Anthropic Messages API

The /v1/messages endpoint, Anthropic-native chat, x-api-key auth, facades, and streaming.

The gateway speaks the Anthropic Messages protocol natively, so Claude-native clients (Claude Code, Claude CoWork, Claude Desktop) can route through the same inspection pipeline as OpenAI-compatible clients.

Endpoint#

Method & pathPurpose
POST /v1/messagesAnthropic-style messages (native, and via facade to OpenAI-style backends)

Authentication#

Claude clients authenticate with either:

  • x-api-key: <key> (console keys), or
  • Authorization: Bearer <key> (OAuth-style tokens).

The gateway accepts both, applies the configured auth mode, and relays to the registered Claude model's backend with anthropic-version: 2023-06-01 and the resolved key (rule → model → global precedence).

Standard request#

POST /v1/messages
{
  "model": "claude-sonnet-4-20250514",
  "max_tokens": 4096,
  "system": "You are a helpful assistant.",
  "messages": [
    { "role": "user", "content": "Hello!" }
  ],
  "stream": false
}

The response uses the standard Anthropic schema (content blocks, stop_reason, usage metadata).

Model protocol metadata#

Each registered model carries an apiStyle, openai (default) or anthropic:

  • Model IDs starting with claude- are registered as anthropic automatically.
  • The protocol column in the Model Inventory can change it per model.

Facades#

The gateway translates between the two protocols transparently:

  • Anthropic client → OpenAI-style model (/v1/messages to a gpt-* backend): the Messages request is translated to chat/completions and the response/SSE back to Messages events (content_block_delta).
  • OpenAI client → Claude backend (/v1/chat/completions to a claude-* backend): the request is translated to the Messages schema, system and developer messages hoisted into the initial system message, max_tokens defaulting to 4096, temperature clamped to 0–1, and the response translated back to the OpenAI shape (tool_use → tool_calls).

Both directions run through the same inspection pipeline; rule enforcement and the semantic tier are protocol-agnostic.

Streaming#

stream: true works with Anthropic content_block_delta frames, scanned token-by-token. A mid-stream violation terminates the SSE stream with a guardrail_violation error event (in the buffered mode, the full response is scanned before delivery and a violation returns 400).

Error shape#

Blocked requests return HTTP 400 with an error translated to the Anthropic error shape (and OpenAI errors → Anthropic shape on the reverse facade), so Anthropic-SDK clients can parse failures the same way they parse provider failures.

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