diff --git a/evals/browser/lib/process.mjs b/evals/browser/lib/process.mjs
index 37cf7937..7f10b49d 100644
--- a/evals/browser/lib/process.mjs
+++ b/evals/browser/lib/process.mjs
@@ -14,10 +14,13 @@ export function runProcess(command, args, { cwd, env, timeoutMs = 300_000 } = {}
     let stderr = "";
     let timedOut = false;
     let settled = false;
+    let forceKillTimeout;
+    const useProcessGroup = process.platform !== "win32";
     const child = spawn(command, args, {
       cwd,
       env: { ...process.env, ...env },
       shell: false,
+      detached: useProcessGroup,
       stdio: ["ignore", "pipe", "pipe"],
     });
 
@@ -28,16 +31,31 @@ export function runProcess(command, args, { cwd, env, timeoutMs = 300_000 } = {}
       stderr = appendCaptured(stderr, chunk);
     });
 
+    function terminate(signal) {
+      if (!useProcessGroup || !child.pid) {
+        child.kill(signal);
+        return;
+      }
+      try {
+        // Descendants may keep the pipes open even after the launcher exits.
+        process.kill(-child.pid, signal);
+      } catch (error) {
+        if (error.code !== "ESRCH") child.kill(signal);
+      }
+    }
+
     const timeout = setTimeout(() => {
       timedOut = true;
-      child.kill("SIGTERM");
-      setTimeout(() => child.kill("SIGKILL"), 2_000).unref();
+      terminate("SIGTERM");
+      forceKillTimeout = setTimeout(() => terminate("SIGKILL"), 2_000);
+      forceKillTimeout.unref();
     }, timeoutMs);
 
     const finish = (result) => {
       if (settled) return;
       settled = true;
       clearTimeout(timeout);
+      clearTimeout(forceKillTimeout);
       resolve({
         command,
         args,
diff --git a/evals/browser/tests/process.test.mjs b/evals/browser/tests/process.test.mjs
index a884d711..62ea6ebc 100644
--- a/evals/browser/tests/process.test.mjs
+++ b/evals/browser/tests/process.test.mjs
@@ -33,3 +33,55 @@ test("process output flushes an incomplete final UTF-8 sequence", async () => {
   assert.equal(result.stdout, "�");
   assert.equal(result.stderr, "�");
 });
+
+for (const parentExits of [false, true]) {
+  test(`process timeout closes inherited pipes when the launcher ${parentExits ? "has exited" : "is running"}`, {
+    skip: process.platform === "win32",
+  }, async () => {
+    const descendant = `
+        process.on("SIGTERM", () => {
+          process.stdout.write("descendant terminated\\n");
+          process.exit(0);
+        });
+        process.stdout.write("descendant ready\\n");
+        setTimeout(() => process.exit(0), 5_000);
+      `;
+    const source = `
+        const { spawn } = require("node:child_process");
+        const child = spawn(process.execPath, ["-e", ${JSON.stringify(descendant)}], {
+          stdio: ["ignore", "inherit", "inherit"],
+        });
+        ${parentExits ? "child.unref();" : ""}
+      `;
+    const result = await runProcess(process.execPath, ["-e", source], { timeoutMs: 500 });
+
+    assert.equal(result.timedOut, true);
+    assert.match(result.stdout, /descendant ready/);
+    assert.match(result.stdout, /descendant terminated/);
+    assert.ok(result.durationMs < 3_000, `timeout took ${result.durationMs} ms`);
+    assert.equal(result.exitCode, parentExits ? 0 : null);
+    assert.equal(result.signal, parentExits ? null : "SIGTERM");
+  });
+}
+
+test("process timeout escalates for descendants that ignore SIGTERM", {
+  skip: process.platform === "win32",
+}, async () => {
+  const descendant = `
+      process.on("SIGTERM", () => {});
+      process.stdout.write("descendant ready\\n");
+      setTimeout(() => process.exit(0), 6_000);
+    `;
+  const source = `
+      const { spawn } = require("node:child_process");
+      spawn(process.execPath, ["-e", ${JSON.stringify(descendant)}], {
+        stdio: ["ignore", "inherit", "inherit"],
+      });
+    `;
+  const result = await runProcess(process.execPath, ["-e", source], { timeoutMs: 500 });
+
+  assert.equal(result.timedOut, true);
+  assert.match(result.stdout, /descendant ready/);
+  assert.ok(result.durationMs < 4_500, `timeout took ${result.durationMs} ms`);
+  assert.equal(result.signal, "SIGTERM");
+});