GPT-5.1 Chat (also known as Instant) is the fast, lightweight member of the 5.1 family, optimized for low-latency chat while retaining strong general intelligence. It uses adaptive reasoning to selectively “think” on harder queries, improving accuracy on math, coding, and multi-step tasks without slowing down typical conversations. The model is warmer and more conversational by default, with better instruction following and more stable short-form reasoning. GPT-5.1 Chat is designed for high-throughput, interactive workloads where responsiveness and consistency matter more than deep deliberation.
Pricing
Pay-as-you-go rates for this model. More details can be found here.
Input Tokens (1M)
$1.25
Cached Input Tokens (1M)
$0.13
Output Tokens (1M)
$10.00
Capabilities
Input Modalities
Text
Image
File
Output Modalities
Text
Supported Parameters
Available parameters for API requests
Frequency Penalty
Function Call
Functions
Logit Bias
Max Completion Tokens
Parallel Tool Calls
Prediction
Presence Penalty
Reasoning Effort
Response Format
Stop
Tool Choice
Tools
Web Search Options