ChatGPT 5.1 vs 5: Differences, API Compatibility, and Pricing

Short answer

GPT-5.1 updated the GPT-5 series, but the differences depend on whether you mean ChatGPT or the API. In ChatGPT, OpenAI described GPT-5.1 Instant as warmer and better at following instructions, and GPT-5.1 Thinking as easier to understand, faster on simple tasks, and more persistent on complex ones. In the API, OpenAI announced adaptive reasoning, a none reasoning-effort setting, extended prompt caching, and new coding tools for GPT-5.1. These are documented changes—not proof that GPT-5.1 performs better on every workload. [1][2]

For a decision now, note that both generations have been retired from ChatGPT, and OpenAI’s API model pages mark gpt-5 and gpt-5.1 as deprecated. Treat them as legacy models rather than default choices for a new production integration. The cited model pages do not give a shutdown date; OpenAI documents a way to check for one below. [3][4][5]

What “ChatGPT 5.1 vs 5” means

The phrase can refer to the ChatGPT experience or to API models. Those are related comparisons, but product options, API model IDs, and their availability are not interchangeable.

• ChatGPT: GPT-5.1 Instant and GPT-5.1 Thinking followed GPT-5’s Instant and Thinking options. OpenAI’s launch description says GPT-5.1 Thinking was easier to understand, faster on simple tasks, and more persistent on complex ones. That is a product-level description, not a controlled head-to-head benchmark. OpenAI’s retirement notice lists GPT-5 Instant and Thinking as retired on February 13, 2026, and GPT-5.1 Instant, Thinking, and Pro as retired on March 11, 2026. [1][3]
• API: The model pages list gpt-5 with snapshot gpt-5-2025-08-07 and gpt-5.1 with snapshot gpt-5.1-2025-11-13. Both pages mark the model and snapshot as deprecated. [4][5]

ChatGPT subscriptions and API usage are separate purchasing channels. The token-price comparison below concerns API usage; it is not a ChatGPT subscription comparison. [3][4][5]

GPT-5.1 vs GPT-5: documented differences

Area	GPT-5	GPT-5.1	What it means
ChatGPT experience at launch	Offered Instant and Thinking experiences.	OpenAI described Instant as warmer and better at following instructions; Thinking as easier to understand, faster on simple tasks, and more persistent on complex ones.	These are OpenAI’s product descriptions, not a guarantee that every user or prompt will prefer 5.1. They are not a controlled benchmark. Both generations are now retired from ChatGPT. [1][3]
API reasoning controls	The model page lists minimal, low, medium, and high for reasoning.effort. OpenAI also documented a verbosity setting with low, medium, and high.	The model page lists none (the default), low, medium, and high for reasoning.effort. OpenAI described GPT-5.1 as adapting its reasoning to task complexity.	The documented settings differ: GPT-5 lists minimal; GPT-5.1 lists none as its default. Do not assume the controls or defaults are identical. [2][4][5]
Task-dependent API behavior	—	In its developer announcement, OpenAI said GPT-5.1 spends fewer tokens thinking on straightforward tasks and remains persistent on difficult tasks requiring extra thinking.	This is an API announcement, distinct from the ChatGPT Thinking product description. It does not establish a universal performance advantage. [2]
Prompt caching	The cited GPT-5 launch material does not establish the 24-hour cache-retention addition.	OpenAI announced prompt caching with retention up to 24 hours, configurable through prompt_cache_retention='24h' on the Responses or Chat Completions API.	This may benefit repeated-context workloads. Savings depend on eligible cached traffic; it is not a guaranteed reduction for every request. [2]
Coding tools	OpenAI positioned GPT-5 for coding and agentic tasks.	OpenAI announced apply_patch and shell as new GPT-5.1 API tools.	These are API-announcement claims, not evidence that every endpoint or replacement model supports the tools. Check the required tool’s documentation before migrating. [2]
Listed API token rates	$1.25 per 1 million input tokens, $0.125 per 1 million cached input tokens, and $10 per 1 million output tokens.	The model page lists the same rates.	These published rates are in USD per 1 million tokens. Equal listed rates do not guarantee equal total bills: input caching, reasoning-token use, output volume, and tools can affect total usage. [4][5][8]

OpenAI’s developer announcement also includes customer-reported GPT-5.1 evaluation and speed results. Those reports reflect the customers’ own workloads and evaluations; they are not a universal, independently controlled benchmark for every application. Benchmark your own task mix before drawing a performance conclusion. [2]

Which should you choose?

If you mean the old ChatGPT options

At launch, GPT-5.1 may have been a better fit for someone who preferred the conversational style OpenAI described for Instant, or the updated Thinking behavior described in its launch announcement. GPT-5 can still serve as a historical baseline when reviewing old outputs. But OpenAI’s retirement notice lists both generations as retired from ChatGPT, so neither is a current standard ChatGPT choice. [1][3]

If you are building or maintaining an API workflow

For a new production integration, do not choose either model solely on the historical comparison: both API model pages mark them as deprecated. For an existing system, check the model’s shutdown information and test a supported replacement against your own evaluation set before migrating. [4][5][6]

For compatibility, the two model pages list Chat Completions and Responses. They also list streaming, function calling, and structured outputs as supported. The pages show the versioned snapshot IDs above. GPT-5 documents minimal reasoning effort; GPT-5.1 documents none as the default, plus low, medium, and high. These settings differ, so a migration should not assume identical defaults. [4][5]

OpenAI recommends Responses for new projects, and its migration guide says reasoning models such as GPT-5 can perform better with Responses than with Chat Completions in OpenAI’s internal evaluations. That is a general endpoint recommendation, not evidence that every GPT-5.1-specific tool or setting behaves identically across endpoints. Confirm support for apply_patch, shell, and any other features your application depends on before switching. [2][7]

Simple API cost example

Using the published rates, aggregate usage of 1 million uncached input tokens and 100,000 output tokens would cost $2.25 USD for either model: (1 × $1.25) + (0.1 × $10). This is accumulated usage across requests, not a claim that one request can contain that many tokens. It excludes tools and other charges. Cached input is listed at $0.125 per million tokens where eligible. These are API token rates, not ChatGPT subscription prices. [4][5][8]

Production API checklist

Before keeping or replacing either model:

1. Check the model ID and status. The published IDs are gpt-5 and gpt-5.1; their pages mark them deprecated and list snapshots gpt-5-2025-08-07 and gpt-5.1-2025-11-13. [4][5]
2. Check shutdown information rather than inferring a date from “deprecated.” OpenAI says to monitor shutdowns programmatically through the List models endpoint (GET /v1/models) and its optional shutdown_date field. The public model pages cited here do not state a shutdown date for these two models. [6]
3. Confirm endpoint and feature compatibility. Both model pages list Chat Completions and Responses, and list streaming, function calling, and structured outputs. Verify the exact endpoint-specific requirements for reasoning settings and any tools—especially apply_patch and shell—before migration. [2][4][5]
4. Compare like-for-like API costs. The model pages list equal input, cached-input, and output token rates in USD per 1 million tokens. Actual usage can still differ if the models consume different token volumes or your workload uses different amounts of caching or tools. [4][5][8]
5. Run regression evaluations. Use representative prompts and compare task success, output format, tool-call correctness, latency, and total usage before routing production traffic. This is a recommended evaluation method, not a test conducted for this article.

Bottom line: GPT-5.1 introduced documented conversational and API changes over GPT-5, but both models are now legacy choices: retired from ChatGPT and marked deprecated in the API model pages. For production, check the exact model status and shutdown information, verify feature compatibility, and evaluate a supported replacement against your workload instead of relying on historical performance claims.

Sources

GPT-5.1: A smarter, more conversational ChatGPT

Introducing GPT-5.1 for developers

Retiring GPT-4o and other ChatGPT models

GPT-5 Model | OpenAI API

GPT-5.1 Model | OpenAI API

Deprecations | OpenAI API

Migrate to the Responses API | OpenAI API

Pricing | OpenAI API