Open-source Stoplight alternatives

Scalar is an open-source API platform built around the OpenAPI standard. One part turns OpenAPI or Swagger documents into an interactive API reference with a built-in testing tool and generated code samples for many languages and frameworks, and it aims for a modern look rather than a dated one. Integrations exist for popular server frameworks, so docs can be mounted straight into an existing project.

The other part is an API client designed to work offline and built for OpenAPI. It is free and open source, treats OpenAPI and Swagger as first-class inputs, works with dozens of API frameworks such as Elysia, Hono and FastAPI, and can stay in sync with a running server through a watch mode. Environment variables, dynamic parameters and the other staples of a REST client are included. You can try it in the browser or download it for Windows, macOS or Linux.

The quickstart even offers a prompt you can hand to a coding agent to set up the reference in your project. Scalar is MIT licensed and written in TypeScript and Vue. It suits API teams that want better-looking documentation and a lighter alternative to Postman-style tools tied to their specs.

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.

Voiden is an offline API client for developers, QA engineers, and technical writers to build, test, collaborate on, and document APIs in one place. APIs are described in plain-text .void files that you keep in Git, with no account and no required cloud sync. It is built with Electron and TypeScript and released under the Apache-2.0 license.

Requests are composed from reusable API blocks that can be replaced and versioned like code, with a slash-command interface and a command-line tool designed to work with AI agents. It supports REST, GraphQL, gRPC, and WebSocket protocols, imports OpenAPI, and topics mention an MCP client. The project says it has no telemetry, no venture funding, and no cloud lock-in.

Voiden is positioned as an alternative to Postman, Bruno, and Insomnia. It suits teams who want their API collections and documentation to live in the repository as files that can be reviewed and shared through normal Git workflows.