""" MCP (Model Context Protocol) client module. Implements JSON-RPC 2.0 over stdio, SSE and Streamable HTTP transports without any external MCP SDK dependency. """ import json import os import queue import subprocess import threading import urllib.request import urllib.error from typing import Optional from common.log import logger # Aliases accepted for the Streamable HTTP transport type _STREAMABLE_HTTP_ALIASES = {"streamable-http", "streamable_http", "streamablehttp", "http"} class McpClient: """Single MCP Server client supporting stdio, SSE and Streamable HTTP transports.""" def __init__(self, config: dict): """ config examples: stdio: {"name": "filesystem", "type": "stdio", "command": "npx", "args": [...]} SSE: {"name": "my-api", "type": "sse", "url": "http://localhost:8000/sse"} streamable-http: {"name": "pubmed", "type": "streamable-http", "url": "https://x/mcp"} """ self.config = config self.name: str = config.get("name", "unknown") raw_transport: str = config.get("type", "stdio") # Per-server timeout for tool calls (default 120s, suitable for data queries) self._timeout: int = int(config.get("timeout", 120)) # Normalize streamable-http aliases to a single internal key self.transport: str = ( "streamable-http" if raw_transport.lower() in _STREAMABLE_HTTP_ALIASES else raw_transport ) # stdio state self._proc: Optional[subprocess.Popen] = None self._read_queue: queue.Queue = queue.Queue() # SSE state self._sse_url: Optional[str] = None self._post_url: Optional[str] = None # endpoint for sending messages (resolved from SSE) # Streamable HTTP state self._http_url: Optional[str] = None self._http_headers: dict = {} # extra headers from user config (e.g. Authorization) self._http_session_id: Optional[str] = None # Mcp-Session-Id assigned by the server # Shared state self._next_id = 1 self._id_lock = threading.Lock() # _call_lock serializes all requests on the single stdio pipe. # SSE and streamable-http use independent HTTP requests, so they # do not acquire this lock (see _send_request). self._call_lock = threading.Lock() # _http_lock protects _http_session_id initialization across # concurrent streamable-http requests. self._http_lock = threading.Lock() self._initialized = False # ------------------------------------------------------------------ # Public interface # ------------------------------------------------------------------ def initialize(self) -> bool: """Connect and perform the MCP handshake. Returns True on success.""" try: if self.transport == "stdio": return self._init_stdio() elif self.transport == "sse": return self._init_sse() elif self.transport == "streamable-http": return self._init_streamable_http() else: logger.warning(f"[MCP:{self.name}] Unknown transport type: {self.transport!r}") return False except Exception as e: logger.warning(f"[MCP:{self.name}] Initialization failed: {e}") return False def list_tools(self) -> list: """Return the tool list from this server. Each item is a dict: {"name": str, "description": str, "inputSchema": dict} """ try: resp = self._send_request("tools/list", {}) tools = resp.get("result", {}).get("tools", []) return [ { "name": t.get("name", ""), "description": t.get("description", ""), "inputSchema": t.get("inputSchema", {}), } for t in tools ] except Exception as e: logger.warning(f"[MCP:{self.name}] list_tools failed: {e}") return [] def call_tool(self, name: str, arguments: dict) -> str: """Call a tool and return the result as a string.""" try: resp = self._send_request("tools/call", {"name": name, "arguments": arguments}) content = resp.get("result", {}).get("content", []) parts = [item.get("text", "") for item in content if item.get("type") == "text"] return "\n".join(parts) except Exception as e: logger.warning(f"[MCP:{self.name}] call_tool({name}) failed: {e}") return f"Error: {e}" def shutdown(self): """Close the connection / terminate the child process.""" if self._proc is not None: try: self._proc.stdin.close() except Exception: pass try: self._proc.terminate() self._proc.wait(timeout=5) except Exception: try: self._proc.kill() except Exception: pass self._proc = None logger.debug(f"[MCP:{self.name}] stdio process terminated") # Best-effort streamable-http session termination if self.transport == "streamable-http" and self._http_session_id and self._http_url: try: req = urllib.request.Request( self._http_url, method="DELETE", headers={"Mcp-Session-Id": self._http_session_id, **self._http_headers}, ) with urllib.request.urlopen(req, timeout=5): pass except Exception: pass self._http_session_id = None self._initialized = False # ------------------------------------------------------------------ # stdio transport # ------------------------------------------------------------------ def _init_stdio(self) -> bool: command = self.config.get("command") if not command: logger.warning(f"[MCP:{self.name}] stdio config missing 'command'") return False args = self.config.get("args", []) extra_env = self.config.get("env", None) env = {**os.environ, **extra_env} if extra_env else None self._proc = subprocess.Popen( [command] + list(args), stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.PIPE, text=True, encoding="utf-8", env=env, ) logger.debug(f"[MCP:{self.name}] stdio process started (pid={self._proc.pid})") threading.Thread( target=self._drain_stderr, daemon=True, name=f"mcp-stderr-{self.name}" ).start() threading.Thread( target=self._drain_stdout, daemon=True, name=f"mcp-stdout-{self.name}" ).start() return self._handshake() def _drain_stderr(self): for line in self._proc.stderr: line = line.strip() if line: logger.warning(f"[MCP:{self.name}] stderr: {line}") def _drain_stdout(self): """Background thread: read lines from stdout and put them into the queue.""" try: for line in self._proc.stdout: self._read_queue.put(line) except Exception: pass finally: try: self._read_queue.put("") except Exception: pass def _readline_with_timeout(self, timeout: Optional[int] = None) -> str: """Read one line from stdio stdout with a hard timeout (cross-platform). Uses the per-server timeout from mcp.json config when no explicit timeout is provided. """ effective = timeout if timeout is not None else self._timeout try: line = self._read_queue.get(timeout=effective) except queue.Empty: raise TimeoutError(f"[MCP:{self.name}] stdio read timed out after {effective}s") if not line: raise IOError(f"[MCP:{self.name}] stdio process closed unexpectedly") return line def _stdio_send(self, message: dict) -> dict: """Send a JSON-RPC message over stdio and read the response.""" raw = json.dumps(message) + "\n" self._proc.stdin.write(raw) self._proc.stdin.flush() expected_id = message.get("id") while True: line = self._readline_with_timeout() if not line: raise IOError(f"[MCP:{self.name}] stdio process closed unexpectedly") line = line.strip() if not line: continue try: data = json.loads(line) except json.JSONDecodeError: continue if "id" not in data: logger.debug(f"[MCP:{self.name}] notification skipped: {data.get('method', '?')}") continue # Verify response id matches request id to avoid consuming a stale # response left over from a previously failed/timed-out request. if data.get("id") != expected_id: logger.warning( f"[MCP:{self.name}] Stale response id={data.get('id')} " f"(expected {expected_id}), skipping" ) continue return data # ------------------------------------------------------------------ # SSE transport # ------------------------------------------------------------------ def _init_sse(self) -> bool: url = self.config.get("url") if not url: logger.warning(f"[MCP:{self.name}] SSE config missing 'url'") return False self._sse_url = url # Read the first SSE event to discover the POST endpoint try: self._post_url = self._sse_discover_endpoint() except Exception as e: logger.warning(f"[MCP:{self.name}] SSE endpoint discovery failed: {e}") return False return self._handshake() def _sse_discover_endpoint(self) -> str: """Open SSE stream and read the 'endpoint' event to learn the POST URL.""" req = urllib.request.Request( self._sse_url, headers={"Accept": "text/event-stream"}, ) with urllib.request.urlopen(req, timeout=10) as resp: for raw_line in resp: line = raw_line.decode("utf-8").rstrip("\n\r") if line.startswith("data:"): data = line[len("data:"):].strip() # Some servers send JSON with a "uri" or plain path if data.startswith("{"): parsed = json.loads(data) return parsed.get("uri") or parsed.get("url") or parsed.get("endpoint") # Plain relative or absolute URL if data.startswith("http"): return data # Relative path: resolve against SSE base from urllib.parse import urljoin return urljoin(self._sse_url, data) raise ValueError(f"[MCP:{self.name}] No endpoint event received from SSE stream") def _sse_send(self, message: dict) -> dict: """POST a JSON-RPC message to the server and return the response.""" body = json.dumps(message).encode("utf-8") req = urllib.request.Request( self._post_url, data=body, method="POST", headers={"Content-Type": "application/json"}, ) with urllib.request.urlopen(req, timeout=30) as resp: raw = resp.read().decode("utf-8") return json.loads(raw) # ------------------------------------------------------------------ # Streamable HTTP transport (MCP spec 2025-03-26) # ------------------------------------------------------------------ def _init_streamable_http(self) -> bool: url = self.config.get("url") if not url: logger.warning(f"[MCP:{self.name}] streamable-http config missing 'url'") return False self._http_url = url # Allow user-provided headers (e.g. {"Authorization": "Bearer xxx"}) extra_headers = self.config.get("headers") or {} if isinstance(extra_headers, dict): self._http_headers = {str(k): str(v) for k, v in extra_headers.items()} return self._handshake() def _streamable_http_send(self, message: dict) -> dict: """POST a JSON-RPC request and return the response (JSON or SSE-wrapped).""" return self._streamable_http_post(message, expect_response=True) def _streamable_http_post(self, message: dict, expect_response: bool) -> dict: """ POST a JSON-RPC message over Streamable HTTP. Per the spec, the response Content-Type can be either: - application/json -> single JSON-RPC response in body - text/event-stream -> SSE stream; we read until we get a matching response """ body = json.dumps(message).encode("utf-8") headers = { "Content-Type": "application/json", "Accept": "application/json, text/event-stream", } # Read session id under lock to avoid racing with the # initialization write below during concurrent requests. with self._http_lock: sid = self._http_session_id if sid: headers["Mcp-Session-Id"] = sid headers.update(self._http_headers) req = urllib.request.Request( self._http_url, data=body, method="POST", headers=headers, ) try: resp = urllib.request.urlopen(req, timeout=30) except urllib.error.HTTPError as e: # Surface the server-provided error body for easier debugging detail = "" try: detail = e.read().decode("utf-8", errors="ignore") except Exception: pass raise IOError( f"[MCP:{self.name}] streamable-http HTTP {e.code}: {detail[:200]}" ) with resp: # Capture session id assigned by the server (if any) session_id = resp.headers.get("Mcp-Session-Id") # Double-checked lock: only the first response sets the # session id, preventing concurrent initializers from # overwriting each other. if session_id and not self._http_session_id: with self._http_lock: if not self._http_session_id: self._http_session_id = session_id status = resp.status if hasattr(resp, "status") else resp.getcode() # Notifications: server may reply with 202 Accepted and no body if not expect_response or status == 202: try: resp.read() except Exception: pass return {} content_type = (resp.headers.get("Content-Type") or "").lower() expected_id = message.get("id") if "text/event-stream" in content_type: return self._read_sse_response(resp, expected_id) raw = resp.read().decode("utf-8") if not raw: return {} return json.loads(raw) def _read_sse_response(self, resp, expected_id) -> dict: """Read an SSE stream and return the first JSON-RPC response with matching id.""" data_buf: list = [] for raw_line in resp: line = raw_line.decode("utf-8").rstrip("\n\r") if line == "": # End of an SSE event, attempt to parse accumulated data if data_buf: payload = "\n".join(data_buf) data_buf = [] try: msg = json.loads(payload) except json.JSONDecodeError: continue # Skip notifications / mismatched ids if "id" not in msg: continue if expected_id is None or msg.get("id") == expected_id: return msg continue if line.startswith(":"): continue # SSE comment / keepalive if line.startswith("data:"): data_buf.append(line[len("data:"):].lstrip()) # Ignore 'event:' / 'id:' lines; we only care about JSON-RPC payloads raise IOError(f"[MCP:{self.name}] streamable-http SSE stream closed before response") # ------------------------------------------------------------------ # Common JSON-RPC helpers # ------------------------------------------------------------------ def _next_request_id(self) -> int: with self._id_lock: rid = self._next_id self._next_id += 1 return rid def _build_request(self, method: str, params: dict) -> dict: return { "jsonrpc": "2.0", "id": self._next_request_id(), "method": method, "params": params, } def _build_notification(self, method: str, params: dict) -> dict: return {"jsonrpc": "2.0", "method": method, "params": params} def _send_request(self, method: str, params: dict) -> dict: """Send a request and return the full response dict.""" if not self._initialized and method != "initialize": raise RuntimeError(f"[MCP:{self.name}] Client not initialized") message = self._build_request(method, params) # stdio transport uses a single pipe and must be serialized. # SSE and streamable-http use independent HTTP requests and # can safely run concurrently across sessions. if self.transport == "stdio": with self._call_lock: return self._stdio_send(message) elif self.transport == "sse": return self._sse_send(message) elif self.transport == "streamable-http": return self._streamable_http_send(message) else: raise ValueError(f"[MCP:{self.name}] Unsupported transport: {self.transport}") def _send_notification(self, method: str, params: dict): """Fire-and-forget notification (no response expected).""" notification = self._build_notification(method, params) raw = json.dumps(notification) + "\n" if self.transport == "stdio": self._proc.stdin.write(raw) self._proc.stdin.flush() elif self.transport == "sse": body = raw.encode("utf-8") req = urllib.request.Request( self._post_url, data=body, method="POST", headers={"Content-Type": "application/json"}, ) try: with urllib.request.urlopen(req, timeout=10): pass except Exception: pass # notifications are fire-and-forget elif self.transport == "streamable-http": try: self._streamable_http_post(notification, expect_response=False) except Exception: pass # notifications are fire-and-forget def _handshake(self) -> bool: """Perform the MCP initialize / notifications/initialized handshake.""" init_params = { "protocolVersion": "2024-11-05", "capabilities": {}, "clientInfo": {"name": "CowAgent", "version": "1.0"}, } # Temporarily mark as initialized so _send_request doesn't block self._initialized = True try: resp = self._send_request("initialize", init_params) except Exception as e: self._initialized = False logger.warning(f"[MCP:{self.name}] Handshake initialize failed: {e}") return False if "error" in resp: self._initialized = False logger.warning(f"[MCP:{self.name}] Handshake error: {resp['error']}") return False self._send_notification("notifications/initialized", {}) logger.debug(f"[MCP:{self.name}] Handshake complete") return True class McpClientRegistry: """Global singleton managing the lifecycle of all MCP Server clients.""" _instance = None _instance_lock = threading.Lock() def __new__(cls): with cls._instance_lock: if cls._instance is None: obj = super().__new__(cls) obj._clients: dict[str, McpClient] = {} obj._registry_lock = threading.Lock() cls._instance = obj return cls._instance def start_all(self, configs: list) -> None: """Initialize McpClient for each config entry; skip failures with a warning.""" if not configs: return for cfg in configs: name = cfg.get("name", "") client = McpClient(cfg) ok = client.initialize() if ok: with self._registry_lock: self._clients[name] = client logger.info(f"[MCP] Server '{name}' initialized successfully") else: logger.warning(f"[MCP] Server '{name}' failed to initialize — skipping") def get(self, server_name: str) -> Optional[McpClient]: """Return the initialized client for server_name, or None.""" with self._registry_lock: return self._clients.get(server_name) def all_clients(self) -> dict: """Return a copy of the {name: McpClient} mapping.""" with self._registry_lock: return dict(self._clients) def shutdown_all(self) -> None: """Shut down all managed clients.""" with self._registry_lock: clients = list(self._clients.values()) self._clients.clear() for client in clients: try: client.shutdown() except Exception as e: logger.warning(f"[MCP] Error shutting down '{client.name}': {e}") logger.info("[MCP] All servers shut down")