OpenAI: GPT-5.1 Chat

Generates conversational responses using selective chain-of-thought reasoning that dynamically allocates compute based on query complexity. The model employs adaptive inference to determine when extended reasoning is necessary versus when direct response generation suffices, reducing latency for straightforward queries while maintaining reasoning depth for complex problems. Optimized for real-time chat interactions with sub-second response times.

Maintains and processes conversation history across multiple turns using a sliding context window with automatic token budgeting. The model tracks conversation state through explicit role-based message formatting (system/user/assistant) and manages context overflow by intelligently truncating or summarizing older messages when approaching token limits. Supports system prompts for behavioral conditioning and maintains coherence across 50+ turn conversations.

Delivers chat completions as server-sent events (SSE) with token-by-token streaming, enabling real-time response rendering in client applications. The implementation uses HTTP/2 streaming with chunked transfer encoding to emit completion tokens as they are generated, reducing perceived latency and enabling progressive UI updates. Supports both streaming and non-streaming modes with identical API signatures.

Enables the model to invoke external tools by generating structured function calls based on a developer-provided schema registry. The model receives tool definitions as JSON schemas, reasons about which tools to invoke and with what parameters, and returns structured function calls that applications can execute. Supports parallel function calls, sequential tool chaining, and automatic retry logic for failed tool invocations.

Processes images alongside text in chat completions, enabling the model to analyze visual content and answer questions about images. The implementation accepts images as base64-encoded data or URLs, supports multiple images per request, and integrates vision understanding with text reasoning in a unified forward pass. Vision tokens are counted separately from text tokens in usage metrics.

Constrains model outputs to conform to developer-specified JSON schemas, ensuring responses are valid, parseable structured data. The model generates responses that strictly adhere to provided schemas, with built-in validation preventing invalid JSON or schema violations. Supports nested objects, arrays, enums, and complex type definitions with automatic schema enforcement during generation.

Provides granular token-level pricing with separate accounting for input, output, and vision tokens, enabling precise cost prediction and optimization. The model returns detailed token usage metrics per request, allowing developers to track costs at request granularity and optimize prompts based on token efficiency. Pricing is lower than GPT-5.1 Preview due to the Instant variant's optimized inference.