Best Free Developer Tools for API Testing, Debugging, and Mocking in 2025

Postman’s 2023 decision to retire its Scratch Pad and push users toward cloud-synced accounts was a wake-up call for many teams: the tools you rely on can change their terms overnight. Since then, developers have been reassessing what “free” really means for API tooling. Some options are open source and self-hostable. Others are free tiers that cap usage. A few are genuinely free with no strings attached.

This guide covers the strongest free tools across three categories—testing, debugging, and mocking—with honest notes on where each one’s limits are. Whether you’re validating a REST endpoint, inspecting gRPC traffic, or standing up a mock server before the backend exists, there’s an option here that won’t require a credit card.

What “Free” Actually Means in 2025

Before comparing tools, it helps to sort them by licensing model, because the differences matter more than feature checklists:

  • Open source (self-hosted): No cost, no vendor lock-in, but you run and maintain it yourself.
  • Open source with a paid cloud: The core is free; convenience features like team sync or hosted runners cost money.
  • Freemium SaaS: Free tier with usage caps—often on API calls, mock requests, or collaborators.
  • Free for individuals, paid for teams: Common in IDE plugins and CLI tools.

A tool that’s “free” but requires a $12/user/month plan the moment a second developer joins isn’t really free for most teams. Keep that lens on as you read.

API Testing Tools

Bruno

Bruno is an open-source, offline-first API client that stores collections as plain-text files (a custom .bru format) directly in your Git repository. That design choice is its biggest selling point: your API tests live alongside your code, get reviewed in pull requests, and don’t depend on a cloud account. It supports REST and GraphQL, has a scripting layer built on JavaScript, and runs on macOS, Windows, and Linux. Because collections are file-based, Bruno sidesteps the export/import friction that plagues cloud-first clients. It’s fully free and open source, with an optional paid tier only for team collaboration features.

Hoppscotch

Hoppscotch is a browser-based, open-source API client with a fast, minimal interface. It handles REST, GraphQL, WebSocket, SSE, and MQTT requests, and its free self-hosted version gives you full control over data. The hosted version has a generous free tier for individuals. If you liked the speed of Postman’s early days and want something lighter, Hoppscotch is worth a look—though very large collections can feel less organized than in dedicated desktop clients.

REST Assured and Karate

For automated, code-based testing, two Java-ecosystem tools dominate the free tier:

  • REST Assured is a mature Java DSL for testing REST services. It integrates cleanly with JUnit and TestNG and is a staple in CI pipelines.
  • Karate combines API testing, mocking, and even performance testing in a single framework, using a Gherkin-like syntax that non-Java developers can read. Both are open source and free.

If your team writes tests in Python, Tavern (built on pytest) or Schemathesis (property-based testing that generates test cases from your OpenAPI or GraphQL schema) are strong free options. Schemathesis in particular is underused—it will find edge cases you’d never write by hand.

k6

k6 is an open-source load testing tool with a scripting API in JavaScript. While it’s primarily for performance testing, its check() assertions make it useful for functional API validation under load. The CLI is free; Grafana Cloud k6 is the paid hosted option.

Debugging Tools

HTTP Toolkit

HTTP Toolkit is an open-source proxy and traffic inspector that intercepts HTTP and HTTPS from browsers, mobile apps, and backend services. Its standout feature is one-click interception: it configures system proxies and installs certificates for you, which removes most of the pain of debugging HTTPS traffic from an Android emulator or a Docker container. The free version covers individual debugging; team features and some advanced rules are paid.

mitmproxy

If you want scriptable, headless interception, mitmproxy is the standard. It’s a command-line proxy with a Python API, so you can write addons that rewrite requests, log traffic, or inject faults. It’s completely free and open source, and it’s the tool many security researchers reach for. The learning curve is steeper than HTTP Toolkit’s, but the ceiling is much higher.

Chrome DevTools and Firefox Developer Tools

Don’t overlook the browser. Chrome DevTools’ Network panel remains one of the fastest ways to inspect XHR/fetch calls, replay requests (right-click → “Copy as fetch”), and throttle conditions. Firefox’s Network panel adds a helpful “Edit and Resend” feature. These are free, always available, and require zero setup. For frontend-adjacent API debugging, they’re often enough.

Wireshark

When the problem is below HTTP—TLS handshakes, TCP retransmits, DNS resolution—Wireshark is the tool. It’s free, open source, and cross-platform. It’s overkill for most API work, but indispensable when you suspect the network itself.

Mocking Tools

WireMock

WireMock is the most widely used open-source API mocking tool. It runs as a standalone server, a Java library, or in Docker, and it lets you stub responses, simulate latency, and inject faults. WireMock Cloud offers a free tier for hosted mocks. Its request-matching engine is expressive enough to handle most real-world scenarios, and the community is large.

Mockoon

Mockoon is a desktop app for creating mock APIs with a GUI—no code required. You define routes, responses, and rules in a visual editor, and it can run multiple mock environments simultaneously. It’s open source, free, and runs locally, which makes it a good fit for frontend teams waiting on backend endpoints. A CLI and cloud option exist for CI and team sharing.

Prism

Prism, from Stoplight, generates a mock server directly from your OpenAPI or JSON Schema document. If you already maintain an OpenAPI spec, Prism turns it into a working mock API in one command. It also validates requests against the spec, which catches contract drift early. The CLI is open source and free.

JSON Server and MSW

For quick, lightweight mocks:

  • JSON Server spins up a full REST API from a JSON file in seconds. Perfect for prototypes.
  • MSW (Mock Service Worker) intercepts requests at the network level in the browser and Node.js, using the same Service Worker API that powers offline web apps. It’s become the default mocking library for many React and Next.js projects, and it’s free and open source.

How to Choose

A practical way to decide:

  1. If you want tests in version control: Bruno or REST Assured/Karate.
  2. If you need to inspect live HTTPS traffic: HTTP Toolkit for ease, mitmproxy for power.
  3. If the backend doesn’t exist yet: Mockoon for GUI-driven mocks, Prism if you have an OpenAPI spec, MSW for frontend integration tests.
  4. If you’re testing at scale: k6 for load, Schemathesis for property-based coverage.

Most teams end up with two or three tools rather than one. That’s normal—the categories overlap but rarely substitute for each other.

The Bottom Line

The free tier of API tooling in 2025 is genuinely strong. Open-source options like Bruno, WireMock, mitmproxy, and Prism cover the vast majority of day-to-day work without a subscription, and they tend to have better longevity than freemium services because their licenses can’t be revoked. The main trade-off is setup and maintenance effort, which is usually a few hours up front. If your team is paying for API tooling mainly out of habit, it’s worth auditing what you actually use—chances are a free alternative now does the job just as well.