Vayu API Client
Free, open source API client with a built-in native C++ load tester. Build requests like Postman and load test the same endpoint at 50k+ req/s in one local app, with no account.
Free, open source API client with a built-in native C++ load tester. Build requests like Postman and load test the same endpoint at 50k+ req/s in one local app, with no account.
This page is designed to help you find out whether Vayu API Client is good and if it is the right choice for you.
Vayu is a free, open source desktop API client that also load tests. Build and send REST and GraphQL requests like you would in Postman, then load test the same endpoint from the same app.
Native load testing. A multi-worker C++ engine sustains 50,000+ requests per second, on par with wrk and ahead of vegeta on the same machine. Throughput, latency percentiles and errors stream live to a dashboard while the test runs.
Easy to switch. Postman collections (v2.0 and v2.1), environments and globals import directly, as do Insomnia exports and OpenAPI or Swagger specs. Most Postman test scripts run unmodified thanks to a QuickJS runtime that implements the pm.* API.
Private by default. Everything runs locally. No account, no telemetry, no cloud sync, so it also works behind corporate firewalls and on air-gapped networks.
Built for coding agents. A built-in MCP server lets Claude Code, Cursor, VS Code, Codex and Zed send requests and run load tests, with a host allowlist, load caps and writes off by default.
Also included: collections with folders, layered environments, Bearer, Basic, API key and OAuth 2.0 auth, SSE streaming requests, data-driven runs from CSV or JSON, and mock servers.
Runs on Windows, macOS and Linux. The engine is AGPL-3.0 and the UI is Apache-2.0. There is no paid tier.
Listed in
Native load testing
Multi-worker C++ engine, 50,000+ req/s, on par with wrk
Live load test dashboard
Throughput, p50/p99 latency and errors stream in real time
Postman import
Collections v2.0/v2.1, environments, globals, plus Insomnia and OpenAPI
Postman-compatible scripts
QuickJS runtime implementing the pm.* API
MCP server for coding agents
Claude Code, Cursor, VS Code, Codex and Zed can run requests and load tests
Fully local
No account, no telemetry, no cloud sync; works offline
It's an API client and a load tester in the same app. You build a REST/GraphQL request the way you would in Postman, then load-test that exact endpoint from the same window - no second tool, no second config. The engine is native C++ (multi-worker libcurl event loop), so it does ~57k req/s on my machine, slightly ahead of wrk, while the dashboard keeps streaming p50/p99 live. It also ships an MCP server, so a coding agent can run the same requests and load tests.
Backend and API developers who want to know how an endpoint holds up under load without setting up k6 or JMeter for it. Also teams that can't use cloud-synced API clients - it works offline and behind corporate firewalls. And more recently, people using Claude Code/Cursor who want the agent to actually test the API it just wrote.
Engine: C++23, libcurl (multi-worker event loop), QuickJS for Postman-compatible scripting, SQLite via sqlite_orm. App: Electron + React 19 + TypeScript, Zustand, TanStack Query, Tailwind CSS v4. Build: CMake + vcpkg for C++, pnpm + Vite for the app. The engine runs as a local sidecar process, so the UI never shares a thread with the load.
Postman, Bruno and Insomnia are good at sending requests, but none of them load-test, so you end up maintaining a separate k6 or JMeter setup with the same endpoints. Vayu does both from one collection. Compared to the Node/Electron clients, the request work runs in a separate C++ process, so the UI doesn't stall under load. It's free and open source (no paid tier), fully local (no account, no telemetry), and Postman collections plus most pm.* scripts import as-is.
Every team I've worked on used two tools for APIs - one to build and send requests, another to load-test them - and keeping the two in sync was always a chore. I wanted one app that does both, without an account or anything leaving my machine. So I started building Vayu as a solo side project, with the load engine in C++ so it could keep up with tools like wrk.
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