@@ -0,0 +1,11 @@
+---
+'@objectstack/cli': patch
+---
+
+`os serve` says once when it cannot write its runtime state file, naming the file and the operating system's error, and goes on booting
+
+Clause-②: no
+
+- **Before:** when `runtime.<environment>.<project>.json` could not be written — an `OS_HOME` under `/proc`, or one below a regular file (`ENOTDIR`) — the write's catch printed nothing. The boot printed its ready banner, an `os dev` parent got its `objectstack:listening` message, and a supervisor that then read the file got `ENOENT` with no reason anywhere.
+- **Now:** one warning on stderr, among the other boot warnings and before the banner, names the file it could not write and carries the operating system's error verbatim (for example `ENOTDIR: not a directory, stat '…'`). It says the server runs on without the file and that `OS_HOME` must point at a directory the process can write.
+- **Unchanged:** the file stays best-effort, so the boot never fails on it; its name, its payload and its removal on exit; the banner and the IPC message; stdout, which stays the stdio MCP transport's alone.
@@ -885,6 +885,8 @@ export interface BoundPortChannels {
   /**
    * Writes `runtime.<environment>.<project>.json`. ⛔ Must COMPLETE before
    * either announcement below: it is the file both of them send a consumer to.
+   * Best-effort: when it cannot be written the boot goes on, and the real
+   * channel says so once, naming the file (#24091).
    */
   writeRuntimeState: (published: { port: number; url: string }) => void;
   /**
@@ -1074,16 +1076,20 @@ export function runtimeStateFileName(environmentId: string, servedAppRoot: strin
 }
 
 /**
- * The real channels: the same three writes this command has always done, with
- * their failure handling unchanged.
+ * The real channels: the same three writes this command has always done.
  *
  * Each leg keeps its OWN `try` — a boot must not die because a supervision file
  * could not be written or because an IPC channel had already closed, and one
- * leg failing must not cost the other two. Only the ORDER changed (commit faff497fd).
+ * leg failing must not cost the other two. The ORDER changed (commit faff497fd),
+ * and a state file that could not be written is now said once on stderr
+ * (#24091); a closed IPC channel is still the ordinary `os serve` case, silent.
  */
 export function runtimeBoundPortChannels(printBanner: () => void): BoundPortChannels {
   return {
     writeRuntimeState: ({ port, url }) => {
+      // Named the moment it is known, so the catch below can say WHICH file
+      // was not written (#24091).
+      let attemptedFile: string | undefined;
       try {
         const environmentId = process.env.OS_ENVIRONMENT_ID ?? 'env_local';
         // Keyed by the SERVED APP'S ROOT as well as the environment, so two
@@ -1095,11 +1101,12 @@ export function runtimeBoundPortChannels(printBanner: () => void): BoundPortChan
           resolveObjectStackHome(),
           runtimeStateFileName(environmentId, servedAppRootOrCwd()),
         );
+        attemptedFile = runtimeFile;
         // Bounded (#23858): an `OS_HOME` that can never be made — a name under
-        // `/proc` — is refused here at once, and the best-effort catch below
-        // lets the boot go on without the file. Node's recursive mkdir looped
-        // on it after the bind and before the banner, holding the event loop,
-        // so the server never announced itself.
+        // `/proc` — is refused here at once, and the catch below lets the boot
+        // go on without the file, saying so. Node's recursive mkdir looped on
+        // it after the bind and before the banner, holding the event loop, so
+        // the server never announced itself.
         makeDirectorySync(path.dirname(runtimeFile));
         fs.writeFileSync(runtimeFile, JSON.stringify({
           pid: process.pid,
@@ -1110,7 +1117,25 @@ export function runtimeBoundPortChannels(printBanner: () => void): BoundPortChan
         }, null, 2));
         const cleanupRuntimeFile = () => { try { fs.rmSync(runtimeFile, { force: true }); } catch { /* noop */ } };
         process.on('exit', cleanupRuntimeFile);
-      } catch { /* non-fatal — supervision file is best-effort */ }
+      } catch (error) {
+        // Best-effort, NOT silent (#24091). The file stays optional — a boot
+        // must not die for a supervision file — but both announcements that
+        // follow (the banner, the IPC message) send a consumer to it, so a
+        // refusal swallowed here told a supervisor "ready" and then answered
+        // its read with nothing. One line, naming the file and the operating
+        // system's own error, carried verbatim rather than paraphrased.
+        //
+        // Said ONCE: `run()` publishes the bound port at exactly one site, so
+        // this leg runs once per boot. Channel: `console.warn`, i.e. stderr,
+        // like every other boot warning in this file — stdout belongs to the
+        // stdio MCP transport's JSON-RPC frames (#7915).
+        const thrown = error instanceof Error ? error.message : String(error);
+        console.warn(chalk.yellow(
+          `  ⚠ Runtime state file not written: ${attemptedFile ?? '(its path could not be resolved)'} — ${thrown}. ` +
+            'The server runs on without it, so a supervisor that reads it will not find this process there; ' +
+            'point OS_HOME at a directory this process can write.',
+        ));
+      }
     },
     announceListening: (message) => {
       try {
 
@@ -4,7 +4,8 @@
  * `os start` and `os create` refuse a directory that can never be made at
  * once, naming the path and the operating system's code, instead of hanging
  * in Node's recursive mkdir (#23858). `os serve` stops hanging on its runtime
- * state file's directory the same way.
+ * state file's directory the same way, and goes on to its banner without the
+ * file — saying so once, naming the file and the error (#24091).
  *
  * ## The defect, measured on `origin/main` 0744f94366 (built CLI, Node v22.22.0)
  *
@@ -31,10 +32,10 @@
  * group with a hard SIGKILL of the whole group at `CHILD_BOUND_MS`, so a
  * reintroduced loop fails this file at the bound instead of hanging the suite.
  * `bin/run.js` with `NODE_ENV` unset is the entry an operator runs, and it
- * starts in about 3 s where the tsx source entry takes about 10 s, so the six
- * children (run concurrently) keep this file under the 10 s line. The two
+ * starts in about 3 s where the tsx source entry takes about 10 s, so the seven
+ * children (run concurrently) keep this file under the 10 s line. The three
  * `serve` children never exit by themselves: each is killed whole as soon as
- * its banner tail is out, and `--port 0` keeps both off any fixed port.
+ * its banner tail is out, and `--port 0` keeps them off any fixed port.
  *
  * ## What is asserted — kind, exit code and named subject, never the prose
  *
@@ -50,14 +51,21 @@
  *   4. CONTROL: `os create plugin --dir <tmp>/a/b/foo` exits 0 and writes the
  *      project.
  *   5. `os serve` under `OS_HOME=/proc/<missing>`: prints its banner tail
- *      inside the bound. The state file cannot be written there, and the
- *      write's best-effort catch lets the boot go on without it.
+ *      inside the bound. The state file cannot be written there; the boot
+ *      goes on without it, and exactly one output line names that file, with
+ *      `ENOENT`.
  *   6. CONTROL: `os serve` under a creatable `OS_HOME` three levels deep
  *      prints its banner tail, and by then that home holds one
- *      `runtime.env_local.<project>.json` naming the child's own pid.
+ *      `runtime.env_local.<project>.json` naming the child's own pid. No
+ *      output line names the file: a written file is not reported.
+ *   7. `os serve` under an `OS_HOME` whose parent is a regular file, the
+ *      portable uncreatable home (#24091): prints its banner tail, and exactly
+ *      one output line names the state file it could not write, with
+ *      `ENOTDIR`. Before #24091 the write's catch printed nothing, so a
+ *      supervisor told "ready" met a missing file and no reason.
  *
- * They need a `/proc` that refuses `mkdir` with ENOENT, so the `/proc` legs
- * run where `/proc/self` exists (Linux, which CI is).
+ * Legs 1, 3 and 5 need a `/proc` that refuses `mkdir` with ENOENT, so they run
+ * where `/proc/self` exists (Linux, which CI is). Leg 7 runs everywhere.
  */
 
 import { describe, it, expect, beforeAll, afterAll } from 'vitest';
@@ -86,6 +94,16 @@ const BANNER_TAIL = 'Press Ctrl+C to stop';
 /** The platform with no application — the cheapest project that reaches a banner. */
 const BARE_CONFIG = 'export default {};\n';
 
+/**
+ * The output lines that name a runtime state file under `home`. The file's
+ * project half is a digest of the served root, so a line is matched on the
+ * home-plus-`runtime.env_local.` prefix, never on a recomputed name.
+ */
+function linesNamingStateFile(output: string, home: string): string[] {
+  const prefix = join(home, 'runtime.env_local.');
+  return output.split('\n').filter((line) => line.includes(prefix));
+}
+
 interface Run {
   code: number | null;
   signal: NodeJS.Signals | null;
@@ -164,10 +182,13 @@ beforeAll(async () => {
   serveDir = join(workdir, 'serve-project');
   mkdirSync(serveDir);
   writeFileSync(join(serveDir, 'objectstack.config.ts'), BARE_CONFIG);
+  // A regular file where the home's parent directory would be: `ENOTDIR`.
+  writeFileSync(join(workdir, 'serve-home-file'), '');
   const legs: Array<[string, Promise<Run>]> = [
     ['start-control', runCli(['start', '-p', UNSERVABLE_PORT], { OS_HOME: join(workdir, 'home', 'a', 'b', 'c') })],
     ['create-control', runCli(['create', 'plugin', 'foo', '--dir', join(workdir, 'create', 'a', 'b', 'foo')], {})],
     ['serve-control', runServe(join(workdir, 'serve-home', 'a', 'b', 'c'))],
+    ['serve-notdir', runServe(join(workdir, 'serve-home-file', 'sub'))],
   ];
   if (RUNS_PROC) {
     legs.push(
@@ -227,6 +248,9 @@ describe('a directory that can never be made is refused at the door, at once (#2
   it.runIf(RUNS_PROC)('os serve under OS_HOME=/proc/<missing> prints its banner instead of looping on the state file', () => {
     const run = runOf('serve-proc');
     expect(run.reached, run.output).toBe(true);
+    const named = linesNamingStateFile(run.output, `${PROC_TOP}-sv`);
+    expect(named, run.output).toHaveLength(1);
+    expect(named[0]).toContain('ENOENT');
   });
 
   it('CONTROL: os serve under a creatable OS_HOME three levels deep writes its state file before the banner', () => {
@@ -236,5 +260,18 @@ describe('a directory that can never be made is refused at the door, at once (#2
     const files = readdirSync(home).filter((name) => /^runtime\.env_local\..+\.json$/.test(name));
     expect(files, `entries in ${home}: ${readdirSync(home).join(', ')}`).toHaveLength(1);
     expect(JSON.parse(readFileSync(join(home, files[0]), 'utf8')).pid).toBe(run.pid);
+    expect(linesNamingStateFile(run.output, home), run.output).toEqual([]);
+  });
+});
+
+describe('a runtime state file that cannot be written is said once, and the boot goes on (#24091)', () => {
+  it('os serve under an OS_HOME below a regular file reaches its banner, and one line names the file and ENOTDIR', () => {
+    const run = runOf('serve-notdir');
+    const home = join(workdir, 'serve-home-file', 'sub');
+    expect(run.reached, run.output).toBe(true);
+    const named = linesNamingStateFile(run.output, home);
+    expect(named, run.output).toHaveLength(1);
+    expect(named[0]).toMatch(/runtime\.env_local\.\S+\.json/);
+    expect(named[0]).toContain('ENOTDIR');
   });
 });