OpenAI hurriedly released GPT-5.1 this week, perhaps moving first to head-off the building hype around the yet to emerge Gemini 3.0. It’s a modest update on paper but will change how to get the most from the model in practice.
Here’s a summary of what’s new.
- Adaptive reasoning now spends less effort on simple tasks and more on complex ones, improving speed where it matters.
- Eight personality presets (Default, Professional, Friendly, Candid, Quirky, Efficient, Nerdy, Cynical) that affect content, not just tone.
- GPT-5.1-Codex supports a 400,000-token context window, making whole-codebase reasoning more realistic.
- A new “none” reasoning-effort setting helps avoid overthinking and saves tokens on simple queries.
- Instruction following is more reliable, and as Sam Altman posted on X: “If you tell ChatGPT not to use em-dashes in your custom instructions, it finally does what it’s supposed to do!”
- Writing quality is on par with or better than Claude for creative work.
- Pricing is unchanged.
- GPT-5 models stay in the legacy menu until February 2026.
- Some users report needing prompt tweaks to stop over-analysis on simple tasks.
OpenAI’s updated prompting guide highlights some specific techniques for 5.1:
- Emphasise persistence, not brevity GPT-5.1 tends to be very concise. Tell it to “persist until the task is fully handled end-to-end” and “bias for action” if you want thorough, practical outputs.
- Control verbosity with clear limits It follows length rules exactly. Use concrete ranges like “2–5 sentences” or “6–10 sentences” and state simple rules such as “no before/after pairs” or “at most 1–2 short snippets”.
- Define agent personas in detail Don’t just say “be friendly”. Describe how it should think and prioritise, e.g. “You care about clarity, momentum, and respect measured by usefulness rather than pleasantries.” The model will adjust its whole approach.
- Use metaprompting to debug prompts Paste your system prompt plus examples of failures and ask GPT-5.1 to explain what went wrong. It is good at spotting conflicts like “be concise” versus “be exhaustive” and suggesting targeted changes.
- Add simple planning scaffolds for complex work For multi-step tasks, have it maintain a small list of milestones (2–5 items) with statuses like pending, in progress, complete. This keeps long runs from ending too early.
- Remove contradictions in your instructions Because it follows instructions closely, conflicting guidance leads to odd behaviour. Clean up tensions between autonomy and clarification, short answers versus complete ones, and tool-usage rules.
Takeaways: GPT-5.1 thinks harder when tasks demand it but can still overcomplicate easy problems if you let it. The personality system now affects what it does, not just how it sounds, and the boosted Codex context window makes genuine multi-file reasoning more practical for developers. To get the best from it, you need tighter prompt design, clearer personas, and simple planning structures baked into your instructions.