diff --git a/test/helpers/fakeBackend.ts b/test/helpers/fakeBackend.ts
new file mode 100644
index 0000000..4b1a18b
--- /dev/null
+++ b/test/helpers/fakeBackend.ts
@@ -0,0 +1,138 @@
+import { writeFile } from 'node:fs/promises';
+
+import { vi } from 'vitest';
+
+import type { RendererBackend } from '../../src/renderer/backend.js';
+import type {
+  ReplayInput,
+  ReplayState,
+  ScreenshotResult,
+  SemanticSnapshot,
+} from '../../src/renderer/types.js';
+
+type MockFn = ReturnType<typeof vi.fn>;
+
+const PNG_HEADER = Buffer.from([0x89, 0x50, 0x4e, 0x47]);
+
+export interface FakeBackendOptions {
+  rendererBackend?: string;
+  bootImplementation?: () => Promise<void>;
+  resultOverrides?: Partial<ScreenshotResult>;
+  writePng?: boolean;
+  fail?: Error;
+  onScreenshot?: (
+    outputPath: string,
+    options?: { showCursor?: boolean },
+  ) => void;
+}
+
+export type FakeRendererBackend = RendererBackend & {
+  bootMock: MockFn;
+  replayToMock: MockFn;
+  snapshotMock: MockFn;
+  screenshotMock: MockFn;
+  getVisibleTextMock: MockFn;
+  disposeMock: MockFn;
+  setBooted: (value: boolean) => void;
+};
+
+export function createFakeBackend(
+  options: FakeBackendOptions = {},
+): FakeRendererBackend {
+  const rendererBackend = options.rendererBackend ?? 'fake-renderer';
+  const writePng = options.writePng ?? true;
+  let booted = false;
+
+  const bootMock = vi.fn((): Promise<void> => {
+    if (options.bootImplementation !== undefined) {
+      return options.bootImplementation();
+    }
+    booted = true;
+    return Promise.resolve();
+  });
+
+  const replayToMock = vi.fn(
+    (input: ReplayInput): Promise<ReplayState> =>
+      Promise.resolve({
+        lastSeq: input.targetSeq,
+        cols: input.initialCols,
+        rows: input.initialRows,
+        cursorRow: 0,
+        cursorCol: 0,
+      }),
+  );
+
+  const snapshotMock = vi.fn(
+    (): Promise<SemanticSnapshot> =>
+      Promise.resolve({
+        sessionId: 'session-01',
+        capturedAtSeq: 0,
+        cols: 80,
+        rows: 24,
+        cursorRow: 0,
+        cursorCol: 0,
+        isAltScreen: false,
+        visibleLines: [],
+      }),
+  );
+
+  const screenshotMock = vi.fn(
+    async (
+      outputPath: string,
+      screenshotOptions?: { showCursor?: boolean },
+    ): Promise<ScreenshotResult> => {
+      options.onScreenshot?.(outputPath, screenshotOptions);
+      if (options.fail !== undefined) {
+        throw options.fail;
+      }
+      if (writePng) {
+        await writeFile(outputPath, PNG_HEADER);
+      }
+      return {
+        sessionId: 'session-01',
+        capturedAtSeq: 5,
+        profileName: 'reference-dark',
+        cols: 80,
+        rows: 24,
+        artifactPath: outputPath,
+        pngSizeBytes: 4,
+        cursorVisible: screenshotOptions?.showCursor === true,
+        rendererBackend,
+        pixelWidth: 800,
+        pixelHeight: 600,
+        sha256: 'a'.repeat(64),
+        renderProfileHash: 'b'.repeat(64),
+        ...options.resultOverrides,
+      };
+    },
+  );
+
+  const getVisibleTextMock = vi.fn((): Promise<string> => Promise.resolve(''));
+
+  const disposeMock = vi.fn((): Promise<void> => {
+    booted = false;
+    return Promise.resolve();
+  });
+
+  return {
+    rendererBackend,
+    get isBooted() {
+      return booted;
+    },
+    setBooted(value: boolean) {
+      booted = value;
+    },
+    boot: bootMock,
+    bootMock,
+    replayTo: replayToMock,
+    replayToMock,
+    snapshot: snapshotMock,
+    snapshotMock,
+    screenshot: screenshotMock,
+    screenshotMock,
+    getVisibleText: getVisibleTextMock,
+    getVisibleTextMock,
+    dispose: disposeMock,
+    disposeMock,
+  };
+}
diff --git a/test/unit/host/renderer.test.ts b/test/unit/host/renderer.test.ts
index 7b9dae1..2c15bd0 100644
--- a/test/unit/host/renderer.test.ts
+++ b/test/unit/host/renderer.test.ts
@@ -10,22 +10,12 @@ import type { RendererBackend } from '../../../src/renderer/backend.js';
 import type {
   RenderProfileConfig,
   ReplayInput,
-  ReplayState,
-  ScreenshotResult,
-  SemanticSnapshot,
 } from '../../../src/renderer/types.js';
 
-type MockFn = ReturnType<typeof vi.fn>;
-
-type FakeRendererBackend = RendererBackend & {
-  bootMock: MockFn;
-  replayToMock: MockFn;
-  snapshotMock: MockFn;
-  screenshotMock: MockFn;
-  getVisibleTextMock: MockFn;
-  disposeMock: MockFn;
-  setBooted: (value: boolean) => void;
-};
+import {
+  createFakeBackend,
+  type FakeRendererBackend,
+} from '../../helpers/fakeBackend.js';
 
 function createProfile(name = 'default'): RenderProfileConfig {
   return {
@@ -57,84 +47,6 @@ function createReplayInput(overrides: Partial<ReplayInput> = {}): ReplayInput {
   };
 }
 
-function createFakeBackend(
-  options: {
-    bootImplementation?: () => Promise<void>;
-  } = {},
-): FakeRendererBackend {
-  let booted = false;
-  const bootMock = vi.fn((): Promise<void> => {
-    if (options.bootImplementation !== undefined) {
-      return options.bootImplementation();
-    }
-
-    booted = true;
-    return Promise.resolve();
-  });
-  const replayToMock = vi.fn(
-    (input: ReplayInput): Promise<ReplayState> =>
-      Promise.resolve({
-        lastSeq: input.targetSeq,
-        cols: input.initialCols,
-        rows: input.initialRows,
-        cursorRow: 0,
-        cursorCol: 0,
-      }),
-  );
-  const snapshotMock = vi.fn(
-    (): Promise<SemanticSnapshot> =>
-      Promise.resolve({
-        sessionId: 'session-01',
-        capturedAtSeq: 0,
-        cols: 80,
-        rows: 24,
-        cursorRow: 0,
-        cursorCol: 0,
-        isAltScreen: false,
-        visibleLines: [],
-      }),
-  );
-  const screenshotMock = vi.fn(
-    (outputPath: string): Promise<ScreenshotResult> =>
-      Promise.resolve({
-        sessionId: 'session-01',
-        capturedAtSeq: 0,
-        profileName: 'default',
-        cols: 80,
-        rows: 24,
-        artifactPath: outputPath,
-        pngSizeBytes: 1,
-      }),
-  );
-  const getVisibleTextMock = vi.fn((): Promise<string> => Promise.resolve(''));
-  const disposeMock = vi.fn((): Promise<void> => {
-    booted = false;
-    return Promise.resolve();
-  });
-
-  return {
-    rendererBackend: 'fake-renderer',
-    boot: bootMock,
-    bootMock,
-    replayTo: replayToMock,
-    replayToMock,
-    snapshot: snapshotMock,
-    snapshotMock,
-    screenshot: screenshotMock,
-    screenshotMock,
-    getVisibleText: getVisibleTextMock,
-    getVisibleTextMock,
-    dispose: disposeMock,
-    disposeMock,
-    get isBooted() {
-      return booted;
-    },
-    setBooted(value: boolean) {
-      booted = value;
-    },
-  };
-}
-
 function createDeferred<T>(): {
   promise: Promise<T>;
   resolve: (value: T | PromiseLike<T>) => void;
diff --git a/test/unit/renderer/libghosttyVtBackend.test.ts b/test/unit/renderer/libghosttyVtBackend.test.ts
index 1de9c7c..6f1145e 100644
--- a/test/unit/renderer/libghosttyVtBackend.test.ts
+++ b/test/unit/renderer/libghosttyVtBackend.test.ts
@@ -1,17 +1,15 @@
 import { describe, expect, it, vi } from 'vitest';
 
-import type { RendererBackend } from '../../../src/renderer/backend.js';
 import { LibghosttyVtBackend } from '../../../src/renderer/libghosttyVt/backend.js';
 import type { LibghosttyVtNativeModule } from '../../../src/renderer/libghosttyVt/backend.js';
 import type {
   RenderProfileConfig,
   ReplayInput,
-  ReplayState,
-  ScreenshotResult,
-  SemanticSnapshot,
 } from '../../../src/renderer/types.js';
 import { createLogger } from '../../../src/util/logger.js';
 
+import { createFakeBackend } from '../../helpers/fakeBackend.js';
+
 function createProfile(): RenderProfileConfig {
   return {
     name: 'reference-dark',
@@ -126,75 +124,6 @@ function createNativeFixture(options: { visibleText?: string } = {}) {
   };
 }
 
-function createFallbackBackend(): RendererBackend & {
-  bootMock: ReturnType<typeof vi.fn>;
-  replayToMock: ReturnType<typeof vi.fn>;
-  screenshotMock: ReturnType<typeof vi.fn>;
-  disposeMock: ReturnType<typeof vi.fn>;
-} {
-  let booted = false;
-  const bootMock = vi.fn(() => {
-    booted = true;
-    return Promise.resolve();
-  });
-  const replayToMock = vi.fn(
-    (input: ReplayInput): Promise<ReplayState> =>
-      Promise.resolve({
-        lastSeq: input.targetSeq,
-        cols: input.initialCols,
-        rows: input.initialRows,
-        cursorRow: 0,
-        cursorCol: 0,
-      }),
-  );
-  const screenshotMock = vi.fn(
-    (outputPath: string): Promise<ScreenshotResult> =>
-      Promise.resolve({
-        sessionId: 'session-01',
-        capturedAtSeq: 2,
-        profileName: 'reference-dark',
-        cols: 12,
-        rows: 5,
-        artifactPath: outputPath,
-        pngSizeBytes: 123,
-        rendererBackend: 'ghostty-web',
-      }),
-  );
-  const disposeMock = vi.fn(() => {
-    booted = false;
-    return Promise.resolve();
-  });
-
-  return {
-    rendererBackend: 'ghostty-web',
-    get isBooted() {
-      return booted;
-    },
-    boot: bootMock,
-    bootMock,
-    replayTo: replayToMock,
-    replayToMock,
-    snapshot: vi.fn(
-      (): Promise<SemanticSnapshot> =>
-        Promise.resolve({
-          sessionId: 'session-01',
-          capturedAtSeq: 2,
-          cols: 12,
-          rows: 5,
-          cursorRow: 0,
-          cursorCol: 0,
-          isAltScreen: false,
-          visibleLines: [],
-        }),
-    ),
-    screenshot: screenshotMock,
-    screenshotMock,
-    getVisibleText: vi.fn().mockResolvedValue(''),
-    dispose: disposeMock,
-    disposeMock,
-  };
-}
-
 function createBackend(
   fixture = createNativeFixture(),
   options: Partial<ConstructorParameters<typeof LibghosttyVtBackend>[2]> = {},
@@ -357,7 +286,17 @@ describe('LibghosttyVtBackend', () => {
 
   it('uses ghostty-web fallback for screenshots and preserves fallback metadata', async () => {
     const fixture = createNativeFixture();
-    const fallback = createFallbackBackend();
+    const fallback = createFakeBackend({
+      rendererBackend: 'ghostty-web',
+      writePng: false,
+      resultOverrides: {
+        capturedAtSeq: 2,
+        cols: 12,
+        rows: 5,
+        pngSizeBytes: 123,
+        rendererBackend: 'ghostty-web',
+      },
+    });
     const backend = createBackend(fixture, {
       fallbackFactory: () => fallback,
     });
@@ -383,7 +322,17 @@ describe('LibghosttyVtBackend', () => {
 
   it('disposes native and fallback resources idempotently', async () => {
     const fixture = createNativeFixture();
-    const fallback = createFallbackBackend();
+    const fallback = createFakeBackend({
+      rendererBackend: 'ghostty-web',
+      writePng: false,
+      resultOverrides: {
+        capturedAtSeq: 2,
+        cols: 12,
+        rows: 5,
+        pngSizeBytes: 123,
+        rendererBackend: 'ghostty-web',
+      },
+    });
     const backend = createBackend(fixture, {
       fallbackFactory: () => fallback,
     });
diff --git a/test/unit/renderer/registry.test.ts b/test/unit/renderer/registry.test.ts
index 4e114b8..ab06cfe 100644
--- a/test/unit/renderer/registry.test.ts
+++ b/test/unit/renderer/registry.test.ts
@@ -1,18 +1,13 @@
 import { describe, expect, it, vi } from 'vitest';
 
-import type { RendererBackend } from '../../../src/renderer/backend.js';
 import {
   DEFAULT_RENDERER_NAME,
   createRendererBackend,
   resolveRendererName,
 } from '../../../src/renderer/index.js';
-import type {
-  RenderProfileConfig,
-  ReplayInput,
-  ReplayState,
-  ScreenshotResult,
-  SemanticSnapshot,
-} from '../../../src/renderer/types.js';
+import type { RenderProfileConfig } from '../../../src/renderer/types.js';
+
+import { createFakeBackend } from '../../helpers/fakeBackend.js';
 
 function createProfile(): RenderProfileConfig {
   return {
@@ -26,51 +21,6 @@ function createProfile(): RenderProfileConfig {
   };
 }
 
-function createFakeBackend(rendererBackend: string): RendererBackend {
-  return {
-    rendererBackend,
-    isBooted: false,
-    boot: vi.fn().mockResolvedValue(undefined),
-    replayTo: vi.fn(
-      (input: ReplayInput): Promise<ReplayState> =>
-        Promise.resolve({
-          lastSeq: input.targetSeq,
-          cols: input.initialCols,
-          rows: input.initialRows,
-          cursorRow: 0,
-          cursorCol: 0,
-        }),
-    ),
-    snapshot: vi.fn(
-      (): Promise<SemanticSnapshot> =>
-        Promise.resolve({
-          sessionId: 'session-01',
-          capturedAtSeq: 0,
-          cols: 80,
-          rows: 24,
-          cursorRow: 0,
-          cursorCol: 0,
-          isAltScreen: false,
-          visibleLines: [],
-        }),
-    ),
-    screenshot: vi.fn(
-      (outputPath: string): Promise<ScreenshotResult> =>
-        Promise.resolve({
-          sessionId: 'session-01',
-          capturedAtSeq: 0,
-          profileName: 'reference-dark',
-          cols: 80,
-          rows: 24,
-          artifactPath: outputPath,
-          pngSizeBytes: 1,
-        }),
-    ),
-    getVisibleText: vi.fn().mockResolvedValue(''),
-    dispose: vi.fn().mockResolvedValue(undefined),
-  };
-}
-
 describe('renderer registry', () => {
   it('resolves the default renderer name', () => {
     expect(DEFAULT_RENDERER_NAME).toBe('ghostty-web');
@@ -100,7 +50,7 @@ describe('renderer registry', () => {
   });
 
   it('lazy-loads the native backend module for libghostty-vt only', async () => {
-    const fakeBackend = createFakeBackend('libghostty-vt');
+    const fakeBackend = createFakeBackend({ rendererBackend: 'libghostty-vt' });
     const LibghosttyVtBackend = vi.fn(function FakeLibghosttyVtBackend() {
       return fakeBackend;
     });
diff --git a/test/unit/screenshot/capture.test.ts b/test/unit/screenshot/capture.test.ts
index b52ae8e..816e460 100644
--- a/test/unit/screenshot/capture.test.ts
+++ b/test/unit/screenshot/capture.test.ts
@@ -1,17 +1,11 @@
 import { access, mkdir, readFile, writeFile } from 'node:fs/promises';
 import { dirname } from 'node:path';
 
-import { describe, expect, it, vi } from 'vitest';
+import { describe, expect, it } from 'vitest';
 
 import { createTemporarySessionDir } from '../../helpers.js';
 
-import type { RendererBackend } from '../../../src/renderer/backend.js';
-import type { ScreenshotResult } from '../../../src/protocol/messages.js';
-import type {
-  ReplayInput,
-  ReplayState,
-  SemanticSnapshot,
-} from '../../../src/renderer/types.js';
+import type { ScreenshotResult } from '../../../src/renderer/types.js';
 
 import {
   captureScreenshotResult,
@@ -23,6 +17,8 @@ import {
   screenshotFilename,
 } from '../../../src/storage/artifactPaths.js';
 
+import { createFakeBackend } from '../../helpers/fakeBackend.js';
+
 const TEST_SHA256 = 'a'.repeat(64);
 const TEST_RENDER_PROFILE_HASH = 'b'.repeat(64);
 // First four bytes of the PNG magic signature (0x89 'P' 'N' 'G'). Enough to
@@ -30,81 +26,6 @@ const TEST_RENDER_PROFILE_HASH = 'b'.repeat(64);
 // produces a real PNG, but these unit tests do not validate the format.
 const PNG_HEADER = Buffer.from([0x89, 0x50, 0x4e, 0x47]);
 
-interface FakeBackendOptions {
-  resultOverrides?: Partial<ScreenshotResult>;
-  writePng?: boolean;
-  fail?: Error;
-  onScreenshot?: (
-    outputPath: string,
-    options?: { showCursor?: boolean },
-  ) => void;
-}
-
-function createFakeBackend(options: FakeBackendOptions = {}): RendererBackend {
-  const writePng = options.writePng ?? true;
-
-  return {
-    rendererBackend: 'fake-renderer',
-    isBooted: false,
-    boot: vi.fn().mockResolvedValue(undefined),
-    replayTo: vi.fn(
-      (input: ReplayInput): Promise<ReplayState> =>
-        Promise.resolve({
-          lastSeq: input.targetSeq,
-          cols: input.initialCols,
-          rows: input.initialRows,
-          cursorRow: 0,
-          cursorCol: 0,
-        }),
-    ),
-    snapshot: vi.fn(
-      (): Promise<SemanticSnapshot> =>
-        Promise.resolve({
-          sessionId: 'session-01',
-          capturedAtSeq: 0,
-          cols: 80,
-          rows: 24,
-          cursorRow: 0,
-          cursorCol: 0,
-          isAltScreen: false,
-          visibleLines: [],
-        }),
-    ),
-    screenshot: vi.fn(
-      async (
-        outputPath: string,
-        screenshotOptions?: { showCursor?: boolean },
-      ): Promise<ScreenshotResult> => {
-        options.onScreenshot?.(outputPath, screenshotOptions);
-        if (options.fail !== undefined) {
-          throw options.fail;
-        }
-        if (writePng) {
-          await writeFile(outputPath, PNG_HEADER);
-        }
-        return {
-          sessionId: 'session-01',
-          capturedAtSeq: 5,
-          profileName: 'reference-dark',
-          cols: 80,
-          rows: 24,
-          artifactPath: outputPath,
-          pngSizeBytes: 4,
-          cursorVisible: screenshotOptions?.showCursor === true,
-          rendererBackend: 'fake-renderer',
-          pixelWidth: 800,
-          pixelHeight: 600,
-          sha256: TEST_SHA256,
-          renderProfileHash: TEST_RENDER_PROFILE_HASH,
-          ...options.resultOverrides,
-        };
-      },
-    ),
-    getVisibleText: vi.fn().mockResolvedValue(''),
-    dispose: vi.fn().mockResolvedValue(undefined),
-  };
-}
-
 async function createSessionDir(sessionId = 'session-01'): Promise<string> {
   return await createTemporarySessionDir(
     'agent-tty-screenshot-capture-',