mcp elicitation (#1094)

Author: pgrayy

src/strands/tools/mcp/mcp_client.py

@@ -20,6 +20,7 @@
 
 import anyio
 from mcp import ClientSession, ListToolsResult
+from mcp.client.session import ElicitationFnT
 from mcp.types import BlobResourceContents, GetPromptResult, ListPromptsResult, TextResourceContents
 from mcp.types import CallToolResult as MCPCallToolResult
 from mcp.types import EmbeddedResource as MCPEmbeddedResource
@@ -98,19 +99,22 @@ def __init__(
         startup_timeout: int = 30,
         tool_filters: ToolFilters | None = None,
         prefix: str | None = None,
-    ):
+        elicitation_callback: Optional[ElicitationFnT] = None,
+    ) -> None:
         """Initialize a new MCP Server connection.
 
         Args:
-            transport_callable: A callable that returns an MCPTransport (read_stream, write_stream) tuple
-            startup_timeout: Timeout after which MCP server initialization should be cancelled
+            transport_callable: A callable that returns an MCPTransport (read_stream, write_stream) tuple.
+            startup_timeout: Timeout after which MCP server initialization should be cancelled.
                 Defaults to 30.
             tool_filters: Optional filters to apply to tools.
             prefix: Optional prefix for tool names.
+            elicitation_callback: Optional callback function to handle elicitation requests from the MCP server.
         """
         self._startup_timeout = startup_timeout
         self._tool_filters = tool_filters
         self._prefix = prefix
+        self._elicitation_callback = elicitation_callback
 
         mcp_instrumentation()
         self._session_id = uuid.uuid4()
@@ -563,7 +567,10 @@ async def _async_background_thread(self) -> None:
             async with self._transport_callable() as (read_stream, write_stream, *_):
                 self._log_debug_with_thread("transport connection established")
                 async with ClientSession(
-                    read_stream, write_stream, message_handler=self._handle_error_message
+                    read_stream,
+                    write_stream,
+                    message_handler=self._handle_error_message,
+                    elicitation_callback=self._elicitation_callback,
                 ) as session:
                     self._log_debug_with_thread("initializing MCP session")
                     await session.initialize()

tests_integ/mcp/elicitation_server.py

@@ -0,0 +1,41 @@
+"""MCP server for testing elicitation.
+
+- Docs: https://modelcontextprotocol.io/specification/draft/client/elicitation
+"""
+
+from mcp.server import FastMCP
+from mcp.types import ElicitRequest, ElicitRequestParams, ElicitResult
+
+
+def server() -> None:
+    """Simulate approval through MCP elicitation."""
+    server_ = FastMCP()
+
+    @server_.tool(description="Tool to request approval")
+    async def approval_tool() -> str:
+        """Simulated approval tool.
+
+        Returns:
+            The elicitation result from the user.
+        """
+        request = ElicitRequest(
+            params=ElicitRequestParams(
+                message="Do you approve",
+                requestedSchema={
+                    "type": "object",
+                    "properties": {
+                        "message": {"type": "string", "description": "request message"},
+                    },
+                    "required": ["message"],
+                },
+            ),
+        )
+        result = await server_.get_context().session.send_request(request, ElicitResult)
+
+        return result.model_dump_json()
+
+    server_.run(transport="stdio")
+
+
+if __name__ == "__main__":
+    server()

tests_integ/mcp/test_mcp_elicitation.py

@@ -0,0 +1,40 @@
+import json
+
+import pytest
+from mcp import StdioServerParameters, stdio_client
+from mcp.types import ElicitResult
+
+from strands import Agent
+from strands.tools.mcp import MCPClient
+
+
+@pytest.fixture
+def callback():
+    async def callback_(_, params):
+        return ElicitResult(action="accept", content={"message": params.message})
+
+    return callback_
+
+
+@pytest.fixture
+def client(callback):
+    return MCPClient(
+        lambda: stdio_client(
+            StdioServerParameters(command="python", args=["tests_integ/mcp/elicitation_server.py"]),
+        ),
+        elicitation_callback=callback,
+    )
+
+
+def test_mcp_elicitation(client):
+    with client:
+        tools = client.list_tools_sync()
+        agent = Agent(tools=tools)
+
+        agent("Can you get approval")
+
+    tool_result = agent.messages[-2]
+
+    tru_result = json.loads(tool_result["content"][0]["toolResult"]["content"][0]["text"])
+    exp_result = {"meta": None, "action": "accept", "content": {"message": "Do you approve"}}
+    assert tru_result == exp_result