OpenAI has rolled out GPT-5.1, and this update is much more than a routine version bump. It brings speed improvements, better accuracy, a friendlier personality, and smarter reasoning for complex tasks. And for the first time, the “Instant” model can decide when to think harder before answering.

In short: GPT-5.1 fixes the biggest issues people had with GPT-5 while making it more enjoyable to use.

The Two New Models: 5.1 Instant and 5.1 Thinking

There are two versions in the 5.1 family:

1. GPT-5.1 Instant

• Designed for fast, conversational interactions
• Warmer, more personable responses
• Much better at following instructions
• Able to “think” when needed using adaptive reasoning

2. GPT-5.1 Thinking

• The heavy-duty model for complex reasoning
• No more over-thinking simple questions
• Now calibrates how long it should think based on difficulty

Together, they create a smoother experience: quick when you need speed, deep when you need accuracy.

A Better Personality (People Really Asked for This)

One of the biggest complaints about GPT-5 was that it felt… bland. many users missed the friendliness and tone of GPT-4.
OpenAI listened.
GPT-5.1 Instant now:
Sounds more natural

• References your memory more fluidly
• Matches texting-style chats more closely
• Feels more human

For example:

GPT-5 response:
Formal, correct, but robotic.

GPT-5.1 Instant response:
“I’ve got you. That’s totally normal with everything going on lately.”
More supportive, more conversational.
OpenAI also introduced new tone presets:

• Friendly
• Efficient
• Professional
• Candid
• Quirky

And you can customize the personality further inside ChatGPT.

Massive Improvements in Instruction Following

This is one area where GPT-5 struggled.
Ask it to “respond with six words”? It might give you sixteen.

GPT-5.1 Instant now follows instructions almost perfectly.

Example:
Prompt: “Always respond in six words.”
5.1 Instant:
“Consider Japan, Italy, Greece, Canada, Iceland.”
Six words. Followed again in the next reply. No hiccups.

For developers, this is a big deal for building deterministic LLM tools.

Adaptive Reasoning: Think When Necessary, Respond Fast When Not

This is one of the most meaningful upgrades.
GPT-5.1 Instant now decides how much reasoning time a question needs.
Easy questions? Fast answers.
Hard questions? Longer thinking.
A benchmark showed:

• Less thinking time on easy prompts
• Similar thinking time on medium ones
• Much more thinking time on hard ones

This leads to:

• Faster responses overall
• Better accuracy on challenging tasks
• Smarter token usage for developers
  

Developer Updates: What’s New in the API

For developers building apps:

• GPT-5.1 available in API today
• Prompt caching extended to 24 hours
• Set reasoning effort to "none" for ultra-fast responses
• Smarter token routing based on question difficulty
• More efficient overall inference costs

This update is one of the most developer-friendly since GPT-4 Turbo.