diff --git a/plugins/codex/scripts/lib/broker-lifecycle.mjs b/plugins/codex/scripts/lib/broker-lifecycle.mjs
index ef763819c..92663a2d7 100644
--- a/plugins/codex/scripts/lib/broker-lifecycle.mjs
+++ b/plugins/codex/scripts/lib/broker-lifecycle.mjs
@@ -7,6 +7,7 @@ import { spawn } from "node:child_process";
 import { fileURLToPath } from "node:url";
 import { createBrokerEndpoint, parseBrokerEndpoint } from "./broker-endpoint.mjs";
 import { resolveStateDir } from "./state.mjs";
+import { terminateProcessTree } from "./process.mjs";
 
 export const PID_FILE_ENV = "CODEX_COMPANION_APP_SERVER_PID_FILE";
 export const LOG_FILE_ENV = "CODEX_COMPANION_APP_SERVER_LOG_FILE";
@@ -123,7 +124,7 @@ export async function ensureBrokerSession(cwd, options = {}) {
       logFile: existing.logFile ?? null,
       sessionDir: existing.sessionDir ?? null,
       pid: existing.pid ?? null,
-      killProcess: options.killProcess ?? null
+      killProcess: options.killProcess ?? terminateProcessTree
     });
     clearBrokerSession(cwd);
   }
@@ -154,7 +155,7 @@ export async function ensureBrokerSession(cwd, options = {}) {
       logFile,
       sessionDir,
       pid: child.pid ?? null,
-      killProcess: options.killProcess ?? null
+      killProcess: options.killProcess ?? terminateProcessTree
     });
     return null;
   }
@@ -170,7 +171,11 @@ export async function ensureBrokerSession(cwd, options = {}) {
   return session;
 }
 
-export function teardownBrokerSession({ endpoint = null, pidFile, logFile, sessionDir = null, pid = null, killProcess = null }) {
+export function teardownBrokerSession({ endpoint = null, pidFile, logFile, sessionDir = null, pid = null, killProcess = terminateProcessTree }) {
+  // The broker is spawned detached and unref'd so it can outlive the process that
+  // started it. That makes termination here mandatory rather than optional: this
+  // function also unlinks the pid file, so a broker left running after its pid file
+  // is gone can never be found again by anything.
   if (Number.isFinite(pid) && killProcess) {
     try {
       killProcess(pid);
diff --git a/tests/broker-lifecycle.test.mjs b/tests/broker-lifecycle.test.mjs
new file mode 100644
index 000000000..637a2c887
--- /dev/null
+++ b/tests/broker-lifecycle.test.mjs
@@ -0,0 +1,111 @@
+import test from "node:test";
+import assert from "node:assert/strict";
+import fs from "node:fs";
+import os from "node:os";
+import path from "node:path";
+import { execFileSync } from "node:child_process";
+
+import {
+  ensureBrokerSession,
+  teardownBrokerSession
+} from "../plugins/codex/scripts/lib/broker-lifecycle.mjs";
+
+// The broker is spawned detached and unref'd, so it outlives the process that
+// started it by design. Teardown also unlinks the pid file. If teardown removes
+// the pid file without terminating the process, the broker is unreachable
+// forever: nothing knows its pid and nothing will ever reap it. These tests pin
+// that termination happens by default rather than only when a caller remembers
+// to inject a killer.
+
+function tempSessionFiles() {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), "cxc-test-"));
+  const pidFile = path.join(dir, "broker.pid");
+  const logFile = path.join(dir, "broker.log");
+  fs.writeFileSync(pidFile, "4242");
+  fs.writeFileSync(logFile, "");
+  return { dir, pidFile, logFile };
+}
+
+test("teardownBrokerSession terminates the broker when no killer is injected", () => {
+  const { pidFile, logFile } = tempSessionFiles();
+  const killed = [];
+
+  teardownBrokerSession({
+    pidFile,
+    logFile,
+    pid: 4242,
+    killProcess: (pid) => killed.push(pid)
+  });
+
+  assert.deepEqual(killed, [4242], "an injected killer must still be honoured");
+  assert.equal(fs.existsSync(pidFile), false, "pid file is removed");
+});
+
+test("teardownBrokerSession does not unlink the pid file while leaving the process alive", () => {
+  // The regression: the default was `killProcess = null`, so the guard
+  // `Number.isFinite(pid) && killProcess` was false and nothing was terminated,
+  // while the pid file was unlinked regardless. Reproduce the shape by asserting
+  // the default parameter is a callable rather than null.
+  const source = fs.readFileSync(
+    new URL("../plugins/codex/scripts/lib/broker-lifecycle.mjs", import.meta.url),
+    "utf8"
+  );
+
+  assert.match(
+    source,
+    /killProcess = terminateProcessTree/,
+    "teardownBrokerSession must default killProcess to a real terminator, not null"
+  );
+  assert.doesNotMatch(
+    source,
+    /killProcess: options\.killProcess \?\? null/,
+    "ensureBrokerSession must not coerce an absent injection to null; that orphans the broker it just tore down"
+  );
+});
+
+test("ensureBrokerSession does not orphan a broker that never becomes ready", async () => {
+  // Behavioural, and deliberately injects nothing: injecting a killer would
+  // exercise the path that always worked. This spawns a REAL process that never
+  // opens the endpoint, so readiness fails and teardown runs with whatever the
+  // default is. Before the fix that default was null and this process survived
+  // with its pid file already unlinked, which is the leak.
+  const cwd = fs.mkdtempSync(path.join(os.tmpdir(), "cxc-cwd-"));
+  const stubPath = path.join(cwd, "never-ready-broker.mjs");
+  fs.writeFileSync(stubPath, "setTimeout(() => {}, 30_000);\n");
+
+  const session = await ensureBrokerSession(cwd, {
+    timeoutMs: 250,
+    scriptPath: stubPath
+    // The real endpoint factory is used deliberately: overriding it with a bare
+    // path makes parseBrokerEndpoint throw before teardown is ever reached, so
+    // the test would fail for an unrelated reason in both directions.
+  });
+
+  assert.equal(session, null, "a broker that never becomes ready yields no session");
+
+  // The pid file is unlinked by teardown, so recover the pid from the process
+  // table instead: any surviving stub is by definition an orphan.
+  let survivors = [];
+  try {
+    const out = execFileSync("pgrep", ["-f", stubPath], { encoding: "utf8" });
+    survivors = out.trim().split("\n").filter(Boolean);
+  } catch {
+    // pgrep exits non-zero when nothing matches, which is the passing case.
+  }
+
+  try {
+    assert.equal(
+      survivors.length,
+      0,
+      `teardown left ${survivors.length} broker process(es) running after unlinking the pid file`
+    );
+  } finally {
+    for (const pid of survivors) {
+      try {
+        process.kill(Number(pid), "SIGKILL");
+      } catch {
+        // Already gone.
+      }
+    }
+  }
+});