diff --git a/docs/development/testing.md b/docs/development/testing.md
index fac0ef580d..f2afb40285 100644
--- a/docs/development/testing.md
+++ b/docs/development/testing.md
@@ -307,7 +307,10 @@ assertion. `vitest.config.ts` applies the deadlines in
 and 120 seconds for built projects, with hooks sharing their project's
 deadline. Coverage doubles every deadline, and so does hosted CI, where
 CPU-bound cases share every core with other workers and their subprocesses.
-Do not pass a per-test timeout to fit a slow host. Tag a bounded workload that needs longer with
+Do not pass a per-test timeout to fit a slow host. A child started through
+the `processTest` registry, including `cli.run`, gets the test's remaining
+deadline less a reporting reserve, so a hung child fails with its captured
+output. Pass `timeoutMs` only when the child's deadline is itself under test. Tag a bounded workload that needs longer with
 `{ tags: ["long-running"] }`, or a whole file with
 `// @module-tag long-running`. Assert elapsed time explicitly when speed is the
 behavior under test. Derive an explicit deadline only when a test must outlive
diff --git a/tests/boundary/process/processFixtureLifecycle.test.ts b/tests/boundary/process/processFixtureLifecycle.test.ts
index d4a897d13b..79f96c7488 100644
--- a/tests/boundary/process/processFixtureLifecycle.test.ts
+++ b/tests/boundary/process/processFixtureLifecycle.test.ts
@@ -6,7 +6,11 @@ import type { ChildProcessWithoutNullStreams } from "node:child_process";
 import { expect, it, vi } from "vitest";
 
 import { awaitAcknowledgement } from "../../fixtures/lifecycleAcknowledgement.js";
-import { createTestProcessRegistry } from "../../support/process/processFixture.js";
+import {
+  createTestProcessRegistry,
+  processTest,
+  remainingProcessDeadline,
+} from "../../support/process/processFixture.js";
 
 const within = async <T>(
   operation: Promise<T>,
@@ -277,3 +281,34 @@ it("terminates a stalled real child at its deadline while retaining captured out
     await processes.shutdown();
   }
 });
+
+it("derives a child deadline from the test deadline, keeping a reporting reserve", () => {
+  expect(remainingProcessDeadline(120_000, 0)).toBe(108_000);
+  expect(remainingProcessDeadline(30_000, 20_000)).toBe(5_000);
+  // The reserve never drops below five seconds, nor the deadline below one.
+  expect(remainingProcessDeadline(8_000, 0)).toBe(3_000);
+  expect(remainingProcessDeadline(8_000, 7_500)).toBe(1_000);
+  expect(remainingProcessDeadline(0, 0)).toBeUndefined();
+});
+
+processTest(
+  "stops a hung child before its test deadline and reports its output",
+  // The test deadline is the behavior under test: the child gets what remains.
+  { timeout: 8_000 },
+  async ({ processes }) => {
+    const started = performance.now();
+    const failure: unknown = await processes
+      .run(process.execPath, [
+        "-e",
+        "process.stdout.write('started'); setInterval(() => {}, 1_000);",
+      ])
+      .catch((error: unknown) => error);
+    expect(failure).toMatchObject({
+      message: expect.stringMatching(
+        /^Test process exceeded its \d+ms deadline$/u,
+      ),
+      cause: { stdout: "started" },
+    });
+    expect(performance.now() - started).toBeLessThan(6_000);
+  },
+);
diff --git a/tests/support/process/processFixture.ts b/tests/support/process/processFixture.ts
index 496ab20ec4..628111fd0a 100644
--- a/tests/support/process/processFixture.ts
+++ b/tests/support/process/processFixture.ts
@@ -8,6 +8,26 @@ import {
 import { test as base } from "vitest";
 
 const SHUTDOWN_GRACE_MS = 1_000;
+/** Minimum time kept back from a test deadline to stop a child and report it. */
+const MIN_REPORT_RESERVE_MS = 5_000;
+
+/**
+ * Default child deadline: the test's remaining deadline, less a reserve for
+ * stopping the child and reporting its captured output. A hung child then fails
+ * with its output instead of an opaque test timeout, without a fixed value that
+ * assumes a host's speed. Undefined when the test itself has no deadline.
+ */
+export const remainingProcessDeadline = (
+  testDeadlineMs: number,
+  elapsedMs: number,
+): number | undefined => {
+  if (!(testDeadlineMs > 0)) return undefined;
+  const reserve = Math.max(MIN_REPORT_RESERVE_MS, testDeadlineMs / 10);
+  return Math.max(
+    SHUTDOWN_GRACE_MS,
+    Math.floor(testDeadlineMs - elapsedMs - reserve),
+  );
+};
 
 /** Captured output and termination state from one child process. */
 export interface ProcessResult {
@@ -70,13 +90,17 @@ const stopChild = async (
   return waitForExit(child, SHUTDOWN_GRACE_MS);
 };
 
-/** Create a child-process registry for a fixture composition root. */
+/**
+ * Create a child-process registry for a fixture composition root. A run without
+ * its own `timeoutMs` uses `defaultTimeoutMs`, when supplied.
+ */
 export const createTestProcessRegistry = (
   spawnChild: (
     command: string,
     arguments_: readonly string[],
     options: SpawnOptionsWithoutStdio,
   ) => ChildProcessWithoutNullStreams = nodeSpawn,
+  defaultTimeoutMs?: () => number | undefined,
 ): TestProcesses => {
   const children = new Set<ChildProcessWithoutNullStreams>();
 
@@ -98,7 +122,11 @@ export const createTestProcessRegistry = (
   return {
     spawn,
     run: async (command, arguments_, options = {}) => {
-      const { input, timeoutMs, ...spawnOptions } = options;
+      const {
+        input,
+        timeoutMs = defaultTimeoutMs?.(),
+        ...spawnOptions
+      } = options;
       const child = spawn(command, arguments_, spawnOptions);
       const stdout: Buffer[] = [];
       const stderr: Buffer[] = [];
@@ -196,11 +224,16 @@ export const createTestProcessRegistry = (
   };
 };
 
-/** Vitest API extended with a bounded child-process registry. */
+/**
+ * Vitest API extended with a child-process registry whose runs are bounded by
+ * the remaining test deadline unless they choose their own.
+ */
 export const processTest = base.extend<{ processes: TestProcesses }>({
-  // oxlint-disable-next-line no-empty-pattern -- Vitest parses fixture dependencies from object destructuring.
-  processes: async ({}, use) => {
-    const processes = createTestProcessRegistry();
+  processes: async ({ task }, use) => {
+    const started = performance.now();
+    const processes = createTestProcessRegistry(nodeSpawn, () =>
+      remainingProcessDeadline(task.timeout, performance.now() - started),
+    );
     try {
       await use(processes);
     } finally {