options accepted when creating a Three.js viewer. */
+export interface CreateThreeViewerOptions {
+  /** Injectable renderer factory (defaults to `THREE.WebGLRenderer`). */
+  createRenderer?: () => ThreeRenderer
+}
+
+/** The imperative 3D viewer surface used by the React component. */
+export interface MolecularViewer {
+  /** Builds (or rebuilds) the structure group for a representation. */
+  setStructure(structure: MolecularStructure, representation: RepresentationId): void
+  /** Rebuilds the structure group in another representation. */
+  setRepresentation(representation: RepresentationId): void
+  /** Shows or hides the structure group. */
+  setVisible(visible: boolean): void
+  /** Frames the camera around a target with the given radius (angstroms). */
+  focusCamera(target: Point3, radius: number): void
+  /** Handles an explicit container resize. */
+  resize(width: number, height: number): void
+  /** Disposes renderer, controls, geometries, materials, and listeners. */
+  dispose(): void
+}
+
+/** Factory signature used to inject a viewer into the React component. */
+export type CreateMolecularViewer = (
+  container: HTMLElement,
+  options?: CreateThreeViewerOptions,
+) => MolecularViewer
diff --git a/apps/web/src/lib/molecular/representations.test.ts b/apps/web/src/lib/molecular/representations.test.ts
new file mode 100644
index 0000000..71ffe71
--- /dev/null
+++ b/apps/web/src/lib/molecular/representations.test.ts
@@ -0,0 +1,53 @@
+import { describe, expect, it } from 'vitest'
+
+import type { RepresentationId } from './representations'
+import {
+  DEFAULT_REPRESENTATION,
+  REPRESENTATIONS,
+  isRepresentationId,
+  representationLabel,
+} from './representations'
+
+describe('REPRESENTATIONS', () => {
+  it('covers every supported representation id with labels and descriptions', () => {
+    expect(REPRESENTATIONS.map((option) => option.id)).toEqual([
+      'cartoon',
+      'ball-and-stick',
+      'space-filling',
+    ])
+    for (const option of REPRESENTATIONS) {
+      expect(option.label.length).toBeGreaterThan(0)
+      expect(option.description.length).toBeGreaterThan(0)
+    }
+  })
+
+  it('defaults to the cartoon representation', () => {
+    expect(DEFAULT_REPRESENTATION).toBe('cartoon')
+    expect(REPRESENTATIONS.some((option) => option.id === DEFAULT_REPRESENTATION)).toBe(true)
+  })
+})
+
+describe('isRepresentationId', () => {
+  it('accepts every catalogued id', () => {
+    for (const option of REPRESENTATIONS) {
+      expect(isRepresentationId(option.id)).toBe(true)
+    }
+  })
+
+  it('rejects unknown values', () => {
+    expect(isRepresentationId('ribbon')).toBe(false)
+    expect(isRepresentationId(undefined)).toBe(false)
+    expect(isRepresentationId(42)).toBe(false)
+  })
+})
+
+describe('representationLabel', () => {
+  it('returns the control label for a known id', () => {
+    expect(representationLabel('ball-and-stick')).toBe('Ball and stick')
+    expect(representationLabel('cartoon')).toBe('Cartoon / ribbon')
+  })
+
+  it('falls back to the id itself when unknown', () => {
+    expect(representationLabel('ribbon' as RepresentationId)).toBe('ribbon')
+  })
+})
diff --git a/apps/web/src/lib/molecular/representations.ts b/apps/web/src/lib/molecular/representations.ts
new file mode 100644
index 0000000..2901309
--- /dev/null
+++ b/apps/web/src/lib/molecular/representations.ts
@@ -0,0 +1,54 @@
+/**
+ * Structure representations (Phase 6.12).
+ *
+ * A small, extensible set of renderings the Molecular Structure Viewer can
+ * draw. Representation ids are the extension point: the builder
+ * (`lib/molecular/render/representationBuilder.ts`) switches on the id, and
+ * the control catalog below drives the labelled representation select. New
+ * representations are added by registering an option here and a builder
+ * branch there.
+ */
+
+/** Identifier of a supported structure representation. */
+export type RepresentationId = 'cartoon' | 'ball-and-stick' | 'space-filling'
+
+/** A representation the viewer offers, with its control label. */
+export interface RepresentationOption {
+  id: RepresentationId
+  /** Short label used in the representation select. */
+  label: string
+  /** Longer description for tooltips / documentation. */
+  description: string
+}
+
+/** The full representation catalog, in control order. */
+export const REPRESENTATIONS: readonly RepresentationOption[] = [
+  {
+    id: 'cartoon',
+    label: 'Cartoon / ribbon',
+    description: 'Trace the polymer backbone as a smooth ribbon through the C-alpha trace.',
+  },
+  {
+    id: 'ball-and-stick',
+    label: 'Ball and stick',
+    description: 'Atoms as coloured spheres with covalent bonds drawn between them.',
+  },
+  {
+    id: 'space-filling',
+    label: 'Space filling',
+    description: 'Atoms as van der Waals spheres, so the molecular surface is visible.',
+  },
+]
+
+/** The representation shown when a structure first loads. */
+export const DEFAULT_REPRESENTATION: RepresentationId = 'cartoon'
+
+/** Type guard for a `RepresentationId`. */
+export function isRepresentationId(value: unknown): value is RepresentationId {
+  return value === 'cartoon' || value === 'ball-and-stick' || value === 'space-filling'
+}
+
+/** Short label for a representation id (falls back to the id itself). */
+export function representationLabel(id: RepresentationId): string {
+  return REPRESENTATIONS.find((option) => option.id === id)?.label ?? id
+}
diff --git a/apps/web/src/lib/molecular/types.ts b/apps/web/src/lib/molecular/types.ts
new file mode 100644
index 0000000..a6ad28c
--- /dev/null
+++ b/apps/web/src/lib/molecular/types.ts
@@ -0,0 +1,95 @@
+/**
+ * TypeScript types for the Molecular Structure Viewer (Phase 6.12).
+ *
+ * ## Coordinate conventions
+ *
+ * Atoms carry **Cartesian coordinates in angstroms**. Atom serials, residue
+ * numbers, and bond endpoints are **one-based**, mirroring the 1-based
+ * residue convention of the Phase 6.5 Protein Viewer. A residue's
+ * `atomIndices` refer to 1-based atom serials; a bond's `atomA`/`atomB`
+ * refer to the same serials.
+ *
+ * ## Data boundary
+ *
+ * These types are the canonical structure model the viewer renders. They are
+ * deliberately independent of any particular file format or backend: the
+ * loader boundary (`lib/molecular/api.ts`) normalizes whatever a future
+ * source returns into this model, so swapping data sources never touches the
+ * viewer or its representations.
+ */
+
+/** Free-form metadata carried by a structure without extra types. */
+export type StructureMetadata = Record<string, string | number | boolean>
+
+/** Molecule class used for default presentation. */
+export type StructureKind = 'protein' | 'nucleic-acid' | 'other'
+
+/** A single atom with its 3D position and residue context. */
+export interface StructureAtom {
+  /** 1-based atom serial; bonds and residues reference this. */
+  index: number
+  /** Chemical element symbol, uppercase (e.g. `C`, `N`, `O`, `S`). */
+  element: string
+  /** Cartesian coordinates in angstroms. */
+  x: number
+  y: number
+  z: number
+  /** 1-based residue number within the chain. */
+  residueIndex: number
+  /** Chain identifier (opaque string). */
+  chainId: string
+  /** Residue / group name, e.g. `ALA`. */
+  residueName?: string
+  /** Atom name, e.g. `CA`, `N`, `C`, `O`. */
+  atomName?: string
+}
+
+/** A covalent bond between two atoms. */
+export interface StructureBond {
+  /** 1-based serial of the first atom. */
+  atomA: number
+  /** 1-based serial of the second atom. */
+  atomB: number
+  /** Bond order (single/double/triple); defaults to `1`. */
+  order?: number
+}
+
+/** A residue (or other group) belonging to a chain. */
+export interface StructureResidue {
+  /** 1-based residue number within the chain. */
+  index: number
+  /** Residue / group name, e.g. `ALA`. */
+  name?: string
+  /** 1-based atom serials belonging to this residue. */
+  atomIndices: number[]
+}
+
+/** A polymer chain made of ordered residues. */
+export interface StructureChain {
+  /** Chain identifier (opaque string). */
+  id: string
+  /** Residues in chain order. */
+  residues: StructureResidue[]
+}
+
+/** The canonical molecular structure model rendered by the viewer. */
+export interface MolecularStructure {
+  /** Stable structure identifier (e.g. a PDB accession or future API id). */
+  id: string
+  /** Molecule name, e.g. `p53 DNA-binding domain`. */
+  name?: string
+  /** Molecule class used for default presentation. */
+  kind?: StructureKind
+  /** Organism, when known. */
+  organism?: string
+  /** Free-text description / function. */
+  description?: string
+  /** Polymer chains in structure order. */
+  chains: StructureChain[]
+  /** All atoms. */
+  atoms: StructureAtom[]
+  /** Covalent bonds, when available. */
+  bonds: StructureBond[]
+  /** Optional free-form metadata. */
+  metadata?: StructureMetadata
+}
diff --git a/apps/web/src/lib/molecular/useMolecularStructureViewer.test.tsx b/apps/web/src/lib/molecular/useMolecularStructureViewer.test.tsx
new file mode 100644
index 0000000..06fa574
--- /dev/null
+++ b/apps/web/src/lib/molecular/useMolecularStructureViewer.test.tsx
@@ -0,0 +1,123 @@
+import { act, cleanup, render, screen, waitFor } from '@testing-library/react'
+import { afterEach, describe, expect, it, vi } from 'vitest'
+
+import { P53_HELIX_STRUCTURE_FIXTURE } from './molecular.fixtures'
+import { useMolecularStructureViewer } from './useMolecularStructureViewer'
+
+function Harness({
+  onResult,
+  options = {},
+}: {
+  onResult: (result: ReturnType<typeof useMolecularStructureViewer>) => void
+  options?: Parameters<typeof useMolecularStructureViewer>[0]
+}) {
+  const result = useMolecularStructureViewer(options)
+  onResult(result)
+  return <output data-testid="status">{result.status}</output>
+}
+
+function renderHook(options: Parameters<typeof useMolecularStructureViewer>[0] = {}) {
+  let result: ReturnType<typeof useMolecularStructureViewer> | undefined
+  render(
+    <Harness
+      options={options}
+      onResult={(next) => {
+        result = next
+      }}
+    />,
+  )
+  return {
+    get result(): ReturnType<typeof useMolecularStructureViewer> {
+      if (result === undefined) {
+        throw new Error('useMolecularStructureViewer did not capture a result')
+      }
+      return result
+    },
+  }
+}
+
+afterEach(() => {
+  cleanup()
+  vi.restoreAllMocks()
+})
+
+describe('useMolecularStructureViewer', () => {
+  it('loads a structure and reports success with a summary', async () => {
+    const loader = vi.fn(async () => P53_HELIX_STRUCTURE_FIXTURE)
+    const captured = renderHook({ loader })
+    await waitFor(() => expect(screen.getByTestId('status').textContent).toBe('success'))
+    expect(loader).toHaveBeenCalledTimes(1)
+    expect(captured.result.structure?.id).toBe('fixture-mini-p53-helix')
+    expect(captured.result.summary?.chains).toBe(1)
+    expect(captured.result.summary?.atoms).toBeGreaterThan(0)
+  })
+
+  it('reports empty for a structure with no atoms', async () => {
+    const loader = vi.fn(async () => ({ id: 'empty', chains: [], atoms: [], bonds: [] }))
+    const captured = renderHook({ loader })
+    await waitFor(() => expect(screen.getByTestId('status').textContent).toBe('empty'))
+    expect(captured.result.structure).toBeDefined()
+    expect(captured.result.summary).toBeUndefined()
+  })
+
+  it('reports error when the loader rejects and refetch retries', async () => {
+    const loader = vi
+      .fn()
+      .mockRejectedValueOnce(new Error('structure down'))
+      .mockResolvedValueOnce(P53_HELIX_STRUCTURE_FIXTURE)
+    const captured = renderHook({ loader })
+    await waitFor(() => expect(screen.getByTestId('status').textContent).toBe('error'))
+    expect(captured.result.error?.message).toBe('structure down')
+
+    captured.result.refetch()
+    await waitFor(() => expect(screen.getByTestId('status').textContent).toBe('success'))
+    expect(loader).toHaveBeenCalledTimes(2)
+  })
+
+  it('defaults to the cartoon representation and updates it', async () => {
+    const loader = vi.fn(async () => P53_HELIX_STRUCTURE_FIXTURE)
+    const captured = renderHook({ loader })
+    await waitFor(() => expect(screen.getByTestId('status').textContent).toBe('success'))
+    expect(captured.result.representation).toBe('cartoon')
+
+    act(() => captured.result.setRepresentation('space-filling'))
+    await waitFor(() => expect(captured.result.representation).toBe('space-filling'))
+  })
+
+  it('tracks visibility, defaulting to visible', async () => {
+    const loader = vi.fn(async () => P53_HELIX_STRUCTURE_FIXTURE)
+    const captured = renderHook({ loader })
+    await waitFor(() => expect(screen.getByTestId('status').textContent).toBe('success'))
+    expect(captured.result.visible).toBe(true)
+
+    act(() => captured.result.setVisible(false))
+    await waitFor(() => expect(captured.result.visible).toBe(false))
+  })
+
+  it('frames the camera around the structure centroid on load', async () => {
+    const loader = vi.fn(async () => P53_HELIX_STRUCTURE_FIXTURE)
+    const captured = renderHook({ loader })
+    await waitFor(() => expect(screen.getByTestId('status').textContent).toBe('success'))
+    expect(captured.result.focus).toBeDefined()
+    expect(captured.result.focus?.radius).toBeGreaterThanOrEqual(1)
+  })
+
+  it('bumps the focus version on reset and fit', async () => {
+    const loader = vi.fn(async () => P53_HELIX_STRUCTURE_FIXTURE)
+    const captured = renderHook({ loader })
+    await waitFor(() => expect(screen.getByTestId('status').textContent).toBe('success'))
+    const initial = captured.result.focus?.version ?? 0
+
+    act(() => captured.result.resetView())
+    await waitFor(() => expect(captured.result.focus?.version).toBe(initial + 1))
+
+    act(() => captured.result.fitToView())
+    await waitFor(() => expect(captured.result.focus?.version).toBe(initial + 2))
+  })
+
+  it('reports an error when no loader or structure id is provided', async () => {
+    const captured = renderHook()
+    await waitFor(() => expect(screen.getByTestId('status').textContent).toBe('error'))
+    expect(captured.result.error?.message).toMatch(/No structure loader provided/)
+  })
+})
diff --git a/apps/web/src/lib/molecular/useMolecularStructureViewer.ts b/apps/web/src/lib/molecular/useMolecularStructureViewer.ts
new file mode 100644
index 0000000..2b3dd9d
--- /dev/null
+++ b/apps/web/src/lib/molecular/useMolecularStructureViewer.ts
@@ -0,0 +1,132 @@
+/**
+ * Molecular Structure Viewer view-model hook (Phase 6.12).
+ *
+ * Composes the Phase 6.1 visualization data lifecycle
+ * (`useVisualizationData`) with representation selection, show/hide state,
+ * and camera framing. The hook owns only **data and intent** — the 3D camera
+ * lives in the renderer, so `focus` is a plain value
+ * (`{ target, radius, version }`) the component applies to its viewer
+ * whenever it changes (on load or on reset/fit).
+ */
+
+import { useCallback, useMemo, useState } from 'react'
+
+import { fetchMolecularStructure } from '@/lib/molecular/api'
+import { cameraFocusForStructure, structureSummary } from '@/lib/molecular/geometry'
+import { DEFAULT_REPRESENTATION, type RepresentationId } from '@/lib/molecular/representations'
+import type { MolecularStructure } from '@/lib/molecular/types'
+import { isUsableStructure } from '@/lib/molecular/validate'
+import type { VisualizationError, VisualizationStatus } from '@/lib/visualization/types'
+import { useVisualizationData } from '@/lib/visualization/useVisualizationData'
+
+/** Camera framing intent handed to the 3D viewer. */
+export interface StructureFocus {
+  target: { x: number; y: number; z: number }
+  radius: number
+  /** Bumped on every explicit reset/fit so the component re-frames. */
+  version: number
+}
+
+/** Human-readable structure summary for status lines and the canvas label. */
+export interface MolecularStructureSummary {
+  name: string
+  chains: number
+  residues: number
+  atoms: number
+  bonds: number
+}
+
+/** Result shape consumed by `MolecularStructureViewer`. */
+export interface MolecularStructureViewerResult {
+  status: VisualizationStatus
+  error: VisualizationError | undefined
+  /** Re-runs the structure load request. */
+  refetch: () => void
+  /** Loaded structure, or `undefined` until success. */
+  structure: MolecularStructure | undefined
+  /** Derived human-readable summary of the loaded structure. */
+  summary: MolecularStructureSummary | undefined
+  /** Active representation. */
+  representation: RepresentationId
+  setRepresentation: (representation: RepresentationId) => void
+  /** Whether the structure group is visible. */
+  visible: boolean
+  setVisible: (visible: boolean) => void
+  /** Current camera framing intent (changes on load and on reset/fit). */
+  focus: StructureFocus | undefined
+  /** Re-frames the camera to fit the structure. */
+  resetView: () => void
+  /** Same as `resetView`; provided as the conventional "fit" control. */
+  fitToView: () => void
+}
+
+export interface UseMolecularStructureViewerOptions {
+  /** Loads a structure (defaults to nothing; a loader or id is required). */
+  loader?: (signal: AbortSignal) => Promise<MolecularStructure>
+  /** Future backend structure id used when no custom loader is provided. */
+  structureId?: string
+}
+
+export function useMolecularStructureViewer(
+  options: UseMolecularStructureViewerOptions = {},
+): MolecularStructureViewerResult {
+  const { loader: customLoader, structureId } = options
+
+  const loader = useCallback(
+    (signal: AbortSignal) => {
+      if (customLoader !== undefined) return customLoader(signal)
+      if (structureId !== undefined) {
+        return fetchMolecularStructure(structureId, signal)
+      }
+      return Promise.reject(
+        new Error('No structure loader provided to useMolecularStructureViewer.'),
+      )
+    },
+    [customLoader, structureId],
+  )
+
+  const { status, data, error, refetch } = useVisualizationData<MolecularStructure>(loader, {
+    isEmpty: (structure) => structure.atoms.length === 0,
+  })
+
+  const structure = data
+  const [representation, setRepresentation] = useState<RepresentationId>(DEFAULT_REPRESENTATION)
+  const [visible, setVisible] = useState(true)
+  const [focusVersion, setFocusVersion] = useState(0)
+
+  const summary = useMemo<MolecularStructureSummary | undefined>(() => {
+    if (structure === undefined || !isUsableStructure(structure)) return undefined
+    const computed = structureSummary(structure)
+    return {
+      name: computed.name,
+      chains: computed.chains,
+      residues: computed.residues,
+      atoms: computed.atoms,
+      bonds: computed.bonds,
+    }
+  }, [structure])
+
+  const focus = useMemo<StructureFocus | undefined>(() => {
+    if (structure === undefined || !isUsableStructure(structure)) return undefined
+    const computed = cameraFocusForStructure(structure)
+    return { target: computed.target, radius: computed.radius, version: focusVersion }
+  }, [structure, focusVersion])
+
+  const resetView = useCallback(() => setFocusVersion((version) => version + 1), [])
+  const fitToView = useCallback(() => setFocusVersion((version) => version + 1), [])
+
+  return {
+    status,
+    error,
+    refetch,
+    structure,
+    summary,
+    representation,
+    setRepresentation,
+    visible,
+    setVisible,
+    focus,
+    resetView,
+    fitToView,
+  }
+}
diff --git a/apps/web/src/lib/molecular/validate.test.ts b/apps/web/src/lib/molecular/validate.test.ts
new file mode 100644
index 0000000..1e2abad
--- /dev/null
+++ b/apps/web/src/lib/molecular/validate.test.ts
@@ -0,0 +1,200 @@
+import { describe, expect, it } from 'vitest'
+
+import type { MolecularStructure } from './types'
+import {
+  firstStructureError,
+  isUsableStructure,
+  isValidStructure,
+  validateStructure,
+} from './validate'
+
+function validStructure(): MolecularStructure {
+  return {
+    id: 'test-1',
+    chains: [{ id: 'A', residues: [{ index: 1, atomIndices: [1, 2] }] }],
+    atoms: [
+      { index: 1, element: 'C', x: 0, y: 0, z: 0, residueIndex: 1, chainId: 'A' },
+      { index: 2, element: 'N', x: 1, y: 1, z: 1, residueIndex: 1, chainId: 'A' },
+    ],
+    bonds: [{ atomA: 1, atomB: 2 }],
+  }
+}
+
+function codes(structure: MolecularStructure): string[] {
+  return validateStructure(structure).map((issue) => issue.code)
+}
+
+describe('validateStructure', () => {
+  it('accepts a structurally consistent structure', () => {
+    expect(isValidStructure(validStructure())).toBe(true)
+    expect(validateStructure(validStructure())).toEqual([])
+  })
+
+  it('rejects a missing structure id', () => {
+    expect(codes({ ...validStructure(), id: '' })).toContain('structure.missing-id')
+  })
+
+  it('rejects an empty structure', () => {
+    const issues = codes({ ...validStructure(), atoms: [], bonds: [] })
+    expect(issues).toContain('structure.no-atoms')
+  })
+
+  it('rejects non-positive and non-integer atom serials', () => {
+    expect(
+      codes({ ...validStructure(), atoms: [{ ...validStructure().atoms[0], index: 0 }] }),
+    ).toContain('atom.invalid-index')
+    expect(
+      codes({ ...validStructure(), atoms: [{ ...validStructure().atoms[0], index: 1.5 }] }),
+    ).toContain('atom.invalid-index')
+  })
+
+  it('rejects duplicate atom serials', () => {
+    const duplicate = {
+      ...validStructure(),
+      atoms: [
+        { ...validStructure().atoms[0], index: 1 },
+        { ...validStructure().atoms[1], index: 1 },
+      ],
+    }
+    expect(codes(duplicate)).toContain('atom.duplicate-index')
+  })
+
+  it('rejects non-finite coordinates', () => {
+    const structure = {
+      ...validStructure(),
+      atoms: [{ ...validStructure().atoms[0], x: Number.NaN }],
+    }
+    expect(codes(structure)).toContain('atom.non-finite-coordinates')
+  })
+
+  it('rejects an invalid residue number on an atom', () => {
+    const structure = {
+      ...validStructure(),
+      atoms: [{ ...validStructure().atoms[0], residueIndex: 0 }],
+    }
+    expect(codes(structure)).toContain('atom.invalid-residue')
+  })
+
+  it('rejects atoms without an element', () => {
+    const structure = {
+      ...validStructure(),
+      atoms: [{ ...validStructure().atoms[0], element: '' }],
+    }
+    expect(codes(structure)).toContain('atom.missing-element')
+  })
+
+  it('rejects a bond referencing a missing atom', () => {
+    const structure = {
+      ...validStructure(),
+      bonds: [{ atomA: 1, atomB: 999 }],
+    }
+    expect(codes(structure)).toContain('bond.dangling')
+  })
+
+  it('rejects a bond that joins an atom to itself', () => {
+    const structure = {
+      ...validStructure(),
+      bonds: [{ atomA: 1, atomB: 1 }],
+    }
+    expect(codes(structure)).toContain('bond.self-loop')
+  })
+
+  it('rejects duplicate bonds regardless of endpoint order', () => {
+    const structure = {
+      ...validStructure(),
+      bonds: [
+        { atomA: 1, atomB: 2 },
+        { atomA: 2, atomB: 1 },
+      ],
+    }
+    expect(codes(structure)).toContain('bond.duplicate')
+  })
+
+  it('rejects a chain without an identifier', () => {
+    const structure = { ...validStructure(), chains: [{ ...validStructure().chains[0], id: '' }] }
+    expect(codes(structure)).toContain('chain.missing-id')
+  })
+
+  it('rejects an invalid residue number', () => {
+    const structure = {
+      ...validStructure(),
+      chains: [{ ...validStructure().chains[0], residues: [{ index: 0, atomIndices: [1, 2] }] }],
+    }
+    expect(codes(structure)).toContain('residue.invalid-index')
+  })
+
+  it('rejects duplicate residue numbers within a chain', () => {
+    const chain = validStructure().chains[0]
+    const structure = {
+      ...validStructure(),
+      chains: [
+        {
+          id: chain.id,
+          residues: [
+            { index: 1, atomIndices: [1] },
+            { index: 1, atomIndices: [2] },
+          ],
+        },
+      ],
+    }
+    expect(codes(structure)).toContain('residue.duplicate-index')
+  })
+
+  it('rejects a residue referencing a missing atom', () => {
+    const structure = {
+      ...validStructure(),
+      chains: [{ ...validStructure().chains[0], residues: [{ index: 1, atomIndices: [999] }] }],
+    }
+    expect(codes(structure)).toContain('residue.dangling-atom')
+  })
+
+  it('rejects atoms that belong to no residue', () => {
+    const structure = {
+      ...validStructure(),
+      chains: [{ ...validStructure().chains[0], residues: [{ index: 1, atomIndices: [1] }] }],
+    }
+    expect(codes(structure)).toContain('atom.unreferenced')
+  })
+
+  it('reports every issue found, not just the first', () => {
+    const structure: MolecularStructure = {
+      id: '',
+      chains: [],
+      atoms: [{ index: 0, element: '', x: Number.NaN, y: 0, z: 0, residueIndex: 0, chainId: '' }],
+      bonds: [],
+    }
+    const found = codes(structure)
+    expect(found).toEqual(
+      expect.arrayContaining([
+        'structure.missing-id',
+        'atom.invalid-index',
+        'atom.non-finite-coordinates',
+        'atom.invalid-residue',
+        'atom.missing-element',
+        'atom.unreferenced',
+      ]),
+    )
+  })
+})
+
+describe('isUsableStructure and firstStructureError', () => {
+  it('is usable for a valid non-empty structure', () => {
+    expect(isUsableStructure(validStructure())).toBe(true)
+  })
+
+  it('is not usable for an empty structure', () => {
+    expect(isUsableStructure({ ...validStructure(), atoms: [], bonds: [] })).toBe(false)
+  })
+
+  it('is not usable for a malformed structure', () => {
+    expect(isUsableStructure({ ...validStructure(), bonds: [{ atomA: 1, atomB: 999 }] })).toBe(
+      false,
+    )
+  })
+
+  it('returns the first issue message or null', () => {
+    expect(firstStructureError(validStructure())).toBeNull()
+    const message = firstStructureError({ ...validStructure(), id: '' })
+    expect(message).toBe('Structure has no identifier.')
+  })
+})
diff --git a/apps/web/src/lib/molecular/validate.ts b/apps/web/src/lib/molecular/validate.ts
new file mode 100644
index 0000000..3ba2072
--- /dev/null
+++ b/apps/web/src/lib/molecular/validate.ts
@@ -0,0 +1,170 @@
+/**
+ * Structure validation (Phase 6.12).
+ *
+ * Pure validation over the canonical `MolecularStructure` model. It reports
+ * structural issues that would break rendering or lie about the data:
+ * non-finite / missing coordinates, broken atom serials, bonds or residue
+ * references that point nowhere, self-bonds, and empty structures. The viewer
+ * uses `isUsableStructure` to decide whether a loaded structure can render;
+ * malformed records are still typed and testable rather than silently fixed.
+ */
+
+import type { MolecularStructure, StructureAtom, StructureBond } from './types'
+
+/** A single validation finding. */
+export interface StructureValidationIssue {
+  /** Stable machine-readable code, e.g. `atom.non-finite-coordinates`. */
+  code: string
+  /** Human-readable description of the problem. */
+  message: string
+}
+
+const MAX_ATOM_INDEX = 1_000_000
+
+/** True when a value is a positive integer usable as an atom serial. */
+function isPositiveInteger(value: number): boolean {
+  return Number.isInteger(value) && value >= 1 && value <= MAX_ATOM_INDEX
+}
+
+function isFinitePoint(atom: StructureAtom): boolean {
+  return Number.isFinite(atom.x) && Number.isFinite(atom.y) && Number.isFinite(atom.z)
+}
+
+function residueAtomIndices(structure: MolecularStructure): number[] {
+  return structure.chains.flatMap((chain) =>
+    chain.residues.flatMap((residue) => residue.atomIndices),
+  )
+}
+
+/**
+ * Validates a structure and returns every issue found. An empty array means
+ * the structure is structurally consistent and renderable.
+ */
+export function validateStructure(structure: MolecularStructure): StructureValidationIssue[] {
+  const issues: StructureValidationIssue[] = []
+
+  if (structure.id.length === 0) {
+    issues.push({ code: 'structure.missing-id', message: 'Structure has no identifier.' })
+  }
+  if (structure.atoms.length === 0) {
+    issues.push({ code: 'structure.no-atoms', message: 'Structure has no atoms to render.' })
+  }
+
+  const atomByIndex = new Map<number, StructureAtom>()
+  for (const atom of structure.atoms) {
+    if (!isPositiveInteger(atom.index)) {
+      issues.push({
+        code: 'atom.invalid-index',
+        message: `Atom has an invalid serial ${atom.index}.`,
+      })
+    } else if (atomByIndex.has(atom.index)) {
+      issues.push({
+        code: 'atom.duplicate-index',
+        message: `Atom serial ${atom.index} appears more than once.`,
+      })
+    } else {
+      atomByIndex.set(atom.index, atom)
+    }
+    if (!isFinitePoint(atom)) {
+      issues.push({
+        code: 'atom.non-finite-coordinates',
+        message: `Atom ${atom.index} has a non-finite coordinate.`,
+      })
+    }
+    if (!isPositiveInteger(atom.residueIndex)) {
+      issues.push({
+        code: 'atom.invalid-residue',
+        message: `Atom ${atom.index} has an invalid residue number ${atom.residueIndex}.`,
+      })
+    }
+    if (atom.element.length === 0) {
+      issues.push({ code: 'atom.missing-element', message: `Atom ${atom.index} has no element.` })
+    }
+  }
+
+  const seenBonds = new Set<string>()
+  for (const bond of structure.bonds) {
+    if (!atomByIndex.has(bond.atomA) || !atomByIndex.has(bond.atomB)) {
+      issues.push({
+        code: 'bond.dangling',
+        message: `Bond ${bond.atomA}-${bond.atomB} references a missing atom.`,
+      })
+    }
+    if (bond.atomA === bond.atomB) {
+      issues.push({
+        code: 'bond.self-loop',
+        message: `Bond ${bond.atomA}-${bond.atomA} joins an atom to itself.`,
+      })
+    }
+    const key =
+      bond.atomA < bond.atomB ? `${bond.atomA}-${bond.atomB}` : `${bond.atomB}-${bond.atomA}`
+    if (seenBonds.has(key)) {
+      issues.push({
+        code: 'bond.duplicate',
+        message: `Bond ${key} appears more than once.`,
+      })
+    }
+    seenBonds.add(key)
+  }
+
+  for (const chain of structure.chains) {
+    if (chain.id.length === 0) {
+      issues.push({ code: 'chain.missing-id', message: 'A chain has no identifier.' })
+    }
+    const residueNumbers = new Set<number>()
+    for (const residue of chain.residues) {
+      if (!isPositiveInteger(residue.index)) {
+        issues.push({
+          code: 'residue.invalid-index',
+          message: `Chain ${chain.id || '?'} has a residue with an invalid number.`,
+        })
+      } else if (residueNumbers.has(residue.index)) {
+        issues.push({
+          code: 'residue.duplicate-index',
+          message: `Chain ${chain.id || '?'} has residue ${residue.index} more than once.`,
+        })
+      }
+      residueNumbers.add(residue.index)
+      for (const atomIndex of residue.atomIndices) {
+        if (!atomByIndex.has(atomIndex)) {
+          issues.push({
+            code: 'residue.dangling-atom',
+            message: `Residue ${residue.index} of chain ${chain.id || '?'} references missing atom ${atomIndex}.`,
+          })
+        }
+      }
+    }
+  }
+
+  const referencedByResidues = new Set(residueAtomIndices(structure))
+  for (const atom of structure.atoms) {
+    if (!referencedByResidues.has(atom.index)) {
+      issues.push({
+        code: 'atom.unreferenced',
+        message: `Atom ${atom.index} belongs to no residue.`,
+      })
+    }
+  }
+
+  return issues
+}
+
+/** True when a structure has no validation issues at all. */
+export function isValidStructure(structure: MolecularStructure): boolean {
+  return validateStructure(structure).length === 0
+}
+
+/**
+ * True when a structure can actually be rendered: it must pass validation and
+ * contain at least one atom. `empty` structures are handled by the data
+ * lifecycle (`isEmpty`), not by rendering.
+ */
+export function isUsableStructure(structure: MolecularStructure): boolean {
+  return structure.atoms.length > 0 && isValidStructure(structure)
+}
+
+/** The first issue's message, or `null` when the structure is valid. */
+export function firstStructureError(structure: MolecularStructure): string | null {
+  const issues = validateStructure(structure)
+  return issues.length > 0 ? issues[0].message : null
+}
diff --git a/apps/web/src/lib/visualization/visualizationModules.test.ts b/apps/web/src/lib/visualization/visualizationModules.test.ts
index 633801b..6122347 100644
--- a/apps/web/src/lib/visualization/visualizationModules.test.ts
+++ b/apps/web/src/lib/visualization/visualizationModules.test.ts
@@ -21,6 +21,7 @@ describe('fetchVisualizationModules', () => {
       'integrated-research-workspace',
       'performance-large-datasets',
       'testing-documentation',
+      'molecular-structure-viewer',
     ])
     for (const module of modules) {
       expect(module.title).toBeTruthy()
@@ -35,7 +36,7 @@ describe('fetchVisualizationModules', () => {
     const second = await fetchVisualizationModules(signal, { delayMs: 0 })
     expect(first).not.toBe(second)
     first.pop()
-    expect((await fetchVisualizationModules(signal, { delayMs: 0 })).length).toBe(10)
+    expect((await fetchVisualizationModules(signal, { delayMs: 0 })).length).toBe(11)
   })
 
   it('honours the simulated latency before resolving', async () => {
diff --git a/apps/web/src/lib/visualization/visualizationModules.ts b/apps/web/src/lib/visualization/visualizationModules.ts
index 914db8a..78b4d4f 100644
--- a/apps/web/src/lib/visualization/visualizationModules.ts
+++ b/apps/web/src/lib/visualization/visualizationModules.ts
@@ -4,7 +4,7 @@ import type { VisualizationDataSource, VisualizationMetadata } from './types'
  * Catalog of the visualization modules delivered across Phase 6.
  *
  * This started as a placeholder catalog for the Phase 6.1 foundation-only
- * demo. It now reflects the modules actually implemented in Phase 6.2–6.11, so
+ * demo. It now reflects the modules actually implemented in Phase 6.2–6.12, so
  * the demo catalog is an accurate map of the platform.
  */
 export interface VisualizationModule extends VisualizationMetadata {
@@ -90,6 +90,14 @@ const MODULES: readonly VisualizationModule[] = [
     milestone: '6.11',
     source: { kind: 'api', reference: '/api/visualization/testing' },
   },
+  {
+    id: 'molecular-structure-viewer',
+    title: 'Molecular Structure Viewer',
+    description:
+      'Interactive 3D molecular structure rendering: cartoon, ball-and-stick, and space-filling representations over a synthetic development fixture.',
+    milestone: '6.12',
+    source: { kind: 'api', reference: '/api/visualization/structures' },
+  },
 ]
 
 export interface FetchVisualizationModulesOptions {
diff --git a/docs/visualization/README.md b/docs/visualization/README.md
index 435140e..89e7378 100644
--- a/docs/visualization/README.md
+++ b/docs/visualization/README.md
@@ -28,6 +28,16 @@ accessibility and component tests for previously untested chart primitives and
 workspace panels, and reconciles the docs with the shipped platform. See
 [Testing](./testing.md) and the [Roadmap](./roadmap.md).
 
+**Phase 6.12 — Molecular Structure Viewer** is implemented: a first
+production-quality 3D molecular structure viewer. It renders structures
+through Three.js behind a pure `MolecularViewer` seam (cartoon / ribbon,
+ball-and-stick, and space-filling representations), with orbit/zoom/pan
+camera controls, reset/fit framing, show/hide, a labelled select and
+`aria-pressed` controls, and full GPU/geometry disposal. The backend exposes
+no structure endpoint yet, so the demo renders a clearly isolated synthetic
+development fixture through the same typed normalizer the future structure
+adapter will use. See [Molecular Structure](./molecular-structure.md).
+
 | Milestone | Description | Status |
 |-----------|-------------|--------|
 | 6.1 | Visualization Foundation | ✅ Implemented |
@@ -41,6 +51,7 @@ workspace panels, and reconciles the docs with the shipped platform. See
 | 6.9 | Integrated Research Workspace | ✅ Implemented |
 | 6.10 | Visualization Performance & Optimization | ✅ Implemented |
 | 6.11 | Visualization Testing & Documentation | ✅ Implemented |
+| 6.12 | Molecular Structure Viewer | ✅ Implemented |
 
 ## What Phase 6.1 Provides
 
@@ -239,6 +250,43 @@ workspace panels, and reconciles the docs with the shipped platform. See
 - The web suite grows from 725 to 813 tests with no flaky or timing-based
   benchmarks.
 
+## What Phase 6.12 Provides
+
+- A **Molecular Structure Viewer** (see
+  [Molecular Structure](./molecular-structure.md)): interactive 3D rendering of
+  a molecular structure — orbit, zoom, and pan the camera; switch between
+  cartoon / ribbon, ball-and-stick, and space-filling representations; reset
+  or fit the view; and show/hide the structure.
+- A **typed canonical structure model** (`lib/molecular/types.ts`) with
+  angstrom coordinates, 1-based atom serials and residue numbers (mirroring the
+  Phase 6.5 protein residue convention), chains/residues/bonds, and a pure
+  validator (`lib/molecular/validate.ts`) that reports structural issues
+  (missing ids, dangling bonds, duplicate serials, non-finite coordinates)
+  instead of silently rendering malformed data.
+- Pure, unit-tested geometry (`lib/molecular/geometry.ts`): CPK element
+  presentation, bounding boxes, centroids, camera framing, backbone traces,
+  and structure summaries — shared by the viewer and its tests.
+- **Three.js** behind a pure seam: the React layer talks only to a
+  `MolecularViewer` interface (`lib/molecular/render/types.ts`); the WebGL
+  implementation (`render/threeViewer.ts`) owns the scene, camera, orbit
+  controls, resize handling, and full disposal, with an injectable renderer so
+  tests never need a GPU. The object-graph builder
+  (`render/representationBuilder.ts`) creates only Three.js objects and is
+  unit-tested in jsdom.
+- A **data boundary**: the backend exposes no structure endpoint yet, so
+  `lib/molecular/api.ts` documents the future `GET /structures/{id}` contract
+  and normalizes whatever a source returns via `toStructure`; the demo renders
+  a clearly isolated, deterministically generated synthetic development
+  fixture routed through the same normalizer as production.
+- A reusable `useMolecularStructureViewer` hook and `MolecularStructureViewer`
+  component (`/visualization/molecular-structure`), with an accessible canvas
+  (`role="img"` + `aria-label`), labelled controls, a live structure summary,
+  and the shared `VisualizationContainer` lifecycle.
+- 84 new tests across the molecular module (validation, geometry,
+  representations, API normalization, hook lifecycle, component lifecycle with
+  an injected fake viewer, and Three.js lifecycle with a fake renderer), plus
+  the catalog entry and data-contract updates for the module catalog.
+
 ## Documents
 
 | Document | Description |
@@ -254,17 +302,21 @@ workspace panels, and reconciles the docs with the shipped platform. See
 | [Research Workspace](workspace.md) | Phase 6.9 Integrated Research Workspace: context, panels, state flow, API usage, fixture boundary, a11y, tests |
 | [Performance](performance.md) | Phase 6.10 Visualization performance & large-dataset handling: data flow, strategies, downsampling limitations, a11y, testing |
 | [Testing](testing.md) | Phase 6.11 Testing & documentation pass: coverage map, test conventions, and how to run the suite |
+| [Molecular Structure](molecular-structure.md) | Phase 6.12 Molecular Structure Viewer: 3D representations, Three.js design decision, data model, API, fixture boundary, a11y, tests |
 | [Roadmap](roadmap.md) | Detailed phase tracking and future work |
 
 ## Technology Notes
 
-The visualization platform is intentionally lightweight. It uses only the
-existing web stack (React, TypeScript, Tailwind CSS) plus SVG layout
-primitives. No C++, WebAssembly, WebGPU, Three.js, Cytoscape.js, or D3.js is
-used — those are introduced only when the milestone that actually requires
-them arrives:
+The visualization platform is intentionally lightweight. It uses the existing
+web stack (React, TypeScript, Tailwind CSS) plus SVG layout primitives, and —
+for Phase 6.12's 3D molecular structure milestone only — **Three.js** (runtime
+`three` + `@types/three`). No C++, WebAssembly, WebGPU, Cytoscape.js, or D3.js
+is used; those remain deferred until a milestone actually requires them:
 
-- Three.js → a future 3D molecular structure milestone
+- Three.js → Phase 6.12 uses Three.js directly (not a molecular-viewer
+  library such as NGL or bio3d-viewer) behind a pure `MolecularViewer` seam;
+  see [molecular-structure.md](./molecular-structure.md) for the design
+  decision
 - Cytoscape.js → deferred: Phase 6.6 ships a deterministic pure-SVG layout
   behind the `createLayout` seam instead (see
   [network-viewer.md](./network-viewer.md)); Cytoscape.js remains available
diff --git a/docs/visualization/architecture.md b/docs/visualization/architecture.md
index 79ae639..9504b38 100644
--- a/docs/visualization/architecture.md
+++ b/docs/visualization/architecture.md
@@ -42,6 +42,7 @@ apps/web/src/
 │   ├── scientific/               ExpressionChart, Heatmap, VolcanoPlot,
 │   │                             CoverageChart, DistributionChart, ChartAxes,
 │   │                             ChartLegend, ChartTooltip
+│   ├── molecular/                MolecularStructureViewer (Three.js-backed)
 │   └── workspace/                ResearchWorkspace + panels + fixture data source
 ├── lib/
 │   ├── visualization/            types, useVisualizationData, module catalog
@@ -51,11 +52,15 @@ apps/web/src/
 │   ├── network/                  types, model/normalize/filter/layout/viewport, API
 │   ├── scientific/               chart types/scales/geometry, statistics, downsample,
 │   │                             heatmap/volcano/coverage/distribution, hooks, API adapters
+│   ├── molecular/                structure types/validation/geometry, representations,
+│   │                             useMolecularStructureViewer, render (Three.js seam), API
 │   └── workspace/                researchContext, dataSources
 └── app/visualization/
     ├── page.tsx                  Route (/visualization) — demo catalog
     ├── VisualizationDemo.tsx     Client demo proving the architecture
-    └── workspace/page.tsx        Research Workspace route (/visualization/workspace)
+    ├── workspace/page.tsx        Research Workspace route (/visualization/workspace)
+    └── molecular-structure/      Molecular Structure Viewer route
+                                  (/visualization/molecular-structure)
 ```
 
 ## Data Flow
diff --git a/docs/visualization/molecular-structure.md b/docs/visualization/molecular-structure.md
new file mode 100644
index 0000000..d408aa2
--- /dev/null
+++ b/docs/visualization/molecular-structure.md
@@ -0,0 +1,226 @@
+# Molecular Structure Viewer
+
+Phase 6.12 introduces the first 3D rendering in the GenomeAI visualization
+platform: an interactive molecular structure viewer at
+`/visualization/molecular-structure` (linked from `/visualization`).
+
+This document covers the scope, the Three.js design decision, the canonical
+data model, the data boundary, the viewer lifecycle, accessibility, testing,
+and future extensions.
+
+## Scope
+
+- Interactive 3D rendering of a molecular structure: orbit, zoom, and pan the
+  camera (mouse drag / wheel / touch, via `OrbitControls`).
+- Three representations, switchable at runtime:
+  - **Cartoon / ribbon** — a smooth tube through the polymer backbone trace.
+  - **Ball and stick** — CPK-coloured atoms as spheres with covalent bonds as
+    cylinders.
+  - **Space filling** — atoms as van der Waals spheres so the molecular
+    surface is visible.
+- Camera controls: **Reset view** and **Fit to structure** re-frame the camera
+  around the structure; **Show/Hide structure** toggles the structure group.
+- A live, human-readable structure summary (name, chains, residues, atoms,
+  bonds) exposed both as a description and as a labelled `role="img"` canvas.
+- The shared Phase 6.1 lifecycle (`loading` / `empty` / `error` + retry) via
+  `VisualizationContainer`.
+
+## Out of scope (deferred by design)
+
+The viewer is a **visualization layer only**. It performs no docking, molecular
+dynamics, folding, prediction, alignment, or analysis. It does not parse PDB /
+MMCIF files directly (a future source adapter can — see
+[Data boundary](#data-boundary)). It makes no claim about the biological
+meaning of coordinates; it renders whatever the loader provides.
+
+## Design decision: Three.js behind a pure seam
+
+Phase 6.1–6.11 deliberately shipped without Three.js, Cytoscape.js, or D3.js,
+deferring each to the milestone that truly requires it. A 3D molecular
+structure viewer genuinely requires a 3D engine, so Phase 6.12 introduces
+**Three.js** (`three`, runtime; `@types/three`, dev) — the only new runtime
+dependency in the phase.
+
+Why Three.js directly rather than a specialized molecular viewer library
+(NGL, bio3d-viewer, Mol*):
+
+- The repo convention is **minimal dependencies behind pure seams** (native
+  scales instead of D3.js, a deterministic layout instead of Cytoscape.js).
+  NGL/bio3d-viewer bring their own component, event, and state models on top
+  of Three.js, which would fight the existing `useVisualizationData` /
+  `VisualizationContainer` lifecycle.
+- The required representations (cartoon ribbon, ball-and-stick, space-filling)
+  are simple to build directly on core Three.js geometry
+  (`TubeGeometry`, `SphereGeometry`, `CylinderGeometry`), giving full control
+  over colors, materials, and — critically — resource disposal.
+- WebGL stays **isolated** behind the `MolecularViewer` seam
+  (`lib/molecular/render/types.ts`): the React layer never imports Three.js.
+  `createThreeViewer` owns the scene, camera, controls, and renderer; tests
+  inject a fake renderer and never touch a GPU, matching the repo's
+  jsdom-testing convention.
+
+Seam layout:
+
+```
+lib/molecular/types.ts                     canonical model (no deps)
+lib/molecular/validate.ts                  pure validation (no deps)
+lib/molecular/geometry.ts                  pure geometry (no deps)
+lib/molecular/representations.ts           representation catalog (no deps)
+lib/molecular/api.ts                       future GET /structures/{id} adapter
+lib/molecular/molecular.fixtures.ts        dev fixture -> toStructure
+lib/molecular/useMolecularStructureViewer.ts  view-model hook (React)
+lib/molecular/render/types.ts              MolecularViewer interface
+lib/molecular/render/representationBuilder.ts  pure Three.js object graph
+lib/molecular/render/threeViewer.ts        WebGL implementation
+components/molecular/MolecularStructureViewer.tsx  React component
+app/visualization/molecular-structure/     route + demo
+```
+
+## Data model
+
+`lib/molecular/types.ts` defines the canonical `MolecularStructure` model:
+
+- `StructureAtom` — 1-based atom serial, element symbol, Cartesian **angstrom**
+  coordinates, 1-based residue number, chain id, optional residue/atom names.
+- `StructureBond` — two 1-based atom serials plus an optional bond order.
+- `StructureResidue` — 1-based residue number, optional name, atom serials.
+- `StructureChain` — chain id plus ordered residues.
+- `MolecularStructure` — id, optional name/kind (`protein` | `nucleic-acid` |
+  `other`)/organism/description/metadata, plus chains/atoms/bonds.
+
+Conventions mirror the Phase 6.5 protein viewer (1-based residue numbering) and
+the genome viewer (1-based, inclusive intervals): atom serials and residue
+numbers are **one-based**, coordinates are in **angstroms**.
+
+## Validation
+
+`lib/molecular/validate.ts` reports structural issues rather than silently
+rendering bad data:
+
+- `structure.missing-id`, `structure.no-atoms`
+- `atom.invalid-index` / `atom.duplicate-index` / `atom.non-finite-coordinates`
+  / `atom.invalid-residue` / `atom.missing-element` / `atom.unreferenced`
+- `bond.dangling` / `bond.self-loop` / `bond.duplicate`
+- `chain.missing-id` / `residue.invalid-index` / `residue.duplicate-index` /
+  `residue.dangling-atom`
+
+`isUsableStructure` gates rendering; the hook only produces a summary and a
+camera focus for usable structures.
+
+## Geometry
+
+`lib/molecular/geometry.ts` is pure and dependency-free:
+
+- `elementPresentation` / `elementColor` — CPK-style colors and
+  van der Waals / ball radii for C, N, O, S, H, P, FE, ZN, CL, BR, CA (with a
+  safe default).
+- `structureBounds` / `structureCentroid` / `structureRadius` /
+  `cameraFocusForStructure` — camera framing (target + radius).
+- `backboneTrace` — per-chain C-alpha-first trace for the cartoon ribbon.
+- `elementCounts` / `structureSummary` — summaries for status lines and the
+  accessible label.
+
+## Data boundary
+
+The GenomeAI backend **does not yet expose any molecular structure endpoint**.
+The module follows the established Phase 6.5/6.6/6.7/6.8 fixture pattern:
+
+- `lib/molecular/api.ts` documents the future contract
+  (`GET /structures/{structure_id}`) and provides `toStructure`, the typed
+  normalizer from raw records to `MolecularStructure`. A real endpoint can be
+  wired in by pointing a loader at `fetchMolecularStructure` without touching
+  the viewer, representations, or geometry.
+- `lib/molecular/molecular.fixtures.ts` provides a **deterministically
+  generated synthetic development fixture** (a small alpha-helix-like trace
+  over TP53 N-terminal residues: backbone N/CA/C/O per residue plus a few side
+  chains, with peptide and covalent bonds). It is generated in code (no random
+  values), flows through the same `toStructure` normalizer production would
+  use, and is clearly marked as synthetic (`metadata.source: 'fixture'`).
+
+This keeps the seam honest: the moment a real structure source exists, the
+fixture is replaced by a loader, and everything else stays.
+
+## Viewer lifecycle
+
+`useMolecularStructureViewer` composes the Phase 6.1 `useVisualizationData`
+lifecycle with:
+
+- `representation` / `setRepresentation` (default `cartoon`)
+- `visible` / `setVisible`
+- `focus` — `{ target, radius, version }` computed from the structure;
+  `resetView` / `fitToView` bump `version` so the component re-frames the
+  camera.
+
+`MolecularStructureViewer` creates the Three.js viewer **once per mount** (and
+only when the success-state container exists) using a `createdRef` guard, then
+updates it in place via separate effects for structure/representation, focus,
+and visibility. The viewer is fully disposed on unmount.
+
+`render/threeViewer.ts` owns:
+
+- `THREE.WebGLRenderer` (antialiased, alpha background) with an injectable
+  factory.
+- A `PerspectiveCamera` framed by `focusCamera` (distance ∝ structure radius,
+  near/far from radius).
+- `OrbitControls` with damping, ambient + key + fill lights.
+- A `ResizeObserver` for responsive sizing and an animation loop.
+- `dispose()` — stops the loop, disposes controls, geometries/materials
+  (`disposeGroup`), the renderer, and detaches the canvas.
+
+`render/representationBuilder.ts` builds only Three.js objects (groups,
+geometries, materials) and is safe to test in jsdom; only the renderer touches
+WebGL.
+
+## Accessibility
+
+The WebGL canvas is a supplementary visual; the controls and the textual
+summary carry the keyboard/assistive interaction:
+
+- The canvas container is `role="img"` with a descriptive `aria-label`
+  (structure name, chains, residues, atoms, bonds), and the same summary is
+  repeated in an `aria-live="polite"` output for updates.
+- All controls are labelled buttons/selects with descriptive names:
+  **Reset view**, **Fit to structure**, a labelled **Representation** select,
+  and a **Show/Hide structure** toggle with `aria-pressed`.
+- Loading / empty / error states reuse the shared `VisualizationContainer`
+  semantics (labelled loading, message, retry button).
+
+Known limitation: orbit/zoom/pan is pointer-driven; there is no keyboard
+camera manipulation, so the textual summary is the assistive window into the
+3D scene. Extending keyboard camera control is a documented future extension.
+
+## Testing
+
+84 new web tests, all running in jsdom (no GPU):
+
+- `validate.test.ts` — every validation code, plus `isUsableStructure` /
+  `firstStructureError`.
+- `geometry.test.ts` — element presentation, bounds, centroid, radius, camera
+  framing, backbone traces, element counts, summaries.
+- `representations.test.ts` — catalog, default, type guard, labels.
+- `api.test.ts` — `toStructure` normalization, malformed-record dropping,
+  field fallbacks, metadata filtering; `fetchMolecularStructure` success /
+  non-2xx / invalid-payload / invalid-structure with a mocked `fetch`.
+- `molecular.fixtures.test.ts` — the fixture is valid, deterministic, and
+  clearly synthetic.
+- `render/representationBuilder.test.ts` — mesh counts per representation and
+  full disposal of geometries/materials.
+- `render/threeViewer.test.ts` — fake-renderer lifecycle: canvas attach, frame
+  loop, camera framing distances, resize, dispose/detach.
+- `useMolecularStructureViewer.test.tsx` — success / empty / error + retry,
+  representation, visibility, focus versions, missing-loader error.
+- `MolecularStructureViewer.test.tsx` — injected fake viewer: create-once,
+  `setStructure`/`focusCamera`, representation select, visibility toggle,
+  reset/fit controls, dispose on unmount, loading/empty/error states.
+
+## Extensions
+
+- Parse real structure formats (PDB / MMCIF) in a future source adapter and
+  normalize through `toStructure`.
+- More representations (licorice, surface/mesh, CA trace) by adding a
+  `RepresentationId` catalog entry and a builder branch.
+- Keyboard camera control and focus/search to pick atoms or residues.
+- Multi-structure comparison or structure alignments (analysis, separate
+  milestone).
+- Render structures from the workspace by reusing this component behind a
+  `WorkspaceDataSource` provider, mirroring the Phase 6.9 panel pattern.
diff --git a/docs/visualization/protein-viewer.md b/docs/visualization/protein-viewer.md
index 5ed1c45..41fa552 100644
--- a/docs/visualization/protein-viewer.md
+++ b/docs/visualization/protein-viewer.md
@@ -35,9 +35,9 @@ Implemented on branch `feat/visualization-protein-viewer`.
 ## Out of scope (later milestones or explicitly excluded)
 
 - **3D molecular structure rendering** — this phase is sequence/annotation
-  only. Structure viewing is a future milestone that would introduce
-  Three.js only if 3D is truly required (see
-  [README](README.md) technology notes).
+  only. 3D structure viewing shipped separately in Phase 6.12 (the
+  [Molecular Structure Viewer](molecular-structure.md), which introduced
+  Three.js exactly because 3D is truly required there).
 - D3.js / Cytoscape.js / WebAssembly / WebGPU / C++
 - Backend annotation-feature support (see
   [Feature data boundary](#feature-data-boundary))
diff --git a/docs/visualization/roadmap.md b/docs/visualization/roadmap.md
index 5de7eb3..6720b13 100644
--- a/docs/visualization/roadmap.md
+++ b/docs/visualization/roadmap.md
@@ -4,7 +4,78 @@ Tracks the Phase 6 visualization platform milestones. See
 [Phase 6 of the project ROADMAP](</ROADMAP.md#phase-6--visualization-platform>) for
 the authoritative milestone list.
 
-## Current Milestone: 6.11 — Visualization Testing & Documentation ✅
+## Current Milestone: 6.12 — Molecular Structure Viewer ✅
+
+The first production-quality 3D molecular structure viewer, on top of the
+whole Phase 6 platform.
+
+Delivered:
+
+- **Typed canonical structure model** (`lib/molecular/types.ts`) — angstrom
+  coordinates, 1-based atom serials and residue numbers (mirroring the Phase
+  6.5 protein residue convention), chains/residues/bonds, and free-form
+  metadata; deliberately independent of any file format or backend.
+- **Pure validation** (`lib/molecular/validate.ts`) — reports structural
+  issues (missing ids, no atoms, invalid/duplicate serials, non-finite
+  coordinates, dangling/self/duplicate bonds, missing chain/residue ids,
+  dangling residue references, unreferenced atoms) instead of silently
+  rendering malformed data; `isUsableStructure` gates rendering.
+- **Pure geometry** (`lib/molecular/geometry.ts`) — CPK element presentation,
+  bounding boxes, centroids, camera framing (`cameraFocusForStructure`),
+  per-chain backbone traces, element counts, and human-readable summaries.
+- **Three.js behind a pure seam** — the React layer talks only to a
+  `MolecularViewer` interface (`render/types.ts`); the WebGL implementation
+  (`render/threeViewer.ts`) owns the scene, perspective camera, lights,
+  `OrbitControls`, `ResizeObserver`, animation loop, and full disposal, with
+  an injectable renderer so tests never need a GPU. The object-graph builder
+  (`render/representationBuilder.ts`) creates only Three.js objects and is
+  unit-tested in jsdom.
+- **Three representations** — cartoon / ribbon (smooth tube through the
+  C-alpha trace), ball-and-stick (CPK-coloured spheres + bond cylinders), and
+  space-filling (van der Waals spheres), behind a small extensible
+  `RepresentationId` catalog.
+- **`useMolecularStructureViewer` hook** — composes the Phase 6.1 data
+  lifecycle with representation, visibility, and camera-framing intent
+  (target/radius/version bumped on reset/fit).
+- **`MolecularStructureViewer` component** — creates the viewer once per mount
+  and updates it in place; labelled `role="img"` canvas, live structure
+  summary, representation select, reset/fit buttons, and show/hide with
+  `aria-pressed`.
+- **Data boundary** — the backend exposes no structure endpoint yet, so
+  `lib/molecular/api.ts` documents the future `GET /structures/{id}` contract
+  and normalizes any source via `toStructure`; the demo renders a clearly
+  isolated, deterministically generated synthetic development fixture routed
+  through the same normalizer as production (mirroring the Phase 6.5/6.6/6.7
+  fixture boundaries).
+- **Route + demo** — `/visualization/molecular-structure` (linked from
+  `/visualization`), a catalog entry (`molecular-structure-viewer`, milestone
+  `6.12`) with its data-contract test updated to 11 modules.
+- **Tests** (84 new web tests) — validation codes, geometry math, catalog
+  contracts, API normalization + malformed records, hook lifecycle (success /
+  empty / error / representation / visibility / focus), component lifecycle
+  with an injected fake viewer (create-once, setStructure/focusCamera,
+  representation select, visibility toggle, dispose on unmount), and Three.js
+  lifecycle with a fake renderer (frame loop, camera framing, resize, dispose).
+- **Docs** — this roadmap, [Molecular Structure](./molecular-structure.md),
+  README milestone table + "What Phase 6.12 Provides" + technology notes
+  (Three.js is now used, behind the seam).
+
+Constraints honored:
+
+- No C++, WebAssembly, WebGPU, CUDA, or a second framework; Three.js is the
+  only new runtime dependency and is isolated behind the `MolecularViewer`
+  seam (WebGL is only touched by `render/threeViewer.ts`)
+- No docking, dynamics, prediction, alignment, or analysis work; the viewer is
+  a visualization layer only
+- No invented backend structure endpoint; the fixture boundary is documented
+- Existing Genome Browser, Workspace, charts, and network viewers keep working
+  unchanged; the visualization UI is not redesigned
+- All new tests pass in jsdom (no GPU required); no flaky or timing-based
+  benchmarks
+
+## Previous milestones
+
+### 6.11 — Visualization Testing & Documentation ✅
 
 A stabilization + docs pass over the whole Phase 6 platform, on top of 6.10.
 
@@ -44,16 +115,14 @@ Delivered:
 
 Constraints honored:
 
-- No new visualization features, architecture redesign, or Phase 6.12 work;
-  no C++/WebAssembly/WebGPU/Three.js/Cytoscape.js/D3.js; no new runtime
-  dependencies; Phase 5 untouched
+- No new visualization features, architecture redesign, or Phase 6.12 work at
+  the time; no C++/WebAssembly/WebGPU/Three.js/Cytoscape.js/D3.js; no new
+  runtime dependencies; Phase 5 untouched
 - No tests deleted, weakened, or skipped; additions are behavior-oriented
   (never implementation-detail fakes)
 - Docs claims are accurate against the current repo (test counts, milestone
   status, component tree)
 
-## Previous milestones
-
 ### 6.9 — Integrated Research Workspace ✅
 
 Implemented on top of 6.8 (and the Phase 6.10 performance work).
@@ -438,4 +507,3 @@ Constraints honored:
 
 | # | Milestone | Notes |
 |---|-----------|-------|
-| 6.12 | Molecular Structure Viewer (3D) | 3D protein structures; Three.js only if 3D is truly required |
\ No newline at end of file
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index e2cb04b..3f66c4a 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -33,6 +33,9 @@ importers:
       react-dom:
         specifier: ^19.0.0
         version: 19.2.8(react@19.2.8)
+      three:
+        specifier: ^0.185.1
+        version: 0.185.1
     devDependencies:
       '@testing-library/dom':
         specifier: ^10.4.0
@@ -52,6 +55,9 @@ importers:
       '@types/react-dom':
         specifier: ^19.0.0
         version: 19.2.3(@types/react@19.2.17)
+      '@types/three':
+        specifier: ^0.185.4
+        version: 0.185.4
       '@vitejs/plugin-react':
         specifier: ^4.4.0
         version: 4.7.0(vite@7.3.6(@types/node@20.19.43)(jiti@1.21.7))
@@ -269,6 +275,9 @@ packages:
     resolution: {integrity: sha512-Vd/9EVDiu6PPJt9yAh6roZP6El1xHrdvIVGjyBsHR0RYwNHgL7FJPyIIW4fANJNG6FtyZfvlRPpFI4ZM/lubvw==}
     engines: {node: '>=18'}
 
+  '@dimforge/rapier3d-compat@0.12.0':
+    resolution: {integrity: sha512-uekIGetywIgopfD97oDL5PfeezkFpNhwlzlaEYNOA0N6ghdsOvh/HYjSMek5Q2O1PYvRSDFcqFVJl4r4ZBwOow==}
+
   '@emnapi/runtime@1.11.2':
     resolution: {integrity: sha512-kyOl3X0DuTiT1h2ft8r2fYO8JYtU9a9Xis/zBSiGArNaagCOWx90N1k2wxp18czFDH+OgcWGb5ZP/XMt3dcyPA==}
 
@@ -837,6 +846,9 @@ packages:
     cpu: [arm64]
     os: [win32]
 
+  '@tweenjs/tween.js@23.1.3':
+    resolution: {integrity: sha512-vJmvvwFxYuGnF2axRtPYocag6Clbb5YS7kLL+SO/TeVFzHqDIWrNKYtcsPMibjDx9O+bu+psAy9NKfWklassUA==}
+
   '@types/aria-query@5.0.4':
     resolution: {integrity: sha512-rfT93uj5s0PRL7EzccGMs3brplhcrghnDoV26NqKhCAS1hVo+WdNsPvE/yb6ilfr5hi2MEk6d5EWJTKdxg8jVw==}
 
@@ -872,6 +884,15 @@ packages:
   '@types/react@19.2.17':
     resolution: {integrity: sha512-MXfmqaVPEVgkBT/aY0aGCkRWWtByiYQXo3xdQ8r5RzuFrPiRn8Gar2tQdXSUQ2GKV3bkXckek89V8wQBY2Q/Aw==}
 
+  '@types/stats.js@0.17.4':
+    resolution: {integrity: sha512-jIBvWWShCvlBqBNIZt0KAshWpvSjhkwkEu4ZUcASoAvhmrgAUI2t1dXrjSL4xXVLB4FznPrIsX3nKXFl/Dt4vA==}
+
+  '@types/three@0.185.4':
+    resolution: {integrity: sha512-gAsBIC07NIFrxjbf7tH2t71c38uulFfk/RFoC7FNBSjMRAQ8J1x/RBvusX0N5PJouaYFJawXQqfCQ0RKUx/1nA==}
+
+  '@types/webxr@0.5.24':
+    resolution: {integrity: sha512-h8fgEd/DpoS9CBrjEQXR+dIDraopAEfu4wYVNY2tEPwk60stPWhvZMf4Foo5FakuQ7HFZoa8WceaWFervK2Ovg==}
+
   '@vitejs/plugin-react@4.7.0':
     resolution: {integrity: sha512-gUu9hwfWvvEDBBmgtAowQCojwZmJ5mcLn3aufeCsitijs3+f2NsrPtlAWIR6OPiqljl96GVCUbLe0HyqIpVaoA==}
     engines: {node: ^14.18.0 || >=16.0.0}
@@ -1099,6 +1120,9 @@ packages:
       picomatch:
         optional: true
 
+  fflate@0.8.3:
+    resolution: {integrity: sha512-tbZNuJrLwGUp3zshBtdy4W+ORxZuIh8a5ilyIEQDC5rY1f3U20JMry0Ll3WBzU58EZKsEuJFXhb5gwv8CsPvgA==}
+
   fill-range@7.1.1:
     resolution: {integrity: sha512-YsGpe3WHLK8ZYi4tWDg2Jy3ebRz2rXowDxnld4bkQB00cc/1Zw9AWnC0i9ztDJitivtQvaI9KaLyKrc+hBW0yg==}
     engines: {node: '>=8'}
@@ -1229,6 +1253,9 @@ packages:
     resolution: {integrity: sha512-8q7VEgMJW4J8tcfVPy8g09NcQwZdbwFEqhe/WZkoIzjn/3TGDwtOCYtXGxA3O8tPzpczCCDgv+P2P5y00ZJOOg==}
     engines: {node: '>= 8'}
 
+  meshoptimizer@1.1.1:
+    resolution: {integrity: sha512-oRFNWJRDA/WTrVj7NWvqa5HqE1t9MYDj2VaWirQCzCCrAd2GHrqR/sQezCxiWATPNlKTcRaPRHPJwIRoPBAp5g==}
+
   micromatch@4.0.8:
     resolution: {integrity: sha512-PXwfBhYu0hBCPw8Dn0E+WDYb7af3dSLVWKi3HGv84IdF4TyFoC0ysxFd0Goxw7nSv4T/PzEJQxsYsEiFCKo2BA==}
     engines: {node: '>=8.6'}
@@ -1510,6 +1537,9 @@ packages:
   thenify@3.3.1:
     resolution: {integrity: sha512-RVZSIV5IG10Hk3enotrhvz0T9em6cyHBLkH/YAZuKqd8hRkKhSfCGIcP2KUY0EPxndzANBmNllzWPwak+bheSw==}
 
+  three@0.185.1:
+    resolution: {integrity: sha512-5aojFCXKwnjBRZvUnt3WFfEcvUJgkN5LlijRFN95hMy8WVkG4I0QNcJE+OuWvuJ0bOdStrbfXn0pkd6/QyiAlg==}
+
   tinybench@2.9.0:
     resolution: {integrity: sha512-0+DUvqWMValLmha6lr4kD8iAMK1HzV0/aKnCtWb9v9641TnP/MFb7Pc2bxoxQjTXAErryXVgUOfv2YqNllqGeg==}
 
@@ -1882,6 +1912,8 @@ snapshots:
 
   '@csstools/css-tokenizer@3.0.4': {}
 
+  '@dimforge/rapier3d-compat@0.12.0': {}
+
   '@emnapi/runtime@1.11.2':
     dependencies:
       tslib: 2.8.1
@@ -2258,6 +2290,8 @@ snapshots:
   '@turbo/windows-arm64@2.10.5':
     optional: true
 
+  '@tweenjs/tween.js@23.1.3': {}
+
   '@types/aria-query@5.0.4': {}
 
   '@types/babel__core@7.20.5':
@@ -2302,6 +2336,19 @@ snapshots:
     dependencies:
       csstype: 3.2.3
 
+  '@types/stats.js@0.17.4': {}
+
+  '@types/three@0.185.4':
+    dependencies:
+      '@dimforge/rapier3d-compat': 0.12.0
+      '@tweenjs/tween.js': 23.1.3
+      '@types/stats.js': 0.17.4
+      '@types/webxr': 0.5.24
+      fflate: 0.8.3
+      meshoptimizer: 1.1.1
+
+  '@types/webxr@0.5.24': {}
+
   '@vitejs/plugin-react@4.7.0(vite@7.3.6(@types/node@20.19.43)(jiti@1.21.7))':
     dependencies:
       '@babel/core': 7.29.7
@@ -2536,6 +2583,8 @@ snapshots:
     optionalDependencies:
       picomatch: 4.0.5
 
+  fflate@0.8.3: {}
+
   fill-range@7.1.1:
     dependencies:
       to-regex-range: 5.0.1
@@ -2660,6 +2709,8 @@ snapshots:
 
   merge2@1.4.1: {}
 
+  meshoptimizer@1.1.1: {}
+
   micromatch@4.0.8:
     dependencies:
       braces: 3.0.3
@@ -2968,6 +3019,8 @@ snapshots:
     dependencies:
       any-promise: 1.3.0
 
+  three@0.185.1: {}
+
   tinybench@2.9.0: {}
 
   tinyexec@0.3.2: {}