Introducing Structured Outputs in the API

2. A new option for the response_format parameter: developers can now supply a JSON Schema via json_schema, a new option for the response_format parameter. This is useful when the model is not calling a tool, but rather, responding to the user in a structured way. This feature works with our newest GPT‑4o models: gpt-4o-2024-08-06, released today, and gpt-4o-mini-2024-07-18. When a response_format is supplied with strict: true, model outputs will match the supplied schema.

The tokens that are valid at the beginning of the output include things like {, {“, {
, etc. However, once the model has already sampled {“val, then { is no longer a valid token. Thus we need to implement dynamic constrained decoding, and determine which tokens are valid after each token is generated, rather than upfront at the beginning of the response.

To do this, we convert the supplied JSON Schema into a context-free grammar (CFG). A grammar is a set of rules that defines a language, and a context-free grammar is a grammar that conforms to specific rules. You can think of JSON and JSON Schema as particular languages with rules to define what is valid within the language. Just as it’s not valid in English to have a sentence with no verb, it is not valid in JSON to have a trailing comma.

Thus, for each JSON Schema, we compute a grammar that represents that schema, and pre-process its components to make it easily accessible during model sampling. This is why the first request with a new schema incurs a latency penalty—we must preprocess the schema to generate this artifact that we can use efficiently during sampling.

Structured Outputs is generally available today in the API. 

Structured Outputs with function calling is available on all models that support function calling in the API. This includes our newest models (

gpt-4o, gpt-4o-mini), all models after and including gpt-4-0613 and gpt-3.5-turbo-0613, and any fine-tuned models that support function calling. This functionality is available on the Chat Completions API, Assistants API, and Batch API. Structured Outputs with function calling is also compatible with vision inputs.

Structured Outputs with response formats is available on

gpt-4o-mini and gpt-4o-2024-08-06 and any fine tunes based on these models. This functionality is available on the Chat Completions API, Assistants API, and Batch API. Structured Outputs with response formats is also compatible with vision inputs. 

By switching to the new

gpt-4o-2024-08-06, developers save 50% on inputs ($2.50/1M input tokens) and 33% on outputs ($10.00/1M output tokens) compared to gpt-4o-2024-05-13.docs⁠(opens in a new window).