fix: harden MCP endpoint after code review

Critical: the MCP SDK auto-enables DNS-rebinding protection when its host
parameter is left at the 127.0.0.1 default, hard-coding a loopback-only Host
allow-list. Every remote client (the only deployment this feature targets) was
refused with HTTP 421 before auth or the tool ran. Now driven by a new
MCP_ALLOWED_HOSTS setting, with '*' as an explicit, logged opt-out.

Also bounds query/top_k to match AskRequest (top_k is amplified 4x downstream,
so an unbounded value was a resource-exhaustion vector), decodes the
Authorization header as latin-1 per the ASGI spec instead of raising a 500 on
malformed bytes, and returns WWW-Authenticate on 401 per RFC 7235.

Moves the psycopg2 import guard into backend/tests/conftest.py: duplicated
across four test modules, it only worked because of alphabetical collection
order, and any earlier-sorting package would have reintroduced a live
connection attempt against the production database.

Registers the mcp module in the authoritative backend architecture doc.

84 backend tests pass. Verified against a live server: allowed remote Host
returns a valid initialize result, unknown Host returns 421, missing token
returns 401 with WWW-Authenticate.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/config/settings.py

@@ -198,6 +198,20 @@ class Settings(BaseSettings):
         description="Comma-separated allowed CORS origins. Never use * in production.",
     )
 
+    # ── MCP ───────────────────────────────────────────────────────────────────
+    # The MCP SDK enables DNS-rebinding protection whenever the transport is
+    # bound to a loopback host, which rejects any Host header not in this list
+    # with HTTP 421. Deployments reachable by a real hostname/IP must list it
+    # here or every remote MCP client is refused before the handler runs.
+    mcp_allowed_hosts: str = Field(
+        default="127.0.0.1:*,localhost:*,[::1]:*",
+        description=(
+            "Comma-separated Host header values accepted by the MCP endpoint. "
+            "A ':*' suffix matches any port. Set to '*' to disable DNS-rebinding "
+            "protection entirely (not recommended)."
+        ),
+    )
+
 @lru_cache
 def get_settings() -> Settings:
     """Return settings."""

backend/app/mcp/server.py

@@ -8,13 +8,20 @@ dict. No new retrieval, ranking, or LLM orchestration logic lives here.
 
 from __future__ import annotations
 
+import logging
+from typing import Annotated
+
 from mcp.server import MCPServer
+from mcp.server.transport_security import TransportSecuritySettings
+from pydantic import Field
 from starlette.responses import PlainTextResponse
 from starlette.types import ASGIApp, Receive, Scope, Send
 
 from app.config.settings import settings
 from app.shared.bootstrap import get_agent_conversation_service, get_jwt_handler
 
+logger = logging.getLogger(__name__)
+
 # Single shared MCPServer instance — analogous to the single shared FastAPI
 # `app` instance in app/api/main.py. Tools registered via @mcp.tool() below.
 # Note: the installed mcp SDK (2.0.0) renamed the older "FastMCP" class to
@@ -24,12 +31,22 @@ mcp = MCPServer("ai-regulations")
 
 
 @mcp.tool()
-def search_regulations(query: str, top_k: int = 5) -> dict:
+def search_regulations(
+    query: Annotated[str, Field(min_length=1, max_length=2000)],
+    top_k: Annotated[int, Field(ge=1, le=20)] = 5,
+) -> dict:
     """Search the compliance knowledge base and return a grounded answer.
 
     query: Natural-language search question, e.g. "国六排放标准最新要求".
-    top_k: Maximum number of cited sources to return (default 5).
+    top_k: Maximum number of cited sources to return (1-20, default 5).
     """
+    # Bounds mirror AskRequest in app/api/models/agent.py so the MCP path cannot
+    # be used to bypass the REST endpoint's limits. They matter more here than
+    # there: KnowledgeRetrievalService amplifies top_k (candidate_k = top_k * 4)
+    # when reranking, and an LLM client can easily hallucinate a huge value.
+    # Declaring them via Annotated puts them in the advertised JSON schema too,
+    # so well-behaved clients never send an out-of-range value in the first place.
+    #
     # No session_id is passed: this keeps each call stateless (no
     # ConversationStore reads/writes), matching "search" semantics rather
     # than multi-turn chat semantics.
@@ -61,19 +78,53 @@ class MCPAuthMiddleware:
             return
 
         headers = dict(scope["headers"])
-        auth_header = headers.get(b"authorization", b"").decode()
+        # ASGI header values are raw bytes specified as latin-1, not UTF-8;
+        # decoding strictly as UTF-8 would raise on a malformed byte and turn a
+        # bad request into an unhandled 500.
+        auth_header = headers.get(b"authorization", b"").decode("latin-1")
         token = auth_header.removeprefix("Bearer ").strip()
         try:
             get_jwt_handler().decode_token(token)
         except ValueError as exc:
             # Reject before the MCP session/protocol layer ever sees the request.
-            response = PlainTextResponse(str(exc), status_code=401)
+            # WWW-Authenticate matches the get_current_user dependency (auth.py)
+            # and is required by RFC 7235 so clients can tell "needs credentials"
+            # apart from a generic failure.
+            response = PlainTextResponse(
+                str(exc), status_code=401, headers={"WWW-Authenticate": "Bearer"}
+            )
             await response(scope, receive, send)
             return
 
         await self.app(scope, receive, send)
 
 
+def _build_transport_security() -> TransportSecuritySettings:
+    """Translate the configured MCP host allow-list into SDK transport settings.
+
+    Without this the SDK infers its own allow-list from the bind host, which
+    defaults to 127.0.0.1 and therefore rejects every remote client with HTTP
+    421 — fatal for a remotely deployed backend.
+    """
+    allowed = [h.strip() for h in settings.mcp_allowed_hosts.split(",") if h.strip()]
+    if "*" in allowed:
+        # Explicit, logged opt-out. Kept as an escape hatch for environments
+        # behind a proxy that rewrites Host unpredictably, but never the default.
+        logger.warning(
+            "MCP DNS-rebinding protection is disabled (mcp_allowed_hosts='*'). "
+            "Set MCP_ALLOWED_HOSTS to the real deployment host(s) instead."
+        )
+        return TransportSecuritySettings(enable_dns_rebinding_protection=False)
+    return TransportSecuritySettings(
+        enable_dns_rebinding_protection=True,
+        allowed_hosts=allowed,
+        # Browser clients send Origin; reuse the already-maintained CORS list so
+        # there is one place to declare trusted web origins. Non-browser MCP
+        # clients send no Origin at all, which the SDK treats as allowed.
+        allowed_origins=[o.strip() for o in settings.cors_allow_origins.split(",") if o.strip()],
+    )
+
+
 def build_mcp_asgi_app() -> ASGIApp:
     """Return the Streamable HTTP ASGI app for the MCP server, auth-guarded.
 
@@ -83,6 +134,9 @@ def build_mcp_asgi_app() -> ASGIApp:
     path to "/", the effective external path would be the confusing "/mcp/mcp"
     instead of "/mcp".
     """
-    asgi_app = mcp.streamable_http_app(streamable_http_path="/")
+    asgi_app = mcp.streamable_http_app(
+        streamable_http_path="/",
+        transport_security=_build_transport_security(),
+    )
     asgi_app.add_middleware(MCPAuthMiddleware)
     return asgi_app

backend/tests/conftest.py

@@ -0,0 +1,27 @@
+"""Shared pytest fixtures and import-time guards for the backend test suite.
+
+pytest imports this file before any test module beneath backend/tests/, which
+makes it the only reliable place to install import-time guards: individual test
+modules cannot guarantee they run first, because collection order follows
+directory names.
+"""
+
+from __future__ import annotations
+
+import sys
+from unittest.mock import MagicMock
+
+# app/shared/bootstrap.py (the composition root) eagerly imports the Postgres
+# store modules, which do `import psycopg2` at their own module scope and later
+# open a real connection pool. Any test that transitively imports bootstrap
+# would therefore bind the real driver and attempt a live TCP connection to the
+# configured production database, surfacing as a multi-second timeout rather
+# than an obvious error. Binding mocks here — before the first test module is
+# imported — makes that impossible regardless of collection order.
+# setdefault (not assignment) keeps a real psycopg2 in place if something has
+# already imported it deliberately.
+_mock_psycopg2 = MagicMock()
+_mock_psycopg2.extras = MagicMock()
+sys.modules.setdefault("psycopg2", _mock_psycopg2)
+sys.modules.setdefault("psycopg2.extras", _mock_psycopg2.extras)
+sys.modules.setdefault("psycopg2.pool", MagicMock())

backend/tests/mcp/test_mcp_auth_middleware.py

@@ -7,23 +7,7 @@ independent of FastMCP internals.
 
 from __future__ import annotations
 
-import sys
-from unittest.mock import MagicMock, patch
-
-# app.mcp.server imports app.shared.bootstrap at module scope (needed for
-# get_agent_conversation_service/get_jwt_handler), which in turn eagerly
-# imports several Postgres store modules that do `import psycopg2` at their
-# own module scope. Since this is the only test file in backend/tests/mcp/
-# that imports app.mcp.server at module scope (not inside a test function),
-# it is the first thing to trigger that chain during pytest collection —
-# guard psycopg2 here the same way backend/tests/observability/
-# test_model_usage_bootstrap.py and test_model_usage_persistence.py already
-# do, so a real (network-connecting) psycopg2 never gets bound first.
-mock_psycopg2 = MagicMock()
-mock_psycopg2.extras = MagicMock()
-sys.modules.setdefault("psycopg2", mock_psycopg2)
-sys.modules.setdefault("psycopg2.extras", mock_psycopg2.extras)
-sys.modules.setdefault("psycopg2.pool", MagicMock())
+from unittest.mock import patch
 
 from starlette.applications import Starlette
 from starlette.responses import PlainTextResponse
@@ -83,3 +67,27 @@ def test_auth_disabled_always_passes_through():
         client = TestClient(_dummy_app())
         response = client.get("/ping")
     assert response.status_code == 200
+
+
+def test_401_includes_www_authenticate_header():
+    """RFC 7235 requires WWW-Authenticate on 401 so clients can tell why they failed."""
+    with patch("app.mcp.server.settings") as fake_settings:
+        fake_settings.auth_enabled = True
+        client = TestClient(_dummy_app())
+        response = client.get("/ping")
+    assert response.status_code == 401
+    assert response.headers["WWW-Authenticate"] == "Bearer"
+
+
+def test_non_utf8_authorization_header_is_rejected_not_crashed():
+    """A non-UTF-8 header byte must yield a clean 401, not an unhandled 500.
+
+    ASGI header values are latin-1 bytes, so any remote client could otherwise
+    trigger a UnicodeDecodeError inside the middleware at will.
+    """
+    with patch("app.mcp.server.settings") as fake_settings:
+        fake_settings.auth_enabled = True
+        client = TestClient(_dummy_app(), raise_server_exceptions=False)
+        # Bypass the http client's own header encoding by writing raw bytes.
+        response = client.get("/ping", headers={"Authorization": b"Bearer \xff\xfe"})
+    assert response.status_code == 401

backend/tests/mcp/test_mcp_transport_security.py

@@ -0,0 +1,107 @@
+"""Tests for the MCP endpoint's DNS-rebinding (Host header) protection.
+
+The MCP SDK auto-enables DNS-rebinding protection and derives its allow-list
+from the bind host, which defaults to 127.0.0.1. Left alone, that rejects every
+request whose Host header is the real deployment address (6.86.80.9:8000) with
+HTTP 421 — before the auth middleware or the tool ever runs. These tests pin
+the configured allow-list behavior so that failure mode cannot come back.
+"""
+
+from __future__ import annotations
+
+import json
+from contextlib import contextmanager
+from unittest.mock import patch
+
+from starlette.testclient import TestClient
+
+from app.mcp.server import _build_transport_security, build_mcp_asgi_app
+
+# A minimal JSON-RPC initialize call. Reaching the MCP handler at all is what
+# matters here; transport security rejects the request long before this body is
+# parsed, so its exact contents only need to be structurally valid.
+_INITIALIZE = {
+    "jsonrpc": "2.0",
+    "id": 1,
+    "method": "initialize",
+    "params": {
+        "protocolVersion": "2025-06-18",
+        "capabilities": {},
+        "clientInfo": {"name": "test", "version": "1.0"},
+    },
+}
+
+_HEADERS = {
+    "Content-Type": "application/json",
+    "Accept": "application/json, text/event-stream",
+}
+
+
+@contextmanager
+def _mcp_client(allowed_hosts: str):
+    """Yield a TestClient over the real MCP app with auth off and hosts configured.
+
+    The settings patch must stay active for the requests themselves, not just
+    for app construction, because MCPAuthMiddleware reads settings per request.
+    Entering the TestClient as a context manager is also required: it runs the
+    app's lifespan, without which the SDK's session manager task group is never
+    initialized and every request raises RuntimeError.
+    """
+    with patch("app.mcp.server.settings") as fake_settings:
+        fake_settings.mcp_allowed_hosts = allowed_hosts
+        fake_settings.cors_allow_origins = "http://localhost:5173"
+        fake_settings.auth_enabled = False
+        with TestClient(build_mcp_asgi_app()) as client:
+            yield client
+
+
+def test_remote_host_allowed_when_configured():
+    """A configured non-loopback Host must reach the MCP handler, not 421."""
+    with _mcp_client("6.86.80.9:*,127.0.0.1:*") as client:
+        response = client.post(
+            "/", json=_INITIALIZE, headers={**_HEADERS, "Host": "6.86.80.9:8000"}
+        )
+    assert response.status_code == 200
+    assert "Invalid Host header" not in response.text
+
+
+def test_unconfigured_host_still_rejected():
+    """Protection must stay on: a Host outside the allow-list is refused with 421."""
+    with _mcp_client("6.86.80.9:*") as client:
+        response = client.post(
+            "/", json=_INITIALIZE, headers={**_HEADERS, "Host": "evil.example.com"}
+        )
+    assert response.status_code == 421
+
+
+def test_initialize_response_is_event_stream():
+    """Sanity check that a permitted request really completes the MCP handshake."""
+    with _mcp_client("6.86.80.9:*") as client:
+        response = client.post(
+            "/", json=_INITIALIZE, headers={**_HEADERS, "Host": "6.86.80.9:8000"}
+        )
+    assert response.status_code == 200
+    # The Streamable HTTP transport replies as SSE; the JSON-RPC result is
+    # embedded in a "data:" line rather than being the whole body.
+    payload = json.loads(response.text.split("data:", 1)[1].strip())
+    assert payload["result"]["serverInfo"]["name"] == "ai-regulations"
+
+
+def test_wildcard_disables_protection_explicitly():
+    """'*' is the documented opt-out; it must disable the check, not allow-list '*'."""
+    with patch("app.mcp.server.settings") as fake_settings:
+        fake_settings.mcp_allowed_hosts = "*"
+        fake_settings.cors_allow_origins = "http://localhost:5173"
+        security = _build_transport_security()
+    assert security.enable_dns_rebinding_protection is False
+
+
+def test_allow_list_is_parsed_into_transport_settings():
+    """Comma-separated config must become the SDK's allowed_hosts list verbatim."""
+    with patch("app.mcp.server.settings") as fake_settings:
+        fake_settings.mcp_allowed_hosts = "6.86.80.9:*, localhost:* ,"
+        fake_settings.cors_allow_origins = "http://localhost:5173"
+        security = _build_transport_security()
+    assert security.enable_dns_rebinding_protection is True
+    assert security.allowed_hosts == ["6.86.80.9:*", "localhost:*"]
+    assert security.allowed_origins == ["http://localhost:5173"]

backend/tests/mcp/test_search_regulations_tool.py

@@ -7,6 +7,7 @@ correct dict shape out.
 
 from __future__ import annotations
 
+import asyncio
 from dataclasses import dataclass
 from unittest.mock import MagicMock, patch
 
@@ -15,6 +16,9 @@ from unittest.mock import MagicMock, patch
 class _FakeSource:
     """Minimal stand-in for a real Source dataclass (only __dict__ is used)."""
 
+    # A dataclass, not a MagicMock: the adapter serializes sources via
+    # source.__dict__, and a MagicMock's __dict__ is full of internal mock
+    # attributes, which would make the assertions meaningless.
     doc_id: str
     doc_title: str
     score: float
@@ -75,3 +79,20 @@ def test_search_regulations_default_top_k():
         search_regulations(query="q")
 
     assert fake_service.ask.call_args.kwargs["top_k"] == 5
+
+
+def test_advertised_schema_bounds_top_k_and_query():
+    """The advertised JSON schema must carry the same bounds as AskRequest.
+
+    Bounds declared via Annotated are what the SDK validates against and what
+    clients see, so asserting on the generated schema is the only way to catch
+    a regression that silently drops them.
+    """
+    from app.mcp.server import mcp
+
+    schema = asyncio.run(mcp.list_tools())[0].input_schema["properties"]
+
+    assert schema["top_k"]["minimum"] == 1
+    assert schema["top_k"]["maximum"] == 20
+    assert schema["query"]["minLength"] == 1
+    assert schema["query"]["maxLength"] == 2000

backend/tests/observability/test_model_usage_bootstrap.py

@@ -12,17 +12,10 @@ is needed anywhere in this file — asyncio.create_task itself is also mocked.
 
 from __future__ import annotations
 
-import sys
 from unittest.mock import MagicMock, patch
 
-# Patch psycopg2 before importing anything that transitively imports it, in
-# case this file is collected before test_model_usage_persistence.py.
-mock_psycopg2 = MagicMock()
-mock_psycopg2.extras = MagicMock()
-sys.modules.setdefault("psycopg2", mock_psycopg2)
-sys.modules.setdefault("psycopg2.extras", mock_psycopg2.extras)
-sys.modules.setdefault("psycopg2.pool", MagicMock())
-
+# psycopg2 is mocked centrally in backend/tests/conftest.py, which pytest
+# imports before any test module regardless of collection order.
 from app.shared import bootstrap
 from app.shared.model_usage_tracker import ModelUsageEntry, ModelUsageTracker
 

backend/tests/observability/test_model_usage_persistence.py

@@ -6,17 +6,11 @@ Mirrors the mocking pattern in backend/tests/perception/test_postgres_event_stor
 
 from __future__ import annotations
 
-import sys
 from datetime import datetime, timezone
 from unittest.mock import MagicMock, patch
 
-# Patch psycopg2 before importing the module under test.
-mock_psycopg2 = MagicMock()
-mock_psycopg2.extras = MagicMock()
-sys.modules.setdefault("psycopg2", mock_psycopg2)
-sys.modules.setdefault("psycopg2.extras", mock_psycopg2.extras)
-sys.modules.setdefault("psycopg2.pool", MagicMock())
-
+# psycopg2 is mocked centrally in backend/tests/conftest.py, so importing the
+# module under test here never binds the real driver.
 from app.shared.model_usage_tracker import ModelUsageEntry
 
 

backend/tests/perception/test_postgres_event_store.py

@@ -4,14 +4,8 @@ import json
 from unittest.mock import MagicMock, patch
 import pytest
 
-# Patch psycopg2 before importing the module under test
-import sys
-mock_psycopg2 = MagicMock()
-mock_psycopg2.extras = MagicMock()
-sys.modules.setdefault("psycopg2", mock_psycopg2)
-sys.modules.setdefault("psycopg2.extras", mock_psycopg2.extras)
-sys.modules.setdefault("psycopg2.pool", MagicMock())
-
+# psycopg2 is mocked centrally in backend/tests/conftest.py, so importing the
+# module under test here never binds the real driver.
 from app.infrastructure.perception.base_event_store import BaseEventStore