Streaming + tools: never JSON.parse a partial arguments delta

TL;DR

It arrives as a run of events, and the only safe moment to parse arguments is the end. Parsing any earlier delta is the bug: the JSON is truncated mid-string and JSON.parse / json.loads throws, or worse, parses a prefix into a wrong object.

Steps

1. With stream=True, a function call does not arrive as one object. It arrives as

a run of events, and the only safe moment to parse arguments is the end.

2. • response.output_item.added announces a function_call item with an empty

   arguments string.

• A series of delta events append fragments to arguments.

• response.output_item.done carries the item with the complete arguments JSON string. Parse it here, and only here.

3. Stream with stream=True. Accumulate nothing for tool calls except routing response.output_text.delta events to the UI.

4. On response.output_item.done with type == "function_call", parse item.arguments (now complete) and run your tool-loop step.

5. Make the follow-up responses.create call non-streaming (or start a second stream) with the appended function_call_output items. Do not try to continue the same stream object; it is finished.

6. • Test with a tool whose arguments are long (a paragraph of text). Short

   arguments fit in one delta and hide the bug; long ones expose it.

• Assert your code has no JSON parse call inside the delta-event branch. Grep for it; the parse must live only in the done-event branch.

When to use

You are seeing this: With stream=True, a function call does not arrive as one object. Use this skill when you run into "Streaming + tools: never JSON.parse a partial arguments delta".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

OpenAI's streaming guide documents typed SSE events including response.outputitem.done carrying completed items. The function calling guide defines arguments as a JSON string on the completed functioncall item.