Simon Willison · 2026-04-25 · notable
Simon Willison: OpenAI Recommends Treating GPT-5.5 as an Entirely New Model Family
Simon Willison highlights OpenAI's official GPT-5.5 prompting guide, which recommends starting from scratch — not porting old prompts. Key changes: outcome-first prompting, streaming preambles for agentic tasks, and a migration tool that can rewrite existing prompt stacks automatically.
OpenAI says old prompts may perform worse on GPT-5.5 — here's what to change and a migration tool to help.
What is it?
Simon Willison's April 25 post covers OpenAI's official GPT-5.5 prompting guide, which is unusual for a model update: OpenAI explicitly recommends treating GPT-5.5 as a new model family rather than a drop-in replacement. The guide warns that prompt templates optimized for GPT-5.4 and earlier may perform worse on GPT-5.5 because the model is 'more opinionated about output format and style.' OpenAI's Codex app includes a migration command that automatically rewrites existing prompt stacks for the new model.
How does it work?
Three key recommendations from the guide: (1) Outcome-first prompting — describe the goal, success criteria, and constraints, then let the model choose the path rather than prescribing steps; (2) Streaming preambles — for multi-step agentic tasks, instruct the model to send a short user-visible acknowledgment before starting tool calls, to prevent UI stall confusion; (3) Start minimal — use the fewest instructions that maintain functionality, then add reasoning depth, verbosity controls, and tool definitions based on real examples. The `text.verbosity` API parameter controls output length.
Why does it matter?
Official prompting guides for frontier models are rare, and a 'start from scratch' recommendation backed by an automated migration tool signals that GPT-5.5's behavioral changes are significant enough to warrant coordinated re-tuning across the ecosystem. For teams running production systems on GPT-5.4, this is actionable immediately.
Who is it for?
Developers and teams with existing GPT-4 and GPT-5.4 production prompts to migrate