Only list MCP tools once, not at every agent run step (#3227)

Author: DouweM

pydantic_ai_slim/pydantic_ai/mcp.py

@@ -113,6 +113,7 @@ class MCPServer(AbstractToolset[Any], ABC):
     _read_stream: MemoryObjectReceiveStream[SessionMessage | Exception]
     _write_stream: MemoryObjectSendStream[SessionMessage]
     _server_info: mcp_types.Implementation
+    _tools: list[mcp_types.Tool] | None
 
     def __init__(
         self,
@@ -148,6 +149,7 @@ def __post_init__(self):
         self._enter_lock = Lock()
         self._running_count = 0
         self._exit_stack = None
+        self._tools = None
 
     @abstractmethod
     @asynccontextmanager
@@ -194,13 +196,14 @@ def server_info(self) -> mcp_types.Implementation:
     async def list_tools(self) -> list[mcp_types.Tool]:
         """Retrieve tools that are currently active on the server.
 
-        Note:
-        - We don't cache tools as they might change.
-        - We also don't subscribe to the server to avoid complexity.
+        Note that we don't subscribe to the server to avoid complexity.
         """
+        if self._tools is not None:
+            return self._tools
         async with self:  # Ensure server is running
             result = await self._client.list_tools()
-        return result.tools
+        self._tools = result.tools
+        return self._tools
 
     async def direct_call_tool(
         self,