feat: add MCP tool call result callback

src/agents/mcp/__init__.py

@@ -17,6 +17,8 @@ if TYPE_CHECKING:
     )
 
 from .util import (
+    MCPToolCallResultCallback,
+    MCPToolCallResultContext,
     MCPToolMetaContext,
     MCPToolMetaResolver,
     MCPUtil,
@@ -50,6 +52,8 @@ __all__ = [
     "MCPServerManager",
     "LocalMCPApprovalCallable",
     "MCPUtil",
+    "MCPToolCallResultCallback",
+    "MCPToolCallResultContext",
     "MCPToolMetaContext",
     "MCPToolMetaResolver",
     "ToolFilter",

src/agents/mcp/server.py

@@ -45,6 +45,7 @@ from ..tool import ToolErrorFunction
 from ..util._types import MaybeAwaitable
 from .util import (
     HttpClientFactory,
+    MCPToolCallResultCallback,
     MCPToolMetaResolver,
     ToolFilter,
     ToolFilterContext,
@@ -229,6 +230,7 @@ class MCPServer(abc.ABC):
         require_approval: RequireApprovalSetting = None,
         failure_error_function: ToolErrorFunction | None | _UnsetType = _UNSET,
         tool_meta_resolver: MCPToolMetaResolver | None = None,
+        tool_call_result_callback: MCPToolCallResultCallback | None = None,
     ):
         """
         Args:
@@ -248,6 +250,9 @@ class MCPServer(abc.ABC):
                 SDK default) will be used.
             tool_meta_resolver: Optional callable that produces MCP request metadata (`_meta`) for
                 tool calls. It is invoked by the Agents SDK before calling `call_tool`.
+            tool_call_result_callback: Optional callback invoked after an MCP tool call returns. The
+                callback receives result metadata and the model-visible tool output, but cannot
+                change the output returned to the model.
         """
         self.use_structured_content = use_structured_content
         self._needs_approval_policy = self._normalize_needs_approval(
@@ -255,6 +260,7 @@ class MCPServer(abc.ABC):
         )
         self._failure_error_function = failure_error_function
         self.tool_meta_resolver = tool_meta_resolver
+        self.tool_call_result_callback = tool_call_result_callback
 
     @abc.abstractmethod
     async def connect(self):
@@ -544,6 +550,7 @@ class _MCPServerWithClientSession(MCPServer, abc.ABC):
         require_approval: RequireApprovalSetting = None,
         failure_error_function: ToolErrorFunction | None | _UnsetType = _UNSET,
         tool_meta_resolver: MCPToolMetaResolver | None = None,
+        tool_call_result_callback: MCPToolCallResultCallback | None = None,
     ):
         """
         Args:
@@ -576,12 +583,16 @@ class _MCPServerWithClientSession(MCPServer, abc.ABC):
                 SDK default) will be used.
             tool_meta_resolver: Optional callable that produces MCP request metadata (`_meta`) for
                 tool calls. It is invoked by the Agents SDK before calling `call_tool`.
+            tool_call_result_callback: Optional callback invoked after an MCP tool call returns. The
+                callback receives result metadata and the model-visible tool output, but cannot
+                change the output returned to the model.
         """
         super().__init__(
             use_structured_content=use_structured_content,
             require_approval=require_approval,
             failure_error_function=failure_error_function,
             tool_meta_resolver=tool_meta_resolver,
+            tool_call_result_callback=tool_call_result_callback,
         )
         self.session: ClientSession | None = None
         self.exit_stack: AsyncExitStack = AsyncExitStack()
@@ -1108,6 +1119,7 @@ class MCPServerStdio(_MCPServerWithClientSession):
         require_approval: RequireApprovalSetting = None,
         failure_error_function: ToolErrorFunction | None | _UnsetType = _UNSET,
         tool_meta_resolver: MCPToolMetaResolver | None = None,
+        tool_call_result_callback: MCPToolCallResultCallback | None = None,
     ):
         """Create a new MCP server based on the stdio transport.
 
@@ -1145,6 +1157,9 @@ class MCPServerStdio(_MCPServerWithClientSession):
                 SDK default) will be used.
             tool_meta_resolver: Optional callable that produces MCP request metadata (`_meta`) for
                 tool calls. It is invoked by the Agents SDK before calling `call_tool`.
+            tool_call_result_callback: Optional callback invoked after an MCP tool call returns. The
+                callback receives result metadata and the model-visible tool output, but cannot
+                change the output returned to the model.
         """
         super().__init__(
             cache_tools_list=cache_tools_list,
@@ -1157,6 +1172,7 @@ class MCPServerStdio(_MCPServerWithClientSession):
             require_approval=require_approval,
             failure_error_function=failure_error_function,
             tool_meta_resolver=tool_meta_resolver,
+            tool_call_result_callback=tool_call_result_callback,
         )
 
         self.params = StdioServerParameters(
@@ -1229,6 +1245,7 @@ class MCPServerSse(_MCPServerWithClientSession):
         require_approval: RequireApprovalSetting = None,
         failure_error_function: ToolErrorFunction | None | _UnsetType = _UNSET,
         tool_meta_resolver: MCPToolMetaResolver | None = None,
+        tool_call_result_callback: MCPToolCallResultCallback | None = None,
     ):
         """Create a new MCP server based on the HTTP with SSE transport.
 
@@ -1268,6 +1285,9 @@ class MCPServerSse(_MCPServerWithClientSession):
                 SDK default) will be used.
             tool_meta_resolver: Optional callable that produces MCP request metadata (`_meta`) for
                 tool calls. It is invoked by the Agents SDK before calling `call_tool`.
+            tool_call_result_callback: Optional callback invoked after an MCP tool call returns. The
+                callback receives result metadata and the model-visible tool output, but cannot
+                change the output returned to the model.
         """
         super().__init__(
             cache_tools_list=cache_tools_list,
@@ -1280,6 +1300,7 @@ class MCPServerSse(_MCPServerWithClientSession):
             require_approval=require_approval,
             failure_error_function=failure_error_function,
             tool_meta_resolver=tool_meta_resolver,
+            tool_call_result_callback=tool_call_result_callback,
         )
 
         self.params = params
@@ -1365,6 +1386,7 @@ class MCPServerStreamableHttp(_MCPServerWithClientSession):
         require_approval: RequireApprovalSetting = None,
         failure_error_function: ToolErrorFunction | None | _UnsetType = _UNSET,
         tool_meta_resolver: MCPToolMetaResolver | None = None,
+        tool_call_result_callback: MCPToolCallResultCallback | None = None,
     ):
         """Create a new MCP server based on the Streamable HTTP transport.
 
@@ -1405,6 +1427,9 @@ class MCPServerStreamableHttp(_MCPServerWithClientSession):
                 SDK default) will be used.
             tool_meta_resolver: Optional callable that produces MCP request metadata (`_meta`) for
                 tool calls. It is invoked by the Agents SDK before calling `call_tool`.
+            tool_call_result_callback: Optional callback invoked after an MCP tool call returns. The
+                callback receives result metadata and the model-visible tool output, but cannot
+                change the output returned to the model.
         """
         super().__init__(
             cache_tools_list=cache_tools_list,
@@ -1417,6 +1442,7 @@ class MCPServerStreamableHttp(_MCPServerWithClientSession):
             require_approval=require_approval,
             failure_error_function=failure_error_function,
             tool_meta_resolver=tool_meta_resolver,
+            tool_call_result_callback=tool_call_result_callback,
         )
 
         self.params = params

src/agents/mcp/util.py

@@ -7,8 +7,9 @@ import hashlib
 import inspect
 import json
 from collections import Counter
-from collections.abc import Awaitable, Callable
+from collections.abc import Awaitable, Callable, Mapping
 from dataclasses import dataclass
+from types import MappingProxyType
 from typing import TYPE_CHECKING, Any, Protocol, Union
 
 import httpx
@@ -149,13 +150,50 @@ class MCPToolMetaContext:
     """The parsed tool arguments."""
 
 
+@dataclass(frozen=True)
+class MCPToolCallResultContext:
+    """Context information available when an MCP tool call returns a result."""
+
+    run_context: RunContextWrapper[Any]
+    """The current run context."""
+
+    server_name: str
+    """The name of the MCP server."""
+
+    tool_name: str
+    """The original MCP tool name invoked on the server."""
+
+    tool_display_name: str
+    """The public tool name exposed through the Agents SDK."""
+
+    arguments: Mapping[str, Any]
+    """The parsed tool arguments."""
+
+    result_meta: Mapping[str, Any] | None
+    """The MCP tool result `_meta` payload, if present."""
+
+    structured_content: Mapping[str, Any] | None
+    """The MCP tool result `structuredContent` payload, if present."""
+
+    is_error: bool | None
+    """The MCP tool result `isError` flag, if present."""
+
+    tool_output: ToolOutput
+    """The model-visible tool output produced by the Agents SDK."""
+
+
 if TYPE_CHECKING:
     MCPToolMetaResolver = Callable[
         [MCPToolMetaContext],
         MaybeAwaitable[dict[str, Any] | None],
     ]
+    MCPToolCallResultCallback = Callable[
+        [MCPToolCallResultContext],
+        MaybeAwaitable[None],
+    ]
 else:
     MCPToolMetaResolver = Callable[..., Any]
+    MCPToolCallResultCallback = Callable[..., Any]
 """A function that produces MCP request metadata for tool calls.
 
 Args:
@@ -164,6 +202,7 @@ Args:
 Returns:
     A dict to send as MCP `_meta`, or None to omit metadata.
 """
+"""A callback that observes MCP tool call results without changing tool output."""
 
 
 def create_static_tool_filter(
@@ -541,6 +580,43 @@ class MCPUtil:
             merged.update(copy.deepcopy(explicit_meta))
         return merged
 
+    @staticmethod
+    def _copy_mapping_proxy(value: Any) -> Mapping[str, Any] | None:
+        if not isinstance(value, dict):
+            return None
+        return MappingProxyType(copy.deepcopy(value))
+
+    @classmethod
+    async def _maybe_call_tool_result_callback(
+        cls,
+        *,
+        server: MCPServer,
+        context: RunContextWrapper[Any],
+        tool_name: str,
+        tool_display_name: str,
+        arguments: dict[str, Any],
+        result: Any,
+        tool_output: ToolOutput,
+    ) -> None:
+        callback = getattr(server, "tool_call_result_callback", None)
+        if callback is None:
+            return
+
+        callback_context = MCPToolCallResultContext(
+            run_context=context,
+            server_name=server.name,
+            tool_name=tool_name,
+            tool_display_name=tool_display_name,
+            arguments=MappingProxyType(copy.deepcopy(arguments)),
+            result_meta=cls._copy_mapping_proxy(getattr(result, "meta", None)),
+            structured_content=cls._copy_mapping_proxy(getattr(result, "structuredContent", None)),
+            is_error=getattr(result, "isError", None),
+            tool_output=copy.deepcopy(tool_output),
+        )
+        callback_result = callback(callback_context)
+        if inspect.isawaitable(callback_result):
+            await callback_result
+
     @classmethod
     async def _resolve_meta(
         cls,
@@ -688,6 +764,16 @@ class MCPUtil:
             else:
                 tool_output = tool_output_list
 
+        await cls._maybe_call_tool_result_callback(
+            server=server,
+            context=context,
+            tool_name=tool.name,
+            tool_display_name=tool_name_for_display,
+            arguments=json_data,
+            result=result,
+            tool_output=tool_output,
+        )
+
         current_span = get_current_span()
         if current_span:
             if isinstance(current_span.span_data, FunctionSpanData):

tests/mcp/helpers.py

@@ -20,7 +20,7 @@ from mcp.types import (
 
 from agents.mcp import MCPServer
 from agents.mcp.server import _UNSET, _MCPServerWithClientSession, _UnsetType
-from agents.mcp.util import MCPToolMetaResolver, ToolFilter
+from agents.mcp.util import MCPToolCallResultCallback, MCPToolMetaResolver, ToolFilter
 from agents.tool import ToolErrorFunction
 
 tee = shutil.which("tee") or ""
@@ -76,12 +76,14 @@ class FakeMCPServer(MCPServer):
         require_approval: object | None = None,
         failure_error_function: ToolErrorFunction | None | _UnsetType = _UNSET,
         tool_meta_resolver: MCPToolMetaResolver | None = None,
+        tool_call_result_callback: MCPToolCallResultCallback | None = None,
     ):
         super().__init__(
             use_structured_content=False,
             require_approval=require_approval,  # type: ignore[arg-type]
             failure_error_function=failure_error_function,
             tool_meta_resolver=tool_meta_resolver,
+            tool_call_result_callback=tool_call_result_callback,
         )
         self.tools: list[MCPTool] = tools or []
         self.tool_calls: list[str] = []

tests/mcp/test_mcp_util.py

@@ -531,6 +531,88 @@ async def test_mcp_meta_resolver_does_not_mutate_arguments():
     assert args == {"foo": "bar"}
 
 
+@pytest.mark.asyncio
+async def test_mcp_tool_call_result_callback_observes_result_without_mutating_output():
+    captured: dict[str, Any] = {}
+
+    def tool_call_result_callback(context):
+        captured["run_context"] = context.run_context
+        captured["server_name"] = context.server_name
+        captured["tool_name"] = context.tool_name
+        captured["tool_display_name"] = context.tool_display_name
+        captured["arguments"] = dict(context.arguments)
+        captured["result_meta"] = dict(context.result_meta or {})
+        captured["structured_content"] = dict(context.structured_content or {})
+        captured["is_error"] = context.is_error
+        captured["tool_output"] = context.tool_output
+
+        with pytest.raises(TypeError):
+            context.arguments["mutated"] = True
+
+        if isinstance(context.tool_output, dict):
+            context.tool_output["text"] = "mutated"
+
+    class ResultMCPServer(FakeMCPServer):
+        async def call_tool(
+            self,
+            tool_name: str,
+            arguments: dict[str, Any] | None,
+            meta: dict[str, Any] | None = None,
+        ) -> CallToolResult:
+            self.tool_calls.append(tool_name)
+            self.tool_metas.append(meta)
+            return CallToolResult(
+                content=[TextContent(text="model summary", type="text")],
+                structuredContent={"rows": 1245},
+                isError=False,
+                _meta={"frontend": {"type": "chart"}},
+            )
+
+    server = ResultMCPServer(
+        server_name="analytics",
+        tool_call_result_callback=tool_call_result_callback,
+    )
+    ctx: RunContextWrapper[dict[str, Any]] = RunContextWrapper(context={"frontend_events": []})
+    tool = MCPTool(name="query_sales", inputSchema={})
+
+    output = await MCPUtil.invoke_mcp_tool(
+        server,
+        tool,
+        ctx,
+        '{"sql": "select 1"}',
+        tool_display_name="analytics__query_sales",
+    )
+
+    assert output == {"type": "text", "text": "model summary"}
+    assert captured == {
+        "run_context": ctx,
+        "server_name": "analytics",
+        "tool_name": "query_sales",
+        "tool_display_name": "analytics__query_sales",
+        "arguments": {"sql": "select 1"},
+        "result_meta": {"frontend": {"type": "chart"}},
+        "structured_content": {"rows": 1245},
+        "is_error": False,
+        "tool_output": {"type": "text", "text": "mutated"},
+    }
+
+
+@pytest.mark.asyncio
+async def test_mcp_tool_call_result_callback_can_be_async():
+    captured: list[str] = []
+
+    async def tool_call_result_callback(context):
+        captured.append(context.tool_name)
+
+    server = FakeMCPServer(tool_call_result_callback=tool_call_result_callback)
+    ctx = RunContextWrapper(context=None)
+    tool = MCPTool(name="test_tool_1", inputSchema={})
+
+    await MCPUtil.invoke_mcp_tool(server, tool, ctx, "{}")
+
+    assert captured == ["test_tool_1"]
+
+
 @pytest.mark.asyncio
 async def test_to_function_tool_passes_static_mcp_meta():
     server = FakeMCPServer()