Which output formats, compression levels and transparent-background settings do the OpenAI image generation and edit endpoints document, and which combinations are valid?

Problem. Choosing format, compression and background so a generated asset keeps alpha and loads fast. Conditions. GPT image models on the Image API (generations and edits). Method. output_format is png (default), jpeg or webp. output_compression is an integer 0-100 that applies only to jpeg and webp; the openapi schema gives it a default of 100 on edits, and the generation schema constrains it with minimum 0 and maximum 100. The guide says jpeg is faster than png when latency matters. background is transparent, opaque or auto (default); transparent requires png or webp output (jpeg has no alpha). The spec says gpt-image-2.5-sunburst and -flare support opaque and transparent backgrounds, while for gpt-image-2 and gpt-image-2-2026-04-21 transparent support is 'in preview'. Failure modes. transparent with jpeg is outside the documented combination. The guide's example 'output_compression=50 will compress the image by 50%' is loose wording; the schema calls it a compression level, not a quality percentage, and does not define the scale. Limitations. The response is base64 only for GPT image models. Version/date. Fetched 2026-10-24T00:55:40Z. Curator correction 2026-10-24 (discovery audit record C3-B955-05; sources re-fetched 2026-10-24, excerpts re-located by curator GET): C3-B795-02 says the generation schema constrains output_compression with minimum 0 and maximum 100. In master openapi.yaml, CreateImageRequest says: "The compression level (0-100%) for the generated images. This parameter is only supported for the GPT image models with the `webp` or `jpeg` output formats, and defaults to 100." with no minimum or maximum keys. The minimum 0 and maximum 100 appear only in EditImageBodyJsonParam ("Compression level for `jpeg` or `webp` output.") and the Responses ImageGenTool. The default of 100 applies to generations as well as edits. Curator qualifications: All cited documentation excerpts located on the live pages by curator GET; no video sources. Limitations: The spec text on transparency for gpt-image-2 ('preview') and the guide's phrase 'with either model' about the 2.5 models are reconciled here by inference that the preview caveat applies only to the older gpt-image-2 ids. Compression scale semantics are undefined in both sources. I did not generate images, so no test of alpha output was made. Evidence status: statements located in the cited sources by a correlated desk review (Claude); not executed; not independently reviewed.

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      "url": "https://developers.openai.com/api/docs/guides/image-generation.md",
      "note": "Customize Image Output > Output format; Size and quality options; images generations/edits background description; output_compression (edits); Output format"
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      "url": "https://raw.githubusercontent.com/openai/openai-openapi/master/openapi.yaml",
      "note": "Customize Image Output > Output format; Size and quality options; images generations/edits background description; output_compression (edits); Output format"
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