diff --git a/src/openai/lib/_parsing/_responses.py b/src/openai/lib/_parsing/_responses.py
index c607587ec1..abf3eb2217 100644
--- a/src/openai/lib/_parsing/_responses.py
+++ b/src/openai/lib/_parsing/_responses.py
@@ -58,7 +58,13 @@ def parse_response(
 ) -> ParsedResponse[TextFormatT]:
     output_list: List[ParsedResponseOutputItem[TextFormatT]] = []
 
-    for output in response.output:
+    # Guard against `response.output` being `None` (observed in the chatgpt.com
+    # Codex backend's consolidated `response.completed` event — see issue #3325).
+    # When the streaming accumulator has already collected output items, it
+    # injects them into the response before calling this function, so reaching
+    # here with `None` means the stream genuinely had no output items and an
+    # empty list is the correct result.
+    for output in response.output or []:
         if output.type == "message":
             content_list: List[ParsedContent[TextFormatT]] = []
             for item in output.content:
diff --git a/src/openai/lib/streaming/responses/_responses.py b/src/openai/lib/streaming/responses/_responses.py
index 6975a9260d..764d54e6a9 100644
--- a/src/openai/lib/streaming/responses/_responses.py
+++ b/src/openai/lib/streaming/responses/_responses.py
@@ -356,12 +356,76 @@ def accumulate_event(self, event: RawResponseStreamEvent) -> ParsedResponseSnaps
             output = snapshot.output[event.output_index]
             if output.type == "function_call":
                 output.arguments += event.delta
+        elif event.type == "response.output_text.done":
+            output = snapshot.output[event.output_index]
+            if output.type == "message":
+                content = output.content[event.content_index]
+                assert content.type == "output_text"
+                content.text = event.text
+        elif event.type == "response.output_item.done":
+            # Replace the item in the snapshot with the finalized item from the
+            # done event. The server sends the authoritative item payload here,
+            # which may include final fields like `results`/`outputs` on tool
+            # items, or a final `status` of `failed`/`incomplete`. Simply
+            # setting `status = "completed"` would discard those fields and
+            # produce a stale final response in the null-output fallback path.
+            if event.output_index < len(snapshot.output):
+                snapshot.output[event.output_index] = construct_type_unchecked(
+                    type_=type(snapshot.output[event.output_index]),
+                    value=event.item.to_dict(),
+                )
+        elif event.type == "response.content_part.done":
+            # Replace the content part in the snapshot with the finalized part
+            # from the done event. The server sends the authoritative part
+            # payload here, which may include metadata like annotations,
+            # logprobs, or finalized text/refusal content that the delta
+            # accumulation may not fully capture.
+            output = snapshot.output[event.output_index]
+            if output.type == "message" and event.content_index < len(output.content):
+                output.content[event.content_index] = construct_type_unchecked(
+                    type_=type(output.content[event.content_index]),
+                    value=event.part.to_dict(),
+                )
+        elif event.type == "response.function_call_arguments.done":
+            # Apply the finalized arguments string from the done event.
+            # The server sends the authoritative arguments payload here, which
+            # may differ from the accumulated deltas. Using the finalized
+            # arguments ensures `parse_response()` can correctly parse
+            # `parsed_arguments` in the null-output fallback path.
+            output = snapshot.output[event.output_index]
+            if output.type == "function_call":
+                output.arguments = event.arguments
+                if hasattr(output, "status"):
+                    output.status = "completed"
         elif event.type == "response.completed":
-            self._completed_response = parse_response(
-                text_format=self._text_format,
-                response=event.response,
-                input_tools=self._input_tools,
-            )
+            # The chatgpt.com Codex backend sometimes sends `response.output: null`
+            # in the consolidated `response.completed` event even when valid
+            # `output_item.done` events were streamed earlier (see issue #3325).
+            # `parse_response()` guards against `None` with `response.output or []`,
+            # but that would discard the already-accumulated `snapshot.output` and
+            # emit an empty final response.  When the completed event has no
+            # output but the snapshot has accumulated items, inject the streamed
+            # items into a shallow copy of the response so `parse_response()` can
+            # still run its text_format / parsed_arguments logic on them.
+            if event.response.output is None and snapshot.output:
+                response_with_output = construct_type_unchecked(
+                    type_=type(event.response),
+                    value={
+                        **event.response.to_dict(),
+                        "output": [item.to_dict() for item in snapshot.output],
+                    },
+                )
+                self._completed_response = parse_response(
+                    text_format=self._text_format,
+                    response=response_with_output,
+                    input_tools=self._input_tools,
+                )
+            else:
+                self._completed_response = parse_response(
+                    text_format=self._text_format,
+                    response=event.response,
+                    input_tools=self._input_tools,
+                )
 
         return snapshot