diff --git a/.nx/version-plans/version-plan-1785423581556.md b/.nx/version-plans/version-plan-1785423581556.md
new file mode 100644
index 0000000..b1f2646
--- /dev/null
+++ b/.nx/version-plans/version-plan-1785423581556.md
@@ -0,0 +1,5 @@
+---
+__default__: patch
+---
+
+The web and Vega platforms start correctly again. Session setup previously threw `Cannot read properties of undefined (reading 'aborted')` because these runners received the harness config in place of their init options.
diff --git a/packages/platform-android/src/runner.ts b/packages/platform-android/src/runner.ts
index 0566bea..a6ef5af 100644
--- a/packages/platform-android/src/runner.ts
+++ b/packages/platform-android/src/runner.ts
@@ -1,7 +1,4 @@
-import {
-  HarnessPlatformRunner,
-  type HarnessPlatformInitOptions,
-} from '@react-native-harness/platforms';
+import { type HarnessPlatformRunnerFactory } from '@react-native-harness/platforms';
 import type { Config as HarnessConfig } from '@react-native-harness/config';
 import {
   AndroidPlatformConfigSchema,
@@ -17,11 +14,10 @@ import {
   initializeAndroidProcessEnv,
 } from './environment.js';
 
-const getAndroidRunner = async (
-  config: AndroidPlatformConfig,
-  harnessConfig: HarnessConfig,
-  init: HarnessPlatformInitOptions
-): Promise<HarnessPlatformRunner> => {
+const getAndroidRunner: HarnessPlatformRunnerFactory<
+  AndroidPlatformConfig,
+  HarnessConfig
+> = async (config, harnessConfig, init) => {
   const parsedConfig = AndroidPlatformConfigSchema.parse(config);
 
   initializeAndroidProcessEnv();
diff --git a/packages/platform-ios/src/runner.ts b/packages/platform-ios/src/runner.ts
index 39bf8f3..818ef99 100644
--- a/packages/platform-ios/src/runner.ts
+++ b/packages/platform-ios/src/runner.ts
@@ -1,7 +1,4 @@
-import {
-  HarnessPlatformRunner,
-  type HarnessPlatformInitOptions,
-} from '@react-native-harness/platforms';
+import { type HarnessPlatformRunnerFactory } from '@react-native-harness/platforms';
 import type { Config as HarnessConfig } from '@react-native-harness/config';
 import {
   ApplePlatformConfigSchema,
@@ -13,11 +10,10 @@ import {
   getAppleSimulatorPlatformInstance,
 } from './instance.js';
 
-const getAppleRunner = async (
-  config: ApplePlatformConfig,
-  harnessConfig: HarnessConfig,
-  init: HarnessPlatformInitOptions
-): Promise<HarnessPlatformRunner> => {
+const getAppleRunner: HarnessPlatformRunnerFactory<
+  ApplePlatformConfig,
+  HarnessConfig
+> = async (config, harnessConfig, init) => {
   const parsedConfig = ApplePlatformConfigSchema.parse(config);
 
   if (isAppleDeviceSimulator(parsedConfig.device)) {
diff --git a/packages/platform-vega/eslint.config.mjs b/packages/platform-vega/eslint.config.mjs
index c334bc0..8c8d168 100644
--- a/packages/platform-vega/eslint.config.mjs
+++ b/packages/platform-vega/eslint.config.mjs
@@ -8,7 +8,11 @@ export default [
       '@nx/dependency-checks': [
         'error',
         {
-          ignoredFiles: ['{projectRoot}/eslint.config.{js,cjs,mjs,ts,cts,mts}'],
+          ignoredDependencies: ['vite', 'vitest'],
+          ignoredFiles: [
+            '{projectRoot}/eslint.config.{js,cjs,mjs,ts,cts,mts}',
+            '{projectRoot}/src/**/__tests__/**',
+          ],
         },
       ],
     },
diff --git a/packages/platform-vega/package.json b/packages/platform-vega/package.json
index 501a3f9..f38fd0f 100644
--- a/packages/platform-vega/package.json
+++ b/packages/platform-vega/package.json
@@ -16,6 +16,7 @@
     }
   },
   "dependencies": {
+    "@react-native-harness/config": "workspace:*",
     "@react-native-harness/platforms": "workspace:*",
     "@react-native-harness/tools": "workspace:*",
     "zod": "^3.25.67",
diff --git a/packages/platform-vega/src/__tests__/runner.test.ts b/packages/platform-vega/src/__tests__/runner.test.ts
new file mode 100644
index 0000000..ddf0910
--- /dev/null
+++ b/packages/platform-vega/src/__tests__/runner.test.ts
@@ -0,0 +1,57 @@
+import { describe, expect, it, vi } from 'vitest';
+import {
+  DEFAULT_METRO_PORT,
+  type Config as HarnessConfig,
+} from '@react-native-harness/config';
+import type { VegaPlatformConfig } from '../config.js';
+
+const stopAppMock = vi.fn().mockResolvedValue(undefined);
+
+vi.mock('../kepler.js', () => ({
+  getVegaDeviceStatus: vi.fn().mockResolvedValue('running'),
+  isAppInstalled: vi.fn().mockResolvedValue(true),
+  isAppRunning: vi.fn().mockResolvedValue(false),
+  startApp: vi.fn().mockResolvedValue(undefined),
+  stopApp: stopAppMock,
+}));
+
+const harnessConfig = { metroPort: DEFAULT_METRO_PORT } as HarnessConfig;
+
+const config: VegaPlatformConfig = {
+  name: 'vega',
+  device: { type: 'emulator', deviceId: 'VegaTV_1' },
+  bundleId: 'com.example.app',
+};
+
+describe('getVegaRunner', () => {
+  it('is invoked with the (config, harnessConfig, init) shape the harness session uses', async () => {
+    // Regression test for the bug where the session called
+    // module.default(config, runtimeConfig, init), but this runner declared
+    // only (config, init?) — so runtimeConfig landed in the init slot and
+    // init.signal.aborted threw. Asserting the arity here catches an
+    // optional-parameter regression, since optional params drop out of
+    // Function.length.
+    const { default: getVegaRunner } = await import('../runner.js');
+    expect(getVegaRunner.length).toBe(3);
+
+    const init = { signal: new AbortController().signal };
+    const runner = await getVegaRunner(config, harnessConfig, init);
+    expect(runner.createAppSession).toBeTypeOf('function');
+    expect(runner.dispose).toBeTypeOf('function');
+  });
+
+  it('stops the app when the init signal aborts after session creation', async () => {
+    const { default: getVegaRunner } = await import('../runner.js');
+    const controller = new AbortController();
+
+    const runner = await getVegaRunner(config, harnessConfig, {
+      signal: controller.signal,
+    });
+    await runner.createAppSession();
+    stopAppMock.mockClear();
+
+    controller.abort();
+
+    expect(stopAppMock).toHaveBeenCalled();
+  });
+});
diff --git a/packages/platform-vega/src/runner.ts b/packages/platform-vega/src/runner.ts
index 9e00577..74ae680 100644
--- a/packages/platform-vega/src/runner.ts
+++ b/packages/platform-vega/src/runner.ts
@@ -4,9 +4,9 @@ import {
   type AppSessionState,
   DeviceNotFoundError,
   AppNotInstalledError,
-  type HarnessPlatformInitOptions,
-  HarnessPlatformRunner,
+  type HarnessPlatformRunnerFactory,
 } from '@react-native-harness/platforms';
+import type { Config as HarnessConfig } from '@react-native-harness/config';
 import { VegaPlatformConfigSchema, type VegaPlatformConfig } from './config.js';
 import * as kepler from './kepler.js';
 
@@ -15,10 +15,10 @@ const APP_EXIT_POLL_INTERVAL_MS = 1000;
 const sleep = (ms: number) =>
   new Promise((resolve) => setTimeout(resolve, ms));
 
-const getVegaRunner = async (
-  config: VegaPlatformConfig,
-  init?: HarnessPlatformInitOptions
-): Promise<HarnessPlatformRunner> => {
+const getVegaRunner: HarnessPlatformRunnerFactory<
+  VegaPlatformConfig,
+  HarnessConfig
+> = async (config, _harnessConfig, init) => {
   const parsedConfig = VegaPlatformConfigSchema.parse(config);
   const deviceId = parsedConfig.device.deviceId;
   const bundleId = parsedConfig.bundleId;
@@ -40,10 +40,10 @@ const getVegaRunner = async (
   // poll loop and the app on session teardown, in addition to the normal
   // dispose() path.
   const disposeCurrentAppSessionOnAbort = () => void currentAppSession?.dispose();
-  if (init?.signal.aborted) {
+  if (init.signal.aborted) {
     disposeCurrentAppSessionOnAbort();
   } else {
-    init?.signal.addEventListener('abort', disposeCurrentAppSessionOnAbort, {
+    init.signal.addEventListener('abort', disposeCurrentAppSessionOnAbort, {
       once: true,
     });
   }
diff --git a/packages/platform-vega/tsconfig.json b/packages/platform-vega/tsconfig.json
index 56b5cd9..0faf279 100644
--- a/packages/platform-vega/tsconfig.json
+++ b/packages/platform-vega/tsconfig.json
@@ -9,6 +9,9 @@
     {
       "path": "../platforms"
     },
+    {
+      "path": "../config"
+    },
     {
       "path": "./tsconfig.lib.json"
     }
diff --git a/packages/platform-vega/tsconfig.lib.json b/packages/platform-vega/tsconfig.lib.json
index 1ff1d7b..1267bfb 100644
--- a/packages/platform-vega/tsconfig.lib.json
+++ b/packages/platform-vega/tsconfig.lib.json
@@ -16,6 +16,9 @@
     },
     {
       "path": "../platforms/tsconfig.lib.json"
+    },
+    {
+      "path": "../config/tsconfig.lib.json"
     }
   ]
 }
diff --git a/packages/platform-vega/vite.config.ts b/packages/platform-vega/vite.config.ts
new file mode 100644
index 0000000..94d9c44
--- /dev/null
+++ b/packages/platform-vega/vite.config.ts
@@ -0,0 +1,18 @@
+/// <reference types='vitest' />
+import { defineConfig } from 'vite';
+
+export default defineConfig(() => ({
+  root: __dirname,
+  cacheDir: '../../node_modules/.vite/packages/platform-vega',
+  test: {
+    watch: false,
+    globals: true,
+    environment: 'node',
+    include: ['{src,tests}/**/*.{test,spec}.{js,mjs,cjs,ts,mts,cts,jsx,tsx}'],
+    reporters: ['default'],
+    coverage: {
+      reportsDirectory: './test-output/vitest/coverage',
+      provider: 'v8' as const,
+    },
+  },
+}));
diff --git a/packages/platform-web/eslint.config.mjs b/packages/platform-web/eslint.config.mjs
index c334bc0..8c8d168 100644
--- a/packages/platform-web/eslint.config.mjs
+++ b/packages/platform-web/eslint.config.mjs
@@ -8,7 +8,11 @@ export default [
       '@nx/dependency-checks': [
         'error',
         {
-          ignoredFiles: ['{projectRoot}/eslint.config.{js,cjs,mjs,ts,cts,mts}'],
+          ignoredDependencies: ['vite', 'vitest'],
+          ignoredFiles: [
+            '{projectRoot}/eslint.config.{js,cjs,mjs,ts,cts,mts}',
+            '{projectRoot}/src/**/__tests__/**',
+          ],
         },
       ],
     },
diff --git a/packages/platform-web/package.json b/packages/platform-web/package.json
index 8d3457c..3624570 100644
--- a/packages/platform-web/package.json
+++ b/packages/platform-web/package.json
@@ -16,6 +16,7 @@
     }
   },
   "dependencies": {
+    "@react-native-harness/config": "workspace:*",
     "@react-native-harness/platforms": "workspace:*",
     "playwright": "^1.50.0",
     "zod": "^3.25.67",
diff --git a/packages/platform-web/src/__tests__/runner.test.ts b/packages/platform-web/src/__tests__/runner.test.ts
new file mode 100644
index 0000000..8d4657e
--- /dev/null
+++ b/packages/platform-web/src/__tests__/runner.test.ts
@@ -0,0 +1,89 @@
+import { describe, expect, it, vi } from 'vitest';
+import {
+  DEFAULT_METRO_PORT,
+  type Config as HarnessConfig,
+} from '@react-native-harness/config';
+import type { WebPlatformConfig } from '../config.js';
+
+const closeMock = vi.fn().mockResolvedValue(undefined);
+const exposeFunctionMock = vi.fn().mockResolvedValue(undefined);
+const gotoMock = vi.fn().mockResolvedValue(undefined);
+const pageOnMock = vi.fn();
+
+const launchMock = vi.fn().mockResolvedValue({
+  close: closeMock,
+  newContext: vi.fn().mockResolvedValue({
+    newPage: vi.fn().mockResolvedValue({
+      exposeFunction: exposeFunctionMock,
+      goto: gotoMock,
+      on: pageOnMock,
+    }),
+  }),
+});
+
+vi.mock('playwright', () => ({
+  chromium: { launch: launchMock },
+  firefox: { launch: launchMock },
+  webkit: { launch: launchMock },
+}));
+
+const harnessConfig = { metroPort: DEFAULT_METRO_PORT } as HarnessConfig;
+
+const config: WebPlatformConfig = {
+  name: 'web',
+  browser: {
+    type: 'chromium',
+    url: 'http://localhost:8081',
+    headless: true,
+  },
+};
+
+describe('getWebRunner', () => {
+  it('is invoked with the (config, harnessConfig, init) shape the harness session uses', async () => {
+    // Regression test for the bug where the session called
+    // module.default(config, runtimeConfig, init), but this runner declared
+    // only (config, init?) — so runtimeConfig landed in the init slot and
+    // init.signal.aborted threw. Asserting the arity here catches an
+    // optional-parameter regression, since optional params drop out of
+    // Function.length.
+    const { default: getWebRunner } = await import('../runner.js');
+    expect(getWebRunner.length).toBe(3);
+
+    const init = { signal: new AbortController().signal };
+    const runner = await getWebRunner(config, harnessConfig, init);
+    expect(runner.createAppSession).toBeTypeOf('function');
+    expect(runner.dispose).toBeTypeOf('function');
+  });
+
+  it('does not throw when the init signal is already aborted', async () => {
+    // This is the exact failure mode from the reported bug: with the wrong
+    // argument order, `init` ended up holding runtimeConfig, and
+    // `init.signal.aborted` threw `TypeError: Cannot read properties of
+    // undefined (reading 'aborted')`.
+    const { default: getWebRunner } = await import('../runner.js');
+    const controller = new AbortController();
+    controller.abort();
+
+    const runner = await getWebRunner(config, harnessConfig, {
+      signal: controller.signal,
+    });
+    const session = await runner.createAppSession();
+
+    expect(session.dispose).toBeTypeOf('function');
+  });
+
+  it('closes the browser when the init signal aborts after session creation', async () => {
+    const { default: getWebRunner } = await import('../runner.js');
+    const controller = new AbortController();
+
+    const runner = await getWebRunner(config, harnessConfig, {
+      signal: controller.signal,
+    });
+    await runner.createAppSession();
+    closeMock.mockClear();
+
+    controller.abort();
+
+    expect(closeMock).toHaveBeenCalled();
+  });
+});
diff --git a/packages/platform-web/src/runner.ts b/packages/platform-web/src/runner.ts
index 314ceeb..c851b69 100644
--- a/packages/platform-web/src/runner.ts
+++ b/packages/platform-web/src/runner.ts
@@ -2,16 +2,16 @@ import {
   createAppSessionEmitter,
   type AppSession,
   type AppSessionState,
-  type HarnessPlatformInitOptions,
-  HarnessPlatformRunner,
+  type HarnessPlatformRunnerFactory,
 } from '@react-native-harness/platforms';
+import type { Config as HarnessConfig } from '@react-native-harness/config';
 import { chromium, firefox, webkit, type Browser, type Page } from 'playwright';
 import { WebPlatformConfigSchema, type WebPlatformConfig } from './config.js';
 
-const getWebRunner = async (
-  config: WebPlatformConfig,
-  init?: HarnessPlatformInitOptions
-): Promise<HarnessPlatformRunner> => {
+const getWebRunner: HarnessPlatformRunnerFactory<
+  WebPlatformConfig,
+  HarnessConfig
+> = async (config, _harnessConfig, init) => {
   const parsedConfig = WebPlatformConfigSchema.parse(config);
 
   let browser: Browser | null = null;
@@ -24,10 +24,10 @@ const getWebRunner = async (
     browser = null;
     page = null;
   };
-  if (init?.signal.aborted) {
+  if (init.signal.aborted) {
     closeBrowserOnAbort();
   } else {
-    init?.signal.addEventListener('abort', closeBrowserOnAbort, { once: true });
+    init.signal.addEventListener('abort', closeBrowserOnAbort, { once: true });
   }
 
   const launchBrowser = async () => {
diff --git a/packages/platform-web/tsconfig.json b/packages/platform-web/tsconfig.json
index 9f9888e..f0cc72d 100644
--- a/packages/platform-web/tsconfig.json
+++ b/packages/platform-web/tsconfig.json
@@ -6,6 +6,9 @@
     {
       "path": "../platforms"
     },
+    {
+      "path": "../config"
+    },
     {
       "path": "./tsconfig.lib.json"
     }
diff --git a/packages/platform-web/tsconfig.lib.json b/packages/platform-web/tsconfig.lib.json
index 595d0aa..167d036 100644
--- a/packages/platform-web/tsconfig.lib.json
+++ b/packages/platform-web/tsconfig.lib.json
@@ -14,6 +14,9 @@
   "references": [
     {
       "path": "../platforms/tsconfig.lib.json"
+    },
+    {
+      "path": "../config/tsconfig.lib.json"
     }
   ]
 }
diff --git a/packages/platform-web/vite.config.ts b/packages/platform-web/vite.config.ts
new file mode 100644
index 0000000..8b86f9c
--- /dev/null
+++ b/packages/platform-web/vite.config.ts
@@ -0,0 +1,18 @@
+/// <reference types='vitest' />
+import { defineConfig } from 'vite';
+
+export default defineConfig(() => ({
+  root: __dirname,
+  cacheDir: '../../node_modules/.vite/packages/platform-web',
+  test: {
+    watch: false,
+    globals: true,
+    environment: 'node',
+    include: ['{src,tests}/**/*.{test,spec}.{js,mjs,cjs,ts,mts,cts,jsx,tsx}'],
+    reporters: ['default'],
+    coverage: {
+      reportsDirectory: './test-output/vitest/coverage',
+      provider: 'v8' as const,
+    },
+  },
+}));
diff --git a/packages/platforms/src/index.ts b/packages/platforms/src/index.ts
index f6eef7e..c5561b7 100644
--- a/packages/platforms/src/index.ts
+++ b/packages/platforms/src/index.ts
@@ -18,6 +18,7 @@ export type {
   CrashArtifactWriter,
   HarnessPlatform,
   HarnessPlatformInitOptions,
+  HarnessPlatformRunnerFactory,
   CollectNativeCoverageOptions,
   HarnessPlatformRunner,
   RunTarget,
diff --git a/packages/platforms/src/types.ts b/packages/platforms/src/types.ts
index c079fe7..80c30f5 100644
--- a/packages/platforms/src/types.ts
+++ b/packages/platforms/src/types.ts
@@ -143,6 +143,21 @@ export type HarnessPlatformInitOptions = {
   diagnostics?: Diagnostics;
 };
 
+/**
+ * Shape every platform runner module's default export must satisfy. The
+ * harness session always invokes it as
+ * `module.default(platform.config, runtimeConfig, init)` — annotate each
+ * runner's default export with this type (parameterized by the platform's
+ * config type and its harness config type) so a signature drift like a
+ * missing `harnessConfig` parameter, or an optional `init`, fails to
+ * typecheck instead of throwing at runtime.
+ */
+export type HarnessPlatformRunnerFactory<TConfig, THarnessConfig> = (
+  config: TConfig,
+  harnessConfig: THarnessConfig,
+  init: HarnessPlatformInitOptions
+) => Promise<HarnessPlatformRunner>;
+
 export type HarnessCliCommandContext = {
   cwd: string;
   projectRoot: string;
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index e36eabc..5ae9b49 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -492,6 +492,9 @@ importers:
 
   packages/platform-vega:
     dependencies:
+      '@react-native-harness/config':
+        specifier: workspace:*
+        version: link:../config
       '@react-native-harness/platforms':
         specifier: workspace:*
         version: link:../platforms
@@ -507,6 +510,9 @@ importers:
 
   packages/platform-web:
     dependencies:
+      '@react-native-harness/config':
+        specifier: workspace:*
+        version: link:../config
       '@react-native-harness/platforms':
         specifier: workspace:*
         version: link:../platforms