feat: add support for disabling output validation for tools

src/mcp/client/session.py

@@ -287,6 +287,7 @@ async def call_tool(
         arguments: dict[str, Any] | None = None,
         read_timeout_seconds: timedelta | None = None,
         progress_callback: ProgressFnT | None = None,
+        validate_output: bool = True,
     ) -> types.CallToolResult:
         """Send a tools/call request with optional progress callback support."""
 
@@ -305,7 +306,7 @@ async def call_tool(
             progress_callback=progress_callback,
         )
 
-        if not result.isError:
+        if not result.isError and validate_output:
             await self._validate_tool_result(name, result)
 
         return result

src/mcp/server/lowlevel/server.py

@@ -422,11 +422,12 @@ async def _get_cached_tool_definition(self, tool_name: str) -> types.Tool | None
 
         return tool
 
-    def call_tool(self, *, validate_input: bool = True):
+    def call_tool(self, *, validate_input: bool = True, validate_output: bool = True):
         """Register a tool call handler.
 
         Args:
             validate_input: If True, validates input against inputSchema. Default is True.
+            validate_output: If True, validates output against outputSchema. Default is True.
 
         The handler validates input against inputSchema (if validate_input=True), calls the tool function,
         and builds a CallToolResult with the results:
@@ -485,10 +486,11 @@ async def handler(req: types.CallToolRequest):
                                 "Output validation error: outputSchema defined but no structured output returned"
                             )
                         else:
-                            try:
-                                jsonschema.validate(instance=maybe_structured_content, schema=tool.outputSchema)
-                            except jsonschema.ValidationError as e:
-                                return self._make_error_result(f"Output validation error: {e.message}")
+                            if validate_output:
+                                try:
+                                    jsonschema.validate(instance=maybe_structured_content, schema=tool.outputSchema)
+                                except jsonschema.ValidationError as e:
+                                    return self._make_error_result(f"Output validation error: {e.message}")
 
                     # result
                     return types.ServerResult(