# Anthropic Claude API token counting API messages count tokens model-specific tokenizer context window docs

Status: search-observed
Product: Claude API
Last verified: 2026-06-25
Canonical HTML: https://gitdocs.org/fix/anthropic-claude-api-token-counting-api-messages-count-tokens-model-specific-tokenizer-context-w
Machine JSON: https://gitdocs.org/api/fixes/anthropic-claude-api-token-counting-api-messages-count-tokens-model-specific-tokenizer-context-w.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 token counting API messages count tokens model-specific tokenizer context window docs
- site:docs.anthropic.com count tokens messages API Claude token counting
- https://docs.anthropic.com/en/docs/build-with-claude/token-counting
- https://platform.claude.com/docs/en/build-with-claude/context-windows

## 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/docs/build-with-claude/token-counting
- Official reference opened by Codex: https://platform.claude.com/docs/en/build-with-claude/context-windows