The chatgpt.com/backend-api/codex backend 400s on every tool_choice
shape for the hosted image_generation tool — it looks up tool_choice as
a function name and never recognizes hosted-tool entries. Removing the
field from _build_responses_payload() lets the host model decide; the
instructions field nudges it toward the tool.
Salvaged from PR #19979 (originally targeted the old
client.responses.stream call, which no longer exists on upstream/main;
the live request now flows through _build_responses_payload + httpx in
_collect_image_b64).
Tranquil-Flow
2bfe9fabbc