feat: add support for passing metadata to MCP tool calls

README.md

@@ -235,6 +235,61 @@ response = await agent.ainvoke({"messages": "what is the weather in nyc?"})
 
 > Only `sse` and `http` transports support runtime headers. These headers are passed with every HTTP request to the MCP server.
 
+## Passing request metadata (`_meta`)
+
+MCP supports passing metadata with each tool invocation via the `_meta` parameter. Per the [MCP specification](https://modelcontextprotocol.io/specification/2025-06-18/basic/index#general-fields), `_meta` is a protocol-level field for attaching additional metadata to interactions. This is useful for:
+
+- Passing request-specific context to servers
+- Custom routing or multi-tenant scenarios
+- Session tracking and correlation IDs
+
+### Example: adding `_meta` via interceptor
+
+Metadata is passed through interceptors, keeping it separate from tool arguments:
+
+```python
+from langchain_mcp_adapters.tools import load_mcp_tools
+from langchain_mcp_adapters.interceptors import MCPToolCallRequest
+
+async def add_context_interceptor(request: MCPToolCallRequest, handler):
+    """Add request context via metadata."""
+    modified = request.override(
+        metaParams={"user_id": "current-user-id", "correlation_id": "trace-123"}
+    )
+    return await handler(modified)
+
+tools = await load_mcp_tools(
+    session,
+    tool_interceptors=[add_context_interceptor],
+)
+
+# Tool arguments remain clean - no metadata mixed in
+result = await tool.ainvoke({"query": "hello"})
+```
+
+### Example: composing multiple metadata interceptors
+
+```python
+async def add_correlation_interceptor(request: MCPToolCallRequest, handler):
+    """Add correlation ID for tracing."""
+    meta = request.metaParams or {}
+    meta["correlation_id"] = "trace-xyz"
+    return await handler(request.override(metaParams=meta))
+
+async def add_user_interceptor(request: MCPToolCallRequest, handler):
+    """Add user context."""
+    meta = request.metaParams or {}
+    meta["user_id"] = "user-123"
+    return await handler(request.override(metaParams=meta))
+
+tools = await load_mcp_tools(
+    session,
+    tool_interceptors=[add_correlation_interceptor, add_user_interceptor],
+)
+```
+
+> Per the [MCP specification](https://modelcontextprotocol.io/specification/2025-06-18/basic/index#general-fields), `_meta` is a separate protocol-level parameter, not part of tool arguments. It enables servers to access metadata without exposing it to language models.
+
 ## Using with LangGraph StateGraph
 
 ```python

langchain_mcp_adapters/interceptors.py

@@ -46,6 +46,7 @@ class _MCPToolCallRequestOverrides(TypedDict, total=False):
     name: NotRequired[str]
     args: NotRequired[dict[str, Any]]
     headers: NotRequired[dict[str, Any] | None]
+    metaParams: NotRequired[dict[str, Any] | None]
 
 
 @dataclass
@@ -60,6 +61,7 @@ class MCPToolCallRequest:
         name: Tool name to invoke.
         args: Tool arguments as key-value pairs.
         headers: HTTP headers for applicable transports (SSE, HTTP).
+        metaParams: Optional metadata to pass to the MCP server.
 
     Context fields (read-only, use for routing/logging):
         server_name: Name of the MCP server handling the tool.
@@ -71,6 +73,7 @@ class MCPToolCallRequest:
     server_name: str  # Context: MCP server name
     headers: dict[str, Any] | None = None  # Modifiable: HTTP headers
     runtime: object | None = None  # Context: LangGraph runtime (if any)
+    metaParams: dict[str, Any] | None = None  # Modifiable: MCP metadata
 
     def override(
         self, **overrides: Unpack[_MCPToolCallRequestOverrides]

langchain_mcp_adapters/tools.py

@@ -67,18 +67,21 @@
 MAX_ITERATIONS = 1000
 
 
-class MCPToolArtifact(TypedDict):
+class MCPToolArtifact(TypedDict, total=False):
     """Artifact returned from MCP tool calls.
 
-    This TypedDict wraps the structured content from MCP tool calls,
+    This TypedDict wraps additional fields returned by MCP tool calls,
     allowing for future extension if MCP adds more fields to tool results.
 
     Attributes:
         structured_content: The structured content returned by the MCP tool,
             corresponding to the structuredContent field in CallToolResult.
+        _meta: The response metadata returned by the MCP tool,
+            corresponding to the _meta field in CallToolResult.
     """
 
     structured_content: dict[str, Any]
+    _meta: dict[str, Any]
 
 
 def _convert_mcp_content_to_lc_block(  # noqa: PLR0911
@@ -155,8 +158,8 @@ def _convert_call_tool_result(
         A tuple containing:
         - The content: either a string (single text), list of content blocks,
           ToolMessage, or Command
-        - The artifact: MCPToolArtifact with structured_content if present,
-          otherwise None
+        - The artifact: MCPToolArtifact with structured_content and/or _meta
+          when present, otherwise None
 
     Raises:
         ToolException: If the tool call resulted in an error.
@@ -188,12 +191,17 @@ def _convert_call_tool_result(
         error_msg = "\n".join(error_parts) if error_parts else str(tool_content)
         raise ToolException(error_msg)
 
-    # Extract structured content and wrap in MCPToolArtifact
+    # Extract artifact fields from MCP result.
     artifact: MCPToolArtifact | None = None
-    if call_tool_result.structuredContent is not None:
-        artifact = MCPToolArtifact(
-            structured_content=call_tool_result.structuredContent
-        )
+    if (
+        call_tool_result.structuredContent is not None
+        or call_tool_result.meta is not None
+    ):
+        artifact = MCPToolArtifact()
+        if call_tool_result.structuredContent is not None:
+            artifact["structured_content"] = call_tool_result.structuredContent
+        if call_tool_result.meta is not None:
+            artifact["_meta"] = call_tool_result.meta
 
     return tool_content, artifact
 
@@ -378,6 +386,7 @@ async def execute_tool(request: MCPToolCallRequest) -> MCPToolCallResult:
                             tool_name,
                             tool_args,
                             progress_callback=mcp_callbacks.progress_callback,
+                            meta=request.metaParams,
                         )
                     except Exception as e:  # noqa: BLE001
                         # Capture exception to re-raise outside context manager
@@ -396,6 +405,7 @@ async def execute_tool(request: MCPToolCallRequest) -> MCPToolCallResult:
                     tool_name,
                     tool_args,
                     progress_callback=mcp_callbacks.progress_callback,
+                    meta=request.metaParams,
                 )
 
             return call_tool_result

pyproject.toml

@@ -14,7 +14,7 @@ readme = "README.md"
 requires-python = ">=3.10"
 dependencies = [
     "langchain-core>=1.0.0,<2.0.0",
-    "mcp>=1.9.2",
+    "mcp>=1.22.0",
     "typing-extensions>=4.14.0",
 ]
 

tests/test_interceptors.py

@@ -321,3 +321,170 @@ async def tool_message_interceptor(
             assert result.content == "Custom ToolMessage response"
             assert result.name == "add"
             assert result.tool_call_id == "test-call-id"
+
+
+class TestMetadataPassthrough:
+    """Tests for metadata passthrough to MCP servers via interceptors."""
+
+    async def test_interceptor_adds_metadata(self, socket_enabled):
+        """Test that interceptors can add metadata to tool calls."""
+        captured_meta = []
+
+        async def capture_meta_interceptor(
+            request: MCPToolCallRequest,
+            handler,
+        ) -> CallToolResult:
+            # Capture the metaParams to verify it was set
+            captured_meta.append(request.metaParams)
+            return await handler(request)
+
+        async def add_meta_interceptor(
+            request: MCPToolCallRequest,
+            handler,
+        ) -> CallToolResult:
+            # Add metaParams through the interceptor
+            modified_request = request.override(
+                metaParams={"session_id": "abc123", "user_id": "test-user"}
+            )
+            return await handler(modified_request)
+
+        with run_streamable_http(_create_math_server, 8210):
+            tools = await load_mcp_tools(
+                None,
+                connection={
+                    "url": "http://localhost:8210/mcp",
+                    "transport": "streamable_http",
+                },
+                tool_interceptors=[add_meta_interceptor, capture_meta_interceptor],
+            )
+
+            add_tool = next(tool for tool in tools if tool.name == "add")
+            result = await add_tool.ainvoke({"a": 2, "b": 3})
+
+            # Tool executes successfully
+            assert result == [{"type": "text", "text": "5", "id": IsLangChainID}]
+
+            # Verify that the metadata was passed through the interceptor
+            assert len(captured_meta) == 1
+            assert captured_meta[0] == {
+                "session_id": "abc123",
+                "user_id": "test-user",
+            }
+
+    async def test_interceptor_modifies_metadata(self, socket_enabled):
+        """Test that interceptors can modify metadata for different requests."""
+        captured_requests = []
+
+        async def capture_request_interceptor(
+            request: MCPToolCallRequest,
+            handler,
+        ) -> CallToolResult:
+            # Capture the request state to verify metadata
+            captured_requests.append(
+                {
+                    "name": request.name,
+                    "args": request.args.copy(),
+                    "metaParams": request.metaParams,
+                }
+            )
+            return await handler(request)
+
+        async def contextual_meta_interceptor(
+            request: MCPToolCallRequest,
+            handler,
+        ) -> CallToolResult:
+            # Add context-specific metadata based on tool name
+            metadata = {
+                "tool": request.name,
+                "timestamp": "2025-02-26T00:00:00Z",
+            }
+
+            modified_request = request.override(metaParams=metadata)
+            return await handler(modified_request)
+
+        with run_streamable_http(_create_math_server, 8211):
+            tools = await load_mcp_tools(
+                None,
+                connection={
+                    "url": "http://localhost:8211/mcp",
+                    "transport": "streamable_http",
+                },
+                tool_interceptors=[
+                    contextual_meta_interceptor,
+                    capture_request_interceptor,
+                ],
+            )
+
+            add_tool = next(tool for tool in tools if tool.name == "add")
+            result = await add_tool.ainvoke({"a": 5, "b": 7})
+
+            # Tool executes successfully
+            assert result == [{"type": "text", "text": "12", "id": IsLangChainID}]
+
+            # Verify the captured request has the metadata
+            assert len(captured_requests) == 1
+            captured = captured_requests[0]
+            assert captured["name"] == "add"
+            assert captured["args"] == {"a": 5, "b": 7}
+            assert captured["metaParams"] == {
+                "tool": "add",
+                "timestamp": "2025-02-26T00:00:00Z",
+            }
+
+    async def test_multiple_interceptors_modify_metadata(self, socket_enabled):
+        """Test that multiple interceptors can compose to build metadata."""
+        captured_meta = []
+
+        async def add_correlation_interceptor(
+            request: MCPToolCallRequest,
+            handler,
+        ) -> CallToolResult:
+            # First interceptor adds correlation ID
+            meta = request.metaParams or {}
+            meta["correlation_id"] = "corr-123"
+            modified = request.override(metaParams=meta)
+            return await handler(modified)
+
+        async def add_user_interceptor(
+            request: MCPToolCallRequest,
+            handler,
+        ) -> CallToolResult:
+            # Second interceptor adds user context
+            meta = request.metaParams or {}
+            meta["user_id"] = "user-456"
+            modified = request.override(metaParams=meta)
+            return await handler(modified)
+
+        async def capture_meta_interceptor(
+            request: MCPToolCallRequest,
+            handler,
+        ) -> CallToolResult:
+            captured_meta.append(request.metaParams)
+            return await handler(request)
+
+        with run_streamable_http(_create_math_server, 8212):
+            tools = await load_mcp_tools(
+                None,
+                connection={
+                    "url": "http://localhost:8212/mcp",
+                    "transport": "streamable_http",
+                },
+                tool_interceptors=[
+                    add_correlation_interceptor,
+                    add_user_interceptor,
+                    capture_meta_interceptor,
+                ],
+            )
+
+            add_tool = next(tool for tool in tools if tool.name == "add")
+            result = await add_tool.ainvoke({"a": 10, "b": 20})
+
+            # Tool executes successfully
+            assert result == [{"type": "text", "text": "30", "id": IsLangChainID}]
+
+            # Verify both interceptors contributed to metadata
+            assert len(captured_meta) == 1
+            assert captured_meta[0] == {
+                "correlation_id": "corr-123",
+                "user_id": "user-456",
+            }