OpenAI API
API access to GPT-5, GPT-5.5, o3/o4 reasoning models, and the Responses API; plus embeddings, image, audio, and Realtime endpoints. The most widely deployed LLM API in production.
Stainless
Accepts an OpenAPI spec and produces high-quality, idiomatic SDKs that feel hand-written across TypeScript, Python, Go, Java, Kotlin, Ruby, PHP, and C#. GitHub integration auto-regenerates SDKs when the spec changes. The team behind OpenAI, Anthropic, Cloudflare, and Meta's official SDKs.