Welcome to Yasumu
Yasumu is a modern API laboratory that puts developers first. Built with Tauri, Next.js, and an embedded JavaScript runtime powered by Deno, it provides a fast, beautiful, and keyboard-friendly environment for designing, testing, and debugging API workflows. Unlike traditional API clients that lock your collections in proprietary formats or require cloud accounts for basic features, Yasumu stores everything locally as plain.ysl (Yasumu Schema Language) files that you can commit, diff, and review like any other code.
Yasumu is currently in active Beta. While the core is functional, you may encounter unfinished features or bugs. We highly value your feedback and contributions as we work toward a stable v1.0 release.
Why Yasumu?
Zero vendor lock-in
Your data lives in your project folder as plain
.ysl files. No proprietary formats, no cloud dependencies, no paid tiers.Git-native collaboration
Treat API collections as code. Commit, push, review PRs, and sync with your team using standard Git workflows.
Powerful scripting
Write pre/post request scripts in TypeScript without external tooling. Scripts run on an embedded JavaScript runtime built on Deno.
Fully offline
No account required. No cloud sync. No telemetry. Just a tool that works offline and stays out of your way.
Key features
REST API client
REST API client
Full HTTP/HTTPS support with request history, environment variables, and variable substitution using
{{VARIABLE_NAME}} syntax.Catch-all SMTP server
Catch-all SMTP server
Integrated local email server for testing email workflows without external services or complicated setup.
Pre/post request scripting
Pre/post request scripting
Execute TypeScript/JavaScript before requests (
onRequest) and after responses (onResponse). Modify headers, URLs, body, or create mock responses.Environment management
Environment management
Manage variables and secrets across multiple environments. Variables are stored in
.ysl files, secrets are injected from environment variables.Project-based workspaces
Project-based workspaces
Initialize Yasumu in any project to create a
yasumu directory. Your API definitions, tests, and workflows live alongside your code.CLI for automation
CLI for automation
Run API tests in CI/CD pipelines with the
yasumu CLI. Execute individual requests or entire collections with environment support.Protocol support
Yasumu is evolving to become a protocol-agnostic powerhouse.| Protocol | Status | Notes |
|---|---|---|
| REST API | ✓ | Full HTTP/HTTPS request support |
| Catch-all SMTP | ✓ | Integrated local email server for testing |
| GraphQL | — | In development |
| WebSocket | — | In development |
| Server-Sent Events | — | In development |
| Socket.IO | — | In development |
Tech stack
Frontend
Next.js (React) - Modern React framework for the desktop UI
Desktop shell
Tauri - Lightweight desktop framework providing OS-level operations
Runtime
Tanxium - Embedded custom JavaScript runtime built on
deno_runtime for request execution and scriptingPlatforms
Windows, Linux, macOS - Cross-platform support with native installers
Get started
Quickstart
Get from zero to your first API request in minutes
Installation
Download and install Yasumu for your platform
CLI reference
Automate API testing with the Yasumu CLI
Schema reference
Learn about the Yasumu Schema Language (.ysl)
Philosophy
- Open source by default - Transparency builds trust. Licensed under AGPL-3.0.
- Zero vendor lock-in - Your data belongs to you, on your disk.
- Developer experience first - Fast, beautiful, and keyboard-friendly.
- Sustainability - Built for long-term maintainability over hype.
Community and support
Yasumu is developed by Neplex and maintained by an open-source community.GitHub
Star the repo, report issues, or contribute code
Documentation
Explore guides, references, and examples