feat(extension): 接入 stdio MCP Plugin Host

backend/app/agent/runtime.py

@@ -491,7 +491,13 @@ class AgentRuntime:
     async def _invoke_tool(self, record: RunRecord, call: ToolCall) -> ToolResult:
         try:
             return await asyncio.wait_for(
-                self.tools.execute(call, ToolExecutionContext(run_id=record.run.run_id)),
+                self.tools.execute(
+                    call,
+                    ToolExecutionContext(
+                        run_id=record.run.run_id,
+                        tool_call_id=call.tool_call_id,
+                    ),
+                ),
                 timeout=record.request.tool_timeout_seconds,
             )
         except TimeoutError:

backend/app/agent/tools.py

@@ -1,6 +1,7 @@
 """Agent 工具注册与执行边界。"""
 
 import inspect
+import threading
 from dataclasses import dataclass
 from time import perf_counter
 from typing import Any, Awaitable, Callable
@@ -17,6 +18,7 @@ ToolExecutor = Callable[[BaseModel, "ToolExecutionContext"], Any | Awaitable[Any
 @dataclass(frozen=True, slots=True)
 class ToolExecutionContext:
     run_id: str
+    tool_call_id: str | None = None
 
 
 @dataclass(slots=True)
@@ -30,11 +32,21 @@ class ToolNotFoundError(LookupError):
     pass
 
 
+class ToolExecutionError(RuntimeError):
+    """Executor 可预期失败，保留领域错误码而不是折叠成通用异常。"""
+
+    def __init__(self, code: str, message: str) -> None:
+        super().__init__(message)
+        self.code = code
+        self.message = message
+
+
 class ToolRegistry:
     """统一校验工具入参并隔离执行异常，避免单个工具击穿 Agent 主循环。"""
 
     def __init__(self) -> None:
         self._tools: dict[str, RegisteredTool] = {}
+        self._lock = threading.RLock()
 
     def register(
         self,
@@ -42,33 +54,38 @@ class ToolRegistry:
         arguments_model: type[BaseModel],
         executor: ToolExecutor,
     ) -> None:
-        if definition.name in self._tools:
+        with self._lock:
-            raise ValueError(f"Tool already registered: {definition.name}")
+            if definition.name in self._tools:
-        self._tools[definition.name] = RegisteredTool(
+                raise ValueError(f"Tool already registered: {definition.name}")
-            definition=definition,
+            self._tools[definition.name] = RegisteredTool(
-            arguments_model=arguments_model,
+                definition=definition,
-            executor=executor,
+                arguments_model=arguments_model,
-        )
+                executor=executor,
+            )
 
     def unregister(self, name: str) -> None:
-        self._tools.pop(name, None)
+        with self._lock:
+            self._tools.pop(name, None)
 
     def contains(self, name: str) -> bool:
-        return name in self._tools
+        with self._lock:
+            return name in self._tools
 
     def get(self, name: str) -> RegisteredTool:
-        try:
+        with self._lock:
-            return self._tools[name]
+            try:
-        except KeyError as exc:
+                return self._tools[name]
-            raise ToolNotFoundError(name) from exc
+            except KeyError as exc:
+                raise ToolNotFoundError(name) from exc
 
     def definitions(self, allowed: list[str] | None = None) -> list[ToolDefinition]:
         names = set(allowed) if allowed is not None else None
-        return [
+        with self._lock:
-            item.definition.model_copy(deep=True)
+            return [
-            for name, item in self._tools.items()
+                item.definition.model_copy(deep=True)
-            if names is None or name in names
+                for name, item in self._tools.items()
-        ]
+                if names is None or name in names
+            ]
 
     async def execute(self, call: ToolCall, context: ToolExecutionContext) -> ToolResult:
         started = perf_counter()
@@ -108,6 +125,15 @@ class ToolRegistry:
                 output=output,
                 duration_ms=round((perf_counter() - started) * 1000),
             )
+        except ToolExecutionError as exc:
+            return ToolResult(
+                tool_call_id=call.tool_call_id,
+                name=call.name,
+                success=False,
+                error_code=exc.code,
+                error_message=exc.message,
+                duration_ms=round((perf_counter() - started) * 1000),
+            )
         except Exception as exc:  # 工具失败转换成结构化结果，由模型决定是否降级或重试。
             return ToolResult(
                 tool_call_id=call.tool_call_id,

backend/app/contracts.py

@@ -417,6 +417,10 @@ class ExtensionInstallRequest(Contract):
 class PluginBackend(Contract):
     type: Literal["mcp", "internal_rpc", "none"] = "none"
     transport: Literal["stdio", "http", "none"] = "none"
+    command: str | None = None
+    args: list[str] = Field(default_factory=list)
+    startup_timeout_seconds: int = Field(default=10, ge=1, le=60)
+    tool_timeout_seconds: int = Field(default=30, ge=1, le=600)
 
 
 class PluginContribution(Contract):
@@ -460,6 +464,28 @@ class PluginListResponse(Contract):
     items: list[Plugin] = Field(default_factory=list)
 
 
+class PluginHostState(str, Enum):
+    stopped = "stopped"
+    starting = "starting"
+    ready = "ready"
+    unhealthy = "unhealthy"
+    error = "error"
+
+
+class PluginHostStatus(Contract):
+    plugin_id: str
+    backend_type: Literal["mcp", "internal_rpc", "none"]
+    transport: Literal["stdio", "http", "none"]
+    status: PluginHostState
+    tools_count: int = 0
+    started_at: datetime | None = None
+    last_seen_at: datetime | None = None
+    protocol_version: str | None = None
+    server_name: str | None = None
+    server_version: str | None = None
+    error: str | None = None
+
+
 class PluginPermissionGrantRequest(Contract):
     permissions: list[str] = Field(default_factory=list)
 

backend/app/extensions/__init__.py

@@ -4,5 +4,13 @@ from app.extensions.runtime import (
     PluginRuntime,
     SkillRuntime,
 )
+from app.extensions.mcp import McpBridge, McpBridgeError
 
-__all__ = ["AgentConfiguration", "ExtensionError", "PluginRuntime", "SkillRuntime"]
+__all__ = [
+    "AgentConfiguration",
+    "ExtensionError",
+    "McpBridge",
+    "McpBridgeError",
+    "PluginRuntime",
+    "SkillRuntime",
+]

backend/app/extensions/mcp.py

@@ -0,0 +1,746 @@
+"""本地 stdio MCP Bridge。
+
+第三方 Server 始终运行在子进程中。Bridge 只把通过校验的 MCP Tool 转换为项目内部
+ToolDefinition/ToolResult，不把 MCP 原始协议泄露给 Agent Runtime 或前端。
+"""
+
+from __future__ import annotations
+
+import asyncio
+import json
+import os
+import queue
+import subprocess
+import threading
+from collections import deque
+from dataclasses import dataclass
+from datetime import datetime, timezone
+from pathlib import Path
+from typing import Any, Callable
+
+from jsonschema import Draft202012Validator
+from jsonschema.exceptions import SchemaError
+
+from app.agent.permissions import KNOWN_PERMISSIONS
+from app.agent.tools import ToolExecutionError
+from app.contracts import (
+    PluginBackend,
+    PluginHostState,
+    PluginHostStatus,
+    ToolDefinition,
+)
+
+MCP_PROTOCOL_VERSION = "2025-11-25"
+SUPPORTED_PROTOCOL_VERSIONS = {
+    MCP_PROTOCOL_VERSION,
+    "2025-06-18",
+    "2025-03-26",
+    "2024-11-05",
+}
+MAX_MCP_MESSAGE_BYTES = 2 * 1024 * 1024
+MAX_MCP_TOOL_RESULT_BYTES = 256 * 1024
+MAX_MCP_TOOLS = 500
+MAX_MCP_LIST_PAGES = 100
+
+
+class McpBridgeError(RuntimeError):
+    def __init__(self, code: str, message: str, *, status_code: int = 502) -> None:
+        super().__init__(message)
+        self.code = code
+        self.message = message
+        self.status_code = status_code
+
+
+@dataclass(frozen=True, slots=True)
+class McpDiscoveredTool:
+    remote_name: str
+    definition: ToolDefinition
+
+
+@dataclass(slots=True)
+class _PendingRequest:
+    response: queue.Queue[dict[str, Any] | BaseException]
+
+
+class McpStdioClient:
+    """线程驱动的换行分隔 JSON-RPC 客户端，避免阻塞 FastAPI 事件循环。"""
+
+    def __init__(
+        self,
+        command: list[str],
+        *,
+        cwd: Path,
+        on_seen: Callable[[], None],
+        on_broken: Callable[[str], None],
+        on_tools_changed: Callable[[], None],
+    ) -> None:
+        self.command = command
+        self.cwd = cwd
+        self.on_seen = on_seen
+        self.on_broken = on_broken
+        self.on_tools_changed = on_tools_changed
+        self.process: subprocess.Popen[str] | None = None
+        self._write_lock = threading.Lock()
+        self._pending_lock = threading.Lock()
+        self._pending: dict[int, _PendingRequest] = {}
+        self._next_id = 1
+        self._stopping = False
+        # stderr 只在 Host 内部保留有限尾部，不进入 API、Trace 或普通日志。
+        self._stderr_tail: deque[str] = deque(maxlen=50)
+
+    def start(self) -> None:
+        if self.process is not None and self.process.poll() is None:
+            return
+        creation_flags = getattr(subprocess, "CREATE_NO_WINDOW", 0) if os.name == "nt" else 0
+        environment = _subprocess_environment()
+        environment.setdefault("PYTHONUNBUFFERED", "1")
+        try:
+            self.process = subprocess.Popen(
+                self.command,
+                cwd=self.cwd,
+                stdin=subprocess.PIPE,
+                stdout=subprocess.PIPE,
+                stderr=subprocess.PIPE,
+                text=True,
+                encoding="utf-8",
+                errors="replace",
+                bufsize=1,
+                shell=False,
+                env=environment,
+                creationflags=creation_flags,
+            )
+        except OSError as exc:
+            raise McpBridgeError(
+                "PLUGIN_HOST_START_FAILED",
+                f"Cannot start MCP server process: {exc}",
+                status_code=503,
+            ) from exc
+        threading.Thread(target=self._stdout_loop, daemon=True).start()
+        threading.Thread(target=self._stderr_loop, daemon=True).start()
+
+    def request(
+        self,
+        method: str,
+        params: dict[str, Any],
+        *,
+        timeout: float,
+        timeout_code: str,
+        response_error_code: str = "MCP_TOOL_CALL_FAILED",
+    ) -> dict[str, Any]:
+        request_id, pending = self.begin_request(method, params)
+        return self.wait_response(
+            request_id,
+            pending,
+            timeout=timeout,
+            timeout_code=timeout_code,
+            response_error_code=response_error_code,
+        )
+
+    def begin_request(
+        self, method: str, params: dict[str, Any]
+    ) -> tuple[int, _PendingRequest]:
+        self._ensure_running()
+        with self._pending_lock:
+            request_id = self._next_id
+            self._next_id += 1
+            pending = _PendingRequest(response=queue.Queue(maxsize=1))
+            self._pending[request_id] = pending
+        try:
+            self._send(
+                {
+                    "jsonrpc": "2.0",
+                    "id": request_id,
+                    "method": method,
+                    "params": params,
+                }
+            )
+        except BaseException:
+            with self._pending_lock:
+                self._pending.pop(request_id, None)
+            raise
+        return request_id, pending
+
+    def wait_response(
+        self,
+        request_id: int,
+        pending: _PendingRequest,
+        *,
+        timeout: float,
+        timeout_code: str,
+        response_error_code: str = "MCP_TOOL_CALL_FAILED",
+    ) -> dict[str, Any]:
+        try:
+            response = pending.response.get(timeout=timeout)
+        except queue.Empty as exc:
+            self.cancel(request_id, "Request timed out.")
+            self.abandon(request_id)
+            raise McpBridgeError(timeout_code, "MCP request timed out.", status_code=504) from exc
+        if isinstance(response, BaseException):
+            raise response
+        if "error" in response:
+            error = response.get("error")
+            message = (
+                str(error.get("message", "MCP JSON-RPC error."))
+                if isinstance(error, dict)
+                else "MCP JSON-RPC error."
+            )
+            raise McpBridgeError(response_error_code, message)
+        result = response.get("result")
+        if not isinstance(result, dict):
+            raise McpBridgeError(
+                "MCP_TOOL_CALL_FAILED", "MCP response result must be an object."
+            )
+        return result
+
+    def notify(self, method: str, params: dict[str, Any] | None = None) -> None:
+        payload: dict[str, Any] = {"jsonrpc": "2.0", "method": method}
+        if params is not None:
+            payload["params"] = params
+        self._send(payload)
+
+    def cancel(self, request_id: int, reason: str = "Cancelled by host.") -> None:
+        try:
+            self.notify(
+                "notifications/cancelled",
+                {"requestId": request_id, "reason": reason},
+            )
+        except McpBridgeError:
+            pass
+
+    def abandon(self, request_id: int) -> None:
+        with self._pending_lock:
+            self._pending.pop(request_id, None)
+
+    def stop(self) -> None:
+        process = self.process
+        if process is None:
+            return
+        self._stopping = True
+        try:
+            if process.stdin:
+                try:
+                    process.stdin.close()
+                except (BrokenPipeError, OSError, ValueError):
+                    pass
+            try:
+                process.wait(timeout=2)
+            except subprocess.TimeoutExpired:
+                process.terminate()
+                try:
+                    process.wait(timeout=2)
+                except subprocess.TimeoutExpired:
+                    process.kill()
+                    process.wait(timeout=2)
+        finally:
+            self._fail_pending(
+                McpBridgeError("PLUGIN_HOST_UNAVAILABLE", "MCP host stopped.", status_code=503)
+            )
+            self.process = None
+
+    def _send(self, message: dict[str, Any]) -> None:
+        self._ensure_running()
+        encoded = json.dumps(message, ensure_ascii=False, separators=(",", ":"))
+        if len(encoded.encode("utf-8")) > MAX_MCP_MESSAGE_BYTES:
+            raise McpBridgeError("MCP_TOOL_CALL_FAILED", "MCP request is too large.")
+        process = self.process
+        assert process is not None and process.stdin is not None
+        try:
+            with self._write_lock:
+                process.stdin.write(encoded + "\n")
+                process.stdin.flush()
+        except (BrokenPipeError, OSError, ValueError) as exc:
+            raise McpBridgeError(
+                "PLUGIN_HOST_UNAVAILABLE", "MCP host input is closed.", status_code=503
+            ) from exc
+
+    def _stdout_loop(self) -> None:
+        process = self.process
+        assert process is not None and process.stdout is not None
+        failure: str | None = None
+        try:
+            for raw_line in process.stdout:
+                if len(raw_line.encode("utf-8")) > MAX_MCP_MESSAGE_BYTES:
+                    failure = "MCP server emitted an oversized protocol message."
+                    break
+                try:
+                    message = json.loads(raw_line)
+                except json.JSONDecodeError:
+                    failure = "MCP server emitted invalid JSON on stdout."
+                    break
+                if not isinstance(message, dict) or message.get("jsonrpc") != "2.0":
+                    failure = "MCP server emitted an invalid JSON-RPC message."
+                    break
+                self.on_seen()
+                if "id" in message and ("result" in message or "error" in message):
+                    request_id = message.get("id")
+                    if isinstance(request_id, int):
+                        with self._pending_lock:
+                            pending = self._pending.pop(request_id, None)
+                        if pending:
+                            pending.response.put(message)
+                    continue
+                method = message.get("method")
+                if method == "notifications/tools/list_changed":
+                    self.on_tools_changed()
+                elif isinstance(method, str) and "id" in message:
+                    self._send(
+                        {
+                            "jsonrpc": "2.0",
+                            "id": message["id"],
+                            "error": {"code": -32601, "message": "Method not supported."},
+                        }
+                    )
+        except (McpBridgeError, OSError, ValueError) as exc:
+            failure = f"MCP stdout closed unexpectedly: {type(exc).__name__}."
+        finally:
+            if failure and process.poll() is None:
+                process.terminate()
+            exit_code = process.poll()
+            if exit_code is None:
+                try:
+                    exit_code = process.wait(timeout=1)
+                except subprocess.TimeoutExpired:
+                    exit_code = None
+            if not self._stopping:
+                message = failure or f"MCP host exited unexpectedly with code {exit_code}."
+                error = McpBridgeError(
+                    "PLUGIN_HOST_UNAVAILABLE", message, status_code=503
+                )
+                self._fail_pending(error)
+                self.on_broken(message)
+
+    def _stderr_loop(self) -> None:
+        process = self.process
+        assert process is not None and process.stderr is not None
+        try:
+            for line in process.stderr:
+                self._stderr_tail.append(line.rstrip()[:1024])
+        except (OSError, ValueError):
+            return
+
+    def _ensure_running(self) -> None:
+        if self.process is None or self.process.poll() is not None:
+            raise McpBridgeError(
+                "PLUGIN_HOST_UNAVAILABLE", "MCP host is not running.", status_code=503
+            )
+
+    def _fail_pending(self, error: BaseException) -> None:
+        with self._pending_lock:
+            pending = list(self._pending.values())
+            self._pending.clear()
+        for item in pending:
+            item.response.put(error)
+
+
+@dataclass(slots=True)
+class _McpHost:
+    backend: PluginBackend
+    client: McpStdioClient
+    status: PluginHostStatus
+
+
+class McpBridge:
+    """管理每个 Plugin 的独立 MCP Client，并执行 Contract 转换。"""
+
+    def __init__(self) -> None:
+        self._hosts: dict[str, _McpHost] = {}
+        self._statuses: dict[str, PluginHostStatus] = {}
+        self._calls: dict[tuple[str, str], int] = {}
+        self._lock = threading.RLock()
+
+    def start(
+        self,
+        plugin_id: str,
+        backend: PluginBackend,
+        package_path: Path,
+        declared_permissions: list[str],
+        on_unavailable: Callable[[str, str], None],
+    ) -> list[McpDiscoveredTool]:
+        if backend.transport != "stdio":
+            raise McpBridgeError(
+                "MCP_CAPABILITY_UNSUPPORTED",
+                "Phase C only supports the MCP stdio transport.",
+                status_code=501,
+            )
+        command = self._resolve_command(package_path, backend)
+        now = datetime.now(timezone.utc)
+        status = PluginHostStatus(
+            plugin_id=plugin_id,
+            backend_type="mcp",
+            transport="stdio",
+            status=PluginHostState.starting,
+            started_at=now,
+            last_seen_at=now,
+        )
+        host_ref: dict[str, _McpHost] = {}
+
+        def seen() -> None:
+            host = host_ref.get("host")
+            if host:
+                host.status.last_seen_at = datetime.now(timezone.utc)
+
+        def broken(message: str) -> None:
+            host = host_ref.get("host")
+            if host:
+                host.status.status = PluginHostState.unhealthy
+                host.status.error = message
+            on_unavailable(plugin_id, message)
+
+        def tools_changed() -> None:
+            broken("MCP tool list changed; restart the Plugin Host to revalidate tools.")
+
+        client = McpStdioClient(
+            command,
+            cwd=package_path,
+            on_seen=seen,
+            on_broken=broken,
+            on_tools_changed=tools_changed,
+        )
+        host = _McpHost(backend=backend, client=client, status=status)
+        host_ref["host"] = host
+        with self._lock:
+            if plugin_id in self._hosts:
+                raise McpBridgeError(
+                    "PLUGIN_HOST_START_FAILED",
+                    f"MCP host is already running: {plugin_id}",
+                    status_code=409,
+                )
+            self._hosts[plugin_id] = host
+            self._statuses[plugin_id] = status
+        try:
+            client.start()
+            initialize = client.request(
+                "initialize",
+                {
+                    "protocolVersion": MCP_PROTOCOL_VERSION,
+                    "capabilities": {},
+                    "clientInfo": {"name": "NotesAgent", "version": "0.1.0"},
+                },
+                timeout=backend.startup_timeout_seconds,
+                timeout_code="MCP_INITIALIZE_FAILED",
+                response_error_code="MCP_INITIALIZE_FAILED",
+            )
+            version = initialize.get("protocolVersion")
+            if version not in SUPPORTED_PROTOCOL_VERSIONS:
+                raise McpBridgeError(
+                    "MCP_INITIALIZE_FAILED",
+                    f"Unsupported MCP protocol version: {version}",
+                )
+            capabilities = initialize.get("capabilities")
+            if not isinstance(capabilities, dict) or not isinstance(
+                capabilities.get("tools"), dict
+            ):
+                raise McpBridgeError(
+                    "MCP_CAPABILITY_UNSUPPORTED",
+                    "MCP server does not declare the tools capability.",
+                )
+            server_info = initialize.get("serverInfo")
+            if not isinstance(server_info, dict):
+                server_info = {}
+            status.protocol_version = str(version)
+            status.server_name = _optional_string(server_info.get("name"))
+            status.server_version = _optional_string(server_info.get("version"))
+            client.notify("notifications/initialized")
+            discovered = self._discover_tools(
+                plugin_id, client, backend, declared_permissions
+            )
+            status.status = PluginHostState.ready
+            status.tools_count = len(discovered)
+            status.last_seen_at = datetime.now(timezone.utc)
+            status.error = None
+            return discovered
+        except McpBridgeError as exc:
+            status.status = PluginHostState.error
+            status.error = exc.message
+            client.stop()
+            with self._lock:
+                self._hosts.pop(plugin_id, None)
+            raise
+        except Exception as exc:
+            status.status = PluginHostState.error
+            status.error = f"MCP initialization failed: {type(exc).__name__}."
+            client.stop()
+            with self._lock:
+                self._hosts.pop(plugin_id, None)
+            raise McpBridgeError("MCP_INITIALIZE_FAILED", status.error) from exc
+
+    async def call_tool(
+        self,
+        plugin_id: str,
+        remote_name: str,
+        arguments: dict[str, Any],
+        *,
+        request_id: str,
+    ) -> Any:
+        host = self._host(plugin_id)
+        rpc_id, pending = host.client.begin_request(
+            "tools/call", {"name": remote_name, "arguments": arguments}
+        )
+        call_key = (plugin_id, request_id)
+        with self._lock:
+            self._calls[call_key] = rpc_id
+        try:
+            result = await asyncio.to_thread(
+                host.client.wait_response,
+                rpc_id,
+                pending,
+                timeout=host.backend.tool_timeout_seconds,
+                timeout_code="MCP_TOOL_CALL_FAILED",
+            )
+        except asyncio.CancelledError:
+            host.client.cancel(rpc_id)
+            host.client.abandon(rpc_id)
+            raise
+        except McpBridgeError as exc:
+            raise ToolExecutionError(exc.code, exc.message) from exc
+        finally:
+            with self._lock:
+                self._calls.pop(call_key, None)
+
+        encoded_size = len(
+            json.dumps(result, ensure_ascii=False, separators=(",", ":")).encode("utf-8")
+        )
+        if encoded_size > MAX_MCP_TOOL_RESULT_BYTES:
+            raise ToolExecutionError(
+                "MCP_TOOL_RESULT_TOO_LARGE",
+                "MCP tool result exceeds the configured size limit.",
+            )
+        if result.get("isError") is True:
+            raise ToolExecutionError(
+                "MCP_TOOL_CALL_FAILED", _mcp_error_message(result.get("content"))
+            )
+        structured = result.get("structuredContent")
+        if structured is not None:
+            if not isinstance(structured, dict):
+                raise ToolExecutionError(
+                    "MCP_TOOL_CALL_FAILED",
+                    "MCP structuredContent must be an object.",
+                )
+            return structured
+        content = result.get("content", [])
+        if not isinstance(content, list):
+            raise ToolExecutionError(
+                "MCP_TOOL_CALL_FAILED", "MCP tool content must be an array."
+            )
+        return {"content": content}
+
+    def cancel(self, plugin_id: str, request_id: str) -> None:
+        with self._lock:
+            rpc_id = self._calls.get((plugin_id, request_id))
+            host = self._hosts.get(plugin_id)
+        if rpc_id is not None and host is not None:
+            host.client.cancel(rpc_id)
+
+    def stop(self, plugin_id: str) -> None:
+        with self._lock:
+            host = self._hosts.pop(plugin_id, None)
+        if host:
+            host.client.stop()
+            host.status.status = PluginHostState.stopped
+            host.status.tools_count = 0
+            host.status.error = None
+
+    def status(self, plugin_id: str, backend: PluginBackend) -> PluginHostStatus:
+        with self._lock:
+            status = self._statuses.get(plugin_id)
+            if status:
+                return status.model_copy(deep=True)
+        return PluginHostStatus(
+            plugin_id=plugin_id,
+            backend_type=backend.type,
+            transport=backend.transport,
+            status=PluginHostState.stopped,
+        )
+
+    def _discover_tools(
+        self,
+        plugin_id: str,
+        client: McpStdioClient,
+        backend: PluginBackend,
+        declared_permissions: list[str],
+    ) -> list[McpDiscoveredTool]:
+        discovered: list[McpDiscoveredTool] = []
+        cursor: str | None = None
+        for _ in range(MAX_MCP_LIST_PAGES):
+            params = {"cursor": cursor} if cursor else {}
+            result = client.request(
+                "tools/list",
+                params,
+                timeout=backend.startup_timeout_seconds,
+                timeout_code="MCP_INITIALIZE_FAILED",
+                response_error_code="MCP_INITIALIZE_FAILED",
+            )
+            raw_tools = result.get("tools")
+            if not isinstance(raw_tools, list):
+                raise McpBridgeError(
+                    "MCP_TOOL_SCHEMA_INVALID", "MCP tools/list must return a tools array."
+                )
+            for raw in raw_tools:
+                discovered.append(
+                    self._map_tool(plugin_id, raw, declared_permissions)
+                )
+                if len(discovered) > MAX_MCP_TOOLS:
+                    raise McpBridgeError(
+                        "MCP_TOOL_SCHEMA_INVALID",
+                        f"MCP server exposes more than {MAX_MCP_TOOLS} tools.",
+                    )
+            next_cursor = result.get("nextCursor")
+            if next_cursor is None:
+                break
+            if not isinstance(next_cursor, str) or not next_cursor:
+                raise McpBridgeError(
+                    "MCP_TOOL_SCHEMA_INVALID", "MCP nextCursor must be a non-empty string."
+                )
+            cursor = next_cursor
+        else:
+            raise McpBridgeError(
+                "MCP_TOOL_SCHEMA_INVALID", "MCP tools/list exceeded the page limit."
+            )
+        names = [item.definition.name for item in discovered]
+        if len(names) != len(set(names)):
+            raise McpBridgeError(
+                "MCP_TOOL_SCHEMA_INVALID", "MCP server returned duplicate tool names."
+            )
+        return discovered
+
+    @staticmethod
+    def _map_tool(
+        plugin_id: str, raw: Any, declared_permissions: list[str]
+    ) -> McpDiscoveredTool:
+        if not isinstance(raw, dict):
+            raise McpBridgeError(
+                "MCP_TOOL_SCHEMA_INVALID", "MCP tool definition must be an object."
+            )
+        remote_name = raw.get("name")
+        if not isinstance(remote_name, str) or not remote_name:
+            raise McpBridgeError(
+                "MCP_TOOL_SCHEMA_INVALID", "MCP tool name must be a non-empty string."
+            )
+        if (
+            len(remote_name) > 128
+            or not remote_name[0].isalnum()
+            or not all(
+                character.islower()
+                or character.isdigit()
+                or character in "._-"
+                for character in remote_name
+            )
+        ):
+            raise McpBridgeError(
+                "MCP_TOOL_SCHEMA_INVALID",
+                f"MCP tool name is not a valid NotesAgent id: {remote_name}",
+            )
+        schema = raw.get("inputSchema", {"type": "object", "properties": {}})
+        if not isinstance(schema, dict) or schema.get("type", "object") != "object":
+            raise McpBridgeError(
+                "MCP_TOOL_SCHEMA_INVALID",
+                f"MCP tool inputSchema must be an object schema: {remote_name}",
+            )
+        try:
+            Draft202012Validator.check_schema(schema)
+        except SchemaError as exc:
+            raise McpBridgeError(
+                "MCP_TOOL_SCHEMA_INVALID",
+                f"Invalid MCP tool schema for {remote_name}: {exc.message}",
+            ) from exc
+        metadata = raw.get("_meta")
+        permission = (
+            metadata.get("notesagent/permission") if isinstance(metadata, dict) else None
+        )
+        if permission is not None and (
+            not isinstance(permission, str) or permission not in KNOWN_PERMISSIONS
+        ):
+            raise McpBridgeError(
+                "MCP_TOOL_SCHEMA_INVALID",
+                f"MCP tool declares an unknown permission: {remote_name}",
+            )
+        if permission and permission not in declared_permissions:
+            raise McpBridgeError(
+                "MCP_TOOL_SCHEMA_INVALID",
+                f"MCP tool permission is missing from Plugin manifest: {permission}",
+            )
+        description = raw.get("description")
+        return McpDiscoveredTool(
+            remote_name=remote_name,
+            definition=ToolDefinition(
+                name=f"{plugin_id}.{remote_name}",
+                description=description if isinstance(description, str) else remote_name,
+                parameters=schema,
+                permission=permission,
+                source="plugin",
+            ),
+        )
+
+    def _host(self, plugin_id: str) -> _McpHost:
+        with self._lock:
+            host = self._hosts.get(plugin_id)
+        if host is None or host.status.status != PluginHostState.ready:
+            raise ToolExecutionError(
+                "PLUGIN_HOST_UNAVAILABLE", f"MCP Plugin Host is not ready: {plugin_id}"
+            )
+        return host
+
+    @staticmethod
+    def _resolve_command(root: Path, backend: PluginBackend) -> list[str]:
+        if not backend.command or not backend.command.strip():
+            raise McpBridgeError(
+                "PLUGIN_HOST_START_FAILED", "MCP stdio backend requires a command."
+            )
+        command = backend.command.strip()
+        if Path(command).is_absolute() or "/" in command or "\\" in command:
+            executable = (
+                (root / command).resolve()
+                if not Path(command).is_absolute()
+                else Path(command).resolve()
+            )
+            try:
+                executable.relative_to(root)
+            except ValueError as exc:
+                raise McpBridgeError(
+                    "PLUGIN_HOST_START_FAILED",
+                    "MCP executable path must stay inside the Plugin package.",
+                ) from exc
+            command = str(executable)
+        return [command, *backend.args]
+
+
+def _mcp_error_message(content: Any) -> str:
+    if isinstance(content, list):
+        texts = [
+            item.get("text")
+            for item in content
+            if isinstance(item, dict)
+            and item.get("type") == "text"
+            and isinstance(item.get("text"), str)
+        ]
+        if texts:
+            return "\n".join(texts)[:4096]
+    return "MCP tool returned an error result."
+
+
+def _optional_string(value: Any) -> str | None:
+    return value if isinstance(value, str) else None
+
+
+def _subprocess_environment() -> dict[str, str]:
+    """只传递启动进程所需的系统变量，隔离 Provider Key、Vault 路径等宿主状态。"""
+
+    allowed = {
+        "PATH",
+        "PATHEXT",
+        "SYSTEMROOT",
+        "WINDIR",
+        "COMSPEC",
+        "TEMP",
+        "TMP",
+        "TMPDIR",
+        "LANG",
+        "LC_ALL",
+        "VIRTUAL_ENV",
+    }
+    environment = {
+        key: value for key, value in os.environ.items() if key.upper() in allowed
+    }
+    environment["PYTHONUNBUFFERED"] = "1"
+    environment["PYTHONIOENCODING"] = "utf-8"
+    return environment

backend/app/extensions/runtime.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import re
+import threading
 from dataclasses import dataclass
 from pathlib import Path
 from typing import Any, Literal
@@ -16,6 +17,7 @@ from app.contracts import (
     ModelCapability,
     Plugin,
     PluginManifest,
+    PluginHostStatus,
     PluginStatus,
     RetrievalConfig,
     Skill,
@@ -23,6 +25,7 @@ from app.contracts import (
     SkillStatus,
     ToolDefinition,
 )
+from app.extensions.mcp import McpBridge, McpBridgeError, McpDiscoveredTool
 
 _EXTENSION_ID = re.compile(r"^[a-z0-9][a-z0-9._-]*$")
 
@@ -242,18 +245,26 @@ class _PluginRecord:
     tools: list[DeclarativeToolSpec]
     package_path: Path
     registered_tools: list[str]
+    mcp_remote_names: dict[str, str]
 
 
 class PluginRuntime:
     """Plugin Manifest、生命周期及 Tool Contribution 注册。"""
 
-    def __init__(self, tools: ToolRegistry, host: DeclarativePluginHost | None = None) -> None:
+    def __init__(
+        self,
+        tools: ToolRegistry,
+        host: DeclarativePluginHost | None = None,
+        mcp_bridge: McpBridge | None = None,
+    ) -> None:
         self.registry = tools
         self.host = host or DeclarativePluginHost()
+        self.mcp = mcp_bridge or McpBridge()
         self._records: dict[str, _PluginRecord] = {}
+        self._lock = threading.RLock()
 
     def install(self, package_path: str | Path) -> Plugin:
-        # 当前只加载声明式清单，不导入或执行插件包中的任意 Python 代码。
+        # 安装阶段只读取清单；MCP 子进程必须在权限授予后的 enable 阶段启动。
         root = _package_dir(package_path)
         raw = _read_yaml(root / "plugin.yaml")
         if "id" in raw and "plugin_id" not in raw:
@@ -271,15 +282,17 @@ class PluginRuntime:
                 status_code=409,
             )
 
-        specs = self._load_tools(root)
+        _validate_backend(manifest)
-        declared = set(manifest.contributes.tools)
+        specs = [] if manifest.backend.type == "mcp" else self._load_tools(root)
-        actual = {spec.name for spec in specs}
+        if manifest.backend.type != "mcp":
-        if declared != actual:
+            declared = set(manifest.contributes.tools)
-            raise ExtensionError(
+            actual = {spec.name for spec in specs}
-                "PLUGIN_CONTRIBUTION_INVALID",
+            if declared != actual:
-                "plugin.yaml tool contributions must exactly match tools.yaml",
+                raise ExtensionError(
-                details={"declared": sorted(declared), "actual": sorted(actual)},
+                    "PLUGIN_CONTRIBUTION_INVALID",
-            )
+                    "plugin.yaml tool contributions must exactly match tools.yaml",
+                    details={"declared": sorted(declared), "actual": sorted(actual)},
+                )
         for spec in specs:
             _validate_id("tool", spec.name)
             _validate_tool_schema(spec)
@@ -302,6 +315,7 @@ class PluginRuntime:
             tools=specs,
             package_path=root,
             registered_tools=[],
+            mcp_remote_names={},
         )
         self._records[manifest.plugin_id] = record
         return record.plugin.model_copy(deep=True)
@@ -313,18 +327,14 @@ class PluginRuntime:
         return self._record(plugin_id).plugin.model_copy(deep=True)
 
     def enable(self, plugin_id: str) -> Plugin:
+        # Host 启动和 Tool 批量注册必须串行，避免并发 enable 产生重复进程或半注册状态。
+        with self._lock:
+            return self._enable(plugin_id)
+
+    def _enable(self, plugin_id: str) -> Plugin:
         record = self._record(plugin_id)
         if record.plugin.enabled:
             return record.plugin.model_copy(deep=True)
-        if record.plugin.manifest.backend.type == "mcp":
-            # TODO(extension): 第二阶段以隔离进程实现 MCP Host，并补充签名与来源校验。
-            record.plugin.status = PluginStatus.dependency_missing
-            raise ExtensionError(
-                "PLUGIN_HOST_UNAVAILABLE",
-                "MCP Plugin Host is reserved for the second development phase.",
-                status_code=501,
-                details={"plugin_id": plugin_id, "backend": "mcp"},
-            )
         missing_grants = sorted(
             set(record.plugin.manifest.permissions) - set(record.plugin.granted_permissions)
         )
@@ -336,7 +346,8 @@ class PluginRuntime:
                 status_code=409,
                 details={"plugin_id": plugin_id, "permissions": missing_grants},
             )
-        conflicts = [spec.name for spec in record.tools if self.registry.contains(spec.name)]
+        declared_tools = list(record.plugin.manifest.contributes.tools)
+        conflicts = [name for name in declared_tools if self.registry.contains(name)]
         if conflicts:
             raise ExtensionError(
                 "PLUGIN_TOOL_CONFLICT",
@@ -346,42 +357,75 @@ class PluginRuntime:
             )
         record.plugin.status = PluginStatus.starting
         try:
-            for spec in record.tools:
+            if record.plugin.manifest.backend.type == "mcp":
-                arguments_model = _arguments_model(spec)
+                discovered = self._start_mcp(record)
+                actual = {item.definition.name for item in discovered}
+                declared = set(declared_tools)
+                if actual != declared:
+                    raise ExtensionError(
+                        "PLUGIN_CONTRIBUTION_INVALID",
+                        "Discovered MCP tools must exactly match Plugin contributions.",
+                        details={"declared": sorted(declared), "actual": sorted(actual)},
+                    )
+                for item in discovered:
+                    self._register_mcp_tool(record, item)
+            else:
+                for spec in record.tools:
+                    arguments_model = _arguments_model(spec)
 
-                async def executor(
+                    async def executor(
-                    arguments: BaseModel,
+                        arguments: BaseModel,
-                    context: ToolExecutionContext,
+                        context: ToolExecutionContext,
-                    _handler: str = spec.handler,
+                        _handler: str = spec.handler,
-                ) -> Any:
+                    ) -> Any:
-                    return await self.host.execute(_handler, arguments, context)
+                        return await self.host.execute(_handler, arguments, context)
 
-                self.registry.register(
+                    self.registry.register(
-                    ToolDefinition(
+                        ToolDefinition(
-                        name=spec.name,
+                            name=spec.name,
-                        description=spec.description,
+                            description=spec.description,
-                        parameters=spec.parameters,
+                            parameters=spec.parameters,
-                        permission=spec.permission,
+                            permission=spec.permission,
-                        source="plugin",
+                            source="plugin",
-                    ),
+                        ),
-                    arguments_model,
+                        arguments_model,
-                    executor,
+                        executor,
-                )
+                    )
-                record.registered_tools.append(spec.name)
+                    record.registered_tools.append(spec.name)
         except Exception as exc:
             # 注册过程必须具备回滚语义，防止半启用插件污染全局工具表。
             for name in record.registered_tools:
                 self.registry.unregister(name)
             record.registered_tools.clear()
+            record.mcp_remote_names.clear()
+            self.mcp.stop(plugin_id)
             record.plugin.status = PluginStatus.error
-            record.plugin.error_message = str(exc)
+            record.plugin.error_message = _safe_extension_message(exc)
-            raise
+            if isinstance(exc, ExtensionError):
+                raise
+            if isinstance(exc, McpBridgeError):
+                raise ExtensionError(
+                    exc.code,
+                    exc.message,
+                    status_code=exc.status_code,
+                    details={"plugin_id": plugin_id},
+                ) from exc
+            raise ExtensionError(
+                "PLUGIN_HOST_START_FAILED",
+                record.plugin.error_message,
+                status_code=503,
+                details={"plugin_id": plugin_id},
+            ) from exc
         record.plugin.enabled = True
         record.plugin.status = PluginStatus.ready
         record.plugin.error_message = None
         return record.plugin.model_copy(deep=True)
 
     def set_permissions(self, plugin_id: str, permissions: list[str]) -> Plugin:
+        with self._lock:
+            return self._set_permissions(plugin_id, permissions)
+
+    def _set_permissions(self, plugin_id: str, permissions: list[str]) -> Plugin:
         record = self._record(plugin_id)
         requested = set(permissions)
         declared = set(record.plugin.manifest.permissions)
@@ -403,15 +447,112 @@ class PluginRuntime:
         return record.plugin.model_copy(deep=True)
 
     def disable(self, plugin_id: str) -> Plugin:
+        with self._lock:
+            return self._disable(plugin_id)
+
+    def _disable(self, plugin_id: str) -> Plugin:
         record = self._record(plugin_id)
         for name in record.registered_tools:
             self.registry.unregister(name)
         record.registered_tools.clear()
+        record.mcp_remote_names.clear()
+        if record.plugin.manifest.backend.type == "mcp":
+            self.mcp.stop(plugin_id)
         record.plugin.enabled = False
         record.plugin.status = PluginStatus.disabled
         return record.plugin.model_copy(deep=True)
 
+    def get_host_status(self, plugin_id: str) -> PluginHostStatus:
+        record = self._record(plugin_id)
+        return self.mcp.status(plugin_id, record.plugin.manifest.backend)
+
+    def restart_host(self, plugin_id: str) -> PluginHostStatus:
+        with self._lock:
+            return self._restart_host(plugin_id)
+
+    def _restart_host(self, plugin_id: str) -> PluginHostStatus:
+        record = self._record(plugin_id)
+        if record.plugin.manifest.backend.type != "mcp":
+            raise ExtensionError(
+                "PLUGIN_HOST_UNAVAILABLE",
+                "Plugin does not use an MCP Host.",
+                status_code=409,
+                details={"plugin_id": plugin_id},
+            )
+        for name in record.registered_tools:
+            self.registry.unregister(name)
+        record.registered_tools.clear()
+        record.mcp_remote_names.clear()
+        self.mcp.stop(plugin_id)
+        record.plugin.enabled = False
+        record.plugin.status = PluginStatus.installed
+        record.plugin.error_message = None
+        self.enable(plugin_id)
+        return self.get_host_status(plugin_id)
+
+    def shutdown(self) -> None:
+        """关闭所有隔离 Host；用于 FastAPI lifespan 和测试清理。"""
+
+        with self._lock:
+            for plugin_id, record in list(self._records.items()):
+                if record.plugin.manifest.backend.type == "mcp":
+                    self.mcp.stop(plugin_id)
+
+    def _start_mcp(self, record: _PluginRecord) -> list[McpDiscoveredTool]:
+        manifest = record.plugin.manifest
+        return self.mcp.start(
+            manifest.plugin_id,
+            manifest.backend,
+            record.package_path,
+            manifest.permissions,
+            self._handle_mcp_unavailable,
+        )
+
+    def _register_mcp_tool(
+        self, record: _PluginRecord, discovered: McpDiscoveredTool
+    ) -> None:
+        definition = discovered.definition
+        arguments_model = _arguments_model_from_schema(
+            definition.name, definition.parameters
+        )
+        plugin_id = record.plugin.manifest.plugin_id
+        remote_name = discovered.remote_name
+
+        async def executor(
+            arguments: BaseModel,
+            context: ToolExecutionContext,
+        ) -> Any:
+            return await self.mcp.call_tool(
+                plugin_id,
+                remote_name,
+                arguments.model_dump(),
+                request_id=context.tool_call_id or f"{context.run_id}:{definition.name}",
+            )
+
+        self.registry.register(definition, arguments_model, executor)
+        record.registered_tools.append(definition.name)
+        record.mcp_remote_names[definition.name] = remote_name
+
+    def _handle_mcp_unavailable(self, plugin_id: str, message: str) -> None:
+        with self._lock:
+            record = self._records.get(plugin_id)
+            if record is None:
+                return
+            for name in record.registered_tools:
+                self.registry.unregister(name)
+            record.registered_tools.clear()
+            record.mcp_remote_names.clear()
+            record.plugin.enabled = False
+            record.plugin.status = PluginStatus.error
+            record.plugin.error_message = message
+
     def uninstall(self, plugin_id: str, dependent_skills: list[str] | None = None) -> None:
+        with self._lock:
+            self._uninstall(plugin_id, dependent_skills)
+
+    def _uninstall(
+        self, plugin_id: str, dependent_skills: list[str] | None = None
+    ) -> None:
         record = self._record(plugin_id)
         if dependent_skills:
             raise ExtensionError(
@@ -422,6 +563,8 @@ class PluginRuntime:
             )
         if record.plugin.enabled:
             self.disable(plugin_id)
+        elif record.plugin.manifest.backend.type == "mcp":
+            self.mcp.stop(plugin_id)
         del self._records[plugin_id]
 
     def _record(self, plugin_id: str) -> _PluginRecord:
@@ -498,6 +641,12 @@ def _manifest_error(kind: str, exc: ValidationError) -> ExtensionError:
 
 def _arguments_model(spec: DeclarativeToolSpec) -> type[BaseModel]:
     schema = spec.parameters or {"type": "object", "properties": {}}
+    return _arguments_model_from_schema(spec.name, schema)
+
+
+def _arguments_model_from_schema(
+    tool_name: str, schema: dict[str, Any]
+) -> type[BaseModel]:
     if schema.get("type", "object") != "object":
         raise ExtensionError("PLUGIN_TOOL_SCHEMA_INVALID", "Tool parameters must be an object schema.")
     properties = schema.get("properties", {})
@@ -514,7 +663,7 @@ def _arguments_model(spec: DeclarativeToolSpec) -> type[BaseModel]:
     for name, field_schema in properties.items():
         annotation = types.get(field_schema.get("type"), Any)
         fields[name] = (annotation, ... if name in required else None)
-    model_name = "PluginArgs_" + re.sub(r"\W+", "_", spec.name)
+    model_name = "PluginArgs_" + re.sub(r"\W+", "_", tool_name)
     return create_model(model_name, __config__=ConfigDict(extra="forbid"), **fields)
 
 
@@ -536,3 +685,30 @@ def _validate_tool_schema(spec: DeclarativeToolSpec) -> None:
             "Tool parameters must be an object schema with object properties.",
             details={"tool": spec.name},
         )
+
+
+def _validate_backend(manifest: PluginManifest) -> None:
+    backend = manifest.backend
+    if backend.type == "mcp":
+        if backend.transport != "stdio":
+            raise ExtensionError(
+                "MCP_CAPABILITY_UNSUPPORTED",
+                "Phase C MCP Plugins must use stdio transport.",
+                status_code=501,
+            )
+        if not backend.command or not backend.command.strip():
+            raise ExtensionError(
+                "EXTENSION_MANIFEST_INVALID",
+                "MCP stdio backend requires a command.",
+            )
+    elif backend.command is not None or backend.args:
+        raise ExtensionError(
+            "EXTENSION_MANIFEST_INVALID",
+            "Only MCP stdio backends may declare command or args.",
+        )
+
+
+def _safe_extension_message(exc: Exception) -> str:
+    if isinstance(exc, (ExtensionError, McpBridgeError)):
+        return exc.message
+    return f"Plugin Host operation failed: {type(exc).__name__}."

backend/app/main.py

@@ -1,19 +1,31 @@
+from contextlib import asynccontextmanager
+
 from fastapi import FastAPI
 from fastapi.exceptions import RequestValidationError
 from fastapi.middleware.cors import CORSMiddleware
 from starlette.exceptions import HTTPException as StarletteHttpException
 
 from app.config import get_settings
+from app.container import container
 from app.errors import ApiError, api_error_handler, http_error_handler, validation_error_handler
 from app.routes import router as api_router
 from app.schemas import HealthResponse, ServiceStatusResponse
 
 settings = get_settings()
 
+
+@asynccontextmanager
+async def lifespan(_: FastAPI):
+    yield
+    # 第三方 MCP Server 必须跟随 AI Core 退出，不能遗留孤儿进程。
+    container.plugins.shutdown()
+
+
 app = FastAPI(
     title=settings.name,
     version=settings.version,
     description="AI 笔记软件的本地 AI Core 与 Agent Core 服务。",
+    lifespan=lifespan,
 )
 
 app.add_middleware(

backend/app/routes.py

@@ -1,3 +1,4 @@
+import asyncio
 from collections.abc import AsyncIterator
 from datetime import datetime, timezone
 from uuid import uuid4
@@ -32,6 +33,7 @@ from app.contracts import (
     PageMeta,
     PermissionDecisionRequest,
     Plugin,
+    PluginHostStatus,
     PluginListResponse,
     PluginPermissionGrantRequest,
     ProviderConfig,
@@ -130,6 +132,15 @@ def extension_call(operation):
         raise ApiError(exc.status_code, exc.code, exc.message, exc.details) from exc
 
 
+async def extension_call_async(operation):
+    """进程启动/关闭可能等待 stdio Host，移出 FastAPI 事件循环。"""
+
+    try:
+        return await asyncio.to_thread(operation)
+    except ExtensionError as exc:
+        raise ApiError(exc.status_code, exc.code, exc.message, exc.details) from exc
+
+
 # Workspace (single configured Vault in Web development mode)
 @router.get("/workspace", response_model=WorkspaceInfo, tags=["Workspace"])
 async def get_workspace() -> WorkspaceInfo:
@@ -483,7 +494,7 @@ async def install_plugin(request: ExtensionInstallRequest) -> Plugin:
     tags=["Plugins"],
 )
 async def enable_plugin(plugin_id: str) -> Plugin:
-    return extension_call(lambda: container.plugins.enable(plugin_id))
+    return await extension_call_async(lambda: container.plugins.enable(plugin_id))
 
 
 @router.post(
@@ -492,7 +503,7 @@ async def enable_plugin(plugin_id: str) -> Plugin:
     tags=["Plugins"],
 )
 async def disable_plugin(plugin_id: str) -> Plugin:
-    return extension_call(lambda: container.plugins.disable(plugin_id))
+    return await extension_call_async(lambda: container.plugins.disable(plugin_id))
 
 
 @router.put(
@@ -503,11 +514,37 @@ async def disable_plugin(plugin_id: str) -> Plugin:
 async def set_plugin_permissions(
     plugin_id: str, request: PluginPermissionGrantRequest
 ) -> Plugin:
-    return extension_call(
+    return await extension_call_async(
         lambda: container.plugins.set_permissions(plugin_id, request.permissions)
     )
 
 
+@router.get(
+    "/plugins/{plugin_id}/host",
+    response_model=PluginHostStatus,
+    tags=["Plugins"],
+)
+async def get_plugin_host_status(plugin_id: str) -> PluginHostStatus:
+    return extension_call(lambda: container.plugins.get_host_status(plugin_id))
+
+
+@router.post(
+    "/plugins/{plugin_id}/host/restart",
+    response_model=OperationResponse,
+    status_code=202,
+    tags=["Plugins"],
+)
+async def restart_plugin_host(plugin_id: str) -> OperationResponse:
+    status = await extension_call_async(
+        lambda: container.plugins.restart_host(plugin_id)
+    )
+    return OperationResponse(
+        status="accepted",
+        resource_id=plugin_id,
+        message=f"Plugin Host status: {status.status.value}",
+    )
+
+
 @router.delete(
     "/plugins/{plugin_id}",
     response_model=OperationResponse,
@@ -516,7 +553,9 @@ async def set_plugin_permissions(
 async def uninstall_plugin(plugin_id: str) -> OperationResponse:
     plugin = extension_call(lambda: container.plugins.get(plugin_id))
     dependent_skills = container.skills.depending_on_tools(plugin.manifest.contributes.tools)
-    extension_call(lambda: container.plugins.uninstall(plugin_id, dependent_skills))
+    await extension_call_async(
+        lambda: container.plugins.uninstall(plugin_id, dependent_skills)
+    )
     return OperationResponse(status="completed", resource_id=plugin_id, message="uninstalled")
 
 

backend/extensions/fixtures/mcp-echo/plugin.yaml

@@ -0,0 +1,21 @@
+id: mcp-fixture
+name: MCP Fixture
+version: 1.0.0
+description: 阶段 C 离线联调 Fixture，覆盖 MCP Tool 生命周期与错误边界。
+permissions:
+  - notes.read
+contributes:
+  tools:
+    - mcp-fixture.echo
+    - mcp-fixture.fail
+    - mcp-fixture.sleep
+    - mcp-fixture.large
+    - mcp-fixture.environment
+    - mcp-fixture.exit
+backend:
+  type: mcp
+  transport: stdio
+  command: python
+  args: [server.py]
+  startup_timeout_seconds: 5
+  tool_timeout_seconds: 1

backend/extensions/fixtures/mcp-echo/server.py

@@ -0,0 +1,183 @@
+"""确定性的 MCP stdio 测试 Server；仅使用标准库，不依赖产品代码。"""
+
+from __future__ import annotations
+
+import json
+import os
+import sys
+import threading
+import time
+from typing import Any
+
+WRITE_LOCK = threading.Lock()
+CANCELLED: dict[int, threading.Event] = {}
+MODE = sys.argv[1] if len(sys.argv) > 1 else "normal"
+
+
+def send(message: dict[str, Any]) -> None:
+    with WRITE_LOCK:
+        sys.stdout.write(json.dumps(message, ensure_ascii=False, separators=(",", ":")) + "\n")
+        sys.stdout.flush()
+
+
+def respond(request_id: int, result: dict[str, Any]) -> None:
+    send({"jsonrpc": "2.0", "id": request_id, "result": result})
+
+
+def tool(name: str, description: str, properties: dict[str, Any] | None = None) -> dict[str, Any]:
+    return {
+        "name": name,
+        "description": description,
+        "inputSchema": {
+            "type": "object",
+            "properties": properties or {},
+            "required": list(properties or {}),
+            "additionalProperties": False,
+        },
+    }
+
+
+TOOLS = {
+    "echo": {
+        **tool("echo", "Return the provided text.", {"text": {"type": "string"}}),
+        "_meta": {"notesagent/permission": "notes.read"},
+    },
+    "fail": tool("fail", "Return an MCP business error."),
+    "sleep": tool("sleep", "Wait until completed or cancelled.", {"seconds": {"type": "number"}}),
+    "large": tool("large", "Return a result larger than the host limit."),
+    "environment": tool("environment", "Report whether host secrets leaked into the process."),
+    "exit": tool("exit", "Terminate the fixture process."),
+}
+
+
+def call_tool(request_id: int, params: dict[str, Any]) -> None:
+    name = params.get("name")
+    arguments = params.get("arguments") or {}
+    if name == "echo":
+        text = str(arguments.get("text", ""))
+        respond(
+            request_id,
+            {
+                "content": [{"type": "text", "text": text}],
+                "structuredContent": {"echo": text},
+                "isError": False,
+            },
+        )
+        return
+    if name == "fail":
+        respond(
+            request_id,
+            {
+                "content": [{"type": "text", "text": "fixture failure"}],
+                "isError": True,
+            },
+        )
+        return
+    if name == "large":
+        respond(
+            request_id,
+            {
+                "content": [{"type": "text", "text": "x" * 300_000}],
+                "isError": False,
+            },
+        )
+        return
+    if name == "environment":
+        respond(
+            request_id,
+            {
+                "content": [{"type": "text", "text": "environment checked"}],
+                "structuredContent": {
+                    "has_openai_key": "OPENAI_API_KEY" in os.environ,
+                    "has_app_db_path": "APP_DB_PATH" in os.environ,
+                },
+                "isError": False,
+            },
+        )
+        return
+    if name == "exit":
+        os._exit(17)
+    if name == "sleep":
+        cancelled = CANCELLED.setdefault(request_id, threading.Event())
+        seconds = max(0.0, min(float(arguments.get("seconds", 0)), 30.0))
+        if cancelled.wait(seconds):
+            respond(
+                request_id,
+                {
+                    "content": [{"type": "text", "text": "cancelled"}],
+                    "isError": True,
+                },
+            )
+        else:
+            respond(
+                request_id,
+                {
+                    "content": [{"type": "text", "text": "completed"}],
+                    "structuredContent": {"slept": seconds},
+                    "isError": False,
+                },
+            )
+        CANCELLED.pop(request_id, None)
+        return
+    send(
+        {
+            "jsonrpc": "2.0",
+            "id": request_id,
+            "error": {"code": -32602, "message": f"Unknown tool: {name}"},
+        }
+    )
+
+
+def main() -> None:
+    for line in sys.stdin:
+        message = json.loads(line)
+        method = message.get("method")
+        request_id = message.get("id")
+        params = message.get("params") or {}
+        if method == "initialize" and isinstance(request_id, int):
+            respond(
+                request_id,
+                {
+                    "protocolVersion": params.get("protocolVersion"),
+                    "capabilities": (
+                        {} if MODE == "no-tools" else {"tools": {"listChanged": False}}
+                    ),
+                    "serverInfo": {"name": "notesagent-mcp-fixture", "version": "1.0.0"},
+                },
+            )
+        elif method == "tools/list" and isinstance(request_id, int):
+            if MODE == "invalid-schema":
+                respond(
+                    request_id,
+                    {
+                        "tools": [
+                            {
+                                "name": "broken",
+                                "description": "invalid schema",
+                                "inputSchema": {"type": "string"},
+                            }
+                        ]
+                    },
+                )
+            elif params.get("cursor") == "page-2":
+                respond(
+                    request_id,
+                    {"tools": [TOOLS["large"], TOOLS["environment"], TOOLS["exit"]]},
+                )
+            else:
+                respond(
+                    request_id,
+                    {"tools": [TOOLS["echo"], TOOLS["fail"], TOOLS["sleep"]], "nextCursor": "page-2"},
+                )
+        elif method == "tools/call" and isinstance(request_id, int):
+            threading.Thread(target=call_tool, args=(request_id, params), daemon=True).start()
+        elif method == "notifications/cancelled":
+            cancelled_id = params.get("requestId")
+            if isinstance(cancelled_id, int):
+                CANCELLED.setdefault(cancelled_id, threading.Event()).set()
+        elif method == "ping" and isinstance(request_id, int):
+            respond(request_id, {})
+
+
+if __name__ == "__main__":
+    main()

backend/tests/test_api.py

@@ -93,6 +93,8 @@ def test_openapi_contains_documented_frontend_interfaces() -> None:
         "/api/skills",
         "/api/plugins",
         "/api/plugins/install",
+        "/api/plugins/{plugin_id}/host",
+        "/api/plugins/{plugin_id}/host/restart",
         "/api/plugins/{plugin_id}/enable",
         "/api/plugins/{plugin_id}/disable",
         "/api/providers/test",

backend/tests/test_extension_core.py

@@ -1,4 +1,6 @@
 import asyncio
+import shutil
+import time
 
 import pytest
 
@@ -13,13 +15,28 @@ from app.contracts import (
 )
 from app.extensions import ExtensionError
 from app.services import note_service
-from app.config import get_settings
+from app.config import BACKEND_DIR, get_settings
+
+
+MCP_FIXTURE = BACKEND_DIR / "extensions" / "fixtures" / "mcp-echo"
 
 
 def run(coroutine):
     return asyncio.run(coroutine)
 
 
+@pytest.fixture
+def mcp_container():
+    container = build_container()
+    installed = container.plugins.install(MCP_FIXTURE)
+    assert installed.status == "permission_required"
+    container.plugins.set_permissions("mcp-fixture", ["notes.read"])
+    try:
+        yield container
+    finally:
+        container.plugins.shutdown()
+
+
 def test_bundled_plugin_registers_tool_and_skill_is_ready() -> None:
     async def scenario() -> None:
         container = build_container()
@@ -297,3 +314,196 @@ def test_attachment_and_transcription_tools_use_host_storage() -> None:
         assert transcription.output["text"] == "会议转写内容"
 
     run(scenario())
+
+
+def test_mcp_stdio_host_discovers_namespaced_tools_and_maps_results(
+    mcp_container, monkeypatch
+) -> None:
+    async def scenario() -> None:
+        monkeypatch.setenv("OPENAI_API_KEY", "must-not-enter-plugin-host")
+        enabled = mcp_container.plugins.enable("mcp-fixture")
+        status = mcp_container.plugins.get_host_status("mcp-fixture")
+        definition = mcp_container.tools.get("mcp-fixture.echo").definition
+        result = await mcp_container.tools.execute(
+            ToolCall(
+                tool_call_id="call_mcp_echo",
+                name="mcp-fixture.echo",
+                arguments={"text": "hello mcp"},
+            ),
+            ToolExecutionContext(
+                run_id="run_mcp_fixture", tool_call_id="call_mcp_echo"
+            ),
+        )
+
+        assert enabled.status == "ready" and enabled.enabled is True
+        assert status.status == "ready"
+        environment = await mcp_container.tools.execute(
+            ToolCall(
+                tool_call_id="call_mcp_environment",
+                name="mcp-fixture.environment",
+                arguments={},
+            ),
+            ToolExecutionContext(run_id="run_mcp_fixture"),
+        )
+
+        assert status.tools_count == 6
+        assert status.protocol_version == "2025-11-25"
+        assert status.server_name == "notesagent-mcp-fixture"
+        assert definition.permission == "notes.read"
+        assert result.success is True
+        assert result.output == {"echo": "hello mcp"}
+        assert environment.success is True
+        assert environment.output == {
+            "has_openai_key": False,
+            "has_app_db_path": False,
+        }
+
+        disabled = mcp_container.plugins.disable("mcp-fixture")
+        assert disabled.status == "disabled"
+        assert mcp_container.plugins.get_host_status("mcp-fixture").status == "stopped"
+        assert not mcp_container.tools.contains("mcp-fixture.echo")
+
+    run(scenario())
+
+
+def test_agent_calls_mcp_tool_through_registry_and_writes_trace(mcp_container) -> None:
+    async def scenario() -> None:
+        mcp_container.plugins.enable("mcp-fixture")
+        created = await mcp_container.agent.create_run(
+            AgentRunCreateRequest(
+                input='/tool mcp-fixture.echo {"text":"agent mcp"}',
+                provider_id="mock",
+                model="mock-1",
+                allowed_tools=["mcp-fixture.echo"],
+            )
+        )
+        completed = await mcp_container.agent.wait(created.run_id)
+        trace = mcp_container.agent.get_trace(
+            created.run_id, after_sequence=-1, limit=100
+        )
+
+        assert completed.status == AgentRunStatus.completed
+        assert completed.tool_results[0].success is True
+        assert completed.tool_results[0].output == {"echo": "agent mcp"}
+        assert any(
+            item.event == "ToolCall" and item.data.get("name") == "mcp-fixture.echo"
+            for item in trace.items
+        )
+
+    run(scenario())
+
+
+def test_mcp_business_error_size_limit_and_timeout_are_structured(mcp_container) -> None:
+    async def scenario() -> None:
+        mcp_container.plugins.enable("mcp-fixture")
+        context = ToolExecutionContext(run_id="run_mcp_errors")
+
+        failed = await mcp_container.tools.execute(
+            ToolCall(tool_call_id="call_fail", name="mcp-fixture.fail", arguments={}),
+            context,
+        )
+        oversized = await mcp_container.tools.execute(
+            ToolCall(tool_call_id="call_large", name="mcp-fixture.large", arguments={}),
+            context,
+        )
+        timed_out = await mcp_container.tools.execute(
+            ToolCall(
+                tool_call_id="call_sleep",
+                name="mcp-fixture.sleep",
+                arguments={"seconds": 5},
+            ),
+            ToolExecutionContext(
+                run_id="run_mcp_errors", tool_call_id="call_sleep"
+            ),
+        )
+        recovered = await mcp_container.tools.execute(
+            ToolCall(
+                tool_call_id="call_after_timeout",
+                name="mcp-fixture.echo",
+                arguments={"text": "still ready"},
+            ),
+            context,
+        )
+
+        assert failed.success is False
+        assert failed.error_code == "MCP_TOOL_CALL_FAILED"
+        assert failed.error_message == "fixture failure"
+        assert oversized.success is False
+        assert oversized.error_code == "MCP_TOOL_RESULT_TOO_LARGE"
+        assert timed_out.success is False
+        assert timed_out.error_code == "MCP_TOOL_CALL_FAILED"
+        assert recovered.success is True
+        assert mcp_container.plugins.get_host_status("mcp-fixture").status == "ready"
+
+    run(scenario())
+
+
+def test_mcp_abnormal_exit_unregisters_tools_and_restart_recovers(mcp_container) -> None:
+    async def scenario() -> None:
+        mcp_container.plugins.enable("mcp-fixture")
+        crashed = await mcp_container.tools.execute(
+            ToolCall(tool_call_id="call_exit", name="mcp-fixture.exit", arguments={}),
+            ToolExecutionContext(run_id="run_mcp_exit", tool_call_id="call_exit"),
+        )
+
+        deadline = time.monotonic() + 2
+        while mcp_container.tools.contains("mcp-fixture.echo") and time.monotonic() < deadline:
+            await asyncio.sleep(0.02)
+
+        plugin = mcp_container.plugins.get("mcp-fixture")
+        status = mcp_container.plugins.get_host_status("mcp-fixture")
+        assert crashed.success is False
+        assert crashed.error_code == "PLUGIN_HOST_UNAVAILABLE"
+        assert plugin.status == "error" and plugin.enabled is False
+        assert status.status == "unhealthy"
+        assert not mcp_container.tools.contains("mcp-fixture.echo")
+
+        restarted = mcp_container.plugins.restart_host("mcp-fixture")
+        assert restarted.status == "ready"
+        assert restarted.tools_count == 6
+        assert mcp_container.tools.contains("mcp-fixture.echo")
+
+    run(scenario())
+
+
+@pytest.mark.parametrize(
+    ("mode", "contributions", "expected_code"),
+    [
+        ("no-tools", "[]", "MCP_CAPABILITY_UNSUPPORTED"),
+        ("invalid-schema", "[mcp-invalid.broken]", "MCP_TOOL_SCHEMA_INVALID"),
+    ],
+)
+def test_mcp_rejects_missing_capability_and_invalid_discovery(
+    tmp_path, mode, contributions, expected_code
+) -> None:
+    package = tmp_path / f"mcp-{mode}"
+    package.mkdir()
+    shutil.copyfile(MCP_FIXTURE / "server.py", package / "server.py")
+    (package / "plugin.yaml").write_text(
+        f"""
+id: mcp-invalid
+name: Invalid MCP Fixture
+version: 1.0.0
+contributes:
+  tools: {contributions}
+backend:
+  type: mcp
+  transport: stdio
+  command: python
+  args: [server.py, {mode}]
+  startup_timeout_seconds: 5
+  tool_timeout_seconds: 1
+""".strip(),
+        encoding="utf-8",
+    )
+    container = build_container()
+    container.plugins.install(package)
+    try:
+        with pytest.raises(ExtensionError) as exc:
+            container.plugins.enable("mcp-invalid")
+        assert exc.value.code == expected_code
+        assert container.plugins.get("mcp-invalid").status == "error"
+        assert container.plugins.get_host_status("mcp-invalid").status == "error"
+        assert not container.tools.contains("mcp-invalid.broken")
+    finally:
+        container.plugins.shutdown()

frontend/src/contracts/index.ts

@@ -255,6 +255,22 @@ export type PluginStatus =
   | 'dependency_missing'
   | 'permission_required'
 
+export type PluginHostState = 'stopped' | 'starting' | 'ready' | 'unhealthy' | 'error'
+
+export interface PluginHostStatus {
+  plugin_id: string
+  backend_type: 'mcp' | 'internal_rpc' | 'none'
+  transport: 'stdio' | 'http' | 'none'
+  status: PluginHostState
+  tools_count: number
+  started_at?: string | null
+  last_seen_at?: string | null
+  protocol_version?: string | null
+  server_name?: string | null
+  server_version?: string | null
+  error?: string | null
+}
+
 export interface PluginContribution {
   type: 'tool' | 'command' | 'importer' | 'exporter' | 'sidebar_panel' | 'settings_section'
   id: string
@@ -539,7 +555,14 @@ export interface ApiPlugin {
       panels: string[]
       settings_sections: string[]
     }
-    backend: { type: 'mcp' | 'internal_rpc' | 'none'; transport: 'stdio' | 'http' | 'none' }
+    backend: {
+      type: 'mcp' | 'internal_rpc' | 'none'
+      transport: 'stdio' | 'http' | 'none'
+      command?: string | null
+      args?: string[]
+      startup_timeout_seconds?: number
+      tool_timeout_seconds?: number
+    }
   }
   status: PluginStatus
   enabled: boolean

frontend/src/services/pluginService.ts

@@ -1,5 +1,5 @@
 import apiClient from './apiClient'
-import type { ApiPlugin, OperationResponse, Plugin, PluginContribution } from '@/contracts'
+import type { ApiPlugin, OperationResponse, Plugin, PluginContribution, PluginHostStatus } from '@/contracts'
 
 function toPlugin(plugin: ApiPlugin): Plugin {
   const { manifest } = plugin
@@ -54,6 +54,14 @@ export async function grantPluginPermissions(pluginId: string, permissions: stri
   return toPlugin(await apiClient.put<ApiPlugin>(`/api/plugins/${pluginId}/permissions`, { permissions }))
 }
 
+export async function getPluginHostStatus(pluginId: string): Promise<PluginHostStatus> {
+  return apiClient.get(`/api/plugins/${pluginId}/host`)
+}
+
+export async function restartPluginHost(pluginId: string): Promise<OperationResponse> {
+  return apiClient.post(`/api/plugins/${pluginId}/host/restart`)
+}
+
 export async function uninstallPlugin(pluginId: string): Promise<OperationResponse> {
   return apiClient.delete(`/api/plugins/${pluginId}`)
 }