# OpenAI strict schema mode

**Source:** https://promtable.com/glossary/openai-strict-schema

> OpenAI strict schema mode is the API flag that constrains GPT output to exactly match the supplied JSON schema — required fields, types, enums — making the model unable to emit invalid output.

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OpenAI strict schema mode is the API flag that constrains GPT output to exactly match the supplied JSON schema — required fields, types, enums — making the model unable to emit invalid output.

Introduced widely by OpenAI in late 2024 and now standard in 2026, strict mode enforces JSON Schema at decode time. Combined with response_format and a tools schema, it guarantees the model's output exactly matches what your downstream code expects — no retries, no partial-JSON parsing errors. Anthropic's tool-use, Google's response_schema, and Mistral's structured output mode are all equivalent patterns. The trade-off: deeply nested unions or recursive schemas can confuse smaller models. For most extraction and tool-use workloads, strict mode is the production default in 2026.

## When to use

- Extraction tasks.
- Tool / function call arguments.
- Any structured output downstream code parses.

## Common mistakes

- Deeply nested optional unions — smaller models drift.
- Schemas with very long enum lists — slows down the model and risks quality drift.

## Related terms

- [strict-mode](https://promtable.com/glossary/strict-mode)
- [json-mode](https://promtable.com/glossary/json-mode)
- [json-schema-mode](https://promtable.com/glossary/json-schema-mode)
- [function-calling](https://promtable.com/glossary/function-calling)

*Last updated: 2026-06-01*
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Original page: https://promtable.com/glossary/openai-strict-schema
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