# Anthropic compatibility

> Use Claude Code and Anthropic SDKs with the Messages API.

`POST /v1/messages` speaks the Anthropic Messages API. Claude Code and the Anthropic SDKs add `/v1/messages` themselves, so their base URL has no `/v1`.

```python
import anthropic

client = anthropic.Anthropic(
    base_url="https://deference.si",
    api_key="sk-df-...",
)

message = client.messages.create(
    model="anthropic/claude-sonnet-5.5",
    max_tokens=1024,
    messages=[{"role": "user", "content": "Say hello in one sentence."}],
)
print(message.content[0].text)
```

## What passes through

* `anthropic-version` and `anthropic-beta` headers pass through as sent. The body keeps its Anthropic format, including thinking and tool-use blocks.
* Streams are relayed as they arrive, including `ping` events. Nothing is buffered.
* `system` and `cache_control` blocks keep their position, so prompt caching works.
* A query string such as `?beta=true` is ignored.
* Error bodies from the provider are returned unchanged, and `retry-after` is a whole number of seconds.
* `anthropic-ratelimit-*` and `x-should-retry` response headers are passed on when the provider sends them.

Set `max_tokens` to an output limit your available credit can cover. Deference never lowers it on Messages. A provider-run `web_search` tool's `max_uses` cannot exceed the request's allowed tool steps, at most 30. If omitted, it uses that allowance. See [Messages](https://deference.si/docs/api-reference/messages#credit-held).

## Credentials

Send the key as `x-api-key: sk-df-...` or as `Authorization: Bearer sk-df-...`. The Anthropic SDK sends the first. Claude Code sends the second when you set `ANTHROPIC_AUTH_TOKEN`.

## Model ids

Use the catalog id, for example `anthropic/claude-sonnet-5.5`. A trailing `[1m]` is accepted: Deference ignores it when it looks up the model and its price, and forwards the body unchanged. Other providers' models are accepted too, but thinking blocks and tool use follow each model's own behavior.

## Errors

Errors use Anthropic's envelope: `type: "error"`, an `error` object with `type` and `message`, and a `request_id`. See [Errors](https://deference.si/docs/api-reference/errors).

## Not available

`POST /v1/messages/count_tokens` is not available. Claude Code falls back to an estimate when it cannot count tokens.