GPT-5.1
GPT-5.1 is an older OpenAI model that you should only use if an existing app depends on it. OpenAI released it on 12 November 2025 as a friendlier, more adaptive update to GPT-5. It came in two modes: GPT-5.1 Instant for quick replies and GPT-5.1 Thinking for harder problems.
OpenAI removed GPT-5.1 from ChatGPT on 11 March 2026. It is still in the API at $1.25 per million input tokens and $10 per million output tokens, with a 400,000-token context window. Newer models such as GPT-6 Sol are far stronger on work tasks and, at $2 / $10, cost about the same.
GPT-5.1 was a solid step when it launched, but in September 2026 it is a legacy model. Its main gains over GPT-5 were tone and control. It sounded warmer, followed instructions better and spent less time thinking on easy questions ("adaptive reasoning"). It also introduced personality presets such as Professional, Candid and Nerdy.
Its weak spot was real office work. On OpenAI's GDPval test of professional tasks, GPT-5.1 Thinking scored 38.8%. Just one month later GPT-5.2 Thinking scored 70.9%, according to OpenAI.
Who should use it: only teams with a working app built on gpt-5.1 who cannot retest yet. Its 2024 knowledge cutoff is also older than any newer model's.
Who should not: anyone starting fresh. GPT-6 Sol costs $2 / $10 per million tokens, only slightly more for input and the same for output, and is a much newer model. For the cheapest jobs, GPT-6 Luna at $0.10 / $0.50 is far less expensive.
Score breakdown
Best for
- Keeping an existing GPT-5.1 integration running
- Comparing old and new model behaviour in tests
- Low-stakes chat features built in late 2025
What we like
- Cheap by frontier standards at $1.25 / $10 per million tokens
- Adaptive reasoning spends less time on easy prompts
- Large 400K-token context window
- Still available in the API for existing apps
Watch out for
- Removed from ChatGPT on 11 March 2026
- Oldest knowledge cutoff of the GPT-5.x line (September 2024)
- Much weaker than GPT-5.2 and later on professional tasks (38.8% GDPval)
Specs at a glance
| Developer | OpenAI |
|---|---|
| Release date | 12 November 2025 (ChatGPT); API later that week |
| Variants | GPT-5.1 Instant (gpt-5.1-chat-latest), GPT-5.1 Thinking (gpt-5.1) |
| API snapshot | gpt-5.1-2025-11-13 |
| Context window | 400,000 tokens |
| Max output | 128,000 tokens |
| Knowledge cutoff | 30 September 2024 |
| Input / output | Text and image in; text out |
| API pricing | $1.25 input, $0.125 cached input, $10 output per 1M tokens |
| Predecessor / successor | GPT-5 / GPT-5.2 |
| ChatGPT retirement | 11 March 2026 |
Benchmarks
Benchmarks are standard tests. Vendor-run results are marked as such; independent results are preferred where they exist.
| Benchmark | Score | Source | Note |
|---|---|---|---|
| GDPval (wins or ties vs experts) | 38.8% | OpenAI (GPT-5.2 launch post) | GPT-5.1 Thinking |
| SWE-bench Verified | 76.3% | OpenAI (GPT-5.2 launch post) | GPT-5.1 Thinking |
| SWE-bench Pro | 50.8% | OpenAI (GPT-5.2 launch post) | |
| GPQA Diamond | 88.1% | OpenAI (GPT-5.2 launch post) | |
| AIME 2025 | 94.0% | OpenAI (GPT-5.2 launch post) | No tools |
| FrontierMath Tier 1–3 | 31.0% | OpenAI (GPT-5.2 launch post) | |
| ARC-AGI-2 | 17.6% | OpenAI (GPT-5.2 launch post) |
Pricing
| Plan / tier | Price | Notes |
|---|---|---|
| API input | $1.25 per 1M tokens | |
| API cached input | $0.125 per 1M tokens | |
| API output | $10 per 1M tokens | |
| ChatGPT | Not available | Retired from all ChatGPT plans on 11 March 2026. |
What GPT-5.1 changed
OpenAI pitched GPT-5.1 as a more pleasant GPT-5, not a smarter one. The launch post listed four main changes (vendor claims):
- Warmer default tone and clearer answers with less jargon.
- Adaptive reasoning: GPT-5.1 Instant could briefly think before answering hard questions, and GPT-5.1 Thinking adjusted its thinking time to each question.
- Better instruction following, for example sticking to a requested word count.
- Personality presets: Default, Professional, Friendly, Candid, Quirky, Efficient, Nerdy and Cynical, plus sliders for things like conciseness and emoji use.
Paid users got it first. GPT-5 stayed in the Legacy menu for three months.
GPT-5.1 vs later OpenAI models
| Model | Released | API price (in / out per 1M) | Context | Status |
|---|---|---|---|---|
| GPT-5.1 | Nov 2025 | $1.25 / $10 | 400K | Superseded |
| GPT-5.2 | Dec 2025 | $1.75 / $14 | 400K | Superseded |
| GPT-5.4 | Mar 2026 | $2.50 / $15 | 1.05M | Superseded |
| GPT-6 Sol | Sep 2026 | $2 / $10 | see model page | Current |
Prices from OpenAI's API docs and launch posts, as of 23 September 2026.
Should you migrate?
Yes, and soon. OpenAI has already removed GPT-5.1 from ChatGPT and has said it will give advance notice before it retires API models. Start by running your test prompts against GPT-6 Sol. If cost is the main concern, try GPT-6 Luna for simple extraction or summarising jobs. Watch for changes in answer length and tone, since GPT-5.1's personality settings do not carry over.
- Pin the snapshot
gpt-5.1-2025-11-13rather than a moving alias so behaviour does not shift before you migrate. - Use cached input for repeated system prompts. It costs $0.125 per million tokens, a tenth of the normal input price.
- When you test GPT-6 Sol as a replacement, compare answer length first. GPT-5.1's warmer style often produced longer replies.
- Turn on web search for anything after September 2024. The model's own knowledge stops there.
Jargon explained
- Adaptive reasoning
- The model decides how long to think based on how hard the question looks, so easy questions get fast answers.
- GDPval
- An OpenAI test where expert judges compare a model's work on real job tasks, like reports or spreadsheets, with work by human professionals.
- Knowledge cutoff
- The date the model's training data ends. It will not know later events unless it searches the web.
- Cached input
- Text the API has seen recently, such as a repeated system prompt. OpenAI charges much less to reuse it.
Alternatives to consider
Frequently asked questions
When was GPT-5.1 released?
OpenAI released GPT-5.1 Instant and GPT-5.1 Thinking in ChatGPT on 12 November 2025, starting with paid users. The API followed later that week.
Is GPT-5.1 still available?
Only through the OpenAI API. It was retired from ChatGPT on 11 March 2026.
How much does GPT-5.1 cost?
$1.25 per million input tokens, $0.125 per million cached input tokens and $10 per million output tokens, per OpenAI's API docs.
What replaced GPT-5.1?
GPT-5.2 on 11 December 2025. As of September 2026 OpenAI's current models are GPT-6 Astra, GPT-6 Sol and GPT-6 Luna.
What is GPT-5.1's knowledge cutoff?
30 September 2024, according to OpenAI's model page. It does not know about events after that unless it searches the web.
Sources
Every fact on this page comes from public information. Vendor figures are labelled as vendor claims.
- GPT-5.1: A smarter, more conversational ChatGPT (OpenAI)
- GPT-5.1 model page (OpenAI API docs)
- Introducing GPT-5.2 (benchmark table incl. GPT-5.1 Thinking) (OpenAI)
- GPT-5.1 (Wikipedia)
- GPT-5.1 on OpenRouter (OpenRouter)
- ChatGPT release notes (OpenAI Help Center)