feat(mcp): bearer-token auth resolver

Thin parser over the existing api_keys lookup. Strips the Bearer
prefix, validates the token via services/api_keys.lookup_key (which
already filters revoked keys and updates last_used_at), and returns
the user_id for the in-flight MCP request.

Tests follow the existing mock-async_session pattern in
test_api_keys.py rather than introducing a real DB fixture.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/fabledassistant/mcp/auth.py

@@ -0,0 +1,19 @@
+"""MCP-side Bearer token resolution. Reuses the existing api_keys infrastructure."""
+from __future__ import annotations
+
+from fabledassistant.services.api_keys import lookup_key
+
+
+async def resolve_bearer_to_user_id(auth_header: str | None) -> int | None:
+    """Parse an `Authorization: Bearer <token>` header and return the user_id.
+
+    Returns None if the header is missing, malformed, or the token is invalid
+    or revoked. The underlying lookup_key already updates last_used_at on hit.
+    """
+    if not auth_header or not auth_header.startswith("Bearer "):
+        return None
+    raw_token = auth_header[len("Bearer "):].strip()
+    if not raw_token:
+        return None
+    api_key = await lookup_key(raw_token)
+    return api_key.user_id if api_key else None

tests/test_mcp_auth.py

@@ -0,0 +1,51 @@
+"""Tests for MCP auth: bearer-token validation that reuses api_keys."""
+from unittest.mock import AsyncMock, MagicMock, patch
+
+import pytest
+
+from fabledassistant.mcp.auth import resolve_bearer_to_user_id
+
+
+@pytest.mark.asyncio
+async def test_resolve_bearer_missing_header_returns_none():
+    assert await resolve_bearer_to_user_id(None) is None
+
+
+@pytest.mark.asyncio
+async def test_resolve_bearer_malformed_header_returns_none():
+    assert await resolve_bearer_to_user_id("Token abc") is None
+    assert await resolve_bearer_to_user_id("Bearer") is None
+    assert await resolve_bearer_to_user_id("Bearer ") is None
+    assert await resolve_bearer_to_user_id("") is None
+
+
+@pytest.mark.asyncio
+async def test_resolve_bearer_unknown_token_returns_none():
+    with patch(
+        "fabledassistant.mcp.auth.lookup_key",
+        AsyncMock(return_value=None),
+    ):
+        assert await resolve_bearer_to_user_id("Bearer fmcp_doesnotexist") is None
+
+
+@pytest.mark.asyncio
+async def test_resolve_bearer_valid_token_returns_user_id():
+    fake_key = MagicMock()
+    fake_key.user_id = 42
+    with patch(
+        "fabledassistant.mcp.auth.lookup_key",
+        AsyncMock(return_value=fake_key),
+    ):
+        uid = await resolve_bearer_to_user_id("Bearer fmcp_validkey")
+    assert uid == 42
+
+
+@pytest.mark.asyncio
+async def test_resolve_bearer_calls_lookup_with_stripped_token():
+    """The Bearer prefix and any trailing whitespace must be stripped before lookup."""
+    fake_key = MagicMock()
+    fake_key.user_id = 1
+    mock_lookup = AsyncMock(return_value=fake_key)
+    with patch("fabledassistant.mcp.auth.lookup_key", mock_lookup):
+        await resolve_bearer_to_user_id("Bearer fmcp_abc123  ")
+    mock_lookup.assert_awaited_once_with("fmcp_abc123")