diff --git a/src/common/seen-ids.mjs b/src/common/seen-ids.mjs
index 534640d..56088b5 100644
--- a/src/common/seen-ids.mjs
+++ b/src/common/seen-ids.mjs
@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: AGPL-3.0-only
 
-import { readFileSync, openSync, writeSync, fsyncSync, closeSync, renameSync } from 'node:fs';
+import { readFileSync, openSync, writeSync, fsyncSync, closeSync, renameSync, mkdirSync } from 'node:fs';
+import { dirname } from 'node:path';
 
 /**
  * Shared seen-ID management for all platform pollers.
@@ -22,6 +23,8 @@ export function loadSeenIds(path, maxIds = 2000) {
 // M5 hermes poller did on 2026-09-01 (issue #90, item 1).
 export function saveSeenIds(path, ids, maxIds = 2000) {
   const arr = [...ids].slice(-maxIds);
+  // The watermark now lives under the user's state dir, which may not exist yet.
+  mkdirSync(dirname(path), { recursive: true });
   const tmp = `${path}.tmp-${process.pid}`;
   const fd = openSync(tmp, 'w');
   try {
diff --git a/src/config.mjs b/src/config.mjs
index 64df2a8..ce53951 100644
--- a/src/config.mjs
+++ b/src/config.mjs
@@ -1,9 +1,42 @@
 // SPDX-License-Identifier: AGPL-3.0-only
 
-import { readFileSync, existsSync } from 'node:fs';
+import { readFileSync, existsSync, mkdirSync, copyFileSync } from 'node:fs';
 import { resolve } from 'node:path';
 import { homedir } from 'node:os';
 
+
+// Seen-id watermarks record what a poller has ALREADY delivered. They were
+// defaulted into /tmp, which macOS clears on boot — so every reboot silently
+// reset every watermark and the next poll replayed history as if it were new.
+// On 2026-09-13 that dumped 48 messages from seven rooms back to March into one
+// agent's inbox, including "get them trading before they close today" from 6
+// March. Noise is the mild failure; an agent acting on a six-month-old
+// instruction is the real one.
+//
+// State that must outlive a reboot belongs under the user's own directory.
+// Honours XDG_STATE_HOME where set, falls back to ~/.local/state.
+const STATE_DIR = process.env.XDG_STATE_HOME
+  ? resolve(process.env.XDG_STATE_HOME, 'iak')
+  : resolve(homedir(), '.local', 'state', 'iak');
+
+const stateFile = (name) => {
+  const target = resolve(STATE_DIR, name);
+  // One-time adoption: a box upgrading from the /tmp defaults still holds a
+  // valid watermark there. Copying it over means the upgrade itself does not
+  // cause the single replay this change exists to prevent.
+  try {
+    if (!existsSync(target)) {
+      const legacy = `/tmp/iak-${name === 'seen-ids.txt' ? 'seen-ids' : name.replace(/\.txt$/, '')}.txt`;
+      mkdirSync(STATE_DIR, { recursive: true });
+      if (existsSync(legacy)) copyFileSync(legacy, target);
+    }
+  } catch {
+    // A read-only or unwritable home is not a reason to fail config load;
+    // the poller will fall back to an empty watermark and simply be noisy once.
+  }
+  return target;
+};
+
 const DEFAULT_CONFIG = {
   listen: { host: '127.0.0.1', port: 8787 },
   queue: { path: './ide-agent-queue.jsonl' },
@@ -13,7 +46,7 @@ const DEFAULT_CONFIG = {
     rooms: '',
     handle: '',
     interval_sec: 30,
-    seen_file: '/tmp/iak-seen-ids.txt',
+    seen_file: stateFile('seen-ids.txt'),
     api_key: '',
     nudge_mode: 'tmux',
     nudge_command: '',
@@ -26,7 +59,7 @@ const DEFAULT_CONFIG = {
     enabled: false,
     handle: '',
     interval_sec: 30,
-    seen_file: '/tmp/iak-dm-seen-ids.txt',
+    seen_file: stateFile('dm-seen-ids.txt'),
     api_key: '',
     human_only: false,
     limit: 100
@@ -36,7 +69,7 @@ const DEFAULT_CONFIG = {
   rate_limit: { message_interval_sec: 30 },
   automation: {
     rules: [],
-    seen_file: '/tmp/iak-automation-seen.txt',
+    seen_file: stateFile('automation-seen.txt'),
     interval_sec: 30,
     cooldown_sec: 5,
     first_match_only: true
@@ -45,12 +78,12 @@ const DEFAULT_CONFIG = {
     moltbook: { posts: [], base_url: 'https://www.moltbook.com' },
     github: { repos: [], token: '' },
     interval_sec: 120,
-    seen_file: '/tmp/iak-comment-seen.txt'
+    seen_file: stateFile('comment-seen.txt')
   },
   discord: {
     channels: [],
     interval_sec: 30,
-    seen_file: '/tmp/iak-discord-seen.txt',
+    seen_file: stateFile('discord-seen.txt'),
     self_id: '',
     skip_bots: false
   },
diff --git a/src/room-poller.mjs b/src/room-poller.mjs
index 94bf95b..05839c2 100644
--- a/src/room-poller.mjs
+++ b/src/room-poller.mjs
@@ -1,6 +1,8 @@
 // SPDX-License-Identifier: AGPL-3.0-only
 
-import { readFileSync, writeFileSync, appendFileSync } from 'node:fs';
+import { readFileSync, writeFileSync, appendFileSync, mkdirSync } from 'node:fs';
+import { dirname, resolve } from 'node:path';
+import { homedir } from 'node:os';
 import { randomUUID } from 'node:crypto';
 import { execSync } from 'node:child_process';
 import { nudgeTmux, nudgeCommand } from './utils.mjs';
@@ -18,7 +20,12 @@ import { resolveSelfHandle, isSelfSender } from './common/handles.mjs';
  * The IDE agent calls `rooms check` to read and clear the notification file.
  */
 
-const SEEN_FILE_DEFAULT = '/tmp/iak-seen-ids.txt';
+// Durable by default: /tmp is cleared on boot, which silently resets the
+// watermark and replays room history as new. See src/config.mjs.
+const SEEN_FILE_DEFAULT = resolve(
+  process.env.XDG_STATE_HOME ? resolve(process.env.XDG_STATE_HOME, 'iak') : resolve(homedir(), '.local', 'state', 'iak'),
+  'seen-ids.txt'
+);
 const NOTIFY_FILE_DEFAULT = '/tmp/iak-new-messages.txt';
 
 function loadSeenIds(path) {
@@ -31,6 +38,7 @@ function loadSeenIds(path) {
 
 function saveSeenIds(path, ids) {
   const arr = [...ids].slice(-1000);
+  mkdirSync(dirname(path), { recursive: true });
   writeFileSync(path, arr.join('\n') + '\n');
 }
 
diff --git a/test/config.test.mjs b/test/config.test.mjs
index f8d923c..6466a30 100644
--- a/test/config.test.mjs
+++ b/test/config.test.mjs
@@ -76,10 +76,16 @@ describe('config', () => {
     assert.deepEqual(cfg.poller.rooms, ['thinkoff-development']);
     assert.equal(cfg.poller.handle, '@CodexMB');
     assert.equal(cfg.poller.interval_sec, 30);
-    assert.equal(cfg.poller.seen_file, '/tmp/iak-seen-ids.txt');
+    // Watermarks must OUTLIVE a reboot: /tmp is cleared on boot, which silently
+    // resets them and replays room history as new (2026-09-13: 48 messages back
+    // to March, including actionable ones). Assert the property, not a literal
+    // path, since the state dir varies by home and XDG_STATE_HOME.
+    assert.ok(!cfg.poller.seen_file.startsWith('/tmp/'), 'seen_file must not live in /tmp');
+    assert.ok(cfg.poller.seen_file.endsWith('seen-ids.txt'), cfg.poller.seen_file);
     assert.equal(cfg.dm_poller.enabled, true);
     assert.equal(cfg.dm_poller.interval_sec, 30);
-    assert.equal(cfg.dm_poller.seen_file, '/tmp/iak-dm-seen-ids.txt');
+    assert.ok(!cfg.dm_poller.seen_file.startsWith('/tmp/'), 'dm seen_file must not live in /tmp');
+    assert.ok(cfg.dm_poller.seen_file.endsWith('dm-seen-ids.txt'), cfg.dm_poller.seen_file);
     assert.equal(cfg.dm_poller.limit, 100);
     assert.equal(cfg.background.enabled, true);
     assert.equal(cfg.background.recent_window_sec, 7200);