GPT 5.1 vs GPT 5: What Actually Changed (2025 Comparison)

OpenAI just fixed GPT 5's biggest problems.

GPT 5 launched with complaints about feeling robotic and less helpful than older models, leading OpenAI to walk back some decisions and blame performance issues on the model's router. GPT 5.1 is their response.

I tested both models for a week.

Here's every meaningful difference.

ALSO READ: How To Use GPT 5.1: Complete Prompting Guide for Developers

Speed: Faster When It Matters, Slower When It Counts

GPT 5.1 Thinking varies its thinking time dynamically—roughly twice as fast on simple tasks and twice as slow on complex ones.

What that actually means:

Simple coding question like "show me an npm command to list global packages"? GPT 5.1 answers in 2 seconds instead of 10.

Complex algorithm design? GPT 5.1 Thinking takes longer because it's actually thinking through edge cases instead of guessing fast.

Real test I ran:

I asked both models to write a function removing duplicates but keeping the last occurrence.

GPT 5 returned code in 3 seconds. It broke on edge cases with empty lists.

GPT 5.1 took 3.2 seconds. The code worked on every edge case I threw at it.

That tiny 200-millisecond hesitation? Worth it. GPT 5.1 finally understands that speed without accuracy is just fancy typing.

Instruction Following: It Actually Listens Now

GPT 5 had a bad habit. You'd say "give me exactly four sentences" and it would give you six.

When tested with arbitrary rules—write exactly four sentences about The Lion King, none starting with 'Simba' or 'The'—GPT 5 missed the rule about 'The' while GPT 5.1 followed every constraint.

I tested this myself:

Prompt: "Describe the best travel destinations in exactly six words."

GPT 5 response: "Beautiful scenery, rich culture, amazing cuisine, friendly locals, great weather, affordable prices." (10 words)

GPT 5.1 response: "Scenery culture cuisine climate friendly locals." (6 words exactly)

This matters for:

• Technical writing with word limits
• Code generation with specific constraints
• Data formatting with exact requirements
• API responses that must match schemas

Tone: Warmer Without Being Fake

GPT 5.1 Instant is warmer by default and more conversational, often surprising people with its playfulness while remaining clear and useful.

Real comparison:

I asked both: "I spilled coffee before a meeting. Do people think I'm an idiot?"

GPT 5: Standard comforting script. Very corporate. Felt like reading a Harvard Business Review article.

GPT 5.1: More raw and personal in response. Felt like talking to someone who gets it.

But here's what matters more—GPT 5.1 lets you customize communication style with personality presets like Professional, Candid, Quirky, Friendly, and Efficient.

The eight personality options:

1. Default
2. Professional
3. Candid
4. Quirky
5. Friendly (formerly Listener)
6. Efficient (formerly Robot)
7. Cynical
8. Nerdy

You can update tone, emoji frequency, or verbosity mid-conversation and the model adjusts instantly. No need to start a new chat.

Coding: Better at Complex Tasks, Smarter About Simple Ones

On SWE-bench Verified, GPT 5.1 reaches 76.3% accuracy compared to GPT 5's performance. That's a benchmark where models get a code repository and must generate a patch to solve real GitHub issues.

What changed:

GPT 5.1 with "no reasoning" mode is better at parallel tool calling, coding tasks, following instructions, and using search tools compared to GPT 5 with minimal reasoning.

Real-world impact from companies using it:

Balyasny Asset Management said GPT 5.1 "outperformed both GPT 4.1 and GPT 5 in their full evaluation suite while running 2-3x faster than GPT 5".

AI insurance company Pace reported their agents run "50% faster on GPT 5.1 while exceeding accuracy of GPT 5 and other leading models".

Adaptive Reasoning: The Hidden Upgrade

This is the feature nobody talks about but matters most.

GPT 5.1 Instant can use adaptive reasoning to decide when to think before responding to challenging questions, resulting in more thorough and accurate answers while still responding quickly.

How it works:

You don't choose when it thinks. The model decides.

Simple question? Fast answer.

Tricky question with edge cases? It pauses briefly to reason through possibilities.

Test I ran:

Asked both models: "Write a function to merge two sorted arrays without using extra space."

GPT 5: Returned code immediately. The solution used O(n) extra space.

GPT 5.1: Took an extra second. Returned an in-place solution with O(1) space complexity and handled all edge cases.

The model recognized this needed actual thinking and allocated time accordingly.

What Didn't Change (And Why That Matters)

Context window: Still the same as GPT 5. No expansion here.

Pricing: GPT 5.1 pricing and rate limits are the same as GPT 5.

Core architecture: This isn't a new model family. It's trained on the same stack and data as GPT 5.

Benchmark ceiling: Independent benchmarks show no significant difference between GPT 5 and GPT 5.1 in AIME, SWE-bench, and Terminal Bench tasks. GPT 5.1 performs better on LiveCodeBench specifically.

The "None" Reasoning Mode Nobody Mentions

GPT 5.1 introduces a "no reasoning" mode by setting reasoning_effort to 'none', making the model behave like a non-reasoning model for latency-sensitive use cases.

When to use it:

• API calls where milliseconds matter
• Real-time chat applications
• Tasks that don't need deep reasoning
• Tool-calling scenarios requiring speed

Performance gain:

Sierra reported GPT 5.1 on "no reasoning" mode showed a "20% improvement on low-latency tool calling performance compared to GPT 5 minimal reasoning".

This mode isn't available in ChatGPT web interface. It's API-only.

Image Generation: Actually Follows Instructions Now

When asked to produce alternate versions of a photo while keeping the person's face identical, GPT 5 changed the face entirely while GPT 5.1 kept the face, clothes, and body the same.

I tested this with profile photos. Asked for "different hairstyle, same face."

GPT 5: Different person entirely.

GPT 5.1: Same person, different hair.

Small detail. Huge difference if you use image generation regularly.

The Migration Reality Check

For teams using GPT 5 via API, GPT 5.1 introduces new endpoints but since it interprets prompts more literally, older prompt templates may need fine-tuning.

What breaks:

Prompts relying on flexible or conversational phrasing might need adjustment. Early testers reported minor differences in output formatting and indentation styles which can affect automation scripts.

What works immediately:

Most pipelines upgrade without code changes if you're already using GPT 5.

Extended Caching: The Cost Saver

GPT 5.1 supports extended prompt caching for up to 24-hour cache retention, driving faster responses for follow-up questions at lower cost.

What that means:

Your prompts stay in cache for 24 hours instead of minutes. Follow-up requests use cached context—lower latency, reduced cost, smoother performance.

Cached input tokens remain 90% cheaper than uncached tokens with no additional charge for cache writes or storage.

Who Should Upgrade (And Who Shouldn't Bother)

Upgrade if you:

• Need better instruction following
• Build customer-facing agents
• Write code with specific constraints
• Care about tone and personality control
• Run agents that call multiple tools

Stay on GPT 5 if you:

• Already have working prompts that don't fail
• Don't need personality customization
• Rarely hit instruction-following issues
• Use legacy models dropdown anyway

The honest take:

GPT 5 models remain available in ChatGPT's model dropdown for three months so paid subscribers can compare versions. You have time to test both.

Real-World Performance: What Companies Report

Augment Code called GPT 5.1 "more deliberate with fewer wasted actions, more efficient reasoning, and better task focus" with "more accurate changes, smoother pull requests, and faster iteration across multi-file projects".

That's from a company building production coding tools. Not a marketing claim.

The Bottom Line

GPT 5.1 fixes what people hated about GPT 5.

It follows instructions better. Sounds more human. Thinks smarter about when to go fast versus when to slow down.

GPT 5.1 doesn't reinvent the wheel; it simply rolls the wagon along more smoothly. And sometimes that's the upgrade that matters most.

Test it yourself. The differences show up fast.