Open-source Stainless alternatives

OpenAPI Generator takes an OpenAPI specification, in version 2 or 3, and produces client SDKs, server stubs, documentation and configuration automatically. Instead of hand-writing the same API wrapper in every language, teams define the API once and generate code for many languages and frameworks.

The project, written in Java and maintained by the OpenAPI Tools organization, offers a migration guide for users coming from Swagger Codegen, a wiki and FAQ, and an eBook introducing code generation for REST APIs. It carries an explicit security warning: if the spec, templates or other inputs such as options and environment variables come from an untrusted source, review them first to avoid problems like code injection. Vulnerabilities should be reported to the maintainers by email.

OpenAPI Generator is Apache-2.0 licensed and runs locally or in CI as a command line tool, Maven or Gradle plugin, or Docker image, so there is no hosted service involved. It suits API providers shipping official SDKs and developers who want type-safe clients generated from a contract.

Cortex turns API specifications and Markdown files into developer-facing outputs from a single project configuration. It reads sources such as OpenAPI, AsyncAPI, GraphQL, Protocol Buffers, OpenRPC, and plain Markdown, and produces interactive documentation for people, typed SDKs for applications, and an MCP server so AI agents can work with the same project context. It is released under the MIT license.

Generated SDKs cover many languages, including TypeScript, Python, Go, Java, Kotlin, Ruby, PHP, Rust, and C++, and can combine several specification files into one client. The tool supports HTTP, WebSocket, GraphQL, gRPC, and JSON-RPC clients, and builds static HTML documentation with interactive API reference pages. The MCP server includes typed tools, embedded specifications, and SDK guides.

You can try a local demo in about a minute, keep documentation in Git, and publish a free site for a public GitHub repository through the project's hosting flow. It suits API providers and developer-experience teams who want one source of truth for docs, client libraries, and agent access.