[HDBR]

Summary

Two small fixes on the Codex hosted image_generation path (the gpt-5.6-*-image catalog entries that #14976 deliberately left on the Responses tool):

• background is now forwarded to the tool. The hosted tool accepts background ("transparent", "opaque", "auto"), but the handler only copied size and quality, so a logo or sticker request with background: "transparent" always came back on an opaque canvas. The dedicated Images API path (fix(providers): route Codex GPT Image models to dedicated Images API #14976) already forwards it; this aligns the hosted path.
• Large inline reference images are downscaled before upload. On /v1/images/edits the backend answers 503 when an inline reference is large (we saw it consistently above ~400 KB, from a self-hosted instance in production use). References above 400 KB are now resized to a 1024 px long edge with the existing optional-sharp helper (normalizeImageBuffer); smaller references, and platforms where sharp fails to load, pass through unchanged.

Changes

• open-sse/handlers/imageGeneration.ts: toolConfig.background from body.background (trimmed, ignored when blank); new exported shrinkCodexReferenceImage() used for each reference image.
• tests/unit/image-generation-handler.test.ts: one test for background forwarding (set and blank), one for shrinkCodexReferenceImage (small passthrough; a >400 KB PNG shrinks to a ≤1024 px long edge and keeps its mime).
• changelog.d/fixes/: fragment.

Tests

node --import tsx/esm ... --test tests/unit/image-generation-handler.test.ts
tests 52, pass 52, fail 0

Injected defect: with the previous handler and the new tests, the two new tests fail (50 pass, 2 fail).
Prettier: clean. ESLint on the two touched files: the same 6 pre-existing findings as the base, none on changed lines.

[diegosouzapw]

Thanks @HDBR — Codex hosted image generation forwards background and shrinks reference images larger than 400 KB. This is in the upcoming release.