# Anthropic Claude API stop_reason refusal stop reason Messages API documentation

Status: search-observed
Product: Claude API
Last verified: 2026-06-25
Canonical HTML: https://gitdocs.org/fix/anthropic-claude-api-stop-reason-refusal-stop-reason-messages-api-documentation
Machine JSON: https://gitdocs.org/api/fixes/anthropic-claude-api-stop-reason-refusal-stop-reason-messages-api-documentation.json

## Exact Symptom

See the observed Codex queries below.

## Diagnosis

SDK types may still accept fields that the selected hosted model rejects; model retirement, tokenizer, thinking, and tool behavior are external and fast-moving.

## Fix

```
Verifier checks current model ID, accepted payload, structured output/tool parsing, and behavior under retries.
```

## Avoid

- Changing local code without checking the current external contract.
- Retrying the same install, build, or API call with no version/source change.
- Applying a broad unsafe bypass when a narrow compatibility fix is available.

## Observed Codex Queries

- Anthropic Claude API stop_reason refusal stop reason Messages API documentation
- site:docs.anthropic.com stop_reason refusal Anthropic Messages API
- Anthropic Messages API stop_reason values docs
- https://docs.anthropic.com/en/api/messages
- https://docs.anthropic.com/llms.txt
- github anthropic openapi messages stop_reason refusal

## Sources

- Claude model migration guide: https://platform.claude.com/docs/en/about-claude/models/migration-guide
- Official reference opened by Codex: https://docs.anthropic.com/en/api/messages
- Official reference opened by Codex: https://docs.anthropic.com/llms.txt