chore: enhance error messaging when MCP tools are used without session initialization

Author: dbschmigelski

src/strands/tools/mcp/mcp_client.py

@@ -41,6 +41,11 @@
     "image/webp": "webp",
 }
 
+CLIENT_SESSION_NOT_RUNNING_ERROR_MESSAGE = (
+    "the client session is not running. Please ensure the agent is used within "
+    "the MCP client context manager ('with mcp_client:')"
+)
+
 
 class MCPClient:
     """Represents a connection to a Model Context Protocol (MCP) server.
@@ -145,7 +150,7 @@ def list_tools_sync(self) -> List[MCPAgentTool]:
         """
         self._log_debug_with_thread("listing MCP tools synchronously")
         if not self._is_session_active():
-            raise MCPClientInitializationError("the client session is not running")
+            raise MCPClientInitializationError(CLIENT_SESSION_NOT_RUNNING_ERROR_MESSAGE)
 
         async def _list_tools_async() -> ListToolsResult:
             return await self._background_thread_session.list_tools()
@@ -180,7 +185,7 @@ def call_tool_sync(
         """
         self._log_debug_with_thread("calling MCP tool '%s' synchronously with tool_use_id=%s", name, tool_use_id)
         if not self._is_session_active():
-            raise MCPClientInitializationError("the client session is not running")
+            raise MCPClientInitializationError(CLIENT_SESSION_NOT_RUNNING_ERROR_MESSAGE)
 
         async def _call_tool_async() -> MCPCallToolResult:
             return await self._background_thread_session.call_tool(name, arguments, read_timeout_seconds)