diff --git a/apps/desktop/e2e/composer-inline-completion.spec.ts b/apps/desktop/e2e/composer-inline-completion.spec.ts
index 81aeadabf3..96f48e84d6 100644
--- a/apps/desktop/e2e/composer-inline-completion.spec.ts
+++ b/apps/desktop/e2e/composer-inline-completion.spec.ts
@@ -1,4 +1,4 @@
-import { FAKE_HOLD_OPEN_PROMPT } from '@maka/runtime/fake-backend';
+import { FAKE_HOLD_OPEN_PROMPT } from '@maka/runtime/test-only/fake-backend';
 import { test, expect, COMPOSER_INPUT } from './fixtures';
 import type { Page } from '@playwright/test';
 
diff --git a/apps/desktop/e2e/slash-command-menu.spec.ts b/apps/desktop/e2e/slash-command-menu.spec.ts
index aa2caff4c5..947782bc78 100644
--- a/apps/desktop/e2e/slash-command-menu.spec.ts
+++ b/apps/desktop/e2e/slash-command-menu.spec.ts
@@ -1,4 +1,4 @@
-import { FAKE_HOLD_OPEN_PROMPT } from '@maka/runtime/fake-backend';
+import { FAKE_HOLD_OPEN_PROMPT } from '@maka/runtime/test-only/fake-backend';
 import { expect, test, COMPOSER_INPUT } from './fixtures';
 
 test('shows only slash commands executable in the current session state', async ({
diff --git a/apps/desktop/e2e/streaming-remount.spec.ts b/apps/desktop/e2e/streaming-remount.spec.ts
index 11b044b922..cb64cbf8e6 100644
--- a/apps/desktop/e2e/streaming-remount.spec.ts
+++ b/apps/desktop/e2e/streaming-remount.spec.ts
@@ -2,7 +2,7 @@ import {
   FAKE_HOLD_OPEN_PROMPT,
   FAKE_HOLD_OPEN_REWRITE_PROMPT,
   FAKE_WAIT_FOR_STEERING_LARGE_RESPONSE_PROMPT,
-} from '@maka/runtime/fake-backend';
+} from '@maka/runtime/test-only/fake-backend';
 import type { Locator } from '@playwright/test';
 import { expect, COMPOSER_INPUT, test } from './fixtures';
 
diff --git a/apps/desktop/electron-builder.config.mjs b/apps/desktop/electron-builder.config.mjs
index 2cabcd3ed6..452335f5a0 100644
--- a/apps/desktop/electron-builder.config.mjs
+++ b/apps/desktop/electron-builder.config.mjs
@@ -6,7 +6,15 @@ export default {
   directories: {
     output: 'release',
   },
-  files: ['dist/**/*', 'dist-renderer/**/*', 'package.json', '!**/__tests__/**'],
+  files: [
+    'dist/**/*',
+    'dist-renderer/**/*',
+    'package.json',
+    '!**/__tests__/**',
+    // FakeBackend and the Desktop E2E candidate bootstrap live under
+    // `test-only/`; they must not reach a packaged app.
+    '!**/test-only/**',
+  ],
   extraResources: [
     {
       from: '../../node_modules/dugite/git',
diff --git a/apps/desktop/src/main/runtime-host-boot.ts b/apps/desktop/src/main/runtime-host-boot.ts
index 819392f667..df4dd1f401 100644
--- a/apps/desktop/src/main/runtime-host-boot.ts
+++ b/apps/desktop/src/main/runtime-host-boot.ts
@@ -456,10 +456,16 @@ runtimeHostManager = await startRuntimeHostDesktopManager(
     rootPath: workspaceRoot,
     clientInstanceId: runtimeHostClientInstanceId,
     generation: runtimeHostGeneration,
+    // The Desktop E2E composition lives behind its own entry module, which
+    // release packaging drops: picking it here is what keeps FakeBackend and
+    // the E2E bootstrap out of the shipped Runtime Host.
     candidateEntrypoint: new URL(
-      import.meta.resolve("@maka/runtime-host/execution-candidate-main"),
+      import.meta.resolve(
+        isE2e
+          ? "@maka/runtime-host/test-only/execution-candidate-e2e-main"
+          : "@maka/runtime-host/execution-candidate-main",
+      ),
     ),
-    ...(isE2e ? { desktopE2e: true } : {}),
     ipcMain,
     workspaceRoot,
     attachmentApprovals,
diff --git a/apps/desktop/src/main/runtime-host-desktop-candidate.ts b/apps/desktop/src/main/runtime-host-desktop-candidate.ts
index c27b3bb589..f301bef2f8 100644
--- a/apps/desktop/src/main/runtime-host-desktop-candidate.ts
+++ b/apps/desktop/src/main/runtime-host-desktop-candidate.ts
@@ -135,7 +135,6 @@ export interface DesktopRuntimeHostCandidateStartInput
   readonly connectTimeoutMs?: number;
   readonly handshakeTimeoutMs?: number;
   readonly candidateEntrypoint: string | URL;
-  readonly desktopE2e?: boolean;
   readonly generation?: string;
   readonly takeoverHostEpoch?: string;
   readonly signal?: AbortSignal;
@@ -695,7 +694,6 @@ function connectInput(
     ...(input.handshakeTimeoutMs === undefined
       ? {}
       : { handshakeTimeoutMs: input.handshakeTimeoutMs }),
-    ...(input.desktopE2e ? { desktopE2e: true } : {}),
     ...(input.signal === undefined ? {} : { signal: input.signal }),
   };
 }
diff --git a/packages/runtime-host/package.json b/packages/runtime-host/package.json
index 47a761f4d1..1abbdce21c 100644
--- a/packages/runtime-host/package.json
+++ b/packages/runtime-host/package.json
@@ -10,7 +10,8 @@
     "./protocol": "./dist/protocol/index.js",
     "./client": "./dist/client/index.js",
     "./execution-candidate-main": "./dist/execution-candidate-main.js",
-    "./server": "./dist/server/index.js"
+    "./server": "./dist/server/index.js",
+    "./test-only/execution-candidate-e2e-main": "./dist/test-only/execution-candidate-e2e-main.js"
   },
   "scripts": {
     "clean": "node ../../scripts/clean-paths.mjs dist tsconfig.tsbuildinfo",
diff --git a/packages/runtime-host/src/__tests__/candidate-cli.test.ts b/packages/runtime-host/src/__tests__/candidate-cli.test.ts
index 929c7e6761..a773f156b9 100644
--- a/packages/runtime-host/src/__tests__/candidate-cli.test.ts
+++ b/packages/runtime-host/src/__tests__/candidate-cli.test.ts
@@ -5,7 +5,7 @@ import { parseInteractiveRuntimeHostCandidateArguments } from '../candidate-cli.
 const ROOT_ID = 'a'.repeat(64);
 const STARTUP_ATTEMPT_ID = '00000000-0000-4000-8000-000000000001';
 
-test('parses the production candidate flags without a desktop E2E override', () => {
+test('parses the production candidate flags', () => {
   const parsed = parseInteractiveRuntimeHostCandidateArguments([
     '--root',
     '/tmp/workspace',
@@ -20,24 +20,11 @@ test('parses the production candidate flags without a desktop E2E override', ()
   assert.equal(parsed.expectedRootId, ROOT_ID);
   assert.equal(parsed.startupAttemptId, STARTUP_ATTEMPT_ID);
   assert.equal(parsed.idleGraceMs, 10_000);
-  assert.equal(parsed.desktopE2e, undefined);
 });
 
-test('parses the desktop E2E composition flag', () => {
-  const parsed = parseInteractiveRuntimeHostCandidateArguments([
-    '--root',
-    '/tmp/workspace',
-    '--expected-root-id',
-    ROOT_ID,
-    '--startup-attempt-id',
-    STARTUP_ATTEMPT_ID,
-    '--desktop-e2e',
-    '1',
-  ]);
-  assert.equal(parsed.desktopE2e, true);
-});
-
-test('rejects an unknown desktop E2E flag value', () => {
+// The Desktop E2E composition is selected by its own entry module, not by a
+// flag on the production CLI — so `--desktop-e2e` is simply unknown here.
+test('rejects the retired desktop E2E flag as an unknown argument', () => {
   assert.throws(
     () =>
       parseInteractiveRuntimeHostCandidateArguments([
@@ -48,8 +35,8 @@ test('rejects an unknown desktop E2E flag value', () => {
         '--startup-attempt-id',
         STARTUP_ATTEMPT_ID,
         '--desktop-e2e',
-        'true',
+        '1',
       ]),
-    /Invalid --desktop-e2e/,
+    /Invalid Runtime Host candidate argument: --desktop-e2e/,
   );
 });
diff --git a/packages/runtime-host/src/__tests__/desktop-e2e-execution.test.ts b/packages/runtime-host/src/__tests__/desktop-e2e-execution.test.ts
index ec5b7ee87b..08f12deaba 100644
--- a/packages/runtime-host/src/__tests__/desktop-e2e-execution.test.ts
+++ b/packages/runtime-host/src/__tests__/desktop-e2e-execution.test.ts
@@ -4,7 +4,7 @@ import type { RuntimeEvent } from '@maka/core/runtime-event';
 import type { SessionHeader } from '@maka/core/session';
 import type { HistoryCompactCheckpoint } from '@maka/runtime/history-compact-checkpoint';
 import type { BackendFactoryContext } from '@maka/runtime/session-manager';
-import { DesktopE2eBackend } from '../desktop-e2e-execution.js';
+import { DesktopE2eBackend } from '../test-only/desktop-e2e-execution.js';
 
 function backendContext(overrides: Partial<BackendFactoryContext> = {}): BackendFactoryContext {
   return {
diff --git a/packages/runtime-host/src/__tests__/execution-candidate-main.test.ts b/packages/runtime-host/src/__tests__/execution-candidate-main.test.ts
index bd0c283601..d87c4ce417 100644
--- a/packages/runtime-host/src/__tests__/execution-candidate-main.test.ts
+++ b/packages/runtime-host/src/__tests__/execution-candidate-main.test.ts
@@ -1,7 +1,7 @@
 import assert from 'node:assert/strict';
 import { spawnSync } from 'node:child_process';
 import { createHash, randomUUID } from 'node:crypto';
-import { mkdir, mkdtemp, rm } from 'node:fs/promises';
+import { mkdir, mkdtemp, readFile, rm } from 'node:fs/promises';
 import { tmpdir } from 'node:os';
 import { dirname, join } from 'node:path';
 import { fileURLToPath } from 'node:url';
@@ -30,14 +30,14 @@ test('classifies invalid candidate arguments as an internal startup failure', ()
       '--startup-attempt-id',
       STARTUP_ATTEMPT_ID,
       '--desktop-e2e',
-      'true',
+      '1',
     ],
     { encoding: 'utf8', timeout: 10_000 },
   );
 
   assert.equal(result.status, 70, result.stderr);
   assert.match(result.stderr, /\[runtime-host\] startup failed:/);
-  assert.match(result.stderr, /Invalid --desktop-e2e/);
+  assert.match(result.stderr, /Invalid Runtime Host candidate argument: --desktop-e2e/);
 });
 
 test('preserves a valid Candidate invocation failure across the detached stderr boundary', async () => {
@@ -75,3 +75,72 @@ test('preserves a valid Candidate invocation failure across the detached stderr
     await rm(controlDirectory, { recursive: true, force: true });
   }
 });
+
+/**
+ * Release packaging drops every `test-only/` module, so the production
+ * candidate entry must not be able to reach one — statically, not merely at
+ * runtime. Walk the built module graph across the bundled `@maka/*` packages
+ * and report every test-only module it can reach.
+ */
+test('the production candidate entry never reaches a test-only module', async () => {
+  const entry = new URL('../execution-candidate-main.js', import.meta.url).href;
+  const seen = new Set<string>([entry]);
+  const queue: string[] = [entry];
+  const reached: string[] = [];
+
+  while (queue.length > 0) {
+    const current = queue.pop();
+    if (current === undefined) break;
+    if (current.includes('/test-only/')) {
+      reached.push(current);
+      continue;
+    }
+    let source: string;
+    try {
+      source = await readFile(new URL(current), 'utf8');
+    } catch {
+      continue;
+    }
+    for (const specifier of staticImportSpecifiers(source)) {
+      const resolved = resolveModule(specifier, current);
+      if (resolved === undefined || seen.has(resolved)) continue;
+      seen.add(resolved);
+      queue.push(resolved);
+    }
+  }
+
+  assert.deepEqual(reached, []);
+  assert.ok(seen.size > 50, `module graph looks truncated: ${seen.size} modules`);
+});
+
+function resolveModule(specifier: string, parent: string): string | undefined {
+  try {
+    if (specifier.startsWith('.')) return new URL(specifier, parent).href;
+    if (specifier.startsWith('@maka/')) {
+      const resolved = import.meta.resolve(specifier);
+      return resolved.startsWith('file:') ? resolved : undefined;
+    }
+  } catch {
+    return undefined;
+  }
+  return undefined;
+}
+
+function staticImportSpecifiers(source: string): string[] {
+  const specifiers: string[] = [];
+  for (const match of source.matchAll(
+    /(?:^|[\s;}])(?:import|export)\b[^'"();]*?from\s*['"]([^'"]+)['"]/g,
+  )) {
+    if (match[1] !== undefined) specifiers.push(match[1]);
+  }
+  for (const match of source.matchAll(/(?:^|[\s;}])import\s*['"]([^'"]+)['"]/g)) {
+    if (match[1] !== undefined) specifiers.push(match[1]);
+  }
+  // Literal dynamic imports are real edges in the shipped graph — the built
+  // `dist` already contains several — so a walk that ignored them could pass
+  // while a production module reached test-only material through `import(…)`.
+  for (const match of source.matchAll(/\bimport\s*\(\s*['"]([^'"]+)['"]/g)) {
+    if (match[1] !== undefined) specifiers.push(match[1]);
+  }
+  return specifiers;
+}
diff --git a/packages/runtime-host/src/__tests__/execution-composition.test.ts b/packages/runtime-host/src/__tests__/execution-composition.test.ts
index 6083587900..1705444b9d 100644
--- a/packages/runtime-host/src/__tests__/execution-composition.test.ts
+++ b/packages/runtime-host/src/__tests__/execution-composition.test.ts
@@ -1,4 +1,5 @@
 import assert from 'node:assert/strict';
+import { parseNoRealConnectionError } from '@maka/core/connection-error-copy';
 import { createRequire } from 'node:module';
 import { mkdir, mkdtemp, rm, stat, writeFile } from 'node:fs/promises';
 import { tmpdir } from 'node:os';
@@ -9,7 +10,7 @@ import type {
   AgentGraphIntentClaimRequest,
 } from '@maka/core/agent-graph-control';
 import type { ShellRunRecord } from '@maka/core/shell-run';
-import { FAKE_ASK_USER_QUESTION_PROMPT, FakeBackend } from '@maka/runtime/fake-backend';
+import { FAKE_ASK_USER_QUESTION_PROMPT, FakeBackend } from '@maka/runtime/test-only/fake-backend';
 import { LOCAL_READ_AGENT_DEFINITION } from '@maka/runtime/agent-catalog';
 import { SessionManager } from '@maka/runtime/session-manager';
 import { fingerprintAgentGraphRunnableIntent } from '@maka/runtime/stream-graph-admission';
@@ -80,7 +81,7 @@ test('production composition closes long-term memory after a later startup failu
     const stores = await openInteractiveExecutionStoresForWrite(owner.lease);
     const session = await stores.sessionStore.create({
       cwd: root,
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
@@ -120,14 +121,14 @@ test('production recovery preserves legacy Automation history and closes an orph
     const stores = await openInteractiveExecutionStoresForWrite(owner.lease);
     const historical = await stores.sessionStore.create({
       cwd: root,
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
     });
     const pending = await stores.sessionStore.create({
       cwd: root,
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
@@ -348,7 +349,7 @@ test('production composition commits automatic titles through Host-owned Session
     try {
       const session = await manager.createSession({
         cwd: root,
-        backend: 'fake',
+        backend: 'ai-sdk',
         llmConnectionSlug: 'fake',
         model: 'fake-model',
         permissionMode: 'ask',
@@ -378,12 +379,51 @@ test('production composition commits automatic titles through Host-owned Session
   });
 });
 
+test('a legacy fake-backend session is refused with the product reason, not a registry error', async () => {
+  await withCompositionRoot(async ({ root, owner }) => {
+    const { composition, manager } = await createCapturedExecutionComposition(owner);
+    try {
+      // Written by an older build: this one never produces `fake`, but the
+      // durable header survives and activation dispatches straight off it.
+      const legacy = await manager.createSession({
+        cwd: root,
+        backend: 'fake',
+        llmConnectionSlug: 'fake',
+        model: 'fake-model',
+        permissionMode: 'ask',
+      });
+      const failure = await composition.handlers['turn.start'](
+        {
+          sessionId: legacy.id,
+          turnId: 'turn-legacy-fake',
+          content: { text: 'resume a retired local simulation' },
+        },
+        {
+          hostEpoch: 'execution-composition-test',
+          connectionId: 'legacy-fake-client',
+          surface: 'tui',
+          principal: 'local_os_user',
+          acquireResidency: () => ({ release() {} }),
+        },
+      ).then(
+        (result) => result,
+        (error: unknown) => error,
+      );
+      const message = failure instanceof Error ? failure.message : JSON.stringify(failure);
+      assert.doesNotMatch(message, /No backend factory registered/);
+      assert.equal(parseNoRealConnectionError(message).reason, 'fake_backend');
+    } finally {
+      await composition.close();
+    }
+  });
+});
+
 test('production composition orphans ownerless ShellRuns before serving Resource queries', async () => {
   await withCompositionRoot(async ({ root, owner }) => {
     const stores = await openInteractiveExecutionStoresForWrite(owner.lease);
     const session = await stores.sessionStore.create({
       cwd: root,
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
@@ -431,7 +471,7 @@ test('production Skill catalog resolves a Graph child durable tool surface', asy
     const stores = await openInteractiveExecutionStoresForWrite(owner.lease);
     const parent = await stores.sessionStore.create({
       cwd: root,
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
@@ -599,7 +639,7 @@ test('Skill capability previews omit unavailable Tavily search surfaces', async
     const stores = await openInteractiveExecutionStoresForWrite(owner.lease);
     const session = await stores.sessionStore.create({
       cwd: root,
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: connection.slug,
       model: 'fake-model',
       permissionMode: 'bypass',
@@ -656,7 +696,7 @@ test('production composition validates graph stop before aborting a claimed chil
     const claims = createAgentGraphControlStore(root);
     const parent = await stores.sessionStore.create({
       cwd: root,
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
@@ -868,7 +908,14 @@ async function createCapturedExecutionComposition(owner: InteractiveRootOwner):
     return originalRecover.call(this, stores);
   };
   try {
-    const composition = await createExecutionRuntimeHostComposition(compositionContext(owner));
+    // The production composition no longer registers a test backend of its
+    // own; the deterministic one arrives through the same `primaryBackendFactory`
+    // seam the Desktop E2E run uses.
+    const composition = await createExecutionRuntimeHostComposition(
+      compositionContext(owner),
+      {},
+      { primaryBackendFactory: (backendContext) => new FakeBackend(backendContext) },
+    );
     await composition.recover();
     if (!manager) throw new Error('Production execution composition did not construct Runtime');
     return { composition, manager };
@@ -889,7 +936,7 @@ async function createClaimedGraphChild(input: {
   const child = await input.stores.sessionStore.createSubagent({
     cwd: input.root,
     name: `Graph operator ${input.suffix}`,
-    backend: 'fake',
+    backend: 'ai-sdk',
     llmConnectionSlug: 'fake',
     model: 'fake-model',
     permissionMode: 'explore',
diff --git a/packages/runtime-host/src/__tests__/execution-host-message.test.ts b/packages/runtime-host/src/__tests__/execution-host-message.test.ts
index a40427ea2b..7bbdf059d6 100644
--- a/packages/runtime-host/src/__tests__/execution-host-message.test.ts
+++ b/packages/runtime-host/src/__tests__/execution-host-message.test.ts
@@ -34,7 +34,7 @@ import {
 import {
   FAKE_ASK_USER_QUESTION_PROMPT,
   FAKE_WAIT_FOR_STEERING_PROMPT,
-} from '@maka/runtime/fake-backend';
+} from '@maka/runtime/test-only/fake-backend';
 import { type MakaTool, type MakaToolContext } from '@maka/runtime/tool-runtime';
 import {
   openInteractiveExecutionStoresForRead,
diff --git a/packages/runtime-host/src/__tests__/execution-host-queue.test.ts b/packages/runtime-host/src/__tests__/execution-host-queue.test.ts
index 31ab553d38..99871e061a 100644
--- a/packages/runtime-host/src/__tests__/execution-host-queue.test.ts
+++ b/packages/runtime-host/src/__tests__/execution-host-queue.test.ts
@@ -34,7 +34,7 @@ import {
 import {
   FAKE_ASK_USER_QUESTION_PROMPT,
   FAKE_WAIT_FOR_STEERING_PROMPT,
-} from '@maka/runtime/fake-backend';
+} from '@maka/runtime/test-only/fake-backend';
 import { type MakaTool, type MakaToolContext } from '@maka/runtime/tool-runtime';
 import {
   openInteractiveExecutionStoresForRead,
diff --git a/packages/runtime-host/src/__tests__/execution-host-recovery.test.ts b/packages/runtime-host/src/__tests__/execution-host-recovery.test.ts
index aee148f2e2..d39180a7ed 100644
--- a/packages/runtime-host/src/__tests__/execution-host-recovery.test.ts
+++ b/packages/runtime-host/src/__tests__/execution-host-recovery.test.ts
@@ -25,7 +25,7 @@ import {
 import {
   FAKE_ASK_USER_QUESTION_PROMPT,
   FAKE_WAIT_FOR_STEERING_PROMPT,
-} from '@maka/runtime/fake-backend';
+} from '@maka/runtime/test-only/fake-backend';
 import { type MakaTool, type MakaToolContext } from '@maka/runtime/tool-runtime';
 import {
   openInteractiveExecutionStoresForRead,
diff --git a/packages/runtime-host/src/__tests__/execution-host.test.ts b/packages/runtime-host/src/__tests__/execution-host.test.ts
index c8cc1383c2..9dcd2f325a 100644
--- a/packages/runtime-host/src/__tests__/execution-host.test.ts
+++ b/packages/runtime-host/src/__tests__/execution-host.test.ts
@@ -36,7 +36,7 @@ import {
   FAKE_ASK_SANDBOX_BOUNDARY_PROMPT,
   FAKE_ASK_USER_QUESTION_PROMPT,
   FAKE_WAIT_FOR_STEERING_PROMPT,
-} from '@maka/runtime/fake-backend';
+} from '@maka/runtime/test-only/fake-backend';
 import { type MakaTool, type MakaToolContext } from '@maka/runtime/tool-runtime';
 import {
   openInteractiveExecutionStoresForRead,
diff --git a/packages/runtime-host/src/__tests__/fixtures/execution-host-suite.ts b/packages/runtime-host/src/__tests__/fixtures/execution-host-suite.ts
index c875ec6e08..c22b242dcb 100644
--- a/packages/runtime-host/src/__tests__/fixtures/execution-host-suite.ts
+++ b/packages/runtime-host/src/__tests__/fixtures/execution-host-suite.ts
@@ -36,7 +36,7 @@ import {
   FAKE_ASK_USER_QUESTION_PROMPT,
   FAKE_WAIT_FOR_STEERING_PROMPT,
   FakeBackend,
-} from '@maka/runtime/fake-backend';
+} from '@maka/runtime/test-only/fake-backend';
 import { type MakaTool, type MakaToolContext } from '@maka/runtime/tool-runtime';
 import {
   openInteractiveExecutionStoresForRead,
@@ -125,7 +125,7 @@ export class ExecutionFixture {
       stores = await openInteractiveExecutionStoresForWrite(owner.lease);
       const session = await stores.sessionStore.create({
         cwd: this.root,
-        backend: 'fake',
+        backend: 'ai-sdk',
         llmConnectionSlug: 'fake',
         model: 'fake-model',
         permissionMode: 'ask',
@@ -271,7 +271,7 @@ export class ExecutionFixture {
       stores = await openInteractiveExecutionStoresForWrite(owner.lease);
       const backends = new BackendRegistry();
       backends.register(
-        'fake',
+        'ai-sdk',
         (ctx) =>
           new FakeBackend({
             sessionId: ctx.sessionId,
@@ -475,7 +475,7 @@ export class ExecutionFixture {
       const child = await stores.sessionStore.createSubagent({
         cwd: this.root,
         name: `${agentName} ${kind}`,
-        backend: 'fake',
+        backend: 'ai-sdk',
         llmConnectionSlug: 'fake',
         model: 'fake-model',
         permissionMode: 'explore',
@@ -1025,7 +1025,7 @@ export async function withExecutionRoot(
     stores = await openInteractiveExecutionStoresForWrite(owner.lease);
     const session = await stores.sessionStore.create({
       cwd: root,
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
diff --git a/packages/runtime-host/src/__tests__/fixtures/execution-host.ts b/packages/runtime-host/src/__tests__/fixtures/execution-host.ts
index e361b1c90c..0af2d2dd77 100644
--- a/packages/runtime-host/src/__tests__/fixtures/execution-host.ts
+++ b/packages/runtime-host/src/__tests__/fixtures/execution-host.ts
@@ -1,7 +1,9 @@
 import { join } from 'node:path';
 import { inspect } from 'node:util';
+import { FakeBackend } from '@maka/runtime/test-only/fake-backend';
 import { createSqliteRuntimeStore } from '@maka/storage';
 import { startExecutionRuntimeHostCandidate } from '../../server/execution-candidate.js';
+import { createExecutionRuntimeHostComposition } from '../../server/execution-composition.js';
 import { runRuntimeHostProcessLifecycle } from '../../server/process-lifecycle.js';
 
 const [rootPath, expectedRootId, idleGraceRaw, recoverySessionId, recoveryRunId] =
@@ -19,11 +21,23 @@ if (!Number.isSafeInteger(idleGraceMs) || idleGraceMs < 0) {
   throw new Error('execution-host requires a non-negative idle grace');
 }
 
-const result = await startExecutionRuntimeHostCandidate({
-  rootPath,
-  expectedRootId,
-  idleGraceMs,
-});
+// The production composition registers no test backend. This fixture is a
+// candidate host in its own right, so it supplies the deterministic one through
+// the composition's `primaryBackendFactory` seam — the same path Desktop E2E
+// takes — and its sessions declare the real `ai-sdk` backend kind.
+const result = await startExecutionRuntimeHostCandidate(
+  {
+    rootPath,
+    expectedRootId,
+    idleGraceMs,
+  },
+  {
+    createComposition: (context, compositionOptions) =>
+      createExecutionRuntimeHostComposition(context, compositionOptions, {
+        primaryBackendFactory: (backendContext) => new FakeBackend(backendContext),
+      }),
+  },
+);
 if (result.kind === 'loser') process.exit(2);
 
 let recoveryOutcome: unknown;
diff --git a/packages/runtime-host/src/__tests__/goal-root-authority.test.ts b/packages/runtime-host/src/__tests__/goal-root-authority.test.ts
index f2bfb0659d..8ca97aafba 100644
--- a/packages/runtime-host/src/__tests__/goal-root-authority.test.ts
+++ b/packages/runtime-host/src/__tests__/goal-root-authority.test.ts
@@ -6,7 +6,7 @@ import { join } from 'node:path';
 import { test } from 'node:test';
 import type { AgentRunHeader } from '@maka/core/agent-run';
 import { BackendRegistry, SessionManager } from '@maka/runtime/session-manager';
-import { FakeBackend } from '@maka/runtime/fake-backend';
+import { FakeBackend } from '@maka/runtime/test-only/fake-backend';
 import { GOAL_SET_TOOL_NAME } from '@maka/runtime/goal-tools';
 import { goalCheckpoint } from '@maka/runtime/goal-state';
 import { type RuntimeHostedRootAuthority } from '@maka/runtime/message-authority';
diff --git a/packages/runtime-host/src/__tests__/plan-two-client-uds.test.ts b/packages/runtime-host/src/__tests__/plan-two-client-uds.test.ts
index 4233a67c4c..00e8f0930b 100644
--- a/packages/runtime-host/src/__tests__/plan-two-client-uds.test.ts
+++ b/packages/runtime-host/src/__tests__/plan-two-client-uds.test.ts
@@ -13,8 +13,9 @@ import {
   type RuntimeHostSessionSubscription,
 } from '../client/index.js';
 import { RUNTIME_HOST_PROTOCOL_VERSION, type SubscriptionFrame } from '../protocol/index.js';
+import { FakeBackend } from '@maka/runtime/test-only/fake-backend';
 import { createExecutionRuntimeHostComposition } from '../server/execution-composition.js';
-import { RuntimeHostKernel } from '../server/host-kernel.js';
+import { RuntimeHostKernel, type RuntimeHostCompositionFactory } from '../server/host-kernel.js';
 
 const PROTOCOL = {
   min: RUNTIME_HOST_PROTOCOL_VERSION,
@@ -36,7 +37,7 @@ test('two Clients and a restarted production Host share one retry-safe Plan auth
     const planStore = await openInteractivePlanStoreForWrite(owner.lease);
     const session = await setupStores.sessionStore.create({
       cwd: root,
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'explore',
@@ -62,7 +63,7 @@ test('two Clients and a restarted production Host share one retry-safe Plan auth
     host = await RuntimeHostKernel.start({
       owner,
       idleGraceMs: 30_000,
-      composition: defineInteractiveRuntimeHostComposition(createExecutionRuntimeHostComposition),
+      composition: defineInteractiveRuntimeHostComposition(deterministicBackendComposition),
     });
     owner = undefined;
     [desktop, tui] = await Promise.all([connect(root, 'desktop'), connect(root, 'tui')]);
@@ -115,7 +116,7 @@ test('two Clients and a restarted production Host share one retry-safe Plan auth
     host = await RuntimeHostKernel.start({
       owner,
       idleGraceMs: 30_000,
-      composition: defineInteractiveRuntimeHostComposition(createExecutionRuntimeHostComposition),
+      composition: defineInteractiveRuntimeHostComposition(deterministicBackendComposition),
     });
     owner = undefined;
     tui = await connect(root, 'tui');
@@ -235,3 +236,14 @@ function withTimeout<T>(promise: Promise<T>, timeoutMs: number, message: string)
     );
   });
 }
+
+/**
+ * The production composition registers no test backend; the deterministic one
+ * rides the same `primaryBackendFactory` seam Desktop E2E uses.
+ */
+const deterministicBackendComposition: RuntimeHostCompositionFactory = (context) =>
+  createExecutionRuntimeHostComposition(
+    context,
+    {},
+    { primaryBackendFactory: (backendContext) => new FakeBackend(backendContext) },
+  );
diff --git a/packages/runtime-host/src/__tests__/root-turn-coordinator.test.ts b/packages/runtime-host/src/__tests__/root-turn-coordinator.test.ts
index ab015a6322..2353c92fa9 100644
--- a/packages/runtime-host/src/__tests__/root-turn-coordinator.test.ts
+++ b/packages/runtime-host/src/__tests__/root-turn-coordinator.test.ts
@@ -11,7 +11,7 @@ import {
   classifyTerminalRuntimeLedger,
   commitTerminalRunWithRuntimeFact,
 } from '@maka/runtime/terminal-run-commit';
-import { FakeBackend, FAKE_ASK_USER_QUESTION_PROMPT } from '@maka/runtime/fake-backend';
+import { FakeBackend, FAKE_ASK_USER_QUESTION_PROMPT } from '@maka/runtime/test-only/fake-backend';
 import {
   IMPLEMENTATION_AGENT_DEFINITION,
   LOCAL_READ_AGENT_PROFILE,
diff --git a/packages/runtime-host/src/__tests__/runtime-policy-coordinator.test.ts b/packages/runtime-host/src/__tests__/runtime-policy-coordinator.test.ts
index 32f37eac2f..3adc25a24f 100644
--- a/packages/runtime-host/src/__tests__/runtime-policy-coordinator.test.ts
+++ b/packages/runtime-host/src/__tests__/runtime-policy-coordinator.test.ts
@@ -10,7 +10,7 @@ import type {
   CredentialLocator,
 } from '@maka/core/runtime-policy';
 import { REQUEST_BODY_OVERLAY_MAX_BYTES } from '@maka/core/runtime-policy';
-import { FAKE_ASK_USER_QUESTION_PROMPT, FakeBackend } from '@maka/runtime/fake-backend';
+import { FAKE_ASK_USER_QUESTION_PROMPT, FakeBackend } from '@maka/runtime/test-only/fake-backend';
 import { type MakaToolContext } from '@maka/runtime/tool-runtime';
 import { openInteractiveExecutionStoresForWrite } from '@maka/storage/execution-stores';
 import { openInteractiveRuntimePolicyStoresForWrite } from '@maka/storage/runtime-policy-stores';
@@ -105,19 +105,23 @@ test('production composition shares one gate across mutation and backend activat
     const setupStores = await openInteractiveExecutionStoresForWrite(owner.lease);
     const session = await setupStores.sessionStore.create({
       cwd: root,
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
     });
 
-    composition = await createExecutionRuntimeHostComposition({
-      owner,
-      hostEpoch: context.hostEpoch,
-      acquireResidency: context.acquireResidency,
-      retainUntilProcessExit: () => undefined,
-      requestDrain: () => undefined,
-    });
+    composition = await createExecutionRuntimeHostComposition(
+      {
+        owner,
+        hostEpoch: context.hostEpoch,
+        acquireResidency: context.acquireResidency,
+        retainUntilProcessExit: () => undefined,
+        requestDrain: () => undefined,
+      },
+      {},
+      { primaryBackendFactory: (backendContext) => new FakeBackend(backendContext) },
+    );
     await composition.recover();
 
     const initial = await composition.handlers['runtime.policy.query']({}, context);
@@ -205,18 +209,22 @@ test('production mutation releases the gate before active-turn backend disposal
     const setupStores = await openInteractiveExecutionStoresForWrite(owner.lease);
     const session = await setupStores.sessionStore.create({
       cwd: root,
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
     });
-    composition = await createExecutionRuntimeHostComposition({
-      owner,
-      hostEpoch: context.hostEpoch,
-      acquireResidency: context.acquireResidency,
-      retainUntilProcessExit: () => undefined,
-      requestDrain: () => undefined,
-    });
+    composition = await createExecutionRuntimeHostComposition(
+      {
+        owner,
+        hostEpoch: context.hostEpoch,
+        acquireResidency: context.acquireResidency,
+        retainUntilProcessExit: () => undefined,
+        requestDrain: () => undefined,
+      },
+      {},
+      { primaryBackendFactory: (backendContext) => new FakeBackend(backendContext) },
+    );
     await composition.recover();
 
     const initial = await composition.handlers['runtime.policy.query']({}, context);
@@ -302,21 +310,25 @@ test('production policy mutation drains and poisons activation when cached backe
     const setupStores = await openInteractiveExecutionStoresForWrite(owner.lease);
     const session = await setupStores.sessionStore.create({
       cwd: root,
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
     });
 
-    composition = await createExecutionRuntimeHostComposition({
-      owner,
-      hostEpoch: context.hostEpoch,
-      acquireResidency: context.acquireResidency,
-      retainUntilProcessExit: () => undefined,
-      requestDrain: () => {
-        drainRequests += 1;
+    composition = await createExecutionRuntimeHostComposition(
+      {
+        owner,
+        hostEpoch: context.hostEpoch,
+        acquireResidency: context.acquireResidency,
+        retainUntilProcessExit: () => undefined,
+        requestDrain: () => {
+          drainRequests += 1;
+        },
       },
-    });
+      {},
+      { primaryBackendFactory: (backendContext) => new FakeBackend(backendContext) },
+    );
     await composition.recover();
 
     const firstTurnId = randomUUID();
diff --git a/packages/runtime-host/src/__tests__/session-revision-two-client-uds.test.ts b/packages/runtime-host/src/__tests__/session-revision-two-client-uds.test.ts
index b0f3dc6163..68021f80ba 100644
--- a/packages/runtime-host/src/__tests__/session-revision-two-client-uds.test.ts
+++ b/packages/runtime-host/src/__tests__/session-revision-two-client-uds.test.ts
@@ -10,7 +10,7 @@ import { type AgentGraphOperatorProvisionRequest } from '@maka/core/agent-graph-
 import { type AgentRunHeader } from '@maka/core/agent-run';
 import { type RuntimeEvent } from '@maka/core/runtime-event';
 import { agentGraphIdForRootSession } from '@maka/runtime/stream-graph-coordinator';
-import { FAKE_ASK_USER_QUESTION_PROMPT } from '@maka/runtime/fake-backend';
+import { FAKE_ASK_USER_QUESTION_PROMPT } from '@maka/runtime/test-only/fake-backend';
 import { createAgentGraphControlStore } from '@maka/storage/agent-graph-control-store';
 import { openInteractiveArtifactStoreForWrite } from '@maka/storage/artifact-stores';
 import { openInteractiveExecutionStoresForWrite } from '@maka/storage/execution-stores';
@@ -584,7 +584,7 @@ async function seedSource(
     const source = await execution.sessionStore.create({
       cwd: root,
       name: 'Source Session',
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
@@ -592,7 +592,7 @@ async function seedSource(
     const busy = await execution.sessionStore.create({
       cwd: root,
       name: 'Busy Session',
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
@@ -600,7 +600,7 @@ async function seedSource(
     const linkedChildSource = await execution.sessionStore.create({
       cwd: root,
       name: 'Linked Child Source Session',
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
@@ -608,7 +608,7 @@ async function seedSource(
     const metadataLinkedSource = await execution.sessionStore.create({
       cwd: root,
       name: 'Metadata-linked Source Session',
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
@@ -616,7 +616,7 @@ async function seedSource(
     const archivedOwnedSource = await execution.sessionStore.create({
       cwd: root,
       name: 'Archived-owned Source Session',
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
@@ -624,7 +624,7 @@ async function seedSource(
     const continuationSource = await execution.sessionStore.create({
       cwd: root,
       name: 'Continuation Source Session',
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
@@ -894,7 +894,7 @@ async function seedSource(
       {
         cwd: root,
         name: 'Graph Worker',
-        backend: 'fake',
+        backend: 'ai-sdk',
         llmConnectionSlug: 'fake',
         model: 'fake-model',
         permissionMode: 'ask',
@@ -1132,7 +1132,7 @@ async function seedSource(
     const ordinaryLinkedChild = await execution.sessionStore.createSubagent({
       cwd: root,
       name: 'Metadata-linked Child Session',
-      backend: 'fake',
+      backend: 'ai-sdk',
       llmConnectionSlug: 'fake',
       model: 'fake-model',
       permissionMode: 'ask',
diff --git a/packages/runtime-host/src/candidate-cli.ts b/packages/runtime-host/src/candidate-cli.ts
index c6c7dc7a4f..dfa5319415 100644
--- a/packages/runtime-host/src/candidate-cli.ts
+++ b/packages/runtime-host/src/candidate-cli.ts
@@ -4,7 +4,6 @@ import { isCandidateStartupAttemptId } from './candidate-startup-failure.js';
 export interface ParsedInteractiveRuntimeHostCandidateArguments
   extends InteractiveRuntimeHostCandidateOptions {
   readonly startupAttemptId: string;
-  readonly desktopE2e?: true;
 }
 
 export function parseInteractiveRuntimeHostCandidateArguments(
@@ -18,7 +17,6 @@ export function parseInteractiveRuntimeHostCandidateArguments(
     'idle-grace-ms',
     'handshake-timeout-ms',
     'generation',
-    'desktop-e2e',
   ]);
   const values = new Map<string, string>();
   for (let index = 0; index < args.length; index += 2) {
@@ -51,7 +49,6 @@ export function parseInteractiveRuntimeHostCandidateArguments(
     idleGraceMs: readOptionalInteger(values, 'idle-grace-ms'),
     handshakeTimeoutMs: readOptionalInteger(values, 'handshake-timeout-ms'),
     ...(values.has('generation') ? { generation: readGeneration(values) } : {}),
-    ...(values.has('desktop-e2e') ? { desktopE2e: readDesktopE2e(values) } : {}),
   };
 }
 
@@ -61,11 +58,6 @@ function readGeneration(values: Map<string, string>): string {
   return value;
 }
 
-function readDesktopE2e(values: Map<string, string>): true {
-  if (values.get('desktop-e2e') !== '1') throw new Error('Invalid --desktop-e2e');
-  return true;
-}
-
 function readOptionalInteger(values: Map<string, string>, key: string): number | undefined {
   const raw = values.get(key);
   if (raw === undefined) return undefined;
diff --git a/packages/runtime-host/src/candidate-entry.ts b/packages/runtime-host/src/candidate-entry.ts
new file mode 100644
index 0000000000..a9a3af2555
--- /dev/null
+++ b/packages/runtime-host/src/candidate-entry.ts
@@ -0,0 +1,75 @@
+import {
+  candidateStartupFailureExitCode,
+  classifyCandidateStartupFailure,
+} from './candidate-startup-failure.js';
+import { parseInteractiveRuntimeHostCandidateArguments } from './candidate-cli.js';
+import { writeCandidateStartupDiagnostic } from './control/startup-diagnostic.js';
+import { installRuntimeHostLogCapture, runtimeHostLogBuffer } from './process-diagnostics.js';
+import {
+  type ExecutionRuntimeHostCandidateDependencies,
+  type ExecutionRuntimeHostCandidateOptions,
+  startExecutionRuntimeHostCandidate,
+} from './server/execution-candidate.js';
+import type { RuntimeHostKernel } from './server/host-kernel.js';
+import { runRuntimeHostProcessLifecycle } from './server/process-lifecycle.js';
+
+export interface ExecutionCandidateEntryHooks {
+  /** Applied to the parsed command line before the candidate starts. */
+  readonly overrideOptions?: (
+    options: ExecutionRuntimeHostCandidateOptions,
+  ) => ExecutionRuntimeHostCandidateOptions;
+  readonly dependencies?: ExecutionRuntimeHostCandidateDependencies;
+  /** Runs once the candidate has won; the returned callback stops the watch. */
+  readonly onWon?: (host: RuntimeHostKernel) => () => void;
+}
+
+/**
+ * The bootstrap both candidate entry modules share. The entry file is the only
+ * thing that differs between the production and Desktop E2E processes, so the
+ * argument parsing, startup diagnostics, and lifecycle live here instead of
+ * being copied — a copy is what silently drifts when either one gains a step.
+ */
+export async function runExecutionCandidateEntry(
+  argv: readonly string[],
+  hooks: ExecutionCandidateEntryHooks = {},
+): Promise<never | void> {
+  installRuntimeHostLogCapture();
+
+  let result: Awaited<ReturnType<typeof startExecutionRuntimeHostCandidate>>;
+  let rootId: string | undefined;
+  let startupAttemptId: string | undefined;
+  try {
+    const parsed = parseInteractiveRuntimeHostCandidateArguments(argv);
+    rootId = parsed.expectedRootId;
+    const { startupAttemptId: parsedStartupAttemptId, ...options } = parsed;
+    startupAttemptId = parsedStartupAttemptId;
+    result = await startExecutionRuntimeHostCandidate(
+      hooks.overrideOptions ? hooks.overrideOptions(options) : options,
+      hooks.dependencies ?? {},
+    );
+  } catch (error) {
+    const failure = classifyCandidateStartupFailure(error);
+    const logs = runtimeHostLogBuffer.snapshot();
+    console.error('[runtime-host] startup failed:', error);
+    if (rootId && startupAttemptId) {
+      await writeCandidateStartupDiagnostic({
+        rootId,
+        startupAttemptId,
+        failure,
+        error,
+        logs,
+      }).catch(() => undefined);
+    }
+    process.exit(candidateStartupFailureExitCode(failure));
+  }
+  if (result.kind === 'loser') process.exit(2);
+
+  const stopWatch = hooks.onWon?.(result.host);
+  try {
+    await runRuntimeHostProcessLifecycle(result.host);
+  } catch {
+    process.exitCode = 1;
+  } finally {
+    stopWatch?.();
+  }
+}
diff --git a/packages/runtime-host/src/client/connect-or-spawn.ts b/packages/runtime-host/src/client/connect-or-spawn.ts
index 511edcea88..10ab806aa9 100644
--- a/packages/runtime-host/src/client/connect-or-spawn.ts
+++ b/packages/runtime-host/src/client/connect-or-spawn.ts
@@ -52,7 +52,6 @@ export interface ConnectOrSpawnRuntimeHostInput {
   connectTimeoutMs?: number;
   handshakeTimeoutMs?: number;
   candidateEntrypoint: string | URL;
-  desktopE2e?: boolean;
   signal?: AbortSignal;
 }
 
@@ -265,7 +264,6 @@ export async function connectOrSpawnRuntimeHostWithDependencies(
             entrypoint: input.candidateEntrypoint,
             initialConnectionTimeoutMs: Math.ceil(remaining),
             ...(input.generation === undefined ? {} : { generation: input.generation }),
-            ...(input.desktopE2e ? { desktopE2e: true } : {}),
           });
           const attempt = await settleBeforeDeadline(launch.spawned, deadline, input.signal);
           if (attempt.startupFailure) {
diff --git a/packages/runtime-host/src/client/launcher.ts b/packages/runtime-host/src/client/launcher.ts
index a68c4dcafb..5059e42b35 100644
--- a/packages/runtime-host/src/client/launcher.ts
+++ b/packages/runtime-host/src/client/launcher.ts
@@ -14,7 +14,6 @@ export interface DetachedCandidateInput {
   initialConnectionTimeoutMs?: number;
   idleGraceMs?: number;
   handshakeTimeoutMs?: number;
-  desktopE2e?: boolean;
   executable?: string;
   entrypoint: string | URL;
   env?: NodeJS.ProcessEnv;
@@ -95,7 +94,6 @@ function spawnCandidate(
   appendArgument(args, '--idle-grace-ms', input.idleGraceMs);
   appendArgument(args, '--handshake-timeout-ms', input.handshakeTimeoutMs);
   appendArgument(args, '--generation', input.generation);
-  if (input.desktopE2e) args.push('--desktop-e2e', '1');
 
   // spawn() commits the side effect synchronously; spawned only reports that commit's outcome.
   const child = spawn(executable, args, {
diff --git a/packages/runtime-host/src/execution-candidate-main.ts b/packages/runtime-host/src/execution-candidate-main.ts
index d492d51045..4c6cf4c39f 100644
--- a/packages/runtime-host/src/execution-candidate-main.ts
+++ b/packages/runtime-host/src/execution-candidate-main.ts
@@ -1,67 +1,4 @@
 #!/usr/bin/env node
-import { parseInteractiveRuntimeHostCandidateArguments } from './candidate-cli.js';
-import {
-  createDesktopE2eExecutionCandidateDependencies,
-  DESKTOP_E2E_IDLE_GRACE_MS,
-  watchDesktopE2eParentProcess,
-} from './desktop-e2e-execution.js';
-import { startExecutionRuntimeHostCandidate } from './server/execution-candidate.js';
-import { runRuntimeHostProcessLifecycle } from './server/process-lifecycle.js';
-import { installRuntimeHostLogCapture, runtimeHostLogBuffer } from './process-diagnostics.js';
-import {
-  candidateStartupFailureExitCode,
-  classifyCandidateStartupFailure,
-} from './candidate-startup-failure.js';
-import { writeCandidateStartupDiagnostic } from './control/startup-diagnostic.js';
+import { runExecutionCandidateEntry } from './candidate-entry.js';
 
-installRuntimeHostLogCapture();
-
-let result: Awaited<ReturnType<typeof startExecutionRuntimeHostCandidate>>;
-let desktopE2e: true | undefined;
-let rootId: string | undefined;
-let startupAttemptId: string | undefined;
-try {
-  const parsed = parseInteractiveRuntimeHostCandidateArguments(process.argv.slice(2));
-  rootId = parsed.expectedRootId;
-  const {
-    desktopE2e: parsedDesktopE2e,
-    startupAttemptId: parsedStartupAttemptId,
-    ...parsedOptions
-  } = parsed;
-  startupAttemptId = parsedStartupAttemptId;
-  desktopE2e = parsedDesktopE2e;
-  const options = desktopE2e
-    ? { ...parsedOptions, idleGraceMs: DESKTOP_E2E_IDLE_GRACE_MS }
-    : parsedOptions;
-  result = await startExecutionRuntimeHostCandidate(
-    options,
-    desktopE2e ? createDesktopE2eExecutionCandidateDependencies() : {},
-  );
-} catch (error) {
-  const failure = classifyCandidateStartupFailure(error);
-  const logs = runtimeHostLogBuffer.snapshot();
-  console.error('[runtime-host] startup failed:', error);
-  if (rootId && startupAttemptId) {
-    await writeCandidateStartupDiagnostic({
-      rootId,
-      startupAttemptId,
-      failure,
-      error,
-      logs,
-    }).catch(() => undefined);
-  }
-  process.exit(candidateStartupFailureExitCode(failure));
-}
-if (result.kind === 'loser') process.exit(2);
-
-const stopParentWatch = desktopE2e
-  ? watchDesktopE2eParentProcess(() => result.host.close())
-  : undefined;
-
-try {
-  await runRuntimeHostProcessLifecycle(result.host);
-} catch {
-  process.exitCode = 1;
-} finally {
-  stopParentWatch?.();
-}
+await runExecutionCandidateEntry(process.argv.slice(2));
diff --git a/packages/runtime-host/src/server/execution-composition.ts b/packages/runtime-host/src/server/execution-composition.ts
index c2ca7fea76..9d1a58fc1f 100644
--- a/packages/runtime-host/src/server/execution-composition.ts
+++ b/packages/runtime-host/src/server/execution-composition.ts
@@ -1,4 +1,8 @@
 import { createHash, randomUUID } from 'node:crypto';
+import {
+  describeChatConfigurationReason,
+  NO_REAL_CONNECTION_CODE,
+} from '@maka/core/connection-error-copy';
 import type { RuntimeExecutionConnection } from '@maka/core/llm-connections';
 import { generalizedErrorMessage } from '@maka/core/redaction';
 import { emptyPlanSessionState } from '@maka/core/plan';
@@ -24,7 +28,6 @@ import {
   createFilesystemWorkerLaunchSpecProvider,
   FilesystemWorkerClient,
 } from '@maka/runtime/filesystem-worker';
-import { FakeBackend } from '@maka/runtime/fake-backend';
 import { isOAuthEnrollmentProviderEnabled } from '@maka/runtime/oauth-provider-contracts';
 import {
   loadHistoryCompactCheckpointsFromRunLedger,
@@ -286,7 +289,18 @@ export async function createExecutionRuntimeHostComposition(
     });
     await stores.messageReceiptStore.beginHostEpoch(context.hostEpoch);
     const backends = new BackendRegistry();
-    backends.register('fake', (backendContext) => new FakeBackend(backendContext));
+    // `fake` is a retired backend kind: this build never writes it, but a
+    // session or Automation persisted by an older one still can, and activation
+    // dispatches straight off that durable value. Registering an explicit
+    // refusal — rather than the test backend, or a read-path rewrite of the
+    // durable header — is what turns "no factory for kind=fake" into the
+    // product's existing answer for these rows: this task came from the retired
+    // local simulation, configure a real model and start a new one.
+    backends.register('fake', () => {
+      throw new Error(
+        `${NO_REAL_CONNECTION_CODE}:fake_backend: ${describeChatConfigurationReason('fake_backend')}`,
+      );
+    });
     const runtimePolicyActivation = new RuntimePolicyActivationGate();
     const runtimePolicy = new HostRuntimePolicyCoordinator(
       runtimePolicyStores,
diff --git a/packages/runtime-host/src/desktop-e2e-execution.ts b/packages/runtime-host/src/test-only/desktop-e2e-execution.ts
similarity index 94%
rename from packages/runtime-host/src/desktop-e2e-execution.ts
rename to packages/runtime-host/src/test-only/desktop-e2e-execution.ts
index f8e3d4895a..0a3aab8e83 100644
--- a/packages/runtime-host/src/desktop-e2e-execution.ts
+++ b/packages/runtime-host/src/test-only/desktop-e2e-execution.ts
@@ -3,10 +3,10 @@ import type {
   BackendCompactHistoryResult,
 } from '@maka/core/backend-types';
 import { buildHistoryCompactCheckpoint } from '@maka/runtime/history-compact-checkpoint';
-import { FakeBackend } from '@maka/runtime/fake-backend';
+import { FakeBackend } from '@maka/runtime/test-only/fake-backend';
 import { type BackendFactoryContext } from '@maka/runtime/session-manager';
-import type { ExecutionRuntimeHostCandidateDependencies } from './server/execution-candidate.js';
-import { createExecutionRuntimeHostComposition } from './server/execution-composition.js';
+import type { ExecutionRuntimeHostCandidateDependencies } from '../server/execution-candidate.js';
+import { createExecutionRuntimeHostComposition } from '../server/execution-composition.js';
 
 /** Fresh Desktop E2E workspaces never reconnect; keep election retry, skip production grace. */
 export const DESKTOP_E2E_IDLE_GRACE_MS = 500;
diff --git a/packages/runtime-host/src/test-only/execution-candidate-e2e-main.ts b/packages/runtime-host/src/test-only/execution-candidate-e2e-main.ts
new file mode 100644
index 0000000000..22c3a9d4f0
--- /dev/null
+++ b/packages/runtime-host/src/test-only/execution-candidate-e2e-main.ts
@@ -0,0 +1,22 @@
+#!/usr/bin/env node
+/**
+ * Desktop E2E candidate entry. It exists so the production entry
+ * (`execution-candidate-main.ts`) carries no import of the E2E composition and
+ * no `--desktop-e2e` branch: the entry file IS the switch, which is what keeps
+ * FakeBackend and this bootstrap out of the release artifacts.
+ *
+ * The E2E run still goes through the real Runtime Host composition — only the
+ * `primaryBackendFactory` seam is substituted.
+ */
+import { runExecutionCandidateEntry } from '../candidate-entry.js';
+import {
+  createDesktopE2eExecutionCandidateDependencies,
+  DESKTOP_E2E_IDLE_GRACE_MS,
+  watchDesktopE2eParentProcess,
+} from './desktop-e2e-execution.js';
+
+await runExecutionCandidateEntry(process.argv.slice(2), {
+  overrideOptions: (options) => ({ ...options, idleGraceMs: DESKTOP_E2E_IDLE_GRACE_MS }),
+  dependencies: createDesktopE2eExecutionCandidateDependencies(),
+  onWon: (host) => watchDesktopE2eParentProcess(() => host.close()),
+});
diff --git a/packages/runtime/README.md b/packages/runtime/README.md
index 02da95c787..bd85a047b2 100644
--- a/packages/runtime/README.md
+++ b/packages/runtime/README.md
@@ -8,7 +8,7 @@ The package root barrel and the subpaths declared in `package.json` are supporte
 
 - `SessionManager` for session and turn orchestration.
 - `BackendRegistry` and `AgentBackend` for backend selection.
-- `AiSdkBackend` and `FakeBackend` for the existing backend implementations.
+- `AiSdkBackend` for the shipped backend implementation. `FakeBackend` is test-only: it lives under `test-only/`, is exported as `@maka/runtime/test-only/fake-backend`, and release packaging drops that directory, so no production module may import it. Tests and the Desktop E2E run reach it through the composition's `primaryBackendFactory` seam.
 - Session execution-boundary APIs for managed sandbox expansion and explicit bypass.
 - `buildBuiltinTools()` and the workspace executor interfaces for tool composition.
 - `RuntimeRunner`, runtime events, projections, and recovery helpers for invocation lifecycle.
diff --git a/packages/runtime/package.json b/packages/runtime/package.json
index e7ce3efa87..71bdc6e8bc 100644
--- a/packages/runtime/package.json
+++ b/packages/runtime/package.json
@@ -21,7 +21,7 @@
     "./model-fetcher": "./dist/model-fetcher.js",
     "./materializer": "./dist/materializer.js",
     "./session-manager": "./dist/session-manager.js",
-    "./fake-backend": "./dist/fake-backend.js",
+    "./test-only/fake-backend": "./dist/test-only/fake-backend.js",
     "./test-only/observation-text-reader": "./dist/__tests__/observation-text-reader.js",
     "./filesystem-worker": "./dist/filesystem-worker/index.js",
     "./sandbox": "./dist/sandbox/index.js",
diff --git a/packages/runtime/src/__tests__/fake-backend.test.ts b/packages/runtime/src/__tests__/fake-backend.test.ts
index 42b072427c..a5a6e724dc 100644
--- a/packages/runtime/src/__tests__/fake-backend.test.ts
+++ b/packages/runtime/src/__tests__/fake-backend.test.ts
@@ -2,7 +2,7 @@ import assert from 'node:assert/strict';
 import { test } from 'node:test';
 import type { SessionEvent } from '@maka/core/events';
 import type { SessionHeader } from '@maka/core/session';
-import { FAKE_ASK_USER_QUESTION_PROMPT, FakeBackend } from '../fake-backend.js';
+import { FAKE_ASK_USER_QUESTION_PROMPT, FakeBackend } from '../test-only/fake-backend.js';
 import {
   RuntimeInteractionInvariantError,
   bindRuntimeInteractionRun,
diff --git a/packages/runtime/src/__tests__/runtime-continuation-crash.test.ts b/packages/runtime/src/__tests__/runtime-continuation-crash.test.ts
index 2ae875d675..013f974376 100644
--- a/packages/runtime/src/__tests__/runtime-continuation-crash.test.ts
+++ b/packages/runtime/src/__tests__/runtime-continuation-crash.test.ts
@@ -15,7 +15,7 @@ import { createSqliteAgentRunStore } from '@maka/storage';
 
 import { type RuntimeContinuationFailpoint } from '../agent-run.js';
 import { BackendRegistry, SessionManager } from '../session-manager.js';
-import { FakeBackend } from '../fake-backend.js';
+import { FakeBackend } from '../test-only/fake-backend.js';
 import { terminateChildProcessTree } from '../process-tree-terminator.js';
 
 const CRASH_CHILD_ENV = 'MAKA_RUNTIME_CONTINUATION_CRASH_CHILD';
diff --git a/packages/runtime/src/__tests__/session-manager.test.ts b/packages/runtime/src/__tests__/session-manager.test.ts
index b294de4031..3606749196 100644
--- a/packages/runtime/src/__tests__/session-manager.test.ts
+++ b/packages/runtime/src/__tests__/session-manager.test.ts
@@ -77,7 +77,7 @@ import {
   RuntimeKernel,
   type RuntimeKernelLike,
 } from '../runtime-kernel.js';
-import { FAKE_ASK_USER_QUESTION_PROMPT, FakeBackend } from '../fake-backend.js';
+import { FAKE_ASK_USER_QUESTION_PROMPT, FakeBackend } from '../test-only/fake-backend.js';
 import { RuntimeReadModel, RuntimeReadModelError } from '../runtime-read-model.js';
 import type { AgentBackend } from '@maka/core/backend-types';
 import type { MakaTool } from '../tool-runtime.js';
diff --git a/packages/runtime/src/__tests__/stream-graph-coordinator.test.ts b/packages/runtime/src/__tests__/stream-graph-coordinator.test.ts
index 9625d44fbc..e554377482 100644
--- a/packages/runtime/src/__tests__/stream-graph-coordinator.test.ts
+++ b/packages/runtime/src/__tests__/stream-graph-coordinator.test.ts
@@ -23,7 +23,7 @@ import {
   OPERATIONAL_STATE_DATABASE_NAME,
 } from '@maka/storage';
 import { createSqliteAgentRunStore, createWorkspaceRuntimeStore } from '@maka/storage';
-import { FakeBackend } from '../fake-backend.js';
+import { FakeBackend } from '../test-only/fake-backend.js';
 import { BackendRegistry, SessionManager } from '../session-manager.js';
 import { SessionActivityRegistry } from '../goal-turn-lifecycle.js';
 import { AgentGraphSupervisorWakeCoordinator } from '../agent-graph-supervisor-wake.js';
diff --git a/packages/runtime/src/fake-backend.ts b/packages/runtime/src/test-only/fake-backend.ts
similarity index 99%
rename from packages/runtime/src/fake-backend.ts
rename to packages/runtime/src/test-only/fake-backend.ts
index 5c7a555641..8bbb9552b6 100644
--- a/packages/runtime/src/fake-backend.ts
+++ b/packages/runtime/src/test-only/fake-backend.ts
@@ -16,8 +16,8 @@ import type { UserQuestionResponse } from '@maka/core/user-question';
 import {
   RuntimeInteractionInvariantError,
   type RuntimeUserQuestionClosureReason,
-} from './interaction-authority.js';
-import type { SessionStore } from './session-manager.js';
+} from '../interaction-authority.js';
+import type { SessionStore } from '../session-manager.js';
 
 const sleep = (ms: number) => new Promise((resolve) => setTimeout(resolve, ms));
 export const FAKE_ASK_USER_QUESTION_PROMPT = '__e2e_ask_user_question__';
diff --git a/scripts/release-cli-file-policy.mjs b/scripts/release-cli-file-policy.mjs
index ac40480439..3252b5d4b3 100644
--- a/scripts/release-cli-file-policy.mjs
+++ b/scripts/release-cli-file-policy.mjs
@@ -23,9 +23,15 @@ export function isThirdPartyDevelopmentArtifact(relativePath) {
   );
 }
 
+// Modules that only a test or E2E entry point may import. They are a
+// Maka-owned convention, so they are not part of DEVELOPMENT_DIRECTORIES: a
+// third-party package is free to ship a directory by that name.
+const MAKA_TEST_ONLY_DIRECTORY = 'test-only';
+
 export function isMakaDevelopmentArtifact(relativePath) {
   const segments = relativePath.split(/[\\/]/).filter(Boolean);
   if (segments.some((segment) => segment === 'src')) return true;
+  if (segments.some((segment) => segment === MAKA_TEST_ONLY_DIRECTORY)) return true;
   if (segments.some((segment) => DEVELOPMENT_DIRECTORIES.has(segment))) return true;
 
   const file = segments.at(-1) ?? '';
diff --git a/scripts/release-cli-file-policy.test.mjs b/scripts/release-cli-file-policy.test.mjs
index 64776b96ba..53ea3d6f3b 100644
--- a/scripts/release-cli-file-policy.test.mjs
+++ b/scripts/release-cli-file-policy.test.mjs
@@ -38,4 +38,19 @@ describe('CLI release file policy', () => {
     assert.equal(isMakaDevelopmentArtifact('dist/__tests__/fixture.js'), true);
     assert.equal(isMakaDevelopmentArtifact('dist/index.js'), false);
   });
+
+  test('rejects Maka test-only modules so no test backend can ship', () => {
+    for (const path of [
+      'dist/test-only/fake-backend.js',
+      'dist/test-only/execution-candidate-e2e-main.js',
+      String.raw`dist\test-only\desktop-e2e-execution.js`,
+    ]) {
+      assert.equal(isMakaDevelopmentArtifact(path), true, path);
+    }
+    assert.equal(isMakaDevelopmentArtifact('dist/execution-candidate-main.js'), false);
+  });
+
+  test('leaves a third-party test-only directory alone', () => {
+    assert.equal(isThirdPartyDevelopmentArtifact('dist/test-only/index.js'), false);
+  });
 });
diff --git a/scripts/release-cli-package.mjs b/scripts/release-cli-package.mjs
index 88bec20310..fe8d2e2e6f 100644
--- a/scripts/release-cli-package.mjs
+++ b/scripts/release-cli-package.mjs
@@ -369,7 +369,12 @@ function copyRuntimeDist(source, destination, options = {}) {
   copyTreeFiles(sourceDist, join(destination, 'dist'), (relativePath) => {
     const segments = relativePath.split(sep);
     const file = segments.at(-1) ?? '';
-    if (segments.some((segment) => segment === '__tests__' || segment === '__fixtures__')) {
+    if (
+      segments.some(
+        (segment) =>
+          segment === '__tests__' || segment === '__fixtures__' || segment === 'test-only',
+      )
+    ) {
       return false;
     }
     if (/(?:^|\.)test\.js$/.test(file) || file.endsWith('.d.ts') || file.endsWith('.map')) {