diff --git a/.changeset/21114-start-signal-forwarding.md b/.changeset/21114-start-signal-forwarding.md
new file mode 100644
index 00000000000..0ef829ee59a
--- /dev/null
+++ b/.changeset/21114-start-signal-forwarding.md
@@ -0,0 +1,31 @@
+---
+'@objectstack/cli': patch
+---
+
+fix(cli): `os start` forwards SIGTERM and SIGINT to its `serve` child and takes the child down when it exits
+
+Clause-②: no
+
+`os start` runs the server as a separate `serve` child process. It used to
+listen for that child's exit and nothing else. A SIGTERM or SIGINT sent to the
+`start` process alone (a plain `kill`, `docker stop`, a systemd stop, a CI step)
+ended `start` and left `serve` running with no parent. The port stayed bound,
+`/health` kept answering 200, and the next start on that port collided with it.
+
+`os start` now supervises its child the way `os dev` already does, through the
+same mechanism:
+
+- SIGTERM or SIGINT to `start` is forwarded to the child. `start` waits for the
+  child to shut down, then exits with the child's exit code, which is 0 after a
+  graceful shutdown. It used to die on the signal at once (shell status 143 for
+  SIGTERM, 130 for SIGINT) while the child kept running.
+- Whatever else ends `start`, the child is sent SIGTERM on the way out.
+- A child that exits on its own still ends `start` with the child's exit code,
+  as before.
+
+One visible side effect, the same one `os dev` already has: Ctrl-C at a terminal
+signals `start` and the child together, so the child now receives SIGINT twice
+and logs one `Shutdown already in progress, ignoring SIGINT` warning. The
+terminal prompt also returns only after the server has stopped, not before.
+
+No flag, port, environment variable, banner line or `os dev` behaviour changes.
diff --git a/packages/cli/src/commands/start.ts b/packages/cli/src/commands/start.ts
index af0f754aba2..b6d246505be 100644
--- a/packages/cli/src/commands/start.ts
+++ b/packages/cli/src/commands/start.ts
@@ -13,6 +13,7 @@ import { redirectStdoutToStderr } from '../utils/json-stdout.js';
 import { redactConnectionUrl } from '../utils/connection-display.js';
 import { databaseDriverFlag } from '../utils/database-driver-flag.js';
 import { childEnvWithResolvedArtifact } from '../utils/internal-artifact-channel.js';
+import { ServeRestartCoordinator } from '../utils/dev-restart.js';
 import { readEnvWithDeprecation } from '@objectstack/types';
 // The ONE port contract, shared with `dev` and with the `serve` child this
 // command spawns (#12673). ⛔ Nothing about ports is declared in this file —
@@ -445,18 +446,50 @@ export default class Start extends Command {
     }
 
     const binPath = process.argv[1];
-    const child = spawn(
-      process.execPath,
-      [
-        binPath,
-        'serve',
-        flags.ui ? '--ui' : '--no-ui',
-        ...(flags.verbose ? ['--verbose'] : []),
-        ...(flags['log-level'] ? ['--log-level', flags['log-level']] : []),
-      ],
-      { stdio: 'inherit', env: localEnv },
-    );
-    child.on('exit', (code) => process.exit(code ?? 0));
+
+    // ── The serve child's lifecycle: `os dev`'s mechanism, not a copy (#21114)
+    // `start` is a supervisor, so a signal sent to ITS pid alone — a plain
+    // `kill`, `docker stop`, a systemd stop, a CI step — has to reach the
+    // child too. This command used to listen for the child's `exit` and
+    // nothing else: a SIGTERM to the parent ended the parent and left `serve`
+    // running, reparented to init, port still bound and `/health` still
+    // answering 200.
+    //
+    // ⭐ Supervised by `ServeRestartCoordinator`, the ONE forwarding mechanism
+    // `os dev` already runs, used as is — `start` never calls
+    // `requestRestart`, so only the parts both commands need are reached:
+    // `beginShutdown` forwards SIGINT / SIGTERM and lets the child's exit end
+    // the parent; `killChildOnParentExit` reaps the child on whatever path
+    // ends the parent; and a child that exits on its own still ends the
+    // parent with `code ?? 0`, exactly the handler this replaces.
+    // ⛔ No forwarding or reaping logic of this command's own: what a signal
+    // DOES belongs in the coordinator, where `dev` gets the same change.
+    // Pinned end to end, with `os dev` as the control, by
+    // `test/start-signal-forwarding.e2e.test.ts`.
+    const coordinator = new ServeRestartCoordinator({
+      spawnChild: () => {
+        // `const child = spawn(` is the anchor
+        // `utils/port-contract-single-source.test.ts` reads to hold the port
+        // door above AHEAD of this spawn — keep the spelling.
+        const child = spawn(
+          process.execPath,
+          [
+            binPath,
+            'serve',
+            flags.ui ? '--ui' : '--no-ui',
+            ...(flags.verbose ? ['--verbose'] : []),
+            ...(flags['log-level'] ? ['--log-level', flags['log-level']] : []),
+          ],
+          { stdio: 'inherit', env: localEnv },
+        );
+        return child;
+      },
+      exitParent: (code) => process.exit(code),
+    });
+    process.on('SIGINT', () => coordinator.beginShutdown('SIGINT'));
+    process.on('SIGTERM', () => coordinator.beginShutdown('SIGTERM'));
+    process.on('exit', () => coordinator.killChildOnParentExit());
+    coordinator.start();
   }
 }
 
diff --git a/packages/cli/test/start-signal-forwarding.e2e.test.ts b/packages/cli/test/start-signal-forwarding.e2e.test.ts
new file mode 100644
index 00000000000..e3004ec236d
--- /dev/null
+++ b/packages/cli/test/start-signal-forwarding.e2e.test.ts
@@ -0,0 +1,286 @@
+// Copyright (c) 2026 ObjectStack. Licensed under the Apache-2.0 license.
+
+/**
+ * A signal to `os start` takes its `serve` child down with it (#21114).
+ *
+ * ## The defect, as measured on `origin/main` before the repair
+ *
+ * `os start` is a supervisor: it spawns `os serve` as a SEPARATE process and
+ * then waits. It listened for the child's `exit` and nothing else, so a SIGTERM
+ * sent to the `start` pid alone (a plain `kill`, a CI step, a process manager)
+ * ended the parent and left the child running, reparented to init, with its
+ * port still bound and `/health` still answering 200. `os dev` never had the
+ * defect: its `ServeRestartCoordinator` forwards SIGINT / SIGTERM to the child
+ * and reaps the child when the parent exits.
+ *
+ * The repair is that `start` supervises its child through that SAME
+ * coordinator, so there is one forwarding mechanism in the CLI, not two.
+ *
+ * ## What is asserted
+ *
+ * The ruling's pin, literally: a SIGTERM (and a SIGINT) to the PARENT leaves no
+ * child process and a free port. `os dev`, booted on the same artifact and sent
+ * the same signal, is the control: it states that the instrument reads the
+ * shipped mechanism correctly, and that `dev`'s behaviour did not move.
+ *
+ * Both readings carry a positive control taken on the same boot before the
+ * signal: the probe sees the child ALIVE and the port BOUND. Without that, "no
+ * child" passes on a probe that cannot see children at all, and "port free"
+ * passes on a child that bound somewhere else.
+ *
+ * ⛔ The signal goes to the parent's pid ALONE, never to its process group. A
+ * group signal reaches the child directly and passes whether or not the parent
+ * forwards anything — it is the shape that HID this defect (the neighbouring
+ * `start-port-banner-agreement.e2e.test.ts` documents why it kills the group,
+ * and for its purpose it is right to).
+ *
+ * ## Spawn shape
+ *
+ * The SOURCE entry, `bin/run-dev.js`, under the tsx loader passed as an
+ * `--import` flag — never the `tsx` CLI, which spawns the script as a child of
+ * its own and relays signals to it, so the pid this file signals would be tsx's
+ * rather than the command's. Loaded this way the spawned pid IS the `os start` /
+ * `os dev` parent, and the `serve` process is its direct child.
+ *
+ * The subject is the supervisor's wiring in `src/commands/start.ts`, which the
+ * built entry runs byte for byte from `dist/`, so the source entry measures the
+ * same code without a build prerequisite. The `development` posture it pins
+ * re-opens `serve`'s port auto-shift; that is harmless here because the port is
+ * read back out of the child's own banner (`boundPortFromBanner`) rather than
+ * assumed from what this file asked for. The built entry was measured too, by
+ * hand, before and after the repair (the PR that landed this records both).
+ *
+ * Each child gets its own process group (`detached: true`) so `afterEach` can
+ * still SIGKILL a survivor — which is exactly what the unrepaired command
+ * leaves behind — instead of leaking it into the container.
+ */
+
+import { describe, it, expect, afterEach } from 'vitest';
+import { execFileSync, spawn, type ChildProcess } from 'node:child_process';
+import { mkdtempSync, rmSync, writeFileSync } from 'node:fs';
+import { tmpdir } from 'node:os';
+import { join, resolve, dirname } from 'node:path';
+import { fileURLToPath } from 'node:url';
+import { PROTOCOL_MAJOR } from '@objectstack/spec/kernel';
+import {
+  childEnv,
+  boundPortFromBanner,
+  portIsFree,
+  reservePort,
+} from './helpers/serve-process.js';
+
+const HERE = dirname(fileURLToPath(import.meta.url));
+const RUN_DEV_JS = resolve(HERE, '../bin/run-dev.js');
+const TSX_LOADER = resolve(HERE, '../../../node_modules/tsx/dist/loader.mjs');
+
+/** The banner tail `printServerReady` ends with — the boot is fully printed. */
+const BANNER_TAIL = /Press Ctrl\+C to stop/;
+
+const BOOT_TIMEOUT_MS = 180_000;
+/** The kernel's graceful shutdown, plus event-loop delivery of the exit. */
+const EXIT_TIMEOUT_MS = 60_000;
+
+/** A minimal but real compiled artifact, booted identically by both commands. */
+const ARTIFACT = JSON.stringify({
+  manifest: {
+    id: 'com.example.signals',
+    name: 'signals',
+    version: '1.0.0',
+    type: 'app',
+    engines: { protocol: `^${PROTOCOL_MAJOR}` },
+  },
+  objects: [
+    { name: 'sig_note', label: 'Note', fields: { name: { type: 'text', label: 'Name' } } },
+  ],
+  views: [],
+  apps: [],
+  flows: [],
+  requires: [],
+});
+
+type Command = 'start' | 'dev';
+
+/** The argv that boots `command` on `artifact` at `port`, with no Console. */
+function argvFor(command: Command, artifact: string, port: number): string[] {
+  // `start` declares `--ui` with `allowNo` (default on); `dev` declares it
+  // default-off and rejects `--no-ui`. Both therefore boot without a Console.
+  return command === 'start'
+    ? ['start', '--artifact', artifact, '--port', String(port), '--no-ui']
+    : ['dev', '--artifact', artifact, '--port', String(port)];
+}
+
+/**
+ * The pids of `ppid`'s `serve` children, read from the process table — a child
+ * whose argv carries the `serve` command token.
+ *
+ * ⚠️ Selected by argv, not "every child", and MEASURED to need it: on a cold
+ * tsx transform cache the loader runs an `esbuild --service` process as a
+ * second child of the CLI parent. It exits on its own a moment AFTER the parent
+ * (alive at the parent's exit, gone 2 s later, 2 of 2 runs with
+ * `TSX_DISABLE_CACHE=1`), so counting it reads as an orphaned server on the
+ * file's first boot and on no other. The ruling's sentence is about the
+ * `serve` child, and so is this probe. `-ww` keeps BSD `ps` from cutting the
+ * argv at a terminal width.
+ */
+function serveChildPidsOf(ppid: number): number[] {
+  const table = execFileSync('ps', ['-A', '-ww', '-o', 'pid=,ppid=,args='], {
+    encoding: 'utf8',
+    env: childEnv(),
+  });
+  return table
+    .split('\n')
+    .map((line) => line.trim().split(/\s+/))
+    .filter(([pid, parent, ...args]) =>
+      Number.isInteger(Number(pid)) && Number(parent) === ppid && args.includes('serve'))
+    .map(([pid]) => Number(pid));
+}
+
+/** Does `pid` name a live process? (`EPERM` = alive, owned by someone else.) */
+function isAlive(pid: number): boolean {
+  try {
+    process.kill(pid, 0);
+    return true;
+  } catch (err) {
+    return (err as NodeJS.ErrnoException).code === 'EPERM';
+  }
+}
+
+/** SIGKILL a whole process group — the survivor sweep, never the measurement. */
+const killGroup = (child: ChildProcess | undefined): void => {
+  if (child?.pid === undefined) return;
+  try { process.kill(-child.pid, 'SIGKILL'); } catch { /* group already gone */ }
+};
+
+interface Exit { code: number | null; signal: NodeJS.Signals | null }
+
+interface Booted {
+  parent: ChildProcess;
+  port: number;
+  output: () => string;
+  exited: Promise<Exit>;
+}
+
+let running: ChildProcess | undefined;
+let workdir: string | undefined;
+
+afterEach(() => {
+  killGroup(running);
+  running = undefined;
+  if (workdir) rmSync(workdir, { recursive: true, force: true });
+  workdir = undefined;
+});
+
+/** Boot a real `os <command>` and resolve once its child printed the ready banner. */
+function boot(command: Command): Promise<Booted> {
+  const dir = mkdtempSync(join(tmpdir(), `os-${command}-signal-`));
+  workdir = dir;
+  const artifact = join(dir, 'objectstack.json');
+  writeFileSync(artifact, ARTIFACT);
+  const port = reservePort();
+
+  return new Promise((resolveBoot, rejectBoot) => {
+    const parent = spawn(
+      process.execPath,
+      ['--import', TSX_LOADER, RUN_DEV_JS, ...argvFor(command, artifact, port)],
+      {
+        cwd: dir,
+        env: childEnv({
+          NO_COLOR: '1',
+          OS_HOME: join(dir, 'home'),
+          OS_LOG_LEVEL: 'error',
+        }),
+        stdio: ['ignore', 'pipe', 'pipe'],
+        detached: true,
+      },
+    );
+    running = parent;
+
+    let output = '';
+    let ready = false;
+    const exited = new Promise<Exit>((settle) => {
+      parent.once('exit', (code, signal) => settle({ code, signal }));
+    });
+
+    const timer = setTimeout(() => {
+      if (ready) return;
+      rejectBoot(new Error(`os ${command} never printed a complete banner.\n--- output ---\n${output}`));
+    }, BOOT_TIMEOUT_MS);
+
+    const onData = (d: unknown) => {
+      output += String(d);
+      if (!ready && BANNER_TAIL.test(output)) {
+        ready = true;
+        clearTimeout(timer);
+        const readback = boundPortFromBanner(output);
+        if (readback.state !== 'bound') {
+          rejectBoot(new Error(`os ${command}: banner unreadable (${readback.state}).\n--- output ---\n${output}`));
+          return;
+        }
+        resolveBoot({ parent, port: readback.port, output: () => output, exited });
+      }
+    };
+    parent.stdout?.on('data', onData);
+    parent.stderr?.on('data', onData);
+    parent.on('error', (err) => { clearTimeout(timer); rejectBoot(err); });
+    void exited.then(({ code, signal }) => {
+      if (ready) return;
+      clearTimeout(timer);
+      rejectBoot(new Error(
+        `os ${command} exited (${signal ?? code}) before its banner.\n--- output ---\n${output}`,
+      ));
+    });
+  });
+}
+
+/** Signal the parent alone, then read what is left of its child and its port. */
+async function signalParent(command: Command, signal: NodeJS.Signals): Promise<void> {
+  const { parent, port, output, exited } = await boot(command);
+  const parentPid = parent.pid!;
+
+  // ── Positive controls, on the same boot, before the signal ─────────────
+  const children = serveChildPidsOf(parentPid);
+  expect(children, `os ${command}: the probe does not see exactly one serve child.\n${output()}`)
+    .toHaveLength(1);
+  expect(children.every(isAlive), `os ${command}: a child reads dead before the signal`).toBe(true);
+  expect(portIsFree(port), `os ${command}: port ${port} reads free while the server is up`).toBe(false);
+
+  // ── The measurement: the PARENT's pid alone, never its group ───────────
+  process.kill(parentPid, signal);
+
+  let timer: ReturnType<typeof setTimeout> | undefined;
+  const fate = await Promise.race([
+    exited,
+    new Promise<'timeout'>((settle) => { timer = setTimeout(() => settle('timeout'), EXIT_TIMEOUT_MS); }),
+  ]);
+  clearTimeout(timer);
+  expect(fate, `os ${command} did not exit within ${EXIT_TIMEOUT_MS}ms of ${signal}.\n${output()}`)
+    .not.toBe('timeout');
+
+  // Soft, both: the two readings are independent facts, and a regression
+  // should report each of them rather than stop at the first.
+  expect.soft(
+    children.filter(isAlive),
+    `os ${command}: ${signal} to the parent left its serve child running (orphaned, ` +
+      `reparented to init).\n--- output ---\n${output()}`,
+  ).toEqual([]);
+  expect.soft(
+    portIsFree(port),
+    `os ${command}: ${signal} to the parent left port ${port} bound.\n--- output ---\n${output()}`,
+  ).toBe(true);
+}
+
+describe('a signal to the CLI parent takes its `serve` child down with it', () => {
+  for (const signal of ['SIGTERM', 'SIGINT'] as const) {
+    it(
+      `os start: ${signal} to the parent leaves no child process and a free port`,
+      () => signalParent('start', signal),
+      BOOT_TIMEOUT_MS + EXIT_TIMEOUT_MS + 30_000,
+    );
+
+    it(
+      `os dev (control): ${signal} to the parent leaves no child process and a free port`,
+      () => signalParent('dev', signal),
+      BOOT_TIMEOUT_MS + EXIT_TIMEOUT_MS + 30_000,
+    );
+  }
+});