POST /v1/messages.
Use a Modellix API key from the console, not an Anthropic Console key. Model IDs use
provider/name (for example anthropic/claude-sonnet-5)—see Models & Pricing for the full list.Configure the client
Bearer alternative
If a tool requires Bearer auth instead ofx-api-key, use ANTHROPIC_AUTH_TOKEN with the same Modellix key. Do not set ANTHROPIC_API_KEY and ANTHROPIC_AUTH_TOKEN to different values.
Messages example
max_tokens is required on Messages. Put system instructions in system, not as a system role inside messages (roles are user or assistant only).
Streaming
Setstream=true (or the SDK equivalent). The gateway returns Anthropic Messages SSE events.
Optional headers
Native Anthropic clients often sendanthropic-version (for example 2023-06-01); Modellix forwards it upstream when present. For multi-turn session affinity you can also send X-Mdlx-Session-Id (see LLM API).
Related
- Claude Code — same base URL pattern with settings persistence
- LLM API guide — protocols, auth, and errors
- Create message — OpenAPI reference