diff --git a/README.md b/README.md
index 633ecff..ac78191 100644
--- a/README.md
+++ b/README.md
@@ -144,8 +144,11 @@ tmux attach -t my-agent
 `pluk watch` and `pluk subscribe` deliberately swallow capture-pane failures,
 input errors and malformed log lines so the pipe-pane process never dies. To
 tell a healthy quiet session from one that is repeatedly failing, add
-`--diagnostics[=secs]` (default every 60 s). It writes one JSON line per period,
-plus a final one on exit, to **stderr** — stdout stays pure event JSONL:
+`--diagnostics[=secs]` (default every 60 s); the value must be a positive number
+of seconds (at most 2147483, the `setInterval` limit), or omitted to use the
+default—anything else is rejected with exit 2.
+It writes one JSON line per period, plus a final one on exit, to **stderr** —
+stdout stays pure event JSONL:
 
 ```json
 {"pluk_diagnostics":1,"command":"watch","uptime_s":120,"final":false,"linesSeen":842,"framesPolled":0,"captureFailures":0,"classifyErrors":0,"inputErrors":0,"eventsEmitted":31,"eventsFiltered":4}
diff --git a/changelog.d/fixed-342.md b/changelog.d/fixed-342.md
new file mode 100644
index 0000000..861a878
--- /dev/null
+++ b/changelog.d/fixed-342.md
@@ -0,0 +1 @@
+- `watch`/`subscribe --diagnostics=SECS`: a non-numeric, non-positive or `false` value (`abc`, `0`, `-5`, `5min`, `false`) used to silently run at the 60s default; it is now rejected with exit 2, as are values above 2147483 s that would overflow `setInterval` (#342).
diff --git a/src/cli.ts b/src/cli.ts
index 99c9a57..07ab9db 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -121,6 +121,24 @@ function parseFilter(raw: string | undefined): PlukEventType[] | undefined {
   return types as PlukEventType[];
 }
 
+// setInterval() stores its delay as a signed 32-bit int; larger values are
+// clamped to 1 ms with a TimeoutOverflowWarning, so reject them up front.
+const MAX_DIAGNOSTICS_SECONDS = Math.floor((2 ** 31 - 1) / 1000);
+
+function parseDiagnosticsFlag(raw: string | undefined): string | undefined {
+  if (raw === undefined || raw === 'true') return raw;
+  const seconds = Number(raw);
+  if (!Number.isFinite(seconds) || seconds <= 0) {
+    console.error(`${paintStderr(ANSI_RED, 'Error:')} --diagnostics expects a positive number of seconds (or a bare --diagnostics for the 60s default)`);
+    process.exit(2);
+  }
+  if (seconds > MAX_DIAGNOSTICS_SECONDS) {
+    console.error(`${paintStderr(ANSI_RED, 'Error:')} --diagnostics must be at most ${MAX_DIAGNOSTICS_SECONDS} seconds`);
+    process.exit(2);
+  }
+  return raw;
+}
+
 function cmdSubscribe(args: string[]): void {
   const { positional, flags } = parseArgs(args);
   const session = positional[0];
@@ -132,6 +150,7 @@ function cmdSubscribe(args: string[]): void {
   }
 
   const filter = parseFilter(flags['filter']);
+  const diagnosticsFlag = parseDiagnosticsFlag(flags['diagnostics']);
 
   const sub = new Subscriber({
     session,
@@ -145,7 +164,7 @@ function cmdSubscribe(args: string[]): void {
     console.log(JSON.stringify(event));
   });
 
-  const stopDiagnostics = startDiagnostics('subscribe', () => ({ ...sub.stats() }), flags['diagnostics']);
+  const stopDiagnostics = startDiagnostics('subscribe', () => ({ ...sub.stats() }), diagnosticsFlag);
 
   sub.on('error', err => {
     console.error(`${paintStderr(ANSI_RED, 'Error:')} ${(err as Error).message}`);
@@ -173,6 +192,7 @@ function cmdWatch(args: string[]): void {
   const cli = flags['cli'] ?? 'claude';
 
   const filter = parseFilter(flags['filter']);
+  const diagnosticsFlag = parseDiagnosticsFlag(flags['diagnostics']);
 
   const noPatternSet = !hasAnyPattern(getPatterns(cli, flags['patterns-dir']));
   if (noPatternSet) {
@@ -237,7 +257,7 @@ function cmdWatch(args: string[]): void {
     // rotation rather than derive a log path from it.
   }
 
-  const stopDiagnostics = startDiagnostics('watch', () => ({ ...watcher.stats() }), flags['diagnostics']);
+  const stopDiagnostics = startDiagnostics('watch', () => ({ ...watcher.stats() }), diagnosticsFlag);
   process.on('exit', stopDiagnostics);
 
   process.on('SIGINT', () => {
diff --git a/src/diagnostics.ts b/src/diagnostics.ts
index a13ebc0..0742fa0 100644
--- a/src/diagnostics.ts
+++ b/src/diagnostics.ts
@@ -18,6 +18,11 @@ const DIAGNOSTICS_INTERVAL_MS = 60_000;
  * no session name, raw output or other user-provided value is included,
  * and nothing is sent anywhere. Returns a function that writes a final
  * summary and stops the timer.
+ *
+ * `flag` is the raw `--diagnostics` value. The CLI validates it before
+ * calling (exit 2 on anything that is not a positive, in-range number);
+ * library callers that pass a non-finite or non-positive value get the
+ * 60 s default instead.
  */
 export function startDiagnostics(
   command: 'watch' | 'subscribe',
diff --git a/test/cli-watch-guard.test.js b/test/cli-watch-guard.test.js
index 21d1b68..fb6e0ac 100644
--- a/test/cli-watch-guard.test.js
+++ b/test/cli-watch-guard.test.js
@@ -94,6 +94,31 @@ for (const bad of ['0', '-5', 'abc', '', '1.5']) {
   });
 }
 
+// --- watch/subscribe: invalid --diagnostics value is rejected ---------------------
+
+for (const cmd of ['watch', 'subscribe']) {
+  for (const bad of ['abc', '0', '-5', '5min', 'false']) {
+    test(`${cmd} --diagnostics=${bad} exits 2 with a usage error`, () => {
+      const { code, stderr } = run(CLI, [cmd, 'x', `--diagnostics=${bad}`]);
+      assert.equal(code, 2);
+      assert.match(stderr, /--diagnostics expects a positive number of seconds/);
+    });
+  }
+
+  test(`${cmd} --diagnostics above the setInterval limit exits 2 with a usage error`, () => {
+    const { code, stderr } = run(CLI, [cmd, 'x', '--diagnostics=2147484']);
+    assert.equal(code, 2);
+    assert.match(stderr, /--diagnostics must be at most 2147483 seconds/);
+    assert.doesNotMatch(stderr, /TimeoutOverflowWarning/);
+  });
+}
+
+test('watch rejects a bad --diagnostics value before consuming stdin or logging', () => {
+  const { code, stderr } = run(CLI, ['watch', 'x', '--cli=claude', '--diagnostics=0'], { input: 'hello\n' });
+  assert.equal(code, 2);
+  assert.doesNotMatch(stderr, /pluk_diagnostics/);
+});
+
 test('watch --pane without --capture warns that --pane is ignored', () => {
   const { code, stderr } = run(CLI, ['watch', 'x', '--pane=%1'], { input: '' });
   assert.equal(code, 0);