OpenAI releases GPT-5.5 API with new prompting guide—developers must rewrite existing prompts from scratch rather than adapt old ones

Simon Willison's Weblog · April 25, 2026

AI Summary

•OpenAI made GPT-5.5 available in its API on April 25, 2026, and published a prompting guide warning developers that the new model is not a drop-in replacement for GPT-5.2 or GPT-5.4. The guide explicitly states: 'treat it as a new model family to tune for' and advises starting with fresh prompts instead of carrying over instructions from older versions.
•One concrete technique OpenAI highlights: for long-running tasks, send a brief user-visible message acknowledging the request and stating the first step before any tool calls—this prevents users from thinking the AI has crashed. OpenAI's own Codex app already implements this pattern.
•For developers maintaining applications built on older GPT models, this means existing prompt optimizations won't automatically work with GPT-5.5—they'll need to re-test and re-tune reasoning effort, verbosity (how much the model explains), tool descriptions, and output format against actual examples. Teams with hundreds of production prompts face weeks of validation work to migrate safely.
•To upgrade existing code, developers can run the command `$openai-docs migrate this project to gpt-5.5` in Codex, which provides a migration guide with instructions for rewriting prompts to fit the new model.

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