diff --git a/src/agents/models/openai_responses.py b/src/agents/models/openai_responses.py
index 36a981404..102179e85 100644
--- a/src/agents/models/openai_responses.py
+++ b/src/agents/models/openai_responses.py
@@ -299,7 +299,12 @@ async def _fetch_response(
             if response_format is not omit:
                 response_format["verbosity"] = model_settings.verbosity  # type: ignore [index]
             else:
-                response_format = {"verbosity": model_settings.verbosity}
+                # When no output_schema is present, `text` needs an explicit format object.
+                # Use the plain text format object with verbosity to produce a valid `text` payload.
+                response_format = {
+                    "format": {"type": "text"},
+                    "verbosity": model_settings.verbosity,
+                }
 
         stream_param: Literal[True] | Omit = True if stream else omit
 
@@ -501,7 +506,7 @@ def _convert_tool(cls, tool: Tool) -> tuple[ToolParam, ResponseIncludable | None
             }
             includes = None
         else:
-            raise UserError(f"Unknown tool type: {type(tool)}, tool")
+            raise UserError(f"Unknown tool type: {type(tool)} for tool {tool!r}")
 
         return converted_tool, includes