diff --git a/package-lock.json b/package-lock.json
index 2e4ca36f..85e5b356 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -16409,7 +16409,7 @@
     },
     "packages/app-core": {
       "name": "@zennotes/app-core",
-      "version": "2.71.0",
+      "version": "2.71.1",
       "dependencies": {
         "@codemirror/autocomplete": "^6.18.3",
         "@codemirror/commands": "^6.7.1",
@@ -16498,14 +16498,14 @@
     },
     "packages/bridge-contract": {
       "name": "@zennotes/bridge-contract",
-      "version": "2.71.0",
+      "version": "2.71.1",
       "devDependencies": {
         "typescript": "^5.7.2"
       }
     },
     "packages/shared-domain": {
       "name": "@zennotes/shared-domain",
-      "version": "2.71.0",
+      "version": "2.71.1",
       "dependencies": {
         "@scure/bip39": "2.4.0",
         "@zennotes/bridge-contract": "*",
diff --git a/packages/app-core/package.json b/packages/app-core/package.json
index a94b271c..21e23d94 100644
--- a/packages/app-core/package.json
+++ b/packages/app-core/package.json
@@ -1,7 +1,7 @@
 {
   "name": "@zennotes/app-core",
   "private": true,
-  "version": "2.71.0",
+  "version": "2.71.1",
   "type": "module",
   "exports": {
     "./main": "./src/main.tsx",
diff --git a/packages/app-core/src/lib/d2-render.test.ts b/packages/app-core/src/lib/d2-render.test.ts
index cecb1234..84bab306 100644
--- a/packages/app-core/src/lib/d2-render.test.ts
+++ b/packages/app-core/src/lib/d2-render.test.ts
@@ -36,7 +36,7 @@ const runtime = vi.hoisted(() => {
   return { calls, D2 }
 })
 
-vi.mock('./d2-runtime', () => ({ D2: runtime.D2 }))
+vi.mock('./d2-runtime', () => ({ D2: runtime.D2, createD2: async () => new runtime.D2() }))
 
 import { d2SourceSetsTheme, formatD2Error, peekD2Svg, renderD2Svg, sanitizeD2Svg, withUniqueD2Ids } from './d2-render'
 
diff --git a/packages/app-core/src/lib/d2-render.ts b/packages/app-core/src/lib/d2-render.ts
index 33e12871..11f53c2f 100644
--- a/packages/app-core/src/lib/d2-render.ts
+++ b/packages/app-core/src/lib/d2-render.ts
@@ -32,7 +32,7 @@ let d2Promise: Promise<D2Instance> | null = null
 
 function loadD2(): Promise<D2Instance> {
   if (!d2Promise) {
-    d2Promise = import('./d2-runtime').then((module) => new module.D2())
+    d2Promise = import('./d2-runtime').then((module) => module.createD2())
     // A failed load (an offline web client that never fetched the chunk) must
     // not poison every later diagram.
     d2Promise.catch(() => {
diff --git a/packages/app-core/src/lib/d2-runtime.test.ts b/packages/app-core/src/lib/d2-runtime.test.ts
new file mode 100644
index 00000000..002c2032
--- /dev/null
+++ b/packages/app-core/src/lib/d2-runtime.test.ts
@@ -0,0 +1,61 @@
+import { createRequire } from 'node:module'
+import { dirname, join } from 'node:path'
+import { pathToFileURL } from 'node:url'
+import { describe, expect, it } from 'vitest'
+import { pageWorkerScript } from './d2-runtime'
+
+describe('pageWorkerScript', () => {
+  it('drops the module exports and hands the handler the port', () => {
+    const source = [
+      'export let BrotliDecode = (() => 1)();',
+      'export function setupMessageHandler(isNode, port, initWasm) {}',
+      'async function initWasmBrowser() { return self.d2 }',
+      'setupMessageHandler(false, self, initWasmBrowser);'
+    ].join('\n')
+    const script = pageWorkerScript(source)
+    expect(script).not.toBeNull()
+    expect(script).not.toMatch(/^\s*export\s/m)
+    expect(script).toContain('setupMessageHandler(false, port, initWasmBrowser);')
+    expect(() => new Function('port', script!)).not.toThrow()
+  })
+
+  it('refuses a script without the entry it expects', () => {
+    expect(pageWorkerScript('setupMessageHandler(true, parentPort, initWasmNode);')).toBeNull()
+  })
+
+  it('turns the worker script of the installed @terrastruct/d2 into a valid function body', async () => {
+    // The browser build assembles its worker from a Blob of strings as a D2
+    // is constructed. Record that Blob, then refuse the Worker, so nothing
+    // starts and the compiler is never decoded.
+    const require = createRequire(import.meta.url)
+    const browserBuild = join(dirname(require.resolve('@terrastruct/d2')), '..', 'browser', 'index.js')
+    const { D2 } = (await import(/* @vite-ignore */ pathToFileURL(browserBuild).href)) as {
+      D2: new () => { ready: Promise<void> }
+    }
+    let source: string | null = null
+    const saved = { Blob: globalThis.Blob, Worker: globalThis.Worker }
+    globalThis.Blob = class extends saved.Blob {
+      constructor(parts?: BlobPart[], options?: BlobPropertyBag) {
+        super(parts, options)
+        if (parts?.every((part) => typeof part === 'string')) source = parts.join('')
+      }
+    }
+    globalThis.Worker = function NoWorker() {
+      throw new Error('not started in tests')
+    } as unknown as typeof Worker
+    let ready: Promise<void>
+    try {
+      ready = new D2().ready
+    } finally {
+      globalThis.Blob = saved.Blob
+      globalThis.Worker = saved.Worker
+    }
+    await expect(ready).rejects.toThrow()
+    expect(source).not.toBeNull()
+    const script = pageWorkerScript(source!)
+    expect(script).not.toBeNull()
+    expect(script).not.toMatch(/^\s*export\s/m)
+    // Parsing only: the Function is built, never called.
+    expect(() => new Function('port', script!)).not.toThrow()
+  })
+})
diff --git a/packages/app-core/src/lib/d2-runtime.ts b/packages/app-core/src/lib/d2-runtime.ts
index d87dd8d7..adcdd217 100644
--- a/packages/app-core/src/lib/d2-runtime.ts
+++ b/packages/app-core/src/lib/d2-runtime.ts
@@ -7,5 +7,129 @@
  * about 8 MB with the compiler's WebAssembly inside it, and the perf harness
  * and the packaged-web browser test assert that this chunk is not fetched
  * before a note needs a D2 diagram.
+ *
+ * It also decides where the compiler runs. `@terrastruct/d2` starts it in a
+ * worker, and in WebKit (the iPhone app, Safari on the self-hosted web app)
+ * the Go runtime overflows a worker's smaller stack while it starts: the
+ * WebAssembly loads, the worker still reports ready, and every diagram then
+ * fails with "undefined is not an object (evaluating 'd2.compile')". On the
+ * page's own thread it starts fine. So a D2 whose worker cannot answer is
+ * replaced by one whose worker script runs on the page.
+ */
+import { D2 } from '@terrastruct/d2'
+
+export { D2 }
+
+/** The worker script's last line: it hands the message handler the worker's
+ *  own global as the port it talks through. */
+const WORKER_ENTRY = 'setupMessageHandler(false, self, initWasmBrowser);'
+
+/**
+ * The worker script `@terrastruct/d2` builds, rewritten to run as a function
+ * on the page: its two `export` keywords dropped (it is a module script, and
+ * a function body cannot export) and its handler given the `port` argument
+ * instead of the worker's global. No blob URL is fetched or imported, which
+ * the self-hosted server's Content-Security-Policy would refuse; running it
+ * needs only the 'unsafe-eval' the script already needs for its layout
+ * library. Null when the script no longer has the entry this expects, so an
+ * update of the package fails loudly instead of running something else.
  */
-export { D2 } from '@terrastruct/d2'
+export function pageWorkerScript(source: string): string | null {
+  if (!source.includes(WORKER_ENTRY)) return null
+  return source
+    .replace(/^(\s*)export\s+(?=(let|const|function|class)\b)/gm, '$1')
+    .replace(WORKER_ENTRY, 'setupMessageHandler(false, port, initWasmBrowser);')
+}
+
+interface MessageTarget {
+  postMessage(data: unknown): void
+  onmessage: ((event: { data: unknown }) => void) | null
+}
+
+/** Stands in for the Worker `@terrastruct/d2` creates: runs its script on the
+ *  page and passes messages both ways a tick later, as a worker would. */
+class PageWorker {
+  onmessage: ((event: { data: unknown }) => void) | null = null
+  onerror: ((event: unknown) => void) | null = null
+  private readonly port: MessageTarget
+
+  constructor(script: string) {
+    this.port = {
+      postMessage: (data) => {
+        setTimeout(() => this.onmessage?.({ data }))
+      },
+      onmessage: null
+    }
+    new Function('port', script)(this.port)
+  }
+
+  postMessage(data: unknown): void {
+    setTimeout(() => this.port.onmessage?.({ data }))
+  }
+
+  terminate(): void {}
+}
+
+/** Globals the Go runtime's `wasm_exec` stubs out when the page lacks them.
+ *  Go reads them once while it starts and keeps its own references, so they
+ *  can leave the page afterwards; a `process` without `env` would otherwise
+ *  mislead every library that checks for Node. `fs` stays: the runtime writes
+ *  its output through the bare name. */
+const GO_STUBS = ['process', 'path'] as const
+
+/** A D2 whose worker script runs on the page. `new D2()` builds its worker
+ *  from a Blob of the script and starts it in the same synchronous step, so
+ *  Blob and Worker are swapped only for that step: the Blob keeps the
+ *  script's text, and the Worker runs it here. */
+async function onThePage(): Promise<D2> {
+  const OriginalBlob = globalThis.Blob
+  const OriginalWorker = globalThis.Worker
+  const absent = GO_STUBS.filter((name) => !(name in globalThis))
+  let script: string | null = null
+  globalThis.Blob = class extends OriginalBlob {
+    constructor(parts?: BlobPart[], options?: BlobPropertyBag) {
+      super(parts, options)
+      if (parts?.every((part) => typeof part === 'string')) {
+        script = pageWorkerScript(parts.join('')) ?? script
+      }
+    }
+  }
+  globalThis.Worker = function PageWorkerFactory() {
+    if (!script) throw new Error('The D2 worker script is not the one this version expects')
+    return new PageWorker(script)
+  } as unknown as typeof Worker
+  let d2: D2
+  try {
+    d2 = new D2()
+  } finally {
+    globalThis.Blob = OriginalBlob
+    globalThis.Worker = OriginalWorker
+  }
+  try {
+    await (d2 as unknown as Probe).ready
+  } finally {
+    for (const name of absent) delete (globalThis as Record<string, unknown>)[name]
+  }
+  return d2
+}
+
+/** What `@terrastruct/d2` has at runtime but leaves out of its typings. */
+interface Probe {
+  ready: Promise<void>
+  version(): Promise<unknown>
+  worker?: { terminate?(): void }
+}
+
+/** A D2 that answers: the package's own, in its worker, when that worker
+ *  starts; otherwise one on the page. The probe is `version()`, the cheapest
+ *  call that reaches the compiler. */
+export async function createD2(): Promise<D2> {
+  const inWorker = new D2()
+  try {
+    await (inWorker as unknown as Probe).version()
+    return inWorker
+  } catch {
+    ;(inWorker as unknown as Probe).worker?.terminate?.()
+    return onThePage()
+  }
+}
diff --git a/packages/bridge-contract/package.json b/packages/bridge-contract/package.json
index 8ccc5da5..9d1311e9 100644
--- a/packages/bridge-contract/package.json
+++ b/packages/bridge-contract/package.json
@@ -1,7 +1,7 @@
 {
   "name": "@zennotes/bridge-contract",
   "private": true,
-  "version": "2.71.0",
+  "version": "2.71.1",
   "type": "module",
   "exports": {
     "./bridge": "./src/bridge.ts",
diff --git a/packages/shared-domain/package.json b/packages/shared-domain/package.json
index baddf73f..2146969c 100644
--- a/packages/shared-domain/package.json
+++ b/packages/shared-domain/package.json
@@ -1,7 +1,7 @@
 {
   "name": "@zennotes/shared-domain",
   "private": true,
-  "version": "2.71.0",
+  "version": "2.71.1",
   "type": "module",
   "exports": {
     "./*": "./src/*.ts"