diff --git a/scripts/lib/browser-nested-source-map-e2e.mjs b/scripts/lib/browser-nested-source-map-e2e.mjs
index 0325b69e48..ff2af88632 100644
--- a/scripts/lib/browser-nested-source-map-e2e.mjs
+++ b/scripts/lib/browser-nested-source-map-e2e.mjs
@@ -1,13 +1,11 @@
 import assert from "node:assert/strict";
 import { execFile } from "node:child_process";
 import { createHash } from "node:crypto";
-import { join } from "node:path";
 import { fileURLToPath } from "node:url";
 import { promisify } from "node:util";
 import { TraceMap, eachMapping } from "@jridgewell/trace-mapping";
 import { CdpBrowserProvider } from "../../dist/browser/CdpBrowserProvider.js";
 import { CdpConnection } from "../../dist/browser/CdpConnection.js";
-import { waitForBrowserDevtoolsPort } from "../../dist/browser/BrowserProcessStartup.js";
 import { parseEvidence } from "../../dist/domain/evidence.js";
 import { traceSourceMap } from "../../dist/javascript/sourceMaps/TraceSourceMap.js";
 import { startBrowserSourceMapSite } from "../fixtures/browser-source-map-site.mjs";
@@ -63,7 +61,6 @@ async function verifyCase(executable, entrypoint, scenario) {
   let fixture;
   let connection;
   let primaryError;
-  let stderr = "";
   let phase = "native-map-admission";
   let browserVersion;
   let expected;
@@ -90,17 +87,7 @@ async function verifyCase(executable, entrypoint, scenario) {
 
     phase = "owned-browser-startup";
     fixture = await startRuntimeFixtureBrowser(executable, site.origin);
-    fixture.browser.stderr.on("data", (chunk) => {
-      if (stderr.length < 65536) stderr += chunk;
-    });
-    const port = await waitForBrowserDevtoolsPort({
-      child: fixture.browser,
-      executable,
-      activePortPath: join(fixture.profile, "DevToolsActivePort"),
-      stderr: () => stderr,
-      timeoutMs: 20_000,
-    });
-    const endpoint = `http://127.0.0.1:${port}`;
+    const endpoint = `http://127.0.0.1:${fixture.port}`;
     const provider = new CdpBrowserProvider();
     const targets = await provider.listTargets({
       cdp_endpoint: endpoint,
diff --git a/scripts/lib/browser-runtime-e2e.mjs b/scripts/lib/browser-runtime-e2e.mjs
index c2ebbed021..c4cb4f09f6 100644
--- a/scripts/lib/browser-runtime-e2e.mjs
+++ b/scripts/lib/browser-runtime-e2e.mjs
@@ -3,14 +3,12 @@ import { spawn, execFile } from "node:child_process";
 import { createHash } from "node:crypto";
 import { once } from "node:events";
 import { access } from "node:fs/promises";
-import { join } from "node:path";
 import { fileURLToPath } from "node:url";
 import { promisify } from "node:util";
 import { Client } from "@modelcontextprotocol/client";
 import { StdioClientTransport } from "@modelcontextprotocol/client/stdio";
 import { CdpBrowserProvider } from "../../dist/browser/CdpBrowserProvider.js";
 import { CdpConnection } from "../../dist/browser/CdpConnection.js";
-import { waitForBrowserDevtoolsPort } from "../../dist/browser/BrowserProcessStartup.js";
 import { parseEvidence } from "../../dist/domain/evidence.js";
 import { webExecutionSchema } from "../../dist/domain/webExecution.js";
 import { webEventListenersSchema } from "../../dist/domain/webEventListeners.js";
@@ -42,23 +40,11 @@ export async function verifyBrowserRuntime(
   const site = await startBrowserRuntimeSite();
   let fixture;
   let primaryError;
-  let stderr = "";
   let action;
   let client;
   try {
     fixture = await startRuntimeFixtureBrowser(executable, site.origin);
-    const { browser, profile } = fixture;
-    browser.stderr.on("data", (chunk) => {
-      if (stderr.length < 65536) stderr += chunk;
-    });
-    const port = await waitForBrowserDevtoolsPort({
-      child: browser,
-      executable,
-      activePortPath: join(profile, "DevToolsActivePort"),
-      stderr: () => stderr,
-      timeoutMs: 20_000,
-    });
-    const endpoint = `http://127.0.0.1:${port}`;
+    const endpoint = `http://127.0.0.1:${fixture.port}`;
     const targets = await new CdpBrowserProvider().listTargets({
       cdp_endpoint: endpoint,
       allowed_origins: [site.origin],
diff --git a/scripts/lib/browser-runtime-fixture-lifecycle.mjs b/scripts/lib/browser-runtime-fixture-lifecycle.mjs
index 24b88c793e..d39617f58c 100644
--- a/scripts/lib/browser-runtime-fixture-lifecycle.mjs
+++ b/scripts/lib/browser-runtime-fixture-lifecycle.mjs
@@ -8,8 +8,9 @@ import { spawnOwnedProviderProcess } from "../../dist/process/ProviderProcess.js
 import { createSystemProcessOwnershipHost } from "../../dist/process/ProcessOwnershipObservation.js";
 import { liveProcesses } from "../../dist/process/ProcessOwnershipProcessTree.js";
 import { CdpConnection } from "../../dist/browser/CdpConnection.js";
+import { waitForBrowserDevtoolsPort } from "../../dist/browser/BrowserProcessStartup.js";
 
-/** Launch only the synthetic fixture browser in its own verified process group. */
+/** Launch the synthetic fixture browser and await CDP in its own verified process group. */
 export async function startRuntimeFixtureBrowser(executable, origin) {
   const profile = await mkdtemp(join(tmpdir(), "rea-real-web-runtime-"));
   let owned;
@@ -38,9 +39,21 @@ export async function startRuntimeFixtureBrowser(executable, origin) {
       expectedCommand: null,
     });
     owned.process.stdout?.resume();
+    let stderr = "";
+    owned.process.stderr?.on("data", (chunk) => {
+      if (stderr.length < 65536) stderr += chunk;
+    });
+    const port = await waitForBrowserDevtoolsPort({
+      child: owned.process,
+      executable,
+      activePortPath: join(profile, "DevToolsActivePort"),
+      stderr: () => stderr,
+      // Match the broader real-browser verifier's allowance for cold Chrome
+      // startup on shared runners; early process failures still reject at once.
+      timeoutMs: 60_000,
+    });
     return {
-      browser: owned.process,
-      profile,
+      port,
       close: async () => {
         await stopOwnedBrowser(owned, profile);
         await rm(profile, {
diff --git a/tests/boundary/browser/runtimeFixtureCleanup.test.ts b/tests/boundary/browser/runtimeFixtureCleanup.test.ts
index be6aca7cff..4aafa317f9 100644
--- a/tests/boundary/browser/runtimeFixtureCleanup.test.ts
+++ b/tests/boundary/browser/runtimeFixtureCleanup.test.ts
@@ -1,6 +1,49 @@
 import { execFile } from "node:child_process";
+import { readdir } from "node:fs/promises";
 import { promisify } from "node:util";
 import { expect, it } from "vitest";
+import { createTestTempDirectory } from "../../fixtures/temporaryDirectory.js";
+
+it.skipIf(process.platform === "win32")(
+  "rejects an exited fixture before returning a lease and removes its profile",
+  async () => {
+    const root = await createTestTempDirectory("rea-runtime-startup-failure-");
+    const helper = new URL(
+      "../../../scripts/lib/browser-runtime-fixture-lifecycle.mjs",
+      import.meta.url,
+    );
+    const script = `
+      const { startRuntimeFixtureBrowser } = await import(${JSON.stringify(helper.href)});
+      // Node rejects Chrome's arguments immediately, exercising actual process exit.
+      try {
+        const fixture = await startRuntimeFixtureBrowser(process.execPath, 'http://127.0.0.1');
+        await fixture.close();
+        throw new Error('Expected startup failure');
+      } catch (error) {
+        if (error._tag !== 'BrowserStartupError') throw error;
+        console.log(JSON.stringify({
+          failure: error.failure,
+          exitCode: error.exitCode,
+          stderr: error.stderr,
+        }));
+      }
+    `;
+    const { stdout } = await promisify(execFile)(
+      process.execPath,
+      ["--input-type=module", "-e", script],
+      {
+        env: { ...process.env, TMPDIR: root, TMP: root, TEMP: root },
+        timeout: 10_000,
+      },
+    );
+    expect(JSON.parse(stdout)).toEqual({
+      failure: "exited",
+      exitCode: expect.any(Number),
+      stderr: expect.stringContaining("bad option"),
+    });
+    expect(await readdir(root)).toEqual([]);
+  },
+);
 
 it("attempts every fixture cleanup and retains the primary capture failure", async () => {
   const helper = new URL(