Anthropic Messages Endpoint

DeltaLLM provides an Anthropic Messages API-compatible endpoint at POST /v1/messages. DeltaLLM converts requests to its standard chat format. It uses the same deployment routing, budgets, rate limits, and failover as /v1/chat/completions. The target model can use a configured provider other than Anthropic.

Authentication

The endpoint uses the same authentication as every other proxy endpoint: Authorization: Bearer YOUR_API_KEY.

The Anthropic SDKs send the API key in an x-api-key header by default. This endpoint does not accept that header.

Configure your client to send a bearer token. For the Python SDK, use Anthropic(auth_token="YOUR_API_KEY", base_url="http://localhost:8000").

Example

curl http://localhost:8000/v1/messages \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-4-5",
    "max_tokens": 1024,
    "messages": [
      {"role": "user", "content": "Hello from DeltaLLM"}
    ]
  }'

The model value is a DeltaLLM public model name, resolved through the gateway's routing configuration like any other endpoint.

Supported

• Text conversations with system prompts (string or text-block form)
• text, tool_use, and tool_result content blocks, including the is_error flag on tool_result (forwarded as an Error: prefix on the tool message content)
• Tool calling: tools, tool_choice (auto, any, tool, none), including full pass-through to Anthropic and Bedrock deployments in their native formats
• Streaming via "stream": true — responses use Anthropic SSE event types (message_start, content_block_start, content_block_delta, content_block_stop, message_delta, message_stop), including streamed tool use
• max_tokens (required), temperature, top_p, stop_sequences, metadata

Not Supported

Unsupported input is rejected with a 400 invalid_request_error — never silently dropped. This includes:

• image and document content blocks (and any other non-text, non-tool block types)
• top_k
• Extended thinking (top-level thinking field), prompt caching (cache_control on any block), citations (citations on text blocks), and any other unrecognized top-level field or block field

Errors

Errors use the Anthropic error envelope:

{
  "type": "error",
  "error": {
    "type": "invalid_request_error",
    "message": "messages[0].content[1]: unsupported content block type 'image'; this endpoint only forwards text and tool blocks"
  }
}

Each failure before the response starts uses this envelope. This includes authentication, request validation, rate limits, budget exhaustion, and upstream or provider failures. DeltaLLM never forwards provider response bodies or messages. It maps the gateway status to an Anthropic error type:

• authentication_error.
• permission_error.
• not_found_error.
• rate_limit_error.
• overloaded_error.
• api_error.

For rate limits, DeltaLLM keeps a valid Retry-After header.

Gateway errors use the same HTTP status codes as the OpenAI-compatible endpoints. An unclassified upstream 401, 403, or 404 is a deployment or configuration failure. DeltaLLM can try another healthy deployment. After all failover attempts fail, the client receives a sanitized 503 overloaded_error. A trusted content-policy or context-window classification remains a terminal 400 invalid_request_error.

If a provider fails after a streaming 200 response has already emitted content, DeltaLLM does not retry or replace the response. It emits one sanitized Anthropic event: error frame and closes the stream without a message_stop event.

message_stop delivery follows the shared text endpoint's configured accounting writer and required audit. Closing at that event does not require draining the HTTP response to EOF. In spend_ingestion_mode: outbox, failed durable charge acceptance leaves the stream incomplete without message_stop; it does not cause a second provider attempt. Required audit failure or finalization timeout also withholds the terminal event.

The default legacy writer can swallow database-write failures, so message_stop in that mode does not prove that billing was persisted. See the shared streaming accounting contract for the mode-specific guarantees.