diff --git a/browse/src/browser-manager.ts b/browse/src/browser-manager.ts
index 03410fda5..e310c515c 100644
--- a/browse/src/browser-manager.ts
+++ b/browse/src/browser-manager.ts
@@ -23,6 +23,7 @@ import { emitActivity } from './activity';
 import { validateNavigationUrl, blockedNavigationReason } from './url-validation';
 import { TabSession, type RefEntry } from './tab-session';
 import { resolveChromiumProfile, cleanSingletonLocks, resolveConfig } from './config';
+import { ensureProjectProfile } from './chromium-profiles';
 import { launchWithXProtectHeal } from './xprotect-heal';
 import { readPidStartTime, shouldSpawnXvfb, pickFreeDisplay, spawnXvfb, xvfbInstallHint, type XvfbHandle } from './xvfb';
 import { withCdpSession } from './cdp-bridge';
@@ -750,6 +751,7 @@ export class BrowserManager {
     const fs = require('fs');
     const path = require('path');
     const userDataDir = resolveChromiumProfile();
+    ensureProjectProfile(userDataDir);
     fs.mkdirSync(userDataDir, { recursive: true });
 
     // Pre-launch cleanup of stale SingletonLock/Socket/Cookie. Chromium's
@@ -1913,6 +1915,7 @@ export class BrowserManager {
       // ignoring $CHROMIUM_PROFILE / $GSTACK_HOME and skipping the lock
       // cleanup — the third shipped drift between the three launch paths.
       const userDataDir = resolveChromiumProfile();
+      ensureProjectProfile(userDataDir);
       fs.mkdirSync(userDataDir, { recursive: true });
       cleanSingletonLocks(userDataDir);
 
diff --git a/browse/src/chromium-profiles.ts b/browse/src/chromium-profiles.ts
new file mode 100644
index 000000000..b3cfe6666
--- /dev/null
+++ b/browse/src/chromium-profiles.ts
@@ -0,0 +1,208 @@
+/**
+ * Per-project headed Chromium profiles (D5, #2492).
+ *
+ * The headed profile used to be machine-wide (<gstack home>/chromium-profile),
+ * so starting headed browse in one project SIGKILLed another project's live
+ * headed Chromium. Each project now gets <project>/.gstack/chromium-profile
+ * (config.resolveChromiumProfile). This module owns the profile lifecycle:
+ * who holds a profile's SingletonLock, first-use seeding from the old
+ * machine-wide profile, and `browse profiles list|prune`.
+ */
+import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
+import { resolveGstackHome } from './config';
+import { isProcessAlive } from './error-handling';
+import { readPidCmdline } from './xvfb';
+import { atomicWriteSync } from '../../lib/fs-atomic';
+
+export type ProfileOwner =
+  | { state: 'free' }
+  | { state: 'orphan'; pid: number }
+  | { state: 'live'; pid: number; detail: string };
+
+/** Processes an orphan is reparented to: init, launchd, and common subreapers. */
+const REAPERS = new Set(['init', 'systemd', 'launchd', 'tini', 'dumb-init', 'docker-init']);
+
+export function legacyProfileDir(): string {
+  return path.join(resolveGstackHome(), 'chromium-profile');
+}
+
+function registryPath(): string {
+  return path.join(resolveGstackHome(), 'browse-profiles.json');
+}
+
+function readParentPid(pid: number): number {
+  try {
+    const stat = fs.readFileSync(`/proc/${pid}/stat`, 'utf-8');
+    return Number(stat.slice(stat.lastIndexOf(')') + 2).split(' ')[1]);
+  } catch {
+    try {
+      const r = Bun.spawnSync(['ps', '-o', 'ppid=', '-p', String(pid)], { stdout: 'pipe', stderr: 'pipe', timeout: 2000, windowsHide: true });
+      return r.exitCode === 0 ? Number(r.stdout.toString().trim()) : 0;
+    } catch {
+      return 0;
+    }
+  }
+}
+
+/**
+ * Who holds this profile. The SingletonLock symlink target is "host-PID".
+ * Only a process on this host that is a Chromium using this exact profile
+ * counts as the holder; a dead or recycled PID means the lock is stale. An
+ * orphan (reparented to init or a subreaper after its daemon died) may be
+ * reaped; anything else, including a lock from another host, is a live owner
+ * that must never be killed.
+ */
+export function profileOwner(profileDir: string): ProfileOwner {
+  let target: string;
+  try {
+    target = fs.readlinkSync(path.join(profileDir, 'SingletonLock'));
+  } catch {
+    return { state: 'free' };
+  }
+  const dash = target.lastIndexOf('-');
+  const host = target.slice(0, dash);
+  const pid = Number(target.slice(dash + 1));
+  if (!Number.isSafeInteger(pid) || pid <= 0) return { state: 'free' };
+  if (host !== os.hostname()) {
+    return { state: 'live', pid, detail: `locked by PID ${pid} on another host (${host})` };
+  }
+  if (!isProcessAlive(pid)) return { state: 'free' };
+  const cmd = readPidCmdline(pid);
+  const dirs = new Set([profileDir]);
+  try { dirs.add(fs.realpathSync(profileDir)); } catch {}
+  const usesProfile = [...dirs].some(d => cmd.includes(`--user-data-dir=${d}`));
+  if (!/chrom/i.test(cmd) || !usesProfile) return { state: 'free' };
+  const ppid = readParentPid(pid);
+  const parent = ppid > 1 ? path.basename(readPidCmdline(ppid).split(' ')[0] || '') : '';
+  if (ppid <= 1 || !isProcessAlive(ppid) || REAPERS.has(parent)) return { state: 'orphan', pid };
+  return { state: 'live', pid, detail: `in use by Chromium PID ${pid} (started by PID ${ppid}: ${parent || 'unknown'})` };
+}
+
+function readRegistry(): string[] {
+  try {
+    const list = JSON.parse(fs.readFileSync(registryPath(), 'utf-8'));
+    return Array.isArray(list) ? list.filter((p): p is string => typeof p === 'string') : [];
+  } catch {
+    return [];
+  }
+}
+
+function writeRegistry(list: string[]): void {
+  fs.mkdirSync(path.dirname(registryPath()), { recursive: true });
+  atomicWriteSync(registryPath(), JSON.stringify([...new Set(list)].sort(), null, 2) + '\n');
+}
+
+/**
+ * Create a project's headed profile on first use. Seeds it from the old
+ * machine-wide profile only when nothing holds that profile, copying into a
+ * temporary 0700 directory without Singleton* files and renaming it into
+ * place, so a concurrent first use or an interrupted copy never leaves a
+ * half-written profile. Says what happened once, through `log`.
+ */
+export function ensureProjectProfile(profileDir: string, log: (msg: string) => void = console.log): void {
+  if (fs.existsSync(profileDir)) return;
+  const legacy = legacyProfileDir();
+  if (path.resolve(profileDir) === path.resolve(legacy)) return;
+  writeRegistry([...readRegistry(), path.resolve(profileDir)]);
+  fs.mkdirSync(path.dirname(profileDir), { recursive: true });
+  const fresh = (why: string) => {
+    fs.mkdirSync(profileDir, { recursive: true, mode: 0o700 });
+    if (why) log(why);
+  };
+  if (!fs.existsSync(legacy)) return fresh('');
+  const before = profileOwner(legacy);
+  if (before.state !== 'free') {
+    return fresh(`[browse] Headed browse now keeps a profile per project (${profileDir}). The old shared profile (${legacy}) is ${'detail' in before ? before.detail : `held by orphaned PID ${before.pid}`}, so this project starts with a fresh profile. To import it later: close that browser, delete ${profileDir}, and start headed browse again.`);
+  }
+  const tmp = `${profileDir}.seed-${process.pid}`;
+  fs.rmSync(tmp, { recursive: true, force: true });
+  try {
+    fs.cpSync(legacy, tmp, {
+      recursive: true,
+      verbatimSymlinks: true,
+      filter: (src) => !path.basename(src).startsWith('Singleton'),
+    });
+    fs.chmodSync(tmp, 0o700);
+    if (profileOwner(legacy).state !== 'free') {
+      fs.rmSync(tmp, { recursive: true, force: true });
+      return fresh(`[browse] The old shared profile (${legacy}) came into use while it was being copied, so this project (${profileDir}) starts with a fresh profile.`);
+    }
+    fs.renameSync(tmp, profileDir);
+  } catch (err) {
+    fs.rmSync(tmp, { recursive: true, force: true });
+    if (fs.existsSync(profileDir)) return;
+    throw err;
+  }
+  log(`[browse] Headed browse now keeps a profile per project. Copied your logins and settings from ${legacy} to ${profileDir} (${formatBytes(dirBytes(profileDir))}). See \`browse profiles\` to list or prune profiles.`);
+}
+
+function dirBytes(dir: string): number {
+  let total = 0;
+  const walk = (d: string) => {
+    for (const e of fs.readdirSync(d, { withFileTypes: true })) {
+      const p = path.join(d, e.name);
+      if (e.isDirectory()) walk(p);
+      else if (e.isFile()) total += fs.statSync(p).size;
+    }
+  };
+  try { walk(dir); } catch {}
+  return total;
+}
+
+function formatBytes(n: number): string {
+  if (n < 1024 * 1024) return `${Math.round(n / 1024)} KB`;
+  return `${(n / (1024 * 1024)).toFixed(1)} MB`;
+}
+
+function describeOwner(owner: ProfileOwner): string {
+  if (owner.state === 'free') return 'idle';
+  if (owner.state === 'orphan') return `held by orphaned PID ${owner.pid}`;
+  return owner.detail;
+}
+
+/** `browse profiles [list]` and `browse profiles prune [--days N]`. Returns the exit code. */
+export function runProfilesCommand(args: string[], out: (line: string) => void = console.log): number {
+  const sub = args[0] ?? 'list';
+  const known = readRegistry();
+  const legacy = legacyProfileDir();
+  if (sub === 'list') {
+    const rows = [...(fs.existsSync(legacy) ? [legacy] : []), ...known];
+    if (rows.length === 0) {
+      out('No headed browser profiles yet. One is created per project on first `browse --headed` or `browse connect`.');
+      return 0;
+    }
+    for (const dir of rows) {
+      if (!fs.existsSync(dir)) { out(`${dir}  (missing: project moved or deleted; \`browse profiles prune\` forgets it)`); continue; }
+      const age = Math.floor((Date.now() - fs.statSync(dir).mtimeMs) / 86_400_000);
+      out(`${dir}  ${formatBytes(dirBytes(dir))}  last used ${age}d ago  ${describeOwner(profileOwner(dir))}${dir === legacy ? '  (old shared profile)' : ''}`);
+    }
+    return 0;
+  }
+  if (sub === 'prune') {
+    const daysIdx = args.indexOf('--days');
+    const days = daysIdx >= 0 ? Number(args[daysIdx + 1]) : 30;
+    if (!Number.isFinite(days) || days < 0) {
+      out('Usage: browse profiles prune [--days N]  (default 30)');
+      return 2;
+    }
+    const keep: string[] = [];
+    let freed = 0;
+    for (const dir of known) {
+      if (!fs.existsSync(dir)) { out(`forgot ${dir} (no longer exists)`); continue; }
+      const idleDays = (Date.now() - fs.statSync(dir).mtimeMs) / 86_400_000;
+      const owner = profileOwner(dir);
+      if (idleDays < days || owner.state !== 'free') { keep.push(dir); continue; }
+      const bytes = dirBytes(dir);
+      fs.rmSync(dir, { recursive: true, force: true });
+      freed += bytes;
+      out(`removed ${dir} (${formatBytes(bytes)}, idle ${Math.floor(idleDays)}d)`);
+    }
+    writeRegistry(keep);
+    out(`Pruned profiles idle for ${days}+ days; freed ${formatBytes(freed)}. Profiles in use are never removed. The old shared profile (${legacy}) is left alone.`);
+    return 0;
+  }
+  out('Usage: browse profiles [list] | browse profiles prune [--days N]');
+  return 2;
+}
diff --git a/browse/src/cli.ts b/browse/src/cli.ts
index cba6c1ab1..5e7c0d0e7 100644
--- a/browse/src/cli.ts
+++ b/browse/src/cli.ts
@@ -19,6 +19,7 @@ import { resolveConfig, ensureStateDir, readVersionHash, isPairAgentEnabled, res
 import { parseProxyConfig, computeConfigHash, ProxyConfigError } from './proxy-config';
 import { redactProxyUrl } from './proxy-redact';
 import { spawnTerminalAgent, BUN_CHILD_FLAGS } from './terminal-agent-control';
+import { profileOwner, ensureProjectProfile, runProfilesCommand } from './chromium-profiles';
 // Zero side effects on import (documented invariant in token-registry.ts) —
 // safe to pull the shared pairing default into the CLI.
 import { DEFAULT_PAIR_SCOPES } from './token-registry';
@@ -285,23 +286,21 @@ function cleanChromiumProfileLocks(profileDir: string = chromiumProfileDir()): v
   }
 }
 
-/** Kill an orphaned Chromium that still holds the profile's SingletonLock. The
- * lock symlink target is "hostname-PID"; killing that PID tears down its
- * renderer tree so the next launch starts clean. No-op when absent/stale. */
+/** Kill an orphaned Chromium that still holds the profile's SingletonLock so
+ * the next launch starts clean (#1781). Only a Chromium on this host that uses
+ * this profile and whose daemon is gone is killed; a profile held by a live
+ * process is never killed — browse stops and names the holder (D5, #2492). */
 async function killOrphanChromium(profileDir: string = chromiumProfileDir()): Promise<void> {
-  try {
-    const lockTarget = fs.readlinkSync(path.join(profileDir, 'SingletonLock')); // "hostname-12345"
-    const orphanPid = parseInt(lockTarget.split('-').pop() || '', 10);
-    if (orphanPid && isProcessAlive(orphanPid)) {
-      safeKill(orphanPid, 'SIGTERM');
-      await new Promise(r => setTimeout(r, 1000));
-      if (isProcessAlive(orphanPid)) {
-        safeKill(orphanPid, 'SIGKILL');
-        await new Promise(r => setTimeout(r, 500));
-      }
-    }
-  } catch (err: any) {
-    if (err?.code !== 'ENOENT' && err?.code !== 'EINVAL') throw err;
+  const owner = profileOwner(profileDir);
+  if (owner.state === 'free') return;
+  if (owner.state === 'live') {
+    throw new Error(`Headed Chromium profile ${profileDir} is ${owner.detail}. Close that browser first, or set CHROMIUM_PROFILE to a different directory.`);
+  }
+  safeKill(owner.pid, 'SIGTERM');
+  await new Promise(r => setTimeout(r, 1000));
+  if (isProcessAlive(owner.pid)) {
+    safeKill(owner.pid, 'SIGKILL');
+    await new Promise(r => setTimeout(r, 500));
   }
 }
 
@@ -621,6 +620,7 @@ async function startServer(extraEnv?: Record<string, string>): Promise<ServerSta
   // blocked by the previous Chromium's SingletonLock — the self-inflicted
   // crash-loop. Previously only the manual connect preamble did this.
   if ((extraEnv?.BROWSE_HEADED ?? process.env.BROWSE_HEADED) === '1') {
+    ensureProjectProfile(chromiumProfileDir());
     await killOrphanChromium();
     cleanChromiumProfileLocks();
   }
@@ -1676,6 +1676,7 @@ Multi-step:     chain (reads JSON from stdin)
 Tabs:           tabs | tab <id> | newtab [url] | closetab [id]
 Server:         status | cookie <n>=<v> | header <n>:<v>
                 useragent <str> | stop | restart
+                profiles [list] | profiles prune [--days N]  (per-project headed profiles)
                 tunnel revoke <name> | tunnel agents  (paired-agent tokens)
                 --force-restart: replace a live-but-busy daemon (any command;
                 LOSES tabs/cookies/logins — never done automatically)
@@ -1693,6 +1694,8 @@ Refs:           After 'snapshot', use @e1, @e2... as selectors:
   const command = args[0];
   const commandArgs = args.slice(1);
 
+  if (command === 'profiles') process.exit(runProfilesCommand(commandArgs));
+
   // ─── Headed Connect (pre-server command) ────────────────────
   // connect must be handled BEFORE ensureServer() because it needs
   // to restart the server in headed mode with the Chrome extension.
diff --git a/browse/src/config.ts b/browse/src/config.ts
index a9080e89e..89cd21418 100644
--- a/browse/src/config.ts
+++ b/browse/src/config.ts
@@ -250,19 +250,21 @@ export function isPairAgentEnabled(): boolean {
 }
 
 /**
- * Resolve the Chromium profile directory.
+ * Resolve the headed Chromium profile directory.
  *
  * Resolution order:
  *   1. `explicit` arg (no production caller passes one today; kept for
  *      direct programmatic use)
  *   2. CHROMIUM_PROFILE env (used by gbrowser's gbd per-workspace)
- *   3. <resolveGstackHome()>/chromium-profile (default)
+ *   3. <project state dir>/chromium-profile (default; per project since D5,
+ *      #2492 — a machine-wide profile let one project's headed launch kill
+ *      another project's live browser)
  */
 export function resolveChromiumProfile(explicit?: string): string {
   if (explicit && explicit.length > 0) return explicit;
   const env = process.env.CHROMIUM_PROFILE;
   if (env && env.length > 0) return env;
-  return path.join(resolveGstackHome(), 'chromium-profile');
+  return path.join(resolveConfig().stateDir, 'chromium-profile');
 }
 
 /**
diff --git a/browse/test/chromium-profile-isolation.test.ts b/browse/test/chromium-profile-isolation.test.ts
index d35eda405..57fdcc036 100644
--- a/browse/test/chromium-profile-isolation.test.ts
+++ b/browse/test/chromium-profile-isolation.test.ts
@@ -52,7 +52,9 @@ describe('Chromium profile isolation cleanup (#2908)', () => {
 describe.skipIf(process.platform === 'win32')('Chromium profile isolation (#2817)', () => {
   let scratch: string;
   let profile: string;
+  let projectProfile: string;
   let stateFile: string;
+  let orphans: number[];
   let env: Record<string, string>;
   let children: ReturnType<typeof Bun.spawn>[];
   let daemonPid: number | undefined;
@@ -61,6 +63,7 @@ describe.skipIf(process.platform === 'win32')('Chromium profile isolation (#2817
     scratch = fs.mkdtempSync(path.join(os.tmpdir(), 'browse-profile-isolation-'));
     profile = path.join(scratch, '.gstack', 'chromium-profile');
     stateFile = path.join(scratch, 'project-b', '.gstack', 'browse.json');
+    projectProfile = path.join(scratch, 'project-b', '.gstack', 'chromium-profile');
     fs.mkdirSync(profile, { recursive: true });
     fs.mkdirSync(path.dirname(stateFile), { recursive: true });
     env = {};
@@ -83,6 +86,7 @@ describe.skipIf(process.platform === 'win32')('Chromium profile isolation (#2817
       BROWSE_START_TIMEOUT: '30000',
     });
     children = [];
+    orphans = [];
     daemonPid = undefined;
   });
 
@@ -92,14 +96,16 @@ describe.skipIf(process.platform === 'win32')('Chromium profile isolation (#2817
     }
     if (daemonPid) killDaemonGroup(daemonPid);
     for (const child of children) child.kill('SIGKILL');
+    for (const pid of orphans) { try { process.kill(pid, 'SIGKILL'); } catch {} }
     await Promise.all(children.map(child => child.exited));
     fs.rmSync(scratch, { recursive: true, force: true });
   });
 
-  async function spawnChromiumHolder() {
+  /** A stand-in Chromium: its command line names Chromium and the profile it uses. */
+  async function spawnChromiumHolder(userDataDir = profile) {
     const script = path.join(scratch, `chromium-holder-${children.length}.ts`);
     fs.writeFileSync(script, 'console.log("ready"); await Bun.sleep(60000);');
-    const child = Bun.spawn([process.execPath, script], {
+    const child = Bun.spawn([process.execPath, script, `--user-data-dir=${userDataDir}`], {
       env,
       stdin: 'ignore',
       stdout: 'pipe',
@@ -113,17 +119,66 @@ describe.skipIf(process.platform === 'win32')('Chromium profile isolation (#2817
     return child;
   }
 
-  function seedLocks(pid: number) {
-    fs.symlinkSync(`${os.hostname()}-${pid}`, path.join(profile, LOCKS[0]));
-    fs.symlinkSync('socket-target', path.join(profile, LOCKS[1]));
-    fs.symlinkSync('cookie-target', path.join(profile, LOCKS[2]));
+  /** A stand-in Chromium whose daemon is gone: reparented to init. */
+  async function spawnOrphanedChromium(userDataDir: string) {
+    const script = path.join(scratch, `chromium-orphan-${orphans.length}.ts`);
+    const ready = `${script}.ready`;
+    fs.writeFileSync(script, `require('node:fs').writeFileSync(${JSON.stringify(ready)}, 'ready'); await Bun.sleep(60000);`);
+    const launcher = Bun.spawnSync(['sh', '-c', `"$0" "$1" "$2" >/dev/null 2>&1 & echo $!`, process.execPath, script, `--user-data-dir=${userDataDir}`], { env, stdout: 'pipe', timeout: 10_000 });
+    const pid = Number(launcher.stdout.toString().trim());
+    orphans.push(pid);
+    const deadline = Date.now() + 10_000;
+    while (!fs.existsSync(ready) && Date.now() < deadline) await Bun.sleep(20);
+    expect(fs.existsSync(ready)).toBe(true);
+    return { pid };
   }
 
-  function expectLocksIntact(pid: number) {
+  function seedLocks(pid: number, dir = profile, host = os.hostname()) {
+    fs.mkdirSync(dir, { recursive: true });
+    fs.symlinkSync(`${host}-${pid}`, path.join(dir, LOCKS[0]));
+    fs.symlinkSync('socket-target', path.join(dir, LOCKS[1]));
+    fs.symlinkSync('cookie-target', path.join(dir, LOCKS[2]));
+  }
+
+  function expectLocksIntact(pid: number, dir = profile, host = os.hostname()) {
     expect(isProcessAlive(pid)).toBe(true);
-    expect(fs.readlinkSync(path.join(profile, LOCKS[0]))).toBe(`${os.hostname()}-${pid}`);
-    expect(fs.readlinkSync(path.join(profile, LOCKS[1]))).toBe('socket-target');
-    expect(fs.readlinkSync(path.join(profile, LOCKS[2]))).toBe('cookie-target');
+    expect(fs.readlinkSync(path.join(dir, LOCKS[0]))).toBe(`${host}-${pid}`);
+    expect(fs.readlinkSync(path.join(dir, LOCKS[1]))).toBe('socket-target');
+    expect(fs.readlinkSync(path.join(dir, LOCKS[2]))).toBe('cookie-target');
+  }
+
+  function writeStubServer() {
+    const serverScript = path.join(scratch, 'stub-server.ts');
+    fs.writeFileSync(serverScript, `
+      import * as fs from 'node:fs';
+      const server = Bun.serve({
+        hostname: '127.0.0.1', port: 0,
+        fetch: () => Response.json({ status: 'healthy' }),
+      });
+      fs.writeFileSync(process.env.BROWSE_STATE_FILE!, JSON.stringify({
+        pid: process.pid, port: server.port, token: 'test-token',
+        mode: process.env.BROWSE_HEADED === '1' ? 'headed' : 'launched',
+      }));
+    `);
+    env.BROWSE_SERVER_SCRIPT = serverScript;
+  }
+
+  async function runCliRaw(args: string[]) {
+    const child = Bun.spawn([process.execPath, CLI, ...args], {
+      cwd: path.join(scratch, 'project-b'),
+      env,
+      stdin: 'ignore',
+      stdout: 'pipe',
+      stderr: 'pipe',
+    });
+    children.push(child);
+    const [code, stdout, stderr] = await Promise.all([
+      child.exited,
+      new Response(child.stdout).text(),
+      new Response(child.stderr).text(),
+    ]);
+    if (fs.existsSync(stateFile)) daemonPid = JSON.parse(fs.readFileSync(stateFile, 'utf-8')).pid;
+    return { code, stdout, stderr };
   }
 
   async function runCli(args: string[]) {
@@ -176,34 +231,108 @@ describe.skipIf(process.platform === 'win32')('Chromium profile isolation (#2817
   }, 60_000);
 
   for (const mode of ['flag', 'environment'] as const) {
-    test(`headed startup via ${mode} still reaps a profile orphan and removes stale locks`, async () => {
-      const orphan = await spawnChromiumHolder();
-      seedLocks(orphan.pid);
+    test(`headed startup via ${mode} reaps an orphaned Chromium on this project's profile and removes stale locks`, async () => {
+      const orphan = await spawnOrphanedChromium(projectProfile);
+      seedLocks(orphan.pid, projectProfile);
       env.BROWSE_HEADED = mode === 'environment' ? '1' : '0';
-      const serverScript = path.join(scratch, 'stub-server.ts');
-      fs.writeFileSync(serverScript, `
-        import * as fs from 'node:fs';
-        const server = Bun.serve({
-          hostname: '127.0.0.1', port: 0,
-          fetch: () => Response.json({ status: 'healthy' }),
-        });
-        fs.writeFileSync(process.env.BROWSE_STATE_FILE!, JSON.stringify({
-          pid: process.pid, port: server.port, token: 'test-token',
-          mode: process.env.BROWSE_HEADED === '1' ? 'headed' : 'launched',
-        }));
-      `);
-      env.BROWSE_SERVER_SCRIPT = serverScript;
+      writeStubServer();
 
       await runCli(mode === 'flag' ? ['--headed', 'status'] : ['status']);
 
       expect(JSON.parse(fs.readFileSync(stateFile, 'utf-8')).mode).toBe('headed');
       expect(isProcessAlive(orphan.pid)).toBe(false);
-      expect(fs.readdirSync(profile)).not.toContain(LOCKS[0]);
-      expect(fs.readdirSync(profile)).not.toContain(LOCKS[1]);
-      expect(fs.readdirSync(profile)).not.toContain(LOCKS[2]);
+      for (const lock of LOCKS) expect(fs.readdirSync(projectProfile)).not.toContain(lock);
     }, 60_000);
   }
 
+  test('headed startup in one project leaves another project\'s live headed Chromium alone (D5, #2492)', async () => {
+    const otherProject = path.join(scratch, 'project-a', '.gstack', 'chromium-profile');
+    const holder = await spawnChromiumHolder(otherProject);
+    seedLocks(holder.pid, otherProject);
+    writeStubServer();
+
+    await runCli(['--headed', 'status']);
+
+    expectLocksIntact(holder.pid, otherProject);
+    expect(fs.existsSync(projectProfile)).toBe(true);
+  }, 60_000);
+
+  test('a live owner of this project\'s profile is never killed; browse names it and stops', async () => {
+    const holder = await spawnChromiumHolder(projectProfile);
+    seedLocks(holder.pid, projectProfile);
+    writeStubServer();
+
+    const r = await runCliRaw(['--headed', 'status']);
+
+    expect(r.code).not.toBe(0);
+    expect(r.stdout + r.stderr).toContain(`in use by Chromium PID ${holder.pid}`);
+    expectLocksIntact(holder.pid, projectProfile);
+  }, 60_000);
+
+  test('a lock written by another host is never acted on', async () => {
+    const holder = await spawnOrphanedChromium(projectProfile);
+    seedLocks(holder.pid, projectProfile, 'some-other-host');
+    writeStubServer();
+
+    const r = await runCliRaw(['--headed', 'status']);
+
+    expect(r.code).not.toBe(0);
+    expect(r.stdout + r.stderr).toContain('on another host (some-other-host)');
+    expectLocksIntact(holder.pid, projectProfile, 'some-other-host');
+  }, 60_000);
+
+  test('first headed use seeds the project profile from a quiescent shared profile, without singleton files', async () => {
+    fs.mkdirSync(path.join(profile, 'Default'), { recursive: true });
+    fs.writeFileSync(path.join(profile, 'Default', 'Cookies'), 'cookie-db');
+    seedLocks(999_999_999);
+    writeStubServer();
+
+    const r = await runCliRaw(['--headed', 'status']);
+
+    expect(r.code, r.stdout + r.stderr).toBe(0);
+    expect(r.stdout).toContain(`Copied your logins and settings from ${profile} to ${projectProfile}`);
+    expect(fs.readFileSync(path.join(projectProfile, 'Default', 'Cookies'), 'utf-8')).toBe('cookie-db');
+    expect(fs.statSync(projectProfile).mode & 0o777).toBe(0o700);
+    expect(fs.readdirSync(projectProfile).filter(f => f.startsWith('Singleton') || f.includes('.seed-'))).toEqual([]);
+    expect(fs.readdirSync(path.dirname(projectProfile)).filter(f => f.includes('.seed-'))).toEqual([]);
+  }, 60_000);
+
+  test('a shared profile in use is not copied; the project starts fresh and says how to import later', async () => {
+    fs.writeFileSync(path.join(profile, 'Cookies'), 'cookie-db');
+    const holder = await spawnChromiumHolder(profile);
+    seedLocks(holder.pid);
+    writeStubServer();
+
+    const r = await runCliRaw(['--headed', 'status']);
+
+    expect(r.code, r.stdout + r.stderr).toBe(0);
+    expect(r.stdout).toContain('starts with a fresh profile. To import it later');
+    expect(fs.existsSync(path.join(projectProfile, 'Cookies'))).toBe(false);
+    expectLocksIntact(holder.pid);
+  }, 60_000);
+
+  test('browse profiles lists per-project profiles and prune removes idle ones but never one in use', async () => {
+    writeStubServer();
+    await runCli(['--headed', 'status']);
+    const listed = await runCliRaw(['profiles']);
+    expect(listed.code).toBe(0);
+    expect(listed.stdout).toContain(projectProfile);
+    expect(listed.stdout).toContain('(old shared profile)');
+
+    const holder = await spawnChromiumHolder(projectProfile);
+    seedLocks(holder.pid, projectProfile);
+    await runCliRaw(['profiles', 'prune', '--days', '0']);
+    expect(fs.existsSync(projectProfile)).toBe(true);
+
+    holder.kill('SIGKILL');
+    await holder.exited;
+    const pruned = await runCliRaw(['profiles', 'prune', '--days', '0']);
+    expect(pruned.code).toBe(0);
+    expect(pruned.stdout).toContain(`removed ${projectProfile}`);
+    expect(fs.existsSync(projectProfile)).toBe(false);
+    expect(fs.existsSync(profile)).toBe(true);
+  }, 60_000);
+
   for (const args of [['stop'], ['--force-restart', 'stop'], ['disconnect']]) {
     test(`headless ${args.join(' ')} preserves another project's headed profile`, async () => {
       await runCli(['status']);
@@ -228,6 +357,7 @@ describe.skipIf(process.platform === 'win32')('Chromium profile isolation (#2817
     test(`${mode} factory shutdown scopes profile cleanup to the active browser mode`, async () => {
       const holder = await spawnChromiumHolder();
       seedLocks(holder.pid);
+      seedLocks(999_999_999, projectProfile);
       const child = Bun.spawn([process.execPath, '-e', `
         import { buildFetchHandler } from ${JSON.stringify(path.resolve(import.meta.dir, '../src/server.ts'))};
         import { resolveConfig } from ${JSON.stringify(path.resolve(import.meta.dir, '../src/config.ts'))};
@@ -249,10 +379,13 @@ describe.skipIf(process.platform === 'win32')('Chromium profile isolation (#2817
       const [code, stderr] = await Promise.all([child.exited, new Response(child.stderr).text()]);
 
       expect(code, stderr).toBe(0);
+      // Headed shutdown cleans only this project's profile; another
+      // project's (here the old shared one) is never touched (D5).
+      expectLocksIntact(holder.pid);
       if (mode === 'headed') {
-        for (const lock of LOCKS) expect(fs.readdirSync(profile)).not.toContain(lock);
+        for (const lock of LOCKS) expect(fs.readdirSync(projectProfile)).not.toContain(lock);
       } else {
-        expectLocksIntact(holder.pid);
+        expect(fs.readdirSync(projectProfile)).toContain(LOCKS[0]);
       }
     }, 60_000);
   }
diff --git a/browse/test/config.test.ts b/browse/test/config.test.ts
index c20bad8f2..53a2081a6 100644
--- a/browse/test/config.test.ts
+++ b/browse/test/config.test.ts
@@ -431,17 +431,21 @@ describe('resolveChromiumProfile', () => {
     }
   });
 
-  test('falls back to resolveGstackHome()/chromium-profile when nothing set', () => {
+  test('falls back to the project state dir, not the machine-wide gstack home (D5, #2492)', () => {
     const origEnv = process.env.CHROMIUM_PROFILE;
     const origHome = process.env.GSTACK_HOME;
+    const origState = process.env.BROWSE_STATE_FILE;
     delete process.env.CHROMIUM_PROFILE;
     process.env.GSTACK_HOME = '/tmp/fallback-gstack';
+    process.env.BROWSE_STATE_FILE = '/tmp/project-a/.gstack/browse.json';
     try {
-      expect(resolveChromiumProfile()).toBe('/tmp/fallback-gstack/chromium-profile');
+      expect(resolveChromiumProfile()).toBe(path.join('/tmp/project-a/.gstack', 'chromium-profile'));
     } finally {
       if (origEnv !== undefined) process.env.CHROMIUM_PROFILE = origEnv;
       if (origHome === undefined) delete process.env.GSTACK_HOME;
       else process.env.GSTACK_HOME = origHome;
+      if (origState === undefined) delete process.env.BROWSE_STATE_FILE;
+      else process.env.BROWSE_STATE_FILE = origState;
     }
   });