diff --git a/browse/src/security.ts b/browse/src/security.ts
index cb46efe12..6475aea73 100644
--- a/browse/src/security.ts
+++ b/browse/src/security.ts
@@ -33,7 +33,8 @@ import { spawn } from 'child_process';
 import * as fs from 'fs';
 import * as path from 'path';
 import * as os from 'os';
-import { writeSecureFile, appendSecureFile, mkdirSecure } from './file-permissions';
+import { restrictFilePermissions, appendSecureFile, mkdirSecure } from './file-permissions';
+import { atomicWriteQuiet } from '../../lib/fs-atomic';
 
 // ─── Thresholds + verdict types ──────────────────────────────
 
@@ -346,17 +347,17 @@ export interface SessionState {
 }
 
 /**
- * Atomic write of session state (temp + rename pattern). Writes are safe
- * across process boundaries.
+ * Atomic write of session state (via lib/fs-atomic). Writes are safe
+ * across process boundaries. Swallow-with-log polarity: a failed write
+ * must never take down the caller (security state is best-effort cache).
  */
 export function writeSessionState(state: SessionState): void {
-  try {
-    mkdirSecure(SECURITY_DIR);
-    const tmp = `${STATE_FILE}.tmp.${process.pid}`;
-    writeSecureFile(tmp, JSON.stringify(state, null, 2));
-    fs.renameSync(tmp, STATE_FILE);
-  } catch (err) {
-    console.error('[security] writeSessionState failed:', (err as Error).message);
+  try { mkdirSecure(SECURITY_DIR); } catch { /* write below fails and logs */ }
+  if (atomicWriteQuiet(STATE_FILE, JSON.stringify(state, null, 2), { mode: 0o600 })) {
+    // Windows ACL hardening (POSIX chmod is redundant with mode above).
+    restrictFilePermissions(STATE_FILE);
+  } else {
+    console.error('[security] writeSessionState failed');
   }
 }
 
diff --git a/browse/src/terminal-agent-control.ts b/browse/src/terminal-agent-control.ts
index 094ba668f..a8c42d63c 100644
--- a/browse/src/terminal-agent-control.ts
+++ b/browse/src/terminal-agent-control.ts
@@ -17,7 +17,8 @@
 import * as fs from 'fs';
 import * as path from 'path';
 import { safeUnlink, safeKill, isProcessAlive } from './error-handling';
-import { writeSecureFile, mkdirSecure } from './file-permissions';
+import { restrictFilePermissions, mkdirSecure } from './file-permissions';
+import { atomicWriteSync } from '../../lib/fs-atomic';
 
 /**
  * Locate the terminal-agent script on disk. In dev (cli.ts running via
@@ -108,13 +109,13 @@ export function readAgentRecord(stateDir: string): AgentRecord | null {
   }
 }
 
-/** Atomic write. Caller must ensure stateDir exists; agent does this at boot. */
+/** Atomic write (throws on failure — boot must not proceed on a bad record). */
 export function writeAgentRecord(stateDir: string, record: AgentRecord): void {
   try { mkdirSecure(stateDir); } catch {}
   const target = agentRecordPath(stateDir);
-  const tmp = `${target}.tmp-${process.pid}`;
-  writeSecureFile(tmp, JSON.stringify(record));
-  fs.renameSync(tmp, target);
+  atomicWriteSync(target, JSON.stringify(record), { mode: 0o600 });
+  // Windows ACL hardening (POSIX chmod is redundant with mode above).
+  restrictFilePermissions(target);
 }
 
 export function clearAgentRecord(stateDir: string): void {
diff --git a/browse/src/terminal-agent.ts b/browse/src/terminal-agent.ts
index 4814c7508..d73fb21c2 100644
--- a/browse/src/terminal-agent.ts
+++ b/browse/src/terminal-agent.ts
@@ -23,7 +23,8 @@
 import * as fs from 'fs';
 import * as path from 'path';
 import * as crypto from 'crypto';
-import { writeSecureFile, mkdirSecure } from './file-permissions';
+import { writeSecureFile, restrictFilePermissions, mkdirSecure } from './file-permissions';
+import { atomicWriteSync, atomicWriteQuiet } from '../../lib/fs-atomic';
 import { safeUnlink } from './error-handling';
 import { writeAgentRecord, clearAgentRecord } from './terminal-agent-control';
 import { extractPtyCookie } from './pty-session-cookie';
@@ -267,12 +268,9 @@ function writeClaudeAvailable(): void {
     checked_at: new Date().toISOString(),
   };
   const target = path.join(stateDir, 'claude-available.json');
-  const tmp = path.join(stateDir, `.tmp-claude-${process.pid}`);
-  try {
-    writeSecureFile(tmp, JSON.stringify(status, null, 2));
-    fs.renameSync(tmp, target);
-  } catch {
-    safeUnlink(tmp);
+  // Fire-and-forget state file: a failed write must not break boot.
+  if (atomicWriteQuiet(target, JSON.stringify(status, null, 2), { mode: 0o600 })) {
+    restrictFilePermissions(target); // Windows ACL hardening
   }
 }
 
@@ -881,12 +879,10 @@ function handleTabState(msg: {
       })),
     };
     const target = path.join(stateDir, 'tabs.json');
-    const tmp = path.join(stateDir, `.tmp-tabs-${process.pid}`);
-    try {
-      writeSecureFile(tmp, JSON.stringify(payload, null, 2));
-      fs.renameSync(tmp, target);
-    } catch {
-      safeUnlink(tmp);
+    // Fire-and-forget state file: atomic write (via lib/fs-atomic) so
+    // claude never reads a half-written JSON document; failures swallowed.
+    if (atomicWriteQuiet(target, JSON.stringify(payload, null, 2), { mode: 0o600 })) {
+      restrictFilePermissions(target); // Windows ACL hardening
     }
   }
 
@@ -896,17 +892,12 @@ function handleTabState(msg: {
   const active = msg.active;
   if (active && active.url && !active.url.startsWith('chrome://') && !active.url.startsWith('chrome-extension://')) {
     const ctxFile = path.join(stateDir, 'active-tab.json');
-    const tmp = path.join(stateDir, `.tmp-tab-${process.pid}`);
-    try {
-      writeSecureFile(tmp, JSON.stringify({
-        tabId: active.tabId ?? null,
-        url: active.url,
-        title: active.title ?? '',
-      }));
-      fs.renameSync(tmp, ctxFile);
-    } catch {
-      safeUnlink(tmp);
-    }
+    const ok = atomicWriteQuiet(ctxFile, JSON.stringify({
+      tabId: active.tabId ?? null,
+      url: active.url,
+      title: active.title ?? '',
+    }), { mode: 0o600 });
+    if (ok) restrictFilePermissions(ctxFile); // Windows ACL hardening
   }
 }
 
@@ -916,17 +907,13 @@ function handleTabSwitch(msg: { tabId?: number; url?: string; title?: string }):
 
   const stateDir = path.dirname(STATE_FILE);
   const ctxFile = path.join(stateDir, 'active-tab.json');
-  const tmp = path.join(stateDir, `.tmp-tab-${process.pid}`);
-  try {
-    writeSecureFile(tmp, JSON.stringify({
-      tabId: msg.tabId ?? null,
-      url,
-      title: msg.title ?? '',
-    }));
-    fs.renameSync(tmp, ctxFile);
-  } catch {
-    safeUnlink(tmp);
-  }
+  // Fire-and-forget: atomic write via lib/fs-atomic, failures swallowed.
+  const ok = atomicWriteQuiet(ctxFile, JSON.stringify({
+    tabId: msg.tabId ?? null,
+    url,
+    title: msg.title ?? '',
+  }), { mode: 0o600 });
+  if (ok) restrictFilePermissions(ctxFile); // Windows ACL hardening
 
   // Best-effort sync to parent server so its activeTabId tracking matches.
   // No await; this is fire-and-forget.
@@ -964,11 +951,11 @@ function main() {
   }
 
   // Write port file atomically so the parent server can pick it up.
+  // Throws on failure — a boot without a discoverable port file is broken.
   const dir = path.dirname(PORT_FILE);
   try { mkdirSecure(dir); } catch {}
-  const tmp = `${PORT_FILE}.tmp-${process.pid}`;
-  writeSecureFile(tmp, String(port));
-  fs.renameSync(tmp, PORT_FILE);
+  atomicWriteSync(PORT_FILE, String(port), { mode: 0o600 });
+  restrictFilePermissions(PORT_FILE); // Windows ACL hardening
 
   // Write identity-based agent record (pid + per-boot gen). Replaces the
   // v1.43- `pkill -f terminal-agent\.ts` regex teardown that could kill
diff --git a/browse/test/terminal-agent.test.ts b/browse/test/terminal-agent.test.ts
index 8dd54e200..e579bd7e3 100644
--- a/browse/test/terminal-agent.test.ts
+++ b/browse/test/terminal-agent.test.ts
@@ -175,11 +175,13 @@ describe('Source-level guard: terminal-agent', () => {
     expect(AGENT_SRC).toContain("msg?.type === 'tabState'");
     expect(AGENT_SRC).toContain('function handleTabState');
     const fn = AGENT_SRC.slice(AGENT_SRC.indexOf('function handleTabState'));
-    // Atomic write via tmp + rename for both files (so claude never reads
-    // a half-written JSON document).
+    // Atomic write for both files (so claude never reads a half-written
+    // JSON document) — via the shared lib/fs-atomic helper, which owns the
+    // tmp + rename dance. Quiet variant: state-file writes are
+    // fire-and-forget and must never take down the agent.
     expect(fn).toContain("'tabs.json'");
     expect(fn).toContain("'active-tab.json'");
-    expect(fn).toContain('renameSync');
+    expect(fn).toContain('atomicWriteQuiet');
     // Skip chrome:// and chrome-extension:// pages — they're not useful
     // targets for browse commands.
     expect(fn).toContain("startsWith('chrome://')");