Found by generating a real image against the live server: a request with
"response_format": "b64_json" came back {url, path} and no bytes.

The request model documents OpenAI's values -- url and b64_json -- and every
OpenAI SDK sends the latter. save_image_response compared against this
codebase's own older internal spelling, "base64", so with --file-path
configured the documented behaviour ("if the client explicitly requested
base64, include it") could never fire for a standards-compliant client. It got
a URL, no bytes, and nothing in the response to say why. Nine call sites were
affected: generations, edits, inpaint, upscale, depth, segment and the rest.

Both spellings are accepted now. The url path is unchanged -- the point of
--file-path is not to ship megabytes inline.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_014T3rZnmWyKjWZerwsb27jZ