diff --git a/packages/mosaic/README.md b/packages/mosaic/README.md
index a616479..6a1b9c9 100644
--- a/packages/mosaic/README.md
+++ b/packages/mosaic/README.md
@@ -47,6 +47,61 @@ export MOSAIC_ADMIN_PASSWORD="securepass123"
 mosaic gateway install
 ```
 
+## Runtime launchers
+
+```bash
+mosaic claude            # Launch Claude Code with Mosaic injection
+mosaic yolo claude       # …with --dangerously-skip-permissions
+mosaic codex | opencode | pi
+```
+
+### `mosaic claudex` (EXPERIMENTAL)
+
+Runs GPT models **inside the Claude Code harness** by pointing Claude Code at a
+local [`claude-code-proxy`](https://github.com/raine/claude-code-proxy) that
+translates the Anthropic Messages API to a ChatGPT-subscription (Codex OAuth)
+backend. This is **not Anthropic Claude** — model behavior, tool use, and output
+quality may differ. Intended for evaluation, not production delivery.
+
+```bash
+mosaic claudex           # launch (prompts through the proxy readiness gate)
+mosaic yolo claudex      # …with --dangerously-skip-permissions
+mosaic claudex --print "hello"   # trailing args are forwarded to Claude Code
+```
+
+**Prerequisite:** the `claude-code-proxy` binary must be installed and
+authenticated (`claude-code-proxy codex auth …`). `mosaic claudex` runs a
+preflight that verifies the binary, the OAuth state (triggering a device re-auth
+if needed), and a trusted local listener before launching; it **fails closed**
+if the proxy cannot be brought up with a verified identity.
+
+**Isolation (never touches your real Claude state).** claudex always launches
+against an isolated `CLAUDE_CONFIG_DIR` (default `~/.config/mosaic/claudex/home`).
+The ambient `CLAUDE_CONFIG_DIR` is deliberately ignored, and a guard proves the
+resolved dir can never be — or live under — the real `~/.claude`. A claudex
+session therefore cannot mutate your normal Claude Code config.
+
+**No token leakage.** claudex never reads the proxy's credential file. Claude
+Code is handed only `ANTHROPIC_AUTH_TOKEN=unused` pointed at the loopback proxy;
+the entire credential-bearing env family (`ANTHROPIC_*`, `AWS_*`, `GOOGLE_CLOUD_*`,
+`GOOGLE_APPLICATION_CREDENTIALS`, `*_TOKEN`, `*_KEY`, `*_SECRET`, …) is stripped
+from the composed environment. The Bedrock/Vertex routing switches
+(`CLAUDE_CODE_USE_BEDROCK`, `CLAUDE_CODE_USE_VERTEX`, and the `_SKIP_*_AUTH`
+pair) are force-removed regardless of value — otherwise their mere presence
+would route Claude Code to the real Anthropic API via AWS/GCP and bypass the
+proxy. The proxy holds the real OAuth credential.
+
+**Model tiers (override via env).**
+
+| Tier                  | Env var                      | Default        |
+| --------------------- | ---------------------------- | -------------- |
+| primary (opus/sonnet) | `ANTHROPIC_MODEL`            | `gpt-5.6-sol`  |
+| small/fast (haiku)    | `ANTHROPIC_SMALL_FAST_MODEL` | `gpt-5.6-luna` |
+
+Operator-provided values win over the defaults. Additional overrides:
+`MOSAIC_CLAUDEX_CONFIG_DIR` (isolated config dir), `ANTHROPIC_BASE_URL` (proxy
+endpoint).
+
 ## Hooks management
 
 After running `mosaic wizard`, Claude hooks are installed in `~/.claude/hooks-config.json`.
diff --git a/packages/mosaic/src/commands/claudex.spec.ts b/packages/mosaic/src/commands/claudex.spec.ts
new file mode 100644
index 0000000..fab7cdc
--- /dev/null
+++ b/packages/mosaic/src/commands/claudex.spec.ts
@@ -0,0 +1,732 @@
+import { describe, it, expect, vi } from 'vitest';
+import { mkdtempSync, mkdirSync, symlinkSync, rmSync, lstatSync, writeFileSync } from 'node:fs';
+import { tmpdir, homedir } from 'node:os';
+import { join } from 'node:path';
+import {
+  CLAUDEX_CONFIG_DIR_ENV,
+  CLAUDEX_DEFAULT_PRIMARY_MODEL,
+  CLAUDEX_DEFAULT_SMALL_FAST_MODEL,
+  CLAUDEX_CREDENTIAL_ENV_RE,
+  defaultClaudexConfigDir,
+  assertIsolatedConfigDir,
+  resolveClaudexConfigDir,
+  resolveClaudexModels,
+  buildClaudexEnv,
+  buildClaudexBanner,
+  buildClaudexContractNote,
+  runClaudexProxyGate,
+  launchClaudex,
+  type ClaudexHarnessAdapter,
+} from './claudex.js';
+import { CLAUDEX_PROXY_URL, type PreflightReport } from './claudex-proxy.js';
+
+// ─── helpers ─────────────────────────────────────────────────────────────────
+
+function makeReport(overrides: Partial<PreflightReport> = {}): PreflightReport {
+  return {
+    binaryPresent: true,
+    binaryPath: '/usr/bin/claude-code-proxy',
+    auth: { state: 'valid' },
+    live: true,
+    listenerVerdict: 'ok',
+    needsReauth: false,
+    ok: true,
+    problems: [],
+    ...overrides,
+  };
+}
+
+function okAdapter(overrides: Partial<ClaudexHarnessAdapter> = {}): ClaudexHarnessAdapter {
+  return {
+    harnessPreflight: () => {},
+    composePrompt: () => '# Composed Claude contract',
+    exec: () => {},
+    ...overrides,
+  };
+}
+
+// Identity canonicalizer + no-op FS deps so config-dir logic is tested purely.
+const idCanon = (p: string): string => p;
+const noFsDeps = { canonicalize: idCanon, mkdir: () => {}, isSymlink: () => false };
+
+// ─── isolated config dir (HARD SECURITY REQ 1 — provable isolation) ───────────
+
+describe('defaultClaudexConfigDir', () => {
+  it('is namespaced under the mosaic home, never ~/.claude', () => {
+    const dir = defaultClaudexConfigDir('/home/agent/.config/mosaic');
+    expect(dir).toBe(join('/home/agent/.config/mosaic', 'claudex', 'home'));
+    expect(dir).not.toBe(join(homedir(), '.claude'));
+  });
+});
+
+describe('assertIsolatedConfigDir — the isolation guard is provable', () => {
+  const realClaude = '/home/agent/.claude';
+
+  it('accepts a dir that does not resolve to ~/.claude', () => {
+    const safe = '/home/agent/.config/mosaic/claudex/home';
+    expect(
+      assertIsolatedConfigDir(safe, { realClaudeDir: realClaude, canonicalize: idCanon }),
+    ).toBe(safe);
+  });
+
+  it('REJECTS a candidate that is literally ~/.claude', () => {
+    expect(() =>
+      assertIsolatedConfigDir(realClaude, { realClaudeDir: realClaude, canonicalize: idCanon }),
+    ).toThrow(/refusing/i);
+  });
+
+  it('REJECTS a descendant of ~/.claude (would pollute the real tree)', () => {
+    expect(() =>
+      assertIsolatedConfigDir('/home/agent/.claude/projects/x', {
+        realClaudeDir: realClaude,
+        canonicalize: idCanon,
+      }),
+    ).toThrow(/refusing/i);
+  });
+
+  it('REJECTS a candidate that canonically resolves to ~/.claude (symlink, both sides canonicalized)', () => {
+    const canon = (p: string): string => (p === '/home/agent/link' ? realClaude : p);
+    expect(() =>
+      assertIsolatedConfigDir('/home/agent/link', {
+        realClaudeDir: realClaude,
+        canonicalize: canon,
+      }),
+    ).toThrow(/refusing/i);
+  });
+
+  it('canonicalizes the ~/.claude side too (real dir itself may be a symlink)', () => {
+    // realClaudeDir is a symlink whose canonical target equals the candidate's target.
+    const canon = (p: string): string =>
+      p === '/home/agent/.claude' || p === '/home/agent/link' ? '/canonical/claude' : p;
+    expect(() =>
+      assertIsolatedConfigDir('/home/agent/link', {
+        realClaudeDir: realClaude,
+        canonicalize: canon,
+      }),
+    ).toThrow(/refusing/i);
+  });
+
+  it('REJECTS an empty or whitespace candidate (fail closed)', () => {
+    expect(() =>
+      assertIsolatedConfigDir('', { realClaudeDir: realClaude, canonicalize: idCanon }),
+    ).toThrow();
+    expect(() =>
+      assertIsolatedConfigDir('   ', { realClaudeDir: realClaude, canonicalize: idCanon }),
+    ).toThrow();
+  });
+
+  it('REJECTS a relative candidate (must be absolute)', () => {
+    expect(() =>
+      assertIsolatedConfigDir('relative/dir', { realClaudeDir: realClaude, canonicalize: idCanon }),
+    ).toThrow(/absolute/i);
+  });
+});
+
+describe('resolveClaudexConfigDir', () => {
+  it('uses the namespaced default and never the ambient CLAUDE_CONFIG_DIR', () => {
+    // Ambient CLAUDE_CONFIG_DIR is deliberately ignored (it could be ~/.claude).
+    const env = { CLAUDE_CONFIG_DIR: join(homedir(), '.claude') };
+    const dir = resolveClaudexConfigDir(env, {
+      mosaicHome: '/home/agent/.config/mosaic',
+      realClaudeDir: '/home/agent/.claude',
+      ...noFsDeps,
+    });
+    expect(dir).toBe(join('/home/agent/.config/mosaic', 'claudex', 'home'));
+  });
+
+  it('honors the dedicated override env when it is safe', () => {
+    const env = { [CLAUDEX_CONFIG_DIR_ENV]: '/home/agent/custom-claudex' };
+    const dir = resolveClaudexConfigDir(env, {
+      mosaicHome: '/home/agent/.config/mosaic',
+      realClaudeDir: '/home/agent/.claude',
+      ...noFsDeps,
+    });
+    expect(dir).toBe('/home/agent/custom-claudex');
+  });
+
+  it('REJECTS a dedicated override that points at ~/.claude (before creating anything)', () => {
+    const mkdir = vi.fn();
+    const env = { [CLAUDEX_CONFIG_DIR_ENV]: '/home/agent/.claude' };
+    expect(() =>
+      resolveClaudexConfigDir(env, {
+        mosaicHome: '/home/agent/.config/mosaic',
+        realClaudeDir: '/home/agent/.claude',
+        canonicalize: idCanon,
+        mkdir,
+        isSymlink: () => false,
+      }),
+    ).toThrow(/refusing/i);
+    expect(mkdir).not.toHaveBeenCalled();
+  });
+
+  it('TOCTOU: REJECTS when the created target is itself a symlink (pre-created race)', () => {
+    const env = {};
+    expect(() =>
+      resolveClaudexConfigDir(env, {
+        mosaicHome: '/home/agent/.config/mosaic',
+        realClaudeDir: '/home/agent/.claude',
+        canonicalize: idCanon,
+        mkdir: () => {},
+        isSymlink: () => true, // the just-ensured dir is a symlink → fail closed
+      }),
+    ).toThrow(/refusing|symlink/i);
+  });
+
+  it('real-FS: creates the isolated dir 0700 and returns its canonical path', () => {
+    const root = mkdtempSync(join(tmpdir(), 'claudex-cfg-'));
+    try {
+      const mosaicHome = join(root, '.config', 'mosaic');
+      const dir = resolveClaudexConfigDir({}, { mosaicHome, realClaudeDir: join(root, '.claude') });
+      expect(dir).toBe(join(mosaicHome, 'claudex', 'home'));
+      const st = lstatSync(dir);
+      expect(st.isDirectory()).toBe(true);
+      // 0700 (owner-only) — mask off the type bits.
+      expect(st.mode & 0o777).toBe(0o700);
+    } finally {
+      rmSync(root, { recursive: true, force: true });
+    }
+  });
+
+  it('real-FS: catches an override whose ancestor symlinks into ~/.claude', () => {
+    const root = mkdtempSync(join(tmpdir(), 'claudex-cfg-'));
+    try {
+      const realClaudeDir = join(root, 'dot-claude');
+      mkdirSync(realClaudeDir, { recursive: true });
+      const link = join(root, 'link'); // link -> dot-claude
+      symlinkSync(realClaudeDir, link, 'dir');
+      const override = join(link, 'sub'); // resolves under ~/.claude
+      expect(() =>
+        resolveClaudexConfigDir(
+          { [CLAUDEX_CONFIG_DIR_ENV]: override },
+          { mosaicHome: join(root, '.config', 'mosaic'), realClaudeDir },
+        ),
+      ).toThrow(/refusing/i);
+    } finally {
+      rmSync(root, { recursive: true, force: true });
+    }
+  });
+
+  it('FAIL CLOSED: default canonicalizer rethrows a non-ENOENT error (ELOOP) instead of a literal fallback', () => {
+    // A symlink loop makes realpathSync throw ELOOP. The guard must NOT swallow
+    // it as "does not exist yet, keep walking up" and return a literal path —
+    // it must fail closed. (REQ 1: fails CLOSED on any uncertainty.)
+    const root = mkdtempSync(join(tmpdir(), 'claudex-loop-'));
+    try {
+      const a = join(root, 'a');
+      const b = join(root, 'b');
+      symlinkSync(b, a, 'dir'); // a -> b
+      symlinkSync(a, b, 'dir'); // b -> a  (loop)
+      const looped = join(a, 'home'); // canonicalizing this hits ELOOP
+      // No canonicalize dep → the real defaultCanonicalizeIntended runs.
+      expect(() =>
+        assertIsolatedConfigDir(looped, { realClaudeDir: join(root, '.claude') }),
+      ).toThrow();
+    } finally {
+      rmSync(root, { recursive: true, force: true });
+    }
+  });
+
+  it('FAIL CLOSED: default isSymlink rethrows a non-ENOENT error (ENOTDIR) rather than reporting "not a symlink"', () => {
+    // A candidate whose parent is a regular FILE makes lstat throw ENOTDIR.
+    // The post-create symlink check must fail closed, not treat it as safe.
+    const root = mkdtempSync(join(tmpdir(), 'claudex-notdir-'));
+    try {
+      const file = join(root, 'afile');
+      writeFileSync(file, 'x');
+      const candidate = join(file, 'child'); // parent is a file → ENOTDIR on lstat
+      expect(() =>
+        // Bypass the guard/mkdir side-effects; only the default isSymlink runs live.
+        resolveClaudexConfigDir(
+          { [CLAUDEX_CONFIG_DIR_ENV]: candidate },
+          {
+            realClaudeDir: join(root, '.claude'),
+            canonicalize: idCanon,
+            mkdir: () => {},
+            // isSymlink omitted → real defaultIsSymlink runs on the ENOTDIR path.
+          },
+        ),
+      ).toThrow();
+    } finally {
+      rmSync(root, { recursive: true, force: true });
+    }
+  });
+
+  it('FAIL CLOSED: an injected canonicalize throwing EACCES is not swallowed', () => {
+    const eacces = Object.assign(new Error('permission denied'), { code: 'EACCES' });
+    expect(() =>
+      resolveClaudexConfigDir(
+        { [CLAUDEX_CONFIG_DIR_ENV]: '/home/agent/custom-claudex' },
+        {
+          realClaudeDir: '/home/agent/.claude',
+          canonicalize: () => {
+            throw eacces;
+          },
+          mkdir: () => {},
+          isSymlink: () => false,
+        },
+      ),
+    ).toThrow(/permission denied/);
+  });
+});
+
+// ─── model-tier map (P3) ──────────────────────────────────────────────────────
+
+describe('resolveClaudexModels', () => {
+  it('defaults primary=sol / smallFast=luna', () => {
+    expect(resolveClaudexModels({})).toEqual({
+      primary: CLAUDEX_DEFAULT_PRIMARY_MODEL,
+      smallFast: CLAUDEX_DEFAULT_SMALL_FAST_MODEL,
+    });
+    expect(CLAUDEX_DEFAULT_PRIMARY_MODEL).toBe('gpt-5.6-sol');
+    expect(CLAUDEX_DEFAULT_SMALL_FAST_MODEL).toBe('gpt-5.6-luna');
+  });
+
+  it('env-provided values WIN over defaults', () => {
+    expect(
+      resolveClaudexModels({ ANTHROPIC_MODEL: 'gpt-x', ANTHROPIC_SMALL_FAST_MODEL: 'gpt-y' }),
+    ).toEqual({ primary: 'gpt-x', smallFast: 'gpt-y' });
+  });
+
+  it('ignores blank env values (falls back to defaults)', () => {
+    expect(
+      resolveClaudexModels({ ANTHROPIC_MODEL: '   ', ANTHROPIC_SMALL_FAST_MODEL: '' }),
+    ).toEqual({
+      primary: CLAUDEX_DEFAULT_PRIMARY_MODEL,
+      smallFast: CLAUDEX_DEFAULT_SMALL_FAST_MODEL,
+    });
+  });
+});
+
+// ─── env injection (HARD SECURITY REQ 2 — zero token leakage) ─────────────────
+
+describe('buildClaudexEnv — zero token leakage', () => {
+  const models = { primary: 'gpt-5.6-sol', smallFast: 'gpt-5.6-luna' };
+  const configDir = '/home/agent/.config/mosaic/claudex/home';
+
+  it('sets only ANTHROPIC_AUTH_TOKEN=unused and points at the loopback proxy', () => {
+    const env = buildClaudexEnv({}, { configDir, models });
+    expect(env.ANTHROPIC_AUTH_TOKEN).toBe('unused');
+    expect(env.ANTHROPIC_BASE_URL).toBe(CLAUDEX_PROXY_URL);
+    expect(env.CLAUDE_CONFIG_DIR).toBe(configDir);
+    expect(env.ANTHROPIC_MODEL).toBe('gpt-5.6-sol');
+    expect(env.ANTHROPIC_SMALL_FAST_MODEL).toBe('gpt-5.6-luna');
+  });
+
+  it('OVERWRITES an inherited real auth token with the literal "unused"', () => {
+    const env = buildClaudexEnv(
+      { ANTHROPIC_AUTH_TOKEN: 'sk-ant-realsecret-should-never-flow' },
+      { configDir, models },
+    );
+    expect(env.ANTHROPIC_AUTH_TOKEN).toBe('unused');
+  });
+
+  it('DELETES ANTHROPIC_API_KEY so no real Anthropic key reaches the local proxy', () => {
+    const env = buildClaudexEnv(
+      { ANTHROPIC_API_KEY: 'sk-ant-api03-realkey' },
+      { configDir, models },
+    );
+    expect('ANTHROPIC_API_KEY' in env).toBe(false);
+    expect(env.ANTHROPIC_API_KEY).toBeUndefined();
+  });
+
+  it('sweeps the WHOLE credential-bearing env family (token/api-key/secret/oauth), not just two', () => {
+    const env = buildClaudexEnv(
+      {
+        ANTHROPIC_API_KEY: 'sk-ant-api03-leak',
+        CLAUDE_CODE_OAUTH_TOKEN: 'oauth-leak',
+        SOME_SERVICE_TOKEN: 'tok-leak',
+        VENDOR_API_KEY: 'key-leak',
+        DB_SECRET: 'secret-leak',
+        HARMLESS: 'kept',
+        PATH: '/usr/bin',
+      },
+      { configDir, models },
+    );
+    expect(env.ANTHROPIC_API_KEY).toBeUndefined();
+    expect(env.CLAUDE_CODE_OAUTH_TOKEN).toBeUndefined();
+    expect(env.SOME_SERVICE_TOKEN).toBeUndefined();
+    expect(env.VENDOR_API_KEY).toBeUndefined();
+    expect(env.DB_SECRET).toBeUndefined();
+    // Non-credential vars the harness needs are preserved.
+    expect(env.HARMLESS).toBe('kept');
+    expect(env.PATH).toBe('/usr/bin');
+  });
+
+  it('neutralizes Bedrock/Vertex provider switches so Claude cannot bypass the proxy (REQ 2)', () => {
+    // CLAUDE_CODE_USE_BEDROCK / _USE_VERTEX are ROUTING switches: their mere
+    // presence makes Claude Code route to AWS Bedrock / GCP Vertex against the
+    // ambient cloud credential chain — reaching the real Anthropic API and
+    // bypassing ANTHROPIC_BASE_URL (the loopback proxy) entirely. They MUST be
+    // gone from the composed env regardless of the launching env.
+    const env = buildClaudexEnv(
+      {
+        CLAUDE_CODE_USE_BEDROCK: '1',
+        CLAUDE_CODE_USE_VERTEX: '1',
+        CLAUDE_CODE_SKIP_BEDROCK_AUTH: '1',
+        CLAUDE_CODE_SKIP_VERTEX_AUTH: '1',
+        AWS_ACCESS_KEY_ID: 'AKIAREAL',
+        AWS_SECRET_ACCESS_KEY: 'realsecret',
+        AWS_SESSION_TOKEN: 'realsession',
+        AWS_BEARER_TOKEN_BEDROCK: 'bearer-bedrock-real',
+        AWS_REGION: 'us-east-1',
+        GOOGLE_APPLICATION_CREDENTIALS: '/home/agent/gcp.json',
+        GOOGLE_CLOUD_ACCESS_TOKEN: 'gcp-token-real',
+        PATH: '/usr/bin',
+      },
+      { configDir, models },
+    );
+    // Routing switches gone by construction.
+    expect('CLAUDE_CODE_USE_BEDROCK' in env).toBe(false);
+    expect('CLAUDE_CODE_USE_VERTEX' in env).toBe(false);
+    expect('CLAUDE_CODE_SKIP_BEDROCK_AUTH' in env).toBe(false);
+    expect('CLAUDE_CODE_SKIP_VERTEX_AUTH' in env).toBe(false);
+    // Cloud credentials swept — none of the Claude-capable creds survive.
+    expect(env.AWS_ACCESS_KEY_ID).toBeUndefined();
+    expect(env.AWS_SECRET_ACCESS_KEY).toBeUndefined();
+    expect(env.AWS_SESSION_TOKEN).toBeUndefined();
+    expect(env.AWS_BEARER_TOKEN_BEDROCK).toBeUndefined();
+    expect(env.AWS_REGION).toBeUndefined();
+    expect(env.GOOGLE_APPLICATION_CREDENTIALS).toBeUndefined();
+    expect(env.GOOGLE_CLOUD_ACCESS_TOKEN).toBeUndefined();
+    // The proxy routing is still the only path.
+    expect(env.ANTHROPIC_BASE_URL).toBe(CLAUDEX_PROXY_URL);
+    expect(env.ANTHROPIC_AUTH_TOKEN).toBe('unused');
+    expect(env.PATH).toBe('/usr/bin');
+  });
+
+  it('closes the mid-string _KEY / _SECRET gap (STRIPE_SECRET_KEY, SSH_PRIVATE_KEY)', () => {
+    const env = buildClaudexEnv(
+      {
+        STRIPE_SECRET_KEY: 'sk-live-real',
+        SSH_PRIVATE_KEY: '-----BEGIN OPENSSH PRIVATE KEY-----',
+        HARMLESS: 'kept',
+      },
+      { configDir, models },
+    );
+    expect(env.STRIPE_SECRET_KEY).toBeUndefined();
+    expect(env.SSH_PRIVATE_KEY).toBeUndefined();
+    expect(env.HARMLESS).toBe('kept');
+  });
+
+  it('no credential-NAMED key in the composed env carries a real-looking value', () => {
+    const env = buildClaudexEnv(
+      {
+        ANTHROPIC_API_KEY: 'sk-ant-api03-leak',
+        ANTHROPIC_AUTH_TOKEN: 'access_token_leak',
+        SOME_JWT_TOKEN: 'eyJhbGciOiJIUzI1NiJ9.payload.sig',
+        REFRESH_SECRET: 'refresh_token_value',
+      },
+      { configDir, models },
+    );
+    for (const [name, value] of Object.entries(env)) {
+      if (CLAUDEX_CREDENTIAL_ENV_RE.test(name)) {
+        // Any surviving credential-named var must carry only a safe sentinel value.
+        expect(value).not.toMatch(/sk-(ant|proj)-/);
+        expect(value).not.toMatch(/access_token|refresh_token/);
+        expect(value).not.toMatch(/eyJ[A-Za-z0-9_-]+\./); // JWT
+      }
+    }
+  });
+
+  it('honors a caller-provided baseUrl override (loopback default otherwise)', () => {
+    const env = buildClaudexEnv({}, { configDir, models, baseUrl: 'http://127.0.0.1:9999' });
+    expect(env.ANTHROPIC_BASE_URL).toBe('http://127.0.0.1:9999');
+  });
+
+  it('returns a fresh object without mutating the base env', () => {
+    const base = { EXISTING: 'kept' };
+    const env = buildClaudexEnv(base, { configDir, models });
+    expect(env.EXISTING).toBe('kept');
+    expect(base).not.toHaveProperty('ANTHROPIC_AUTH_TOKEN');
+  });
+});
+
+// ─── EXPERIMENTAL classification (P4) ─────────────────────────────────────────
+
+describe('buildClaudexBanner / buildClaudexContractNote', () => {
+  const models = { primary: 'gpt-5.6-sol', smallFast: 'gpt-5.6-luna' };
+
+  it('banner marks EXPERIMENTAL and names the models + proxy', () => {
+    const banner = buildClaudexBanner(models);
+    expect(banner).toMatch(/EXPERIMENTAL/);
+    expect(banner).toMatch(/gpt-5\.6-sol/);
+    expect(banner).toMatch(/gpt-5\.6-luna/);
+    expect(banner).toMatch(/claude-code-proxy/);
+    expect(banner).not.toMatch(/unused/); // no token material in the banner
+  });
+
+  it('contract note classifies the runtime as EXPERIMENTAL GPT-via-proxy', () => {
+    const note = buildClaudexContractNote(models);
+    expect(note).toMatch(/EXPERIMENTAL/);
+    expect(note).toMatch(/gpt-5\.6-sol/);
+    expect(note).toMatch(/not.*Anthropic/i);
+  });
+});
+
+// ─── proxy gate ───────────────────────────────────────────────────────────────
+
+describe('runClaudexProxyGate', () => {
+  it('is ok when the first preflight already passes', async () => {
+    const preflight = vi.fn().mockResolvedValue(makeReport());
+    const ensureProxy = vi.fn();
+    const reauth = vi.fn();
+    const gate = await runClaudexProxyGate({ preflight, ensureProxy, reauth });
+    expect(gate.ok).toBe(true);
+    expect(ensureProxy).not.toHaveBeenCalled();
+    expect(reauth).not.toHaveBeenCalled();
+  });
+
+  it('fails fast when the binary is missing (no reauth, no start)', async () => {
+    const preflight = vi
+      .fn()
+      .mockResolvedValue(
+        makeReport({ binaryPresent: false, ok: false, problems: ['binary not found'] }),
+      );
+    const ensureProxy = vi.fn();
+    const reauth = vi.fn();
+    const gate = await runClaudexProxyGate({ preflight, ensureProxy, reauth });
+    expect(gate.ok).toBe(false);
+    expect(reauth).not.toHaveBeenCalled();
+    expect(ensureProxy).not.toHaveBeenCalled();
+  });
+
+  it('runs device reauth then re-preflights when OAuth needs it', async () => {
+    const preflight = vi
+      .fn()
+      .mockResolvedValueOnce(
+        makeReport({
+          auth: { state: 'expired' },
+          needsReauth: true,
+          ok: false,
+          problems: ['expired'],
+        }),
+      )
+      .mockResolvedValueOnce(makeReport());
+    const reauth = vi.fn().mockReturnValue(0);
+    const gate = await runClaudexProxyGate({ preflight, reauth, ensureProxy: vi.fn() });
+    expect(reauth).toHaveBeenCalledTimes(1);
+    expect(preflight).toHaveBeenCalledTimes(2);
+    expect(gate.ok).toBe(true);
+  });
+
+  it('does NOT reauth when auth is already valid', async () => {
+    const preflight = vi.fn().mockResolvedValue(makeReport());
+    const reauth = vi.fn();
+    await runClaudexProxyGate({ preflight, reauth, ensureProxy: vi.fn() });
+    expect(reauth).not.toHaveBeenCalled();
+  });
+
+  it('aborts when device reauth fails', async () => {
+    const preflight = vi.fn().mockResolvedValue(
+      makeReport({
+        auth: { state: 'unauthenticated' },
+        needsReauth: true,
+        ok: false,
+        problems: ['unauth'],
+      }),
+    );
+    const reauth = vi.fn().mockReturnValue(1);
+    const gate = await runClaudexProxyGate({ preflight, reauth, ensureProxy: vi.fn() });
+    expect(gate.ok).toBe(false);
+    expect(gate.problems.join(' ')).toMatch(/re-auth/i);
+  });
+
+  it('starts the proxy then re-preflights when nothing is live', async () => {
+    const preflight = vi
+      .fn()
+      .mockResolvedValueOnce(
+        makeReport({ live: false, listenerVerdict: 'unknown', ok: false, problems: ['dead'] }),
+      )
+      .mockResolvedValueOnce(makeReport());
+    const ensureProxy = vi.fn().mockResolvedValue({ live: true, method: 'systemd' });
+    const gate = await runClaudexProxyGate({ preflight, ensureProxy, reauth: vi.fn() });
+    expect(ensureProxy).toHaveBeenCalledTimes(1);
+    expect(gate.ok).toBe(true);
+  });
+
+  it('aborts (non-sensitive) when the proxy cannot come up trusted', async () => {
+    const preflight = vi
+      .fn()
+      .mockResolvedValue(
+        makeReport({ live: false, listenerVerdict: 'unknown', ok: false, problems: ['dead'] }),
+      );
+    const ensureProxy = vi.fn().mockResolvedValue({ live: false, method: 'untrusted' });
+    const gate = await runClaudexProxyGate({ preflight, ensureProxy, reauth: vi.fn() });
+    expect(gate.ok).toBe(false);
+    expect(gate.problems.join(' ')).toMatch(/untrusted/);
+    // Non-sensitive: no token material in surfaced problems.
+    expect(gate.problems.join(' ')).not.toMatch(/access_token|refresh_token|sk-/);
+  });
+});
+
+// ─── launch orchestration (fail-closed ordering) ──────────────────────────────
+
+describe('launchClaudex', () => {
+  const baseDeps = {
+    baseEnv: {},
+    proxyGate: () => Promise.resolve({ ok: true, report: makeReport(), problems: [] }),
+    resolveConfigDir: () => '/home/agent/.config/mosaic/claudex/home',
+    log: () => {},
+    errorLog: () => {},
+    fail: (() => {
+      throw new Error('exit');
+    }) as (code: number) => never,
+  };
+
+  it('yolo=true passes --dangerously-skip-permissions + injected env to claude', async () => {
+    const exec = vi.fn();
+    await launchClaudex(['--print', 'hi'], true, okAdapter({ exec }), baseDeps);
+    expect(exec).toHaveBeenCalledTimes(1);
+    const [cmd, args, env] = exec.mock.calls[0]!;
+    expect(cmd).toBe('claude');
+    expect(args[0]).toBe('--dangerously-skip-permissions');
+    expect(args).toContain('--append-system-prompt');
+    expect(args).toContain('--print');
+    expect(args).toContain('hi');
+    expect(env.ANTHROPIC_AUTH_TOKEN).toBe('unused');
+    expect(env.ANTHROPIC_BASE_URL).toBe(CLAUDEX_PROXY_URL);
+    expect(env.CLAUDE_CONFIG_DIR).toBe('/home/agent/.config/mosaic/claudex/home');
+    expect(env.ANTHROPIC_MODEL).toBe('gpt-5.6-sol');
+  });
+
+  it('non-yolo omits --dangerously-skip-permissions but still injects the proxy env', async () => {
+    const exec = vi.fn();
+    await launchClaudex([], false, okAdapter({ exec }), baseDeps);
+    const [, args, env] = exec.mock.calls[0]!;
+    expect(args).not.toContain('--dangerously-skip-permissions');
+    expect(args[0]).toBe('--append-system-prompt');
+    expect(env.ANTHROPIC_AUTH_TOKEN).toBe('unused');
+  });
+
+  it('appends the EXPERIMENTAL contract note to the composed prompt', async () => {
+    const exec = vi.fn();
+    await launchClaudex([], true, okAdapter({ exec, composePrompt: () => '# BASE' }), baseDeps);
+    const args = exec.mock.calls[0]![1] as string[];
+    const promptIdx = args.indexOf('--append-system-prompt') + 1;
+    expect(args[promptIdx]).toContain('# BASE');
+    expect(args[promptIdx]).toMatch(/EXPERIMENTAL/);
+  });
+
+  it('runs the harness preflight BEFORE the proxy gate and exec', async () => {
+    const order: string[] = [];
+    const adapter = okAdapter({
+      harnessPreflight: () => order.push('preflight'),
+      exec: () => order.push('exec'),
+    });
+    await launchClaudex([], true, adapter, {
+      ...baseDeps,
+      proxyGate: () => {
+        order.push('gate');
+        return Promise.resolve({ ok: true, report: makeReport(), problems: [] });
+      },
+    });
+    expect(order).toEqual(['preflight', 'gate', 'exec']);
+  });
+
+  it('FAIL CLOSED: exits WITHOUT exec when the proxy gate fails', async () => {
+    const exec = vi.fn();
+    const errors: string[] = [];
+    await expect(
+      launchClaudex([], true, okAdapter({ exec }), {
+        ...baseDeps,
+        proxyGate: () =>
+          Promise.resolve({ ok: false, report: makeReport({ ok: false }), problems: ['no proxy'] }),
+        errorLog: (m: string) => errors.push(m),
+      }),
+    ).rejects.toThrow('exit');
+    expect(exec).not.toHaveBeenCalled();
+    expect(errors.join('\n')).toMatch(/no proxy/);
+  });
+
+  it('FAIL CLOSED: exits WITHOUT exec when the config-dir guard throws', async () => {
+    const exec = vi.fn();
+    await expect(
+      launchClaudex([], true, okAdapter({ exec }), {
+        ...baseDeps,
+        resolveConfigDir: () => {
+          throw new Error('refusing to use ~/.claude');
+        },
+      }),
+    ).rejects.toThrow('exit');
+    expect(exec).not.toHaveBeenCalled();
+  });
+
+  it('FAIL CLOSED: reports a non-Error throw via String() and still aborts', async () => {
+    const exec = vi.fn();
+    const errors: string[] = [];
+    await expect(
+      launchClaudex([], true, okAdapter({ exec }), {
+        ...baseDeps,
+        resolveConfigDir: () => {
+          // A non-Error throw exercises the String(err) branch of the catch.
+          throw { toString: () => 'string-shaped failure' };
+        },
+        errorLog: (m: string) => errors.push(m),
+      }),
+    ).rejects.toThrow('exit');
+    expect(exec).not.toHaveBeenCalled();
+    expect(errors.join('\n')).toMatch(/string-shaped failure/);
+  });
+});
+
+// ─── production DI defaults (fallback-branch coverage; no real proxy touched) ──
+
+describe('production dependency defaults', () => {
+  it('assertIsolatedConfigDir defaults realClaudeDir to ~/.claude', () => {
+    const safe = join(tmpdir(), 'mosaic-claudex-default-real', 'home');
+    // Only canonicalize injected; realClaudeDir falls back to ~/.claude.
+    expect(assertIsolatedConfigDir(safe, { canonicalize: idCanon })).toBe(safe);
+  });
+
+  it('assertIsolatedConfigDir default canonicalizer resolves a non-existent path', () => {
+    // No canonicalize dep → exercises the real realpath-longest-ancestor walk
+    // (including the not-yet-existing tail), on a path safely outside ~/.claude.
+    const safe = join(tmpdir(), 'mosaic-claudex-canon', 'nested', 'home');
+    expect(assertIsolatedConfigDir(safe)).toContain('mosaic-claudex-canon');
+  });
+
+  it('resolveClaudexConfigDir defaults mosaicHome when not injected', () => {
+    const dir = mkdtempSync(join(tmpdir(), 'claudex-cfg-'));
+    try {
+      const target = join(dir, 'home');
+      // Override env points elsewhere; mosaicHome dep omitted → MOSAIC_HOME default path is exercised.
+      const out = resolveClaudexConfigDir(
+        { [CLAUDEX_CONFIG_DIR_ENV]: target },
+        { canonicalize: idCanon },
+      );
+      expect(out).toBe(target);
+      expect(lstatSync(target).isDirectory()).toBe(true);
+    } finally {
+      rmSync(dir, { recursive: true, force: true });
+    }
+  });
+
+  it('runClaudexProxyGate defaults ensureProxy/reauth/log without invoking them on a missing binary', async () => {
+    // Only preflight injected; binary missing → returns before the default
+    // ensureProxy/reauth thunks could ever reach the real proxy.
+    const preflight = vi
+      .fn()
+      .mockResolvedValue(makeReport({ binaryPresent: false, ok: false, problems: ['missing'] }));
+    const gate = await runClaudexProxyGate({ preflight });
+    expect(gate.ok).toBe(false);
+  });
+
+  it('launchClaudex defaults log/errorLog/fail/baseEnv/models/buildEnv on the success path', async () => {
+    const logSpy = vi.spyOn(console, 'log').mockImplementation(() => {});
+    try {
+      const exec = vi.fn();
+      // Inject only the boundaries that would touch the real proxy/FS; let the
+      // rest default. Success path never calls fail/errorLog.
+      await launchClaudex([], true, okAdapter({ exec }), {
+        proxyGate: () => Promise.resolve({ ok: true, report: makeReport(), problems: [] }),
+        resolveConfigDir: () => join(tmpdir(), 'claudex-default-launch'),
+      });
+      expect(exec).toHaveBeenCalledTimes(1);
+      const [, , env] = exec.mock.calls[0]!;
+      expect(env.ANTHROPIC_AUTH_TOKEN).toBe('unused');
+      expect(env.ANTHROPIC_MODEL).toBe(CLAUDEX_DEFAULT_PRIMARY_MODEL);
+    } finally {
+      logSpy.mockRestore();
+    }
+  });
+});
diff --git a/packages/mosaic/src/commands/claudex.ts b/packages/mosaic/src/commands/claudex.ts
new file mode 100644
index 0000000..f6f47d6
--- /dev/null
+++ b/packages/mosaic/src/commands/claudex.ts
@@ -0,0 +1,464 @@
+/**
+ * Claudex launch composition (P2–P4 of `mosaic yolo claudex`).
+ *
+ * Builds the isolated launch environment for running GPT models inside the
+ * Claude Code harness via `raine/claude-code-proxy` (ChatGPT-subscription OAuth).
+ * PR-1 (`claudex-proxy.ts`) owns the proxy preflight/lifecycle; this module owns
+ * the *composition* the launcher hands to Claude Code:
+ *
+ *   P2  isolated CLAUDE_CONFIG_DIR (provably never the real ~/.claude) + env
+ *       injection that leaks ZERO token material;
+ *   P3  the model-tier map (primary → gpt-5.6-sol, small/fast → gpt-5.6-luna,
+ *       operator env values win);
+ *   P4  the EXPERIMENTAL classification banner + composed-contract note.
+ *
+ * Two hard security invariants (secrev-enforced):
+ *   REQ 1 — Provable isolation. {@link assertIsolatedConfigDir} makes the
+ *           CLAUDE_CONFIG_DIR seam incapable of resolving to `~/.claude` (or any
+ *           descendant of it); it canonicalizes both sides, rejects descendants,
+ *           and — after ensuring the dir — re-checks and rejects a symlinked
+ *           target (TOCTOU). Fails CLOSED on any uncertainty.
+ *   REQ 2 — Zero token leakage. This module never reads the proxy's
+ *           `auth.json`; {@link buildClaudexEnv} strips the ENTIRE
+ *           credential-bearing env family and hands Claude Code only
+ *           `ANTHROPIC_AUTH_TOKEN=unused`. The proxy holds the real credential.
+ *
+ * Every side-effecting boundary is dependency-injected so the launch path is
+ * unit-testable without spawning Claude Code or touching a real config dir.
+ */
+
+import { lstatSync, mkdirSync, realpathSync } from 'node:fs';
+import { homedir } from 'node:os';
+import { dirname, isAbsolute, join, relative, resolve } from 'node:path';
+import {
+  CLAUDEX_PROXY_URL,
+  ensureProxyRunning,
+  runDeviceReauth,
+  runProxyPreflight,
+  type EnsureProxyResult,
+  type PreflightReport,
+} from './claudex-proxy.js';
+
+const MOSAIC_HOME = process.env['MOSAIC_HOME'] ?? join(homedir(), '.config', 'mosaic');
+
+// ─── Isolated CLAUDE_CONFIG_DIR (HARD SECURITY REQ 1) ─────────────────────────
+
+/** Dedicated override env for the isolated config dir. The ambient
+ * `CLAUDE_CONFIG_DIR` is deliberately NOT honored — it may already point at the
+ * real `~/.claude` of the launching session. */
+export const CLAUDEX_CONFIG_DIR_ENV = 'MOSAIC_CLAUDEX_CONFIG_DIR';
+
+/** The default isolated config dir — structurally under the mosaic home, so it
+ * can never equal `~/.claude`. */
+export function defaultClaudexConfigDir(mosaicHome: string = MOSAIC_HOME): string {
+  return join(mosaicHome, 'claudex', 'home');
+}
+
+export interface ConfigDirDeps {
+  /** The real Claude state dir to protect (default `~/.claude`). */
+  realClaudeDir?: string;
+  /** Resolve a path to canonical form, resolving symlinks on the longest
+   * existing ancestor (so a not-yet-created dir still canonicalizes). */
+  canonicalize?: (p: string) => string;
+  /** Ensure the isolated dir exists (mkdir -p, owner-only 0700). */
+  mkdir?: (p: string) => void;
+  /** Whether a path is itself a symlink (lstat). */
+  isSymlink?: (p: string) => boolean;
+}
+
+/** Resolve a path to canonical form, resolving symlinks on the LONGEST EXISTING
+ * ancestor and re-appending the not-yet-existing tail. A symlinked ancestor that
+ * points into `~/.claude` is therefore caught even before the leaf exists. */
+function defaultCanonicalizeIntended(p: string): string {
+  const abs = resolve(p);
+  let existing = abs;
+  const tail: string[] = [];
+  // Walk up until we hit an existing ancestor (or the filesystem root).
+  for (;;) {
+    try {
+      const real = realpathSync(existing);
+      return tail.length > 0 ? join(real, ...tail) : real;
+    } catch (err) {
+      // ONLY a genuine "does not exist yet" (ENOENT) justifies walking up to an
+      // existing ancestor. Any other errno (ELOOP, EACCES, ENOTDIR, …) means we
+      // cannot establish the canonical form — fail CLOSED rather than fall back
+      // to a possibly-wrong literal path.
+      if ((err as NodeJS.ErrnoException).code !== 'ENOENT') throw err;
+      const parent = dirname(existing);
+      if (parent === existing) return abs; // reached root without an existing prefix
+      tail.unshift(existing.slice(parent.length + 1));
+      existing = parent;
+    }
+  }
+}
+
+function defaultMkdir(p: string): void {
+  mkdirSync(p, { recursive: true, mode: 0o700 });
+}
+
+function defaultIsSymlink(p: string): boolean {
+  try {
+    return lstatSync(p).isSymbolicLink();
+  } catch (err) {
+    // A missing path is genuinely "not a symlink"; anything else (EACCES, ELOOP,
+    // ENOTDIR, …) is uncertainty the guard must not swallow — fail CLOSED.
+    if ((err as NodeJS.ErrnoException).code === 'ENOENT') return false;
+    throw err;
+  }
+}
+
+/** True when `child` is `parent` itself or a descendant of it (path-wise). */
+function isWithin(child: string, parent: string): boolean {
+  if (child === parent) return true;
+  const rel = relative(parent, child);
+  return rel.length > 0 && !rel.startsWith('..') && !isAbsolute(rel);
+}
+
+/**
+ * Guard: prove that `candidate` is a legitimate ISOLATED config dir and can
+ * never be, resolve to, or live under the real `~/.claude`. Canonicalizes BOTH
+ * sides (either may be a symlink), rejects `~/.claude` and any descendant, and
+ * fails CLOSED (throws) on an empty/relative candidate. Returns the canonical
+ * isolated path on success.
+ */
+export function assertIsolatedConfigDir(candidate: string, deps: ConfigDirDeps = {}): string {
+  const canonicalize = deps.canonicalize ?? defaultCanonicalizeIntended;
+  const realClaudeDir = deps.realClaudeDir ?? join(homedir(), '.claude');
+
+  if (typeof candidate !== 'string' || candidate.trim() === '') {
+    throw new Error('claudex: isolated CLAUDE_CONFIG_DIR must be a non-empty path (fail closed).');
+  }
+  if (!isAbsolute(candidate)) {
+    throw new Error(
+      `claudex: isolated CLAUDE_CONFIG_DIR must be an absolute path: ${JSON.stringify(candidate)}`,
+    );
+  }
+
+  const canonCandidate = canonicalize(candidate);
+  const canonReal = canonicalize(realClaudeDir);
+
+  // Compare canonical forms AND raw resolved forms — belt and suspenders so a
+  // canonicalizer that no-ops on a nonexistent real dir still catches the literal.
+  if (isWithin(canonCandidate, canonReal) || isWithin(resolve(candidate), resolve(realClaudeDir))) {
+    throw new Error(
+      `claudex: refusing to use the real Claude config dir (or a descendant of it) as the ` +
+        `isolated CLAUDE_CONFIG_DIR. Resolved to ${JSON.stringify(canonCandidate)}.`,
+    );
+  }
+
+  return canonCandidate;
+}
+
+/**
+ * Resolve the isolated CLAUDE_CONFIG_DIR: pick the dedicated override or the
+ * namespaced default, run the pre-create guard, ensure the dir (0700), then
+ * RE-CHECK after creation — reject a symlinked target and re-run the guard on
+ * the now-existing (fully canonicalizable) path. This closes the pre-created
+ * symlink race (TOCTOU). Every failure throws (fail closed).
+ */
+export function resolveClaudexConfigDir(
+  env: NodeJS.ProcessEnv = process.env,
+  deps: ConfigDirDeps & { mosaicHome?: string } = {},
+): string {
+  const mosaicHome = deps.mosaicHome ?? MOSAIC_HOME;
+  const mkdir = deps.mkdir ?? defaultMkdir;
+  const isSymlink = deps.isSymlink ?? defaultIsSymlink;
+
+  const override = env[CLAUDEX_CONFIG_DIR_ENV]?.trim();
+  const candidate =
+    override && override.length > 0 ? override : defaultClaudexConfigDir(mosaicHome);
+
+  // Pre-create guard (before touching the filesystem).
+  assertIsolatedConfigDir(candidate, deps);
+
+  // Ensure the dir, then re-verify against the post-create reality.
+  mkdir(candidate);
+  if (isSymlink(candidate)) {
+    throw new Error(
+      'claudex: refusing to use the isolated CLAUDE_CONFIG_DIR — the target is a symlink ' +
+        '(possible pre-created race). Fail closed.',
+    );
+  }
+  // Re-run the guard now that the leaf exists so canonicalization reflects any
+  // symlinked ancestor introduced between the pre-check and mkdir.
+  return assertIsolatedConfigDir(candidate, deps);
+}
+
+// ─── Model-tier map (P3) ──────────────────────────────────────────────────────
+
+export const CLAUDEX_DEFAULT_PRIMARY_MODEL = 'gpt-5.6-sol';
+export const CLAUDEX_DEFAULT_SMALL_FAST_MODEL = 'gpt-5.6-luna';
+
+export interface ClaudexModels {
+  /** Primary tier (opus/sonnet) → ANTHROPIC_MODEL. */
+  primary: string;
+  /** Small/fast tier (haiku) → ANTHROPIC_SMALL_FAST_MODEL. */
+  smallFast: string;
+}
+
+/** Resolve the model-tier map. Operator-provided env values WIN over defaults;
+ * blank values fall back to the defaults. */
+export function resolveClaudexModels(env: NodeJS.ProcessEnv = process.env): ClaudexModels {
+  const primary = env['ANTHROPIC_MODEL']?.trim() || CLAUDEX_DEFAULT_PRIMARY_MODEL;
+  const smallFast = env['ANTHROPIC_SMALL_FAST_MODEL']?.trim() || CLAUDEX_DEFAULT_SMALL_FAST_MODEL;
+  return { primary, smallFast };
+}
+
+// ─── Env injection (HARD SECURITY REQ 2 — zero token leakage) ─────────────────
+
+/**
+ * Names of env vars considered credential-bearing. The whole family is stripped
+ * from the composed env so no real Anthropic key, OAuth token, or third-party /
+ * cloud credential can reach the local proxy or be used by Claude Code to bypass
+ * it. We then re-add ONLY the safe claudex vars (`ANTHROPIC_MODEL`,
+ * `_SMALL_FAST_MODEL`, `_BASE_URL`, `_AUTH_TOKEN=unused`). A name-pattern sweep
+ * can't miss a specific var a short denylist forgot, while still preserving the
+ * arbitrary non-credential env the harness/MCP/hooks require (PATH, HOME, XDG,
+ * terminal, proxies, …), which a strict allowlist would fragilely drop.
+ *
+ * The cloud-provider families (`AWS_*`, `GOOGLE_APPLICATION_CREDENTIALS`,
+ * `GOOGLE_CLOUD_*`, `GCP_*`) are included because Claude Code can route to the
+ * real Anthropic API via AWS Bedrock / GCP Vertex using the ambient cloud
+ * credential chain — a Claude-capable credential that must never survive into a
+ * claudex launch. `_KEY$` / `_SECRET` (not just the `_API_KEY$` tail) close the
+ * mid-string gap (`STRIPE_SECRET_KEY`, `SSH_PRIVATE_KEY`, `AWS_SECRET_ACCESS_KEY`).
+ */
+export const CLAUDEX_CREDENTIAL_ENV_RE =
+  /^ANTHROPIC_|^CLAUDE_CODE_OAUTH|^AWS_|^GOOGLE_APPLICATION_CREDENTIALS$|^GOOGLE_CLOUD_|^GCP_|_API_?KEY$|_KEY$|_TOKEN$|_SECRET/i;
+
+/**
+ * Provider ROUTING switches whose mere PRESENCE (independent of any credential)
+ * makes Claude Code bypass `ANTHROPIC_BASE_URL` (the loopback proxy) and talk to
+ * the real Anthropic API via Bedrock/Vertex. A name-pattern is the wrong model
+ * for a boolean switch, so these are force-deleted by exact name — REGARDLESS of
+ * value — after the credential sweep. (REQ 2: isolation must hold for any
+ * launching env, including a Bedrock/Vertex-configured enterprise host.)
+ */
+export const CLAUDEX_FORCED_UNSET_ENV = [
+  'CLAUDE_CODE_USE_BEDROCK',
+  'CLAUDE_CODE_USE_VERTEX',
+  'CLAUDE_CODE_SKIP_BEDROCK_AUTH',
+  'CLAUDE_CODE_SKIP_VERTEX_AUTH',
+] as const;
+
+export interface BuildClaudexEnvOptions {
+  configDir: string;
+  models: ClaudexModels;
+  /** Override the proxy base URL (defaults to the PR-1 loopback constant). */
+  baseUrl?: string;
+}
+
+/**
+ * Compose the launch env for Claude Code. Returns a FRESH object (never mutates
+ * the base env). Strips the entire credential-bearing family AND force-deletes
+ * the Bedrock/Vertex routing switches (REQ 2), then sets the isolated config dir
+ * and the proxy routing. Claude Code sees only `ANTHROPIC_AUTH_TOKEN=unused`
+ * pointed at the loopback proxy; the proxy holds the real OAuth credential, which
+ * this module never reads.
+ */
+export function buildClaudexEnv(
+  baseEnv: NodeJS.ProcessEnv,
+  opts: BuildClaudexEnvOptions,
+): NodeJS.ProcessEnv {
+  const env: NodeJS.ProcessEnv = {};
+  for (const [name, value] of Object.entries(baseEnv)) {
+    if (CLAUDEX_CREDENTIAL_ENV_RE.test(name)) continue; // drop the whole credential family
+    env[name] = value;
+  }
+
+  // Force-delete routing switches by exact name — their presence (not their
+  // value) is what would route Claude Code off the proxy to the real API.
+  for (const name of CLAUDEX_FORCED_UNSET_ENV) delete env[name];
+
+  env['CLAUDE_CONFIG_DIR'] = opts.configDir;
+  env['ANTHROPIC_BASE_URL'] = opts.baseUrl ?? CLAUDEX_PROXY_URL;
+  env['ANTHROPIC_AUTH_TOKEN'] = 'unused';
+  env['ANTHROPIC_MODEL'] = opts.models.primary;
+  env['ANTHROPIC_SMALL_FAST_MODEL'] = opts.models.smallFast;
+  return env;
+}
+
+// ─── EXPERIMENTAL classification (P4) ─────────────────────────────────────────
+
+/** Console banner shown at launch. Contains no token material by construction. */
+export function buildClaudexBanner(models: ClaudexModels): string {
+  return [
+    '',
+    '  ┌─────────────────────────────────────────────────────────────────────┐',
+    '  │  ⚠  EXPERIMENTAL — mosaic claudex                                     │',
+    '  └─────────────────────────────────────────────────────────────────────┘',
+    `  Running GPT models inside the Claude Code harness via claude-code-proxy`,
+    `  (ChatGPT-subscription OAuth). This is NOT Anthropic Claude.`,
+    `    primary    : ${models.primary}`,
+    `    small/fast : ${models.smallFast}`,
+    `  Model behavior, tool use, and output quality may differ from Claude.`,
+    '',
+  ].join('\n');
+}
+
+/** Markdown note appended to the composed runtime contract so the model itself
+ * knows it is running the EXPERIMENTAL GPT-via-proxy configuration. */
+export function buildClaudexContractNote(models: ClaudexModels): string {
+  return [
+    '# EXPERIMENTAL Runtime — claudex (GPT via claude-code-proxy)',
+    '',
+    'You are running in Mosaic **claudex** mode: the Claude Code harness is wired to',
+    'GPT models through a local `claude-code-proxy` (ChatGPT-subscription OAuth). This',
+    "runtime is NOT Anthropic's Claude API and is not Claude.",
+    '',
+    `- Primary model: \`${models.primary}\``,
+    `- Small/fast model: \`${models.smallFast}\``,
+    '',
+    'Some Claude-specific harness assumptions may not hold under GPT models — verify',
+    'tool output carefully. This classification is EXPERIMENTAL and is not intended for',
+    'production delivery without explicit operator sign-off.',
+  ].join('\n');
+}
+
+// ─── Proxy gate ───────────────────────────────────────────────────────────────
+
+export interface ProxyGateResult {
+  ok: boolean;
+  report: PreflightReport;
+  /** Non-sensitive problems suitable for surfacing to the operator. */
+  problems: string[];
+}
+
+export interface ProxyGateDeps {
+  preflight?: () => Promise<PreflightReport>;
+  ensureProxy?: () => Promise<EnsureProxyResult>;
+  reauth?: () => number;
+  log?: (message: string) => void;
+}
+
+/**
+ * Run the proxy readiness gate: preflight → (device reauth if OAuth needs it) →
+ * (start the proxy if nothing trusted is live) → re-preflight. Returns `ok` only
+ * when the final preflight passes (binary present, OAuth valid, a TRUSTED-live
+ * listener — identity verified by PR-1's `verifyListenerIdentity`). All surfaced
+ * problems are non-sensitive (port + verdict only; never a token).
+ */
+export async function runClaudexProxyGate(deps: ProxyGateDeps = {}): Promise<ProxyGateResult> {
+  const preflight = deps.preflight ?? (() => runProxyPreflight());
+  const ensureProxy = deps.ensureProxy ?? (() => ensureProxyRunning());
+  const reauth = deps.reauth ?? (() => runDeviceReauth());
+  const log = deps.log ?? (() => {});
+
+  let report = await preflight();
+
+  // A missing binary is unrecoverable here — don't attempt reauth or a start.
+  if (!report.binaryPresent) {
+    return { ok: false, report, problems: report.problems };
+  }
+
+  if (report.needsReauth) {
+    log('claudex: claude-code-proxy OAuth needs re-authentication — starting device flow…');
+    const code = reauth();
+    if (code !== 0) {
+      return {
+        ok: false,
+        report,
+        problems: [...report.problems, 'claudex: device re-authentication did not complete.'],
+      };
+    }
+    report = await preflight();
+  }
+
+  if (!report.live) {
+    log('claudex: no trusted claude-code-proxy responding — starting it…');
+    const started = await ensureProxy();
+    if (!started.live) {
+      return {
+        ok: false,
+        report,
+        problems: [
+          ...report.problems,
+          `claudex: could not bring up a trusted claude-code-proxy (${started.method}).`,
+        ],
+      };
+    }
+    report = await preflight();
+  }
+
+  return { ok: report.ok, report, problems: report.problems };
+}
+
+// ─── Launch orchestration ─────────────────────────────────────────────────────
+
+/**
+ * The launch.ts-provided seam. Keeps `claudex.ts` free of a circular import back
+ * into `launch.ts` while letting the orchestration reuse the harness preflight,
+ * the composed runtime contract, and the process-replacing exec.
+ */
+export interface ClaudexHarnessAdapter {
+  /** Runs the Claude-harness preflight (mosaic home, SOUL, `claude` on PATH,
+   * sequential-thinking). May terminate the process on a hard failure. */
+  harnessPreflight: () => void;
+  /** Compose the full Claude runtime contract (== `composeContract('claude')`). */
+  composePrompt: () => string;
+  /** Replace the current process with `claude` using the composed env. */
+  exec: (cmd: string, args: string[], env: NodeJS.ProcessEnv) => void;
+}
+
+export interface LaunchClaudexDeps {
+  baseEnv?: NodeJS.ProcessEnv;
+  proxyGate?: () => Promise<ProxyGateResult>;
+  resolveConfigDir?: () => string;
+  models?: () => ClaudexModels;
+  buildEnv?: (base: NodeJS.ProcessEnv, opts: BuildClaudexEnvOptions) => NodeJS.ProcessEnv;
+  log?: (message: string) => void;
+  errorLog?: (message: string) => void;
+  fail?: (code: number) => never;
+}
+
+/**
+ * Orchestrate a `mosaic [yolo] claudex` launch. Runs the harness preflight, the
+ * proxy gate, composes the isolated env (REQ 1 + REQ 2), appends the EXPERIMENTAL
+ * note, and exec's Claude Code. FAIL CLOSED: on any gate failure or guard throw
+ * it reports non-sensitive detail and exits WITHOUT reaching exec.
+ */
+export async function launchClaudex(
+  args: string[],
+  yolo: boolean,
+  adapter: ClaudexHarnessAdapter,
+  deps: LaunchClaudexDeps = {},
+): Promise<void> {
+  const log = deps.log ?? ((m: string) => console.log(m));
+  const errorLog = deps.errorLog ?? ((m: string) => console.error(m));
+  const fail = deps.fail ?? ((code: number) => process.exit(code));
+  const baseEnv = deps.baseEnv ?? process.env;
+  const proxyGate = deps.proxyGate ?? (() => runClaudexProxyGate({ log }));
+  const resolveConfigDir = deps.resolveConfigDir ?? (() => resolveClaudexConfigDir(baseEnv));
+  const models = deps.models ?? (() => resolveClaudexModels(baseEnv));
+  const buildEnv = deps.buildEnv ?? buildClaudexEnv;
+
+  try {
+    // Harness readiness first (claude on PATH, mosaic home, sequential-thinking).
+    adapter.harnessPreflight();
+
+    // Proxy readiness (binary, OAuth, trusted-live listener).
+    const gate = await proxyGate();
+    if (!gate.ok) {
+      errorLog('[mosaic] claudex preflight failed:');
+      for (const problem of gate.problems) errorLog(`  - ${problem}`);
+      return fail(1);
+    }
+
+    // Compose the isolated launch env (guard throws → caught below, fail closed).
+    const resolvedModels = models();
+    const configDir = resolveConfigDir();
+    const env = buildEnv(baseEnv, { configDir, models: resolvedModels });
+    const prompt = `${adapter.composePrompt()}\n\n${buildClaudexContractNote(resolvedModels)}`;
+
+    log(buildClaudexBanner(resolvedModels));
+
+    const cliArgs = yolo ? ['--dangerously-skip-permissions'] : [];
+    cliArgs.push('--append-system-prompt', prompt, ...args);
+    adapter.exec('claude', cliArgs, env);
+  } catch (err) {
+    errorLog(
+      `[mosaic] claudex launch aborted: ${err instanceof Error ? err.message : String(err)}`,
+    );
+    return fail(1);
+  }
+}
diff --git a/packages/mosaic/src/commands/launch.spec.ts b/packages/mosaic/src/commands/launch.spec.ts
index c6d47fb..d468c9b 100644
--- a/packages/mosaic/src/commands/launch.spec.ts
+++ b/packages/mosaic/src/commands/launch.spec.ts
@@ -9,6 +9,7 @@ import {
   piForceSkillNames,
   registerRuntimeLaunchers,
   type RuntimeLaunchHandler,
+  type ClaudexLaunchHandler,
 } from './launch.js';
 
 /**
@@ -31,6 +32,16 @@ function buildProgram(handler: RuntimeLaunchHandler): Command {
   return program;
 }
 
+function buildProgramWithClaudex(
+  handler: RuntimeLaunchHandler,
+  claudexHandler: ClaudexLaunchHandler,
+): Command {
+  const program = new Command();
+  program.exitOverride();
+  registerRuntimeLaunchers(program, handler, claudexHandler);
+  return program;
+}
+
 const fakeSkills = ['--skill', '/skills/test-driven-development', '--skill', '/skills/pdf'];
 const fakeForced = ['--skill', '/skills/mosaic-tools'];
 
@@ -280,3 +291,61 @@ describe('registerRuntimeLaunchers — yolo <runtime>', () => {
     expect(mockExit).toHaveBeenCalledWith(1);
   });
 });
+
+describe('registerRuntimeLaunchers — claudex (EXPERIMENTAL overlay)', () => {
+  let mockExit: MockInstance<typeof process.exit>;
+  let mockError: MockInstance<typeof console.error>;
+
+  beforeEach(() => {
+    mockExit = vi.spyOn(process, 'exit').mockImplementation(exitThrows);
+    mockError = vi.spyOn(console, 'error').mockImplementation(() => {});
+  });
+
+  afterEach(() => {
+    mockExit.mockRestore();
+    mockError.mockRestore();
+  });
+
+  it('dispatches `claudex` to the claudex handler (yolo=false), not the runtime handler', () => {
+    const handler = vi.fn();
+    const claudex = vi.fn();
+    const program = buildProgramWithClaudex(handler, claudex);
+    program.parse(['node', 'mosaic', 'claudex']);
+
+    expect(claudex).toHaveBeenCalledTimes(1);
+    expect(claudex).toHaveBeenCalledWith([], false);
+    expect(handler).not.toHaveBeenCalled();
+  });
+
+  it('forwards excess args after `claudex`', () => {
+    const handler = vi.fn();
+    const claudex = vi.fn();
+    const program = buildProgramWithClaudex(handler, claudex);
+    program.parse(['node', 'mosaic', 'claudex', '--print', 'hi']);
+
+    expect(claudex).toHaveBeenCalledWith(['--print', 'hi'], false);
+  });
+
+  it('dispatches `yolo claudex` with yolo=true and slices off the runtime name (#454)', () => {
+    const handler = vi.fn();
+    const claudex = vi.fn();
+    const program = buildProgramWithClaudex(handler, claudex);
+    program.parse(['node', 'mosaic', 'yolo', 'claudex']);
+
+    expect(claudex).toHaveBeenCalledTimes(1);
+    // extraArgs must be empty — the positional 'claudex' must not leak through.
+    expect(claudex).toHaveBeenCalledWith([], true);
+    expect(handler).not.toHaveBeenCalled();
+    expect(mockExit).not.toHaveBeenCalled();
+  });
+
+  it('forwards true excess args after `yolo claudex`', () => {
+    const handler = vi.fn();
+    const claudex = vi.fn();
+    const program = buildProgramWithClaudex(handler, claudex);
+    program.parse(['node', 'mosaic', 'yolo', 'claudex', '--model', 'gpt-5.6-sol']);
+
+    expect(claudex).toHaveBeenCalledWith(['--model', 'gpt-5.6-sol'], true);
+    expect(mockExit).not.toHaveBeenCalled();
+  });
+});
diff --git a/packages/mosaic/src/commands/launch.ts b/packages/mosaic/src/commands/launch.ts
index c56cc27..b51bcb1 100644
--- a/packages/mosaic/src/commands/launch.ts
+++ b/packages/mosaic/src/commands/launch.ts
@@ -27,6 +27,7 @@ import {
 import { readRegularFileSecure } from '../fleet/secure-file.js';
 import { readPersonaContractBlock } from '../fleet/persona-contract.js';
 import { canonicalizeRoleClass } from './fleet-personas.js';
+import { launchClaudex, type ClaudexHarnessAdapter } from './claudex.js';
 
 const MOSAIC_HOME = process.env['MOSAIC_HOME'] ?? join(homedir(), '.config', 'mosaic');
 const MAX_INSTALLED_TOOLS_BYTES = 256 * 1024;
@@ -806,12 +807,12 @@ function launchRuntime(runtime: RuntimeName, args: string[], yolo: boolean): nev
 }
 
 /** exec into the runtime, replacing the current process. */
-function execRuntime(cmd: string, args: string[]): void {
+function execRuntime(cmd: string, args: string[], env: NodeJS.ProcessEnv = process.env): void {
   try {
     // Use execFileSync with inherited stdio to replace the process
     const result = spawnSync(cmd, args, {
       stdio: 'inherit',
-      env: process.env,
+      env,
     });
     process.exit(result.status ?? 0);
   } catch (err) {
@@ -820,6 +821,29 @@ function execRuntime(cmd: string, args: string[]): void {
   }
 }
 
+/**
+ * Production glue for `mosaic [yolo] claudex` (EXPERIMENTAL — GPT models inside
+ * the Claude Code harness via claude-code-proxy). Assembles the real harness
+ * adapter and delegates the security-critical composition + fail-closed
+ * orchestration to `launchClaudex` in `claudex.ts`. Kept thin so the tested
+ * logic lives in the DI module, not here.
+ */
+function launchClaudexProduction(args: string[], yolo: boolean): void {
+  writeSessionLock('claude');
+  const adapter: ClaudexHarnessAdapter = {
+    harnessPreflight: () => {
+      checkMosaicHome();
+      checkFile(join(MOSAIC_HOME, 'AGENTS.md'), 'AGENTS.md');
+      checkSoul();
+      checkRuntime('claude');
+      checkSequentialThinking('claude');
+    },
+    composePrompt: () => buildRuntimePrompt('claude'),
+    exec: (cmd, cmdArgs, env) => execRuntime(cmd, cmdArgs, env),
+  };
+  void launchClaudex(args, yolo, adapter);
+}
+
 // ─── Framework script/tool delegation ───────────────────────────────────────
 
 function delegateToScript(scriptPath: string, args: string[], env?: Record<string, string>): never {
@@ -1034,12 +1058,25 @@ export type RuntimeLaunchHandler = (
   yolo: boolean,
 ) => void;
 
+/**
+ * Handler invoked for `claudex` / `yolo claudex`. Kept separate from
+ * `RuntimeLaunchHandler` because claudex is an EXPERIMENTAL harness overlay
+ * (GPT-via-proxy), not one of the first-class runtimes. Exposed + injectable so
+ * the commander wiring can be exercised without composing a real launch.
+ */
+export type ClaudexLaunchHandler = (extraArgs: string[], yolo: boolean) => void;
+
 /**
  * Wire `<runtime>` and `yolo <runtime>` subcommands onto `program` using a
  * pluggable launch handler. Separated from `registerLaunchCommands` so tests
  * can inject a spy and verify argument forwarding.
  */
-export function registerRuntimeLaunchers(program: Command, handler: RuntimeLaunchHandler): void {
+export function registerRuntimeLaunchers(
+  program: Command,
+  handler: RuntimeLaunchHandler,
+  claudexHandler: ClaudexLaunchHandler = (extraArgs, yolo) =>
+    launchClaudexProduction(extraArgs, yolo),
+): void {
   for (const runtime of ['claude', 'codex', 'opencode', 'pi'] as const) {
     program
       .command(runtime)
@@ -1051,16 +1088,37 @@ export function registerRuntimeLaunchers(program: Command, handler: RuntimeLaunc
       });
   }
 
+  // claudex — EXPERIMENTAL: GPT models inside the Claude Code harness via
+  // claude-code-proxy (ChatGPT-subscription OAuth). Isolated CLAUDE_CONFIG_DIR
+  // + zero-token-leak env injection live in claudex.ts.
+  program
+    .command('claudex')
+    .description('EXPERIMENTAL: launch Claude Code harness against GPT via claude-code-proxy')
+    .allowUnknownOption(true)
+    .allowExcessArguments(true)
+    .action((_opts: unknown, cmd: Command) => {
+      claudexHandler(cmd.args, false);
+    });
+
   program
     .command('yolo <runtime>')
-    .description('Launch a runtime in dangerous-permissions mode (claude|codex|opencode|pi)')
+    .description(
+      'Launch a runtime in dangerous-permissions mode (claude|codex|opencode|pi|claudex)',
+    )
     .allowUnknownOption(true)
     .allowExcessArguments(true)
     .action((runtime: string, _opts: unknown, cmd: Command) => {
+      // claudex is an EXPERIMENTAL overlay, not a RuntimeName — dispatch it
+      // before the runtime allowlist check. Slice off the positional runtime
+      // name for the same reason as below (#454).
+      if (runtime === 'claudex') {
+        claudexHandler(cmd.args.slice(1), true);
+        return;
+      }
       const valid: RuntimeName[] = ['claude', 'codex', 'opencode', 'pi'];
       if (!valid.includes(runtime as RuntimeName)) {
         console.error(
-          `[mosaic] ERROR: Unsupported yolo runtime '${runtime}'. Use: ${valid.join('|')}`,
+          `[mosaic] ERROR: Unsupported yolo runtime '${runtime}'. Use: ${valid.join('|')}|claudex`,
         );
         process.exit(1);
       }