> ## Documentation Index
> Fetch the complete documentation index at: https://langwatch.ai/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Anthropic

> Configure Anthropic (Claude) as a provider behind the LangWatch AI Gateway.

Anthropic is one of the two core providers for the gateway (alongside OpenAI). The `/v1/messages` endpoint is Anthropic-native and designed to be Claude-Code-compatible.

## Configure the provider credential

Under **Settings → Model Providers**:

1. Click **Add provider** → **Anthropic**.
2. Paste your Anthropic API key (starts `sk-ant-...`).
3. Save.

The same credential serves the gateway, the evaluators' litellm path, and any other place in LangWatch that needs an Anthropic call.

## Bind to a VK

Pick Anthropic as a primary or fallback on any VK. When a VK uses Anthropic as its primary, its clients should ideally call `/v1/messages` (Anthropic-native shape). But `/v1/chat/completions` also works, the gateway uses bifrost/core to translate the request shape to Anthropic's Messages API and back.

## Supported endpoints

| Gateway route                           | Upstream                         | Notes                                                                         |
| --------------------------------------- | -------------------------------- | ----------------------------------------------------------------------------- |
| `POST /v1/messages`                     | `POST /v1/messages`              | Native shape. Preferred for Claude Code, Anthropic SDK, any tool-using agent. |
| `POST /v1/chat/completions`             | `POST /v1/messages` (translated) | OpenAI SDKs work transparently via translation.                               |
| `POST /v1/embeddings`                   | ❌ not supported                  | Anthropic has no embeddings endpoint.                                         |
| `POST /v1/messages` with `stream: true` | `POST /v1/messages` streaming    | SSE pass-through byte-for-byte (tool-call deltas preserved).                  |

## `cache_control` passthrough, the 90-percent discount

Anthropic supports prompt caching via `cache_control` blocks on content. Ephemeral (5-min TTL) and persistent (1-hour TTL) caches are both honoured.

The gateway's hard invariant: **`cache_control` blocks are forwarded byte-for-byte in `cache.mode = respect`** (default). If you see cache-hit costs not matching expectations, first check `X-LangWatch-Cache` response header, `miss` means no cache was hit; `hit` with non-zero `cache_read_input_tokens` means the discount applied.

Usage reporting (in the response body and trace) separates three counters:

* `cache_read_input_tokens`, tokens served from cache (priced at \~10%).
* `cache_creation_input_tokens`, tokens written to cache (priced at 125%).
* `input_tokens`, regular cold input tokens.

The gateway sums these and computes per-request cost for the budget ledger.

See [Caching Passthrough](/docs/ai-gateway/caching-passthrough) for the full decision matrix and override headers.

## Known quirks

* **`max_tokens` required**, Anthropic requires `max_tokens` on every Messages request. The gateway does not default-insert a value; a client that omits it gets the upstream 400 back.
* **System prompt structure**: Anthropic Messages uses a top-level `system` field (string or content blocks), not a message role. OpenAI-SDK clients that send a `system` role message get it rewritten into the top-level field by the translator.
* **Tool-call streaming deltas**: Anthropic emits `content_block_delta` events with `input_json_delta` partial JSON. The gateway proxies these byte-for-byte post-first-chunk so Claude Code (and any other agent) can reconstruct the tool call incrementally.
* **`thinking` (extended-reasoning) blocks**, available on Claude Opus, Sonnet extended-thinking models. Blocks appear in the response; pricing is separate. Thinking-token counts roll into `langwatch.usage.output_tokens` + debit the budget; a dedicated `langwatch.reasoning_tokens` span attribute is a v1.1 observability follow-up.

## Using via Claude Code

See [Claude Code](/docs/ai-gateway/cli/claude-code): set `ANTHROPIC_BASE_URL=https://gateway.langwatch.ai` and `ANTHROPIC_AUTH_TOKEN=vk-lw-…` and it works.