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>

.env.example

@@ -166,3 +166,13 @@ AGENTIC_GROUNDING_MAX_TOKENS=250
 # 逗号分隔的允许跨域来源列表，生产环境绝不能使用 *
 CORS_ALLOW_ORIGINS=http://localhost:5173
 
+# ===== MCP (Model Context Protocol) =====
+# MCP 端点（/mcp/）的 Host 头白名单，逗号分隔。MCP SDK 默认开启 DNS rebinding
+# 防护，任何不在此列表中的 Host 都会被直接返回 HTTP 421，请求根本到不了鉴权和
+# 工具逻辑。因此**远程部署必须把真实访问地址写进来**，否则所有外部 MCP 客户端
+# （Claude Desktop / IDE 等）100% 连不上。
+# 语法：`:*` 后缀表示匹配任意端口；填 `*` 表示彻底关闭该防护（不推荐）。
+# 例如部署在 6.86.80.9:8000 时：
+# MCP_ALLOWED_HOSTS=6.86.80.9:*,127.0.0.1:*,localhost:*
+MCP_ALLOWED_HOSTS=127.0.0.1:*,localhost:*,[::1]:*
+

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
 
 

docs/architecture/backend-project-architecture.md

@@ -206,6 +206,7 @@
 ```text
 backend/app/
   api/
+  mcp/
   application/
     documents/
     knowledge/
@@ -314,6 +315,25 @@ backend/app/
 - `backend/app/shared/bootstrap.py` 是现阶段的 composition root，负责把端口实现、基础设施适配器和 application service 连接起来。
 - 后续如果新增 wiring 入口，应继续保持在同一类装配边界内，不要把依赖装配拆回各个路由或 service 构造函数中。
 
+### 4.6 `mcp`
+
+职责：
+
+- 以 Model Context Protocol 对外暴露平台已有能力
+- MCP tool 注册与入参 schema 绑定
+- MCP 专用鉴权（复用现有 JWT）与 Streamable HTTP 子 ASGI 应用装配
+
+非职责：
+
+- 不实现任何新的检索、问答或业务编排逻辑
+- 不直接访问 Milvus、MinIO、LLM SDK
+
+说明：
+
+- `mcp` 与 `api` 是并列的两个 transport 适配层：`api` 面向 HTTP REST 客户端，`mcp` 面向 MCP 客户端（Claude Desktop、IDE 等）。二者共用同一套 application service。
+- 它独立成顶层模块而不是放进 `api/routes/`，因为 MCP 使用装饰器式 tool 注册和自带的子 ASGI 应用，与 `APIRouter` 是不同的传输机制。
+- 当前实现见 `backend/app/mcp/server.py`，只暴露 `search_regulations` 一个 tool，内部直接调用 `get_agent_conversation_service()`。
+
 ## 5. Module Responsibilities
 
 ### 5.1 `api`
@@ -637,6 +657,7 @@ infrastructure -> external systems
 具体规则如下：
 
 - `api` 可以依赖 `application` 和 API 自己的 request/response models
+- `mcp` 与 `api` 同级，只能依赖 `application` 和 composition root，不能依赖 `infrastructure` 或反过来被 `application` 依赖
 - `application` 只能依赖 `domain`、端口接口，以及通过 composition root 注入进来的实现实例
 - `domain` 不能依赖 `api` 或 `infrastructure`
 - `infrastructure` 可以依赖 `domain` 定义的端口和数据模型，但不能反向驱动 application 逻辑

docs/superpowers/plans/2026-07-29-mcp-search-regulations.md

@@ -469,3 +469,18 @@ Confirm `search_regulations` appears in the tool list and returns a real answer
 | 1 | `app/mcp/__init__.py`, `app/mcp/server.py`, `tests/mcp/__init__.py`, `tests/mcp/test_search_regulations_tool.py` | — | 3 |
 | 2 | `tests/mcp/test_mcp_auth_middleware.py` | `app/mcp/server.py` | 4 |
 | 3 | — | `requirements.txt`, `api/main.py` | 0 (full-suite regression check + manual e2e) |
+
+## Task 4 (post-review): code-review fixes
+
+Added after a code review of the three implementation commits. Findings and resolutions:
+
+- [x] **Critical — every remote client rejected with HTTP 421.** 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; a client at `http://6.86.80.9:8000/mcp/` was refused before auth or the tool ran, making the feature non-functional in the only deployment it targets. Fixed by passing an explicit `TransportSecuritySettings` built from a new `MCP_ALLOWED_HOSTS` setting (`app/config/settings.py`, documented in `.env.example`), with `*` as a logged, explicit opt-out. Deliberately *not* fixed by passing `host="0.0.0.0"`, which would silently disable the protection.
+- [x] **Important — `top_k` unbounded on the MCP path.** `AskRequest` constrains the same parameter to 1–20, but the tool accepted any integer and `KnowledgeRetrievalService` amplifies it (`top_k * 4`), so `top_k=100000` would request 400,000 Milvus candidates. Fixed with `Annotated[int, Field(ge=1, le=20)]` (and `query` bounded to 1–2000 chars), which also publishes the bounds in the advertised JSON schema.
+- [x] **Important — order-dependent `psycopg2` test guard.** The guard was duplicated across four test modules and only worked because of pytest's alphabetical collection order; any new test package sorting earlier would have reintroduced a multi-second TCP timeout against the production database. Moved into a single `backend/tests/conftest.py` (imported before any test module regardless of order) and the four in-file copies deleted. `bootstrap.py`'s eager imports were left alone — restructuring the composition root every route depends on is disproportionate to a test-harness ordering problem.
+- [x] **Minor — non-UTF-8 `Authorization` header caused a 500.** ASGI header values are latin-1; strict UTF-8 decoding let any remote client trigger an unhandled `UnicodeDecodeError`. Now decoded as latin-1, yielding a clean 401.
+- [x] **Minor — 401 missing `WWW-Authenticate`.** Added `WWW-Authenticate: Bearer`, matching `get_current_user` and RFC 7235.
+- [x] **Minor — missing `#` comment** in `tests/mcp/test_search_regulations_tool.py` (AGENTS.md requires at least one per file). Added.
+
+Reviewer-confirmed as correct, no change needed: the `AsyncExitStack` lifespan wiring (including its failure path), the absence of auth-bypass vectors, and the statelessness of `ask()` without a `session_id`.
+
+New tests: `tests/mcp/test_mcp_transport_security.py` (5, exercising the real MCP app end-to-end) plus 3 more across the existing two files — 84 backend tests pass. Verified against a live server: `Host: 6.86.80.9:8000` → 200 with a valid `initialize` result, `Host: evil.example.com` → 421, no token → 401 with `WWW-Authenticate: Bearer`.

docs/superpowers/specs/2026-07-29-mcp-search-regulations-design.md

@@ -157,19 +157,40 @@ class MCPAuthMiddleware:
 
 JWTs expire after `expire_minutes` (480 by default, see `JWTHandler`) — long-lived external tool connections will need a token refresh story, but that is an existing limitation of the JWT scheme generally (not new to MCP), so it is not addressed differently here.
 
+### Transport security (Host allow-list)
+
+> **Added post-design after code review.** This was missed in the original design and would have made the feature 100% non-functional in the target deployment.
+
+The MCP SDK enables DNS-rebinding protection automatically whenever the transport's bind host is a loopback address (its `host` parameter defaults to `127.0.0.1`), and then hard-codes the allow-list to `127.0.0.1:*`, `localhost:*`, `[::1]:*`. `TransportSecurityMiddleware` rejects any request whose `Host` header is not on that list with **HTTP 421**, *before* `MCPAuthMiddleware` or the tool runs. A client pointed at `http://6.86.80.9:8000/mcp/` sends `Host: 6.86.80.9:8000` and is therefore refused every time.
+
+The module resolves this by passing an explicit `TransportSecuritySettings` built from a new setting, `MCP_ALLOWED_HOSTS` (comma-separated, `:*` suffix matches any port, documented in `.env.example`):
+
+- Default `127.0.0.1:*,localhost:*,[::1]:*` — safe for local development.
+- Deployments must add their real address, e.g. `MCP_ALLOWED_HOSTS=6.86.80.9:*,127.0.0.1:*,localhost:*`.
+- The literal value `*` disables the protection entirely. This is deliberately an explicit, log-warned opt-out rather than the default, since binding to `0.0.0.0` to sidestep the check would silently switch DNS-rebinding protection off.
+- `allowed_origins` reuses the existing `CORS_ALLOW_ORIGINS` list, so trusted browser origins are declared in exactly one place. Non-browser MCP clients send no `Origin` header, which the SDK treats as allowed.
+
+### Tool input bounds
+
+`search_regulations` declares `query` as 1–2000 characters and `top_k` as 1–20 via `Annotated[..., Field(...)]`, matching `AskRequest` in `app/api/models/agent.py`. This is load-bearing rather than cosmetic: `KnowledgeRetrievalService.retrieve()` amplifies the value (`candidate_k = max(top_k * 4, 20)`) when reranking is active, so an unbounded `top_k` is a cheap resource-exhaustion vector — and an LLM client hallucinating a large value is the likelier trigger than an attacker. Declaring the bounds via `Annotated` also publishes them in the advertised JSON schema, so well-behaved clients never send an out-of-range value at all.
+
 ---
 
 ## Error Handling
 
-- **Auth failure** (missing/expired/invalid token, when `auth_enabled=True`): HTTP 401 from `MCPAuthMiddleware`, before the MCP protocol layer is invoked at all.
+- **Auth failure** (missing/expired/invalid token, when `auth_enabled=True`): HTTP 401 from `MCPAuthMiddleware`, before the MCP protocol layer is invoked at all. The response carries `WWW-Authenticate: Bearer`, matching the `get_current_user` dependency and RFC 7235.
+- **Malformed `Authorization` header bytes**: ASGI header values are latin-1, so the middleware decodes as latin-1; a non-UTF-8 byte yields a normal 401 rather than an unhandled `UnicodeDecodeError`/500.
+- **Rejected `Host` header**: HTTP 421 from the SDK's transport-security middleware (see above), before auth.
 - **Tool execution failure** (e.g. the underlying retrieval/LLM call raises): FastMCP's own tool-call error handling catches exceptions raised inside `@mcp.tool()`-decorated functions and returns them as a normal MCP tool-error result to the calling client — no special handling needed in `search_regulations` itself, consistent with how `/agent/ask`'s REST handler already lets the global FastAPI exception handler in `main.py` catch unexpected errors.
 - **Lifespan startup failure** (e.g. `mcp_app`'s session manager fails to start): surfaces the same way any other `lifespan()` failure does today — the app fails to start, visible immediately in logs, not a silent partial-degradation.
 
 ## Testing
 
-- `backend/tests/mcp/test_search_regulations_tool.py` — unit test for the tool function with a mocked `AgentConversationService` (same mocking style as existing `application/agent` tests): asserts `search_regulations()` calls `.ask(query=..., top_k=...)` with no `session_id`, and shapes the returned dict correctly (`answer`, `sources`).
-- `backend/tests/mcp/test_mcp_auth_middleware.py` — unit test for `MCPAuthMiddleware`: no token → 401; invalid/expired token → 401; valid token → request passed through to the wrapped app; `auth_enabled=False` → always passed through. Uses Starlette's `TestClient` against a minimal dummy inner ASGI app, no real MCP protocol handshake needed.
-- **Manual end-to-end verification** (not automated): use the official `mcp` Python client (`mcp.client.streamable_http.streamablehttp_client` + `mcp.ClientSession`) to connect to a locally running instance, call `list_tools()`, then call `search_regulations` with a real query, confirming a real answer + sources come back. This is a one-time manual check, not a CI test — full MCP protocol handshake testing would require standing up the `mcp` client SDK as a test dependency for marginal additional confidence beyond the two unit tests above.
+- `backend/tests/mcp/test_search_regulations_tool.py` — unit test for the tool function with a mocked `AgentConversationService` (same mocking style as existing `application/agent` tests): asserts `search_regulations()` calls `.ask(query=..., top_k=...)` with no `session_id`, shapes the returned dict correctly (`answer`, `sources`), and that the advertised JSON schema carries the `query`/`top_k` bounds.
+- `backend/tests/mcp/test_mcp_auth_middleware.py` — unit test for `MCPAuthMiddleware`: no token → 401; invalid/expired token → 401; valid token → request passed through to the wrapped app; `auth_enabled=False` → always passed through; 401 carries `WWW-Authenticate`; non-UTF-8 header bytes → 401 not 500. Uses Starlette's `TestClient` against a minimal dummy inner ASGI app, no real MCP protocol handshake needed.
+- `backend/tests/mcp/test_mcp_transport_security.py` — exercises the **real** MCP app over `TestClient`: a configured remote `Host` completes a real JSON-RPC `initialize` handshake; an unconfigured `Host` is refused with 421; `*` disables protection; the comma-separated setting parses correctly. These run the app's lifespan via `with TestClient(...)`, without which the SDK's session-manager task group is uninitialized.
+- `backend/tests/conftest.py` — mocks `psycopg2` at import time for the whole suite. Individual test modules cannot do this reliably, because whether a module runs before the one that needs the mock depends on alphabetical collection order; `conftest.py` is imported before any test module in the tree.
+- **Manual end-to-end verification** (not automated): use the official `mcp` Python client (`mcp.client.streamable_http.streamable_http_client` + `mcp.ClientSession`) to connect to a locally running instance, call `list_tools()`, then call `search_regulations` with a real query, confirming a real answer + sources come back. This is a one-time manual check, not a CI test.
 
 ## Dependencies