feat: add MCP (Model Context Protocol) client support
Connect to external MCP servers via stdio transport, discover their tools
at startup, and register them into the hermes-agent tool registry.
- New tools/mcp_tool.py: config loading, server connection via background
event loop, tool handler factories, discovery, and graceful shutdown
- model_tools.py: trigger MCP discovery after built-in tool imports
- cli.py: call shutdown_mcp_servers in _run_cleanup
- pyproject.toml: add mcp>=1.2.0 as optional dependency
- 27 unit tests covering config, schema conversion, handlers, registration,
SDK interaction, toolset injection, graceful fallback, and shutdown
Config format (in ~/.hermes/config.yaml):
mcp_servers:
filesystem:
command: "npx"
args: ["-y", "@modelcontextprotocol/server-filesystem", "/tmp"]
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tools/mcp_tool.py
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380
tools/mcp_tool.py
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#!/usr/bin/env python3
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"""
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MCP (Model Context Protocol) Client Support
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Connects to external MCP servers via stdio transport, discovers their tools,
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and registers them into the hermes-agent tool registry so the agent can call
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them like any built-in tool.
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Configuration is read from ~/.hermes/config.yaml under the ``mcp_servers`` key.
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The ``mcp`` Python package is optional -- if not installed, this module is a
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no-op and logs a debug message.
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Example config::
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mcp_servers:
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filesystem:
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command: "npx"
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args: ["-y", "@modelcontextprotocol/server-filesystem", "/tmp"]
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env: {}
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github:
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command: "npx"
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args: ["-y", "@modelcontextprotocol/server-github"]
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env:
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GITHUB_PERSONAL_ACCESS_TOKEN: "ghp_..."
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Architecture:
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A dedicated background event loop (_mcp_loop) runs in a daemon thread.
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All MCP connections live on this loop. Tool handlers schedule coroutines
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onto it via run_coroutine_threadsafe(), so they work from any thread.
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"""
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import asyncio
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import json
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import logging
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import threading
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from typing import Any, Dict, List, Optional
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logger = logging.getLogger(__name__)
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# ---------------------------------------------------------------------------
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# Graceful import -- MCP SDK is an optional dependency
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# ---------------------------------------------------------------------------
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_MCP_AVAILABLE = False
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try:
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from mcp import ClientSession, StdioServerParameters
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from mcp.client.stdio import stdio_client
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from contextlib import AsyncExitStack
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_MCP_AVAILABLE = True
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except ImportError:
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logger.debug("mcp package not installed -- MCP tool support disabled")
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# ---------------------------------------------------------------------------
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# Connection tracking
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# ---------------------------------------------------------------------------
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class MCPConnection:
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"""Holds a live MCP server connection and its async resource stack."""
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__slots__ = ("server_name", "session", "stack")
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def __init__(self, server_name: str, session: Any, stack: Any):
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self.server_name = server_name
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self.session: Optional[Any] = session
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self.stack: Optional[Any] = stack
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_connections: Dict[str, MCPConnection] = {}
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# Dedicated event loop running in a background daemon thread.
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# All MCP async operations (connect, call_tool, shutdown) run here.
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_mcp_loop: Optional[asyncio.AbstractEventLoop] = None
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_mcp_thread: Optional[threading.Thread] = None
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def _ensure_mcp_loop():
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"""Start the background event loop thread if not already running."""
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global _mcp_loop, _mcp_thread
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if _mcp_loop is not None and _mcp_loop.is_running():
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return
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_mcp_loop = asyncio.new_event_loop()
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_mcp_thread = threading.Thread(
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target=_mcp_loop.run_forever,
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name="mcp-event-loop",
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daemon=True,
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)
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_mcp_thread.start()
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def _run_on_mcp_loop(coro, timeout: float = 30):
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"""Schedule a coroutine on the MCP event loop and block until done."""
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if _mcp_loop is None or not _mcp_loop.is_running():
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raise RuntimeError("MCP event loop is not running")
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future = asyncio.run_coroutine_threadsafe(coro, _mcp_loop)
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return future.result(timeout=timeout)
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# ---------------------------------------------------------------------------
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# Config loading
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# ---------------------------------------------------------------------------
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def _load_mcp_config() -> Dict[str, dict]:
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"""Read ``mcp_servers`` from the Hermes config file.
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Returns a dict of ``{server_name: {command, args, env}}`` or empty dict.
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"""
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try:
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from hermes_cli.config import load_config
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config = load_config()
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servers = config.get("mcp_servers")
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if not servers or not isinstance(servers, dict):
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return {}
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return servers
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except Exception as exc:
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logger.debug("Failed to load MCP config: %s", exc)
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return {}
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# ---------------------------------------------------------------------------
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# Server connection
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# ---------------------------------------------------------------------------
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async def _connect_server(name: str, config: dict) -> MCPConnection:
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"""Start an MCP server subprocess and initialize a ClientSession.
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Args:
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name: Logical server name (e.g. "filesystem").
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config: Dict with ``command``, ``args``, and optional ``env``.
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Returns:
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An ``MCPConnection`` with a live session.
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Raises:
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Exception on connection or initialization failure.
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"""
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command = config.get("command")
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args = config.get("args", [])
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env = config.get("env")
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if not command:
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raise ValueError(f"MCP server '{name}' has no 'command' in config")
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server_params = StdioServerParameters(
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command=command,
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args=args,
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env=env if env else None,
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)
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stack = AsyncExitStack()
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stdio_transport = await stack.enter_async_context(stdio_client(server_params))
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read_stream, write_stream = stdio_transport
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session = await stack.enter_async_context(ClientSession(read_stream, write_stream))
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await session.initialize()
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return MCPConnection(server_name=name, session=session, stack=stack)
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# ---------------------------------------------------------------------------
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# Handler / check-fn factories
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# ---------------------------------------------------------------------------
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def _make_tool_handler(server_name: str, tool_name: str):
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"""Return a sync handler that calls an MCP tool via the background loop.
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The handler conforms to the registry's dispatch interface:
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``handler(args_dict, **kwargs) -> str``
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"""
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def _handler(args: dict, **kwargs) -> str:
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conn = _connections.get(server_name)
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if not conn or not conn.session:
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return json.dumps({
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"error": f"MCP server '{server_name}' is not connected"
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})
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async def _call():
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result = await conn.session.call_tool(tool_name, arguments=args)
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# MCP CallToolResult has .content (list of content blocks) and .isError
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if result.isError:
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error_text = ""
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for block in (result.content or []):
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if hasattr(block, "text"):
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error_text += block.text
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return json.dumps({"error": error_text or "MCP tool returned an error"})
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# Collect text from content blocks
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parts: List[str] = []
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for block in (result.content or []):
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if hasattr(block, "text"):
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parts.append(block.text)
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return json.dumps({"result": "\n".join(parts) if parts else ""})
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try:
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return _run_on_mcp_loop(_call(), timeout=120)
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except Exception as exc:
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logger.error("MCP tool %s/%s call failed: %s", server_name, tool_name, exc)
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return json.dumps({"error": f"MCP call failed: {type(exc).__name__}: {exc}"})
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return _handler
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def _make_check_fn(server_name: str):
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"""Return a check function that verifies the MCP connection is alive."""
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def _check() -> bool:
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conn = _connections.get(server_name)
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return conn is not None and conn.session is not None
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return _check
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# ---------------------------------------------------------------------------
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# Discovery & registration
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# ---------------------------------------------------------------------------
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def _convert_mcp_schema(server_name: str, mcp_tool) -> dict:
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"""Convert an MCP tool listing to the Hermes registry schema format.
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Args:
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server_name: The logical server name for prefixing.
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mcp_tool: An MCP ``Tool`` object with ``.name``, ``.description``,
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and ``.inputSchema``.
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Returns:
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A dict suitable for ``registry.register(schema=...)``.
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"""
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# Sanitize: replace hyphens and dots with underscores for LLM API compatibility
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safe_tool_name = mcp_tool.name.replace("-", "_").replace(".", "_")
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safe_server_name = server_name.replace("-", "_").replace(".", "_")
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prefixed_name = f"mcp_{safe_server_name}_{safe_tool_name}"
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return {
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"name": prefixed_name,
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"description": mcp_tool.description or f"MCP tool {mcp_tool.name} from {server_name}",
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"parameters": mcp_tool.inputSchema if mcp_tool.inputSchema else {
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"type": "object",
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"properties": {},
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},
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}
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async def _discover_and_register_server(name: str, config: dict) -> List[str]:
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"""Connect to a single MCP server, discover tools, and register them.
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Returns list of registered tool names.
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"""
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from tools.registry import registry
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from toolsets import create_custom_toolset
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conn = await _connect_server(name, config)
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_connections[name] = conn
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# Discover tools
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tools_result = await conn.session.list_tools()
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tools = tools_result.tools if hasattr(tools_result, "tools") else []
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registered_names: List[str] = []
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toolset_name = f"mcp-{name}"
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for mcp_tool in tools:
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schema = _convert_mcp_schema(name, mcp_tool)
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tool_name_prefixed = schema["name"]
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registry.register(
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name=tool_name_prefixed,
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toolset=toolset_name,
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schema=schema,
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handler=_make_tool_handler(name, mcp_tool.name),
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check_fn=_make_check_fn(name),
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is_async=False,
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description=schema["description"],
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)
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registered_names.append(tool_name_prefixed)
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# Create a custom toolset so these tools are discoverable
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if registered_names:
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create_custom_toolset(
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name=toolset_name,
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description=f"MCP tools from {name} server",
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tools=registered_names,
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)
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logger.info(
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"MCP server '%s': registered %d tool(s): %s",
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name, len(registered_names), ", ".join(registered_names),
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)
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return registered_names
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# ---------------------------------------------------------------------------
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# Public API
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# ---------------------------------------------------------------------------
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def discover_mcp_tools() -> List[str]:
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"""Entry point: load config, connect to MCP servers, register tools.
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Called from ``model_tools._discover_tools()``. Safe to call even when
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the ``mcp`` package is not installed (returns empty list).
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Returns:
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List of all registered MCP tool names.
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"""
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if not _MCP_AVAILABLE:
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logger.debug("MCP SDK not available -- skipping MCP tool discovery")
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return []
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servers = _load_mcp_config()
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if not servers:
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logger.debug("No MCP servers configured")
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return []
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# Start the background event loop for MCP connections
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_ensure_mcp_loop()
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all_tools: List[str] = []
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async def _discover_all():
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for name, cfg in servers.items():
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try:
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registered = await _discover_and_register_server(name, cfg)
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all_tools.extend(registered)
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except Exception as exc:
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logger.warning("Failed to connect to MCP server '%s': %s", name, exc)
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_run_on_mcp_loop(_discover_all(), timeout=60)
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if all_tools:
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# Add MCP tools to hermes-cli and other platform toolsets
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from toolsets import TOOLSETS
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for ts_name in ("hermes-cli", "hermes-telegram", "hermes-discord",
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"hermes-whatsapp", "hermes-slack"):
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ts = TOOLSETS.get(ts_name)
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if ts:
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for tool_name in all_tools:
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if tool_name not in ts["tools"]:
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ts["tools"].append(tool_name)
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return all_tools
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def shutdown_mcp_servers():
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"""Close all MCP server connections and stop the background loop."""
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global _mcp_loop, _mcp_thread
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if not _connections:
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_stop_mcp_loop()
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return
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async def _shutdown():
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for name, conn in list(_connections.items()):
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try:
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if conn.stack:
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await conn.stack.aclose()
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except Exception as exc:
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logger.debug("Error closing MCP server '%s': %s", name, exc)
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finally:
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conn.session = None
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conn.stack = None
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_connections.clear()
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if _mcp_loop is not None and _mcp_loop.is_running():
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try:
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future = asyncio.run_coroutine_threadsafe(_shutdown(), _mcp_loop)
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future.result(timeout=10)
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except Exception as exc:
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logger.debug("Error during MCP shutdown: %s", exc)
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_stop_mcp_loop()
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def _stop_mcp_loop():
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"""Stop the background event loop and join its thread."""
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global _mcp_loop, _mcp_thread
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if _mcp_loop is not None:
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_mcp_loop.call_soon_threadsafe(_mcp_loop.stop)
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if _mcp_thread is not None:
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_mcp_thread.join(timeout=5)
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_mcp_thread = None
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_mcp_loop.close()
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_mcp_loop = None
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