OpenAI: GPT 5.1 Thinking (Fast)
GPT 5.1 Thinking (Fast) is a reasoning model from OpenAI released on Nov 12, 2025. It reads text, images and files and writes text, with a 400K token context window and up to 128K tokens per reply.
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It costs $2.50 per million input tokens and $20 per million output tokens. Cached input bills at $0.25, 90% less than fresh input.
It supports tool calling and prompt caching.
- Modalities
- In / out price
- $2.50 / $20 per 1M
- Context
- 400K
- Max output
- 128K
- Released
- Nov 12, 2025
- Knowledge cutoff
- n/a
- Tool calling
- Structured output
- Reasoning
- Prompt caching
- Open weights
- Free to use
GPT 5.1 Thinking (Fast) pricing
USD, as listed on Oct 15, 2026. Most bills are input heavy, so the cached input rate matters as much as the headline price.
| Input | $2.50 / 1M tokens | |
|---|---|---|
| Output | $20 / 1M tokens | |
| Cached input (read) | $0.25 / 1M tokens | 90% off input |
What GPT 5.1 Thinking (Fast) costs per month
Pick a workload close to yours or type your own volumes.
Supported request parameters
What you can set when you call GPT 5.1 Thinking (Fast) through an OpenAI compatible API.
- Tools
tools - Reasoning
reasoning
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GPT 5.1 Thinking (Fast) questions
How much does the GPT 5.1 Thinking (Fast) API cost?
$2.50 per million input tokens and $20 per million output tokens, with cached input at $0.25. The estimator on this page turns that into a monthly bill for your own workload.
What is the context window of GPT 5.1 Thinking (Fast)?
400,000 tokens of input, and it can write up to 128,000 tokens in one reply. As a rough guide, a million tokens is around 750,000 English words.
Does GPT 5.1 Thinking (Fast) support tool calling?
Yes. It accepts tool definitions, which is what an agent needs to call your APIs.
Is GPT 5.1 Thinking (Fast) open source?
No. OpenAI serves it through an API only, so you cannot download the weights or host it yourself.
When was GPT 5.1 Thinking (Fast) released?
Nov 12, 2025.
Catalog data from models.dev (MIT licensed), refreshed daily, last on Oct 15, 2026. The summary is written from these facts.
Building on GPT 5.1 Thinking (Fast)?
I test your real tasks on it and the two closest alternatives, then ship the agent on whichever wins on cost and accuracy.
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