An API operated by the same lab that creates the model family it serves.
A direct lab API usually exposes a model's native features first and provides the shortest support path when behavior changes. OpenAI, Anthropic, Google, Mistral, xAI, Cohere, DeepSeek, and Z.ai all operate direct model APIs.
The cost of that directness is tighter dependency on one vendor's request format, product decisions, and availability.