feat(client): add validate_output flag to call_tool to skip output-schema revalidation

Author: Mitulagr

src/mcp/client/client.py

@@ -758,6 +758,7 @@ async def call_tool(
         input_responses: InputResponses | None = None,
         request_state: str | None = None,
         meta: RequestParamsMeta | None = None,
+        validate_output: bool = True,
     ) -> CallToolResult:
         """Call a tool on the server.
 
@@ -784,6 +785,9 @@ async def call_tool(
                 resuming from a persisted `InputRequiredResult`).
             request_state: Opaque state to seed the first call with.
             meta: Additional metadata for the request.
+            validate_output: When `True` (default), the tool's output schema is
+                validated against the returned structured content. When `False`,
+                the result is returned without schema validation.
 
         Returns:
             The tool result.
@@ -807,6 +811,7 @@ async def retry(r: InputResponses | None, s: str | None) -> CallToolResult | Inp
                 allow_input_required=True,
                 # Input rounds resolve before a claimed result, so a claim may end any round.
                 allow_claimed=True,
+                validate_output=validate_output,
             )
 
         result = await self._drive_input_required(await retry(input_responses, request_state), retry)
@@ -818,7 +823,7 @@ async def retry(r: InputResponses | None, s: str | None) -> CallToolResult | Inp
             result,
             ClaimContext(session=self.session, tool_name=name, read_timeout_seconds=read_timeout_seconds),
         )
-        if not final.is_error:
+        if validate_output and not final.is_error:
             # Match the direct path: revalidate the output schema, but never for isError results.
             await self.session.validate_tool_result(name, final)
         return final

src/mcp/client/session.py

@@ -964,6 +964,7 @@ async def call_tool(
         meta: RequestParamsMeta | None = None,
         allow_input_required: Literal[False] = False,
         allow_claimed: Literal[False] = False,
+        validate_output: bool = True,
     ) -> types.CallToolResult: ...
 
     @overload
@@ -979,6 +980,7 @@ async def call_tool(
         meta: RequestParamsMeta | None = None,
         allow_input_required: bool,
         allow_claimed: Literal[False] = False,
+        validate_output: bool = True,
     ) -> types.CallToolResult | types.InputRequiredResult: ...
 
     @overload
@@ -994,6 +996,7 @@ async def call_tool(
         meta: RequestParamsMeta | None = None,
         allow_input_required: Literal[False] = False,
         allow_claimed: bool,
+        validate_output: bool = True,
     ) -> types.CallToolResult | types.Result: ...
 
     @overload
@@ -1009,6 +1012,7 @@ async def call_tool(
         meta: RequestParamsMeta | None = None,
         allow_input_required: bool,
         allow_claimed: bool,
+        validate_output: bool = True,
     ) -> types.CallToolResult | types.InputRequiredResult | types.Result: ...
 
     async def call_tool(
@@ -1023,6 +1027,7 @@ async def call_tool(
         meta: RequestParamsMeta | None = None,
         allow_input_required: bool = False,
         allow_claimed: bool = False,
+        validate_output: bool = True,
     ) -> types.CallToolResult | types.InputRequiredResult | types.Result:
         """Send a tools/call request with optional progress callback support.
 
@@ -1039,6 +1044,9 @@ async def call_tool(
                 so the caller can resolve the requests and retry.
             allow_claimed: When `False` (default), a claimed extension result raises
                 `UnexpectedClaimedResult`; when `True`, the parsed claim model is returned.
+            validate_output: When `True` (default), the client's cached tool output schema
+                is validated against the returned structured content. When `False`, the
+                result is returned without schema validation.
 
         Raises:
             RuntimeError: If the server returns an `InputRequiredResult` and
@@ -1060,7 +1068,7 @@ async def call_tool(
             progress_callback=progress_callback,
         )
 
-        if isinstance(result, types.CallToolResult) and not result.is_error:
+        if validate_output and isinstance(result, types.CallToolResult) and not result.is_error:
             await self.validate_tool_result(name, result)
 
         # The input_required arm stays first; a claimed shape is terminal for the multi-round-trip driver.

tests/client/test_output_schema_validation.py

@@ -163,3 +163,30 @@ async def on_call_tool(ctx: ServerRequestContext, params: CallToolRequestParams)
         assert result.is_error is False
 
         assert "Tool mystery_tool not listed" in caplog.text
+
+
+@pytest.mark.anyio
+async def test_call_tool_validate_output_false_skips_validation():
+    """Test that validate_output=False bypasses client-side output-schema revalidation."""
+    output_schema = {
+        "type": "object",
+        "properties": {"result": {"type": "integer", "title": "Result"}},
+        "required": ["result"],
+        "title": "calculate_Output",
+    }
+
+    server = _make_server(
+        tools=[
+            Tool(
+                name="calculate",
+                description="Calculate something",
+                input_schema={"type": "object"},
+                output_schema=output_schema,
+            )
+        ],
+        structured_content={"result": "not_a_number"},  # Invalid: should be int
+    )
+
+    async with Client(server) as client:
+        result = await client.call_tool("calculate", {}, validate_output=False)
+        assert result.structured_content == {"result": "not_a_number"}