Mockttp

Mockttp is for JavaScript developers who want programmable HTTP mocks inside integration tests or custom proxies. It is a strong fit when tests need traffic interception, HTTPS, or parallel runs; browser administration and careful certificate handling make it less turnkey than a standalone mock service.

Overview

Mockttp intercepts, transforms, and tests HTTP requests and responses. It can stub replies, verify requests, and capture, inspect, or rewrite traffic through a custom proxy, with mocking both inside and outside its own process. Dynamic responses and support for HTTP, HTTPS, WebSocket, and JSON-RPC broaden its use beyond fixed HTTP fixtures.

As a free, open-source JavaScript library with local deployment, Mockttp suits teams that want to keep mocking in their own code and test environment. It is not the same offering as HTTP Toolkit’s Professional product, which includes automated HTTP mocking and rewriting; Mockttp is the library those capabilities can be built around.

Key features

Integration testing and proxying

Mockttp supports HTTP integration tests in Node.js and browsers, including transparent proxying and HTTPS. It can also serve as the basis for a custom proxy that captures or rewrites real traffic. This flexibility helps when a test must exercise requests as they pass through a proxy, but requires developers to own the setup rather than rely on a ready-made hosted workflow.

Parallel runs and integrations

Automatically configured ports, mock URLs, and proxy settings let parallel tests use separate mocks without manually coordinating those values. The README demonstrates Mocha, Chai, and Superagent, but these are optional: Mockttp can work with other promise-capable test tools and HTTP clients. That makes it adaptable to an existing JavaScript stack instead of tying it to one test framework.

Browser and HTTPS setup

Browser tests need an admin server running outside the browser, since browsers cannot start an HTTP server or proxy themselves. That requirement is a real infrastructure consideration for browser suites. HTTPS mocking uses generated self-signed certificates, and the setup guidance warns against trusting a test CA system-wide or sharing its private key; mishandling either can let an attacker impersonate secure connections.

The latest published version is actively supported, while the preceding major version receives security fixes for six months after the next major release. Vulnerabilities can be reported privately through GitHub, with confirmed issues to be fixed and published as advisories within 90 days. These policies provide a defined security process, though teams still need to manage certificate trust safely in their own environments.

Pricing

Mockttp itself is free and open source, with no paid library tiers or usage caps described. Its commercial relation, HTTP Toolkit, has separate plans:

• Hobbyist: free, billed Free. Includes basic traffic viewing and rewriting, manual breakpoints, and a built-in HTTP client. It is the no-cost option for individual traffic inspection and manual work; it does not include the automated mocking and rewriting in Professional.
• Professional: custom pricing, billed per month. Includes automated mocking and rewriting, import/export, and advanced debugging and configuration. It fits users who want those HTTP Toolkit features without building around the library.
• Team: custom pricing, billed per user, per month. Includes team licensing and centralized billing; team workspaces are coming soon, and additional options are available on request. It suits organizations that need shared licensing and billing, with per-user billing to account for.

Platforms

Mockttp supports Linux, macOS, and Windows, as well as web and self-hosted use. The library can be deployed locally. HTTP Toolkit separately provides desktop installation packages for Windows, Mac, and Linux. Its operator, Toolshed Labs SLU, is based in Barcelona, Spain, and says HTTP request and response contents are never collected and remain stored locally on the user’s machine.

Who it's for

Mockttp is best for JavaScript developers building integration tests, parallel test suites, or custom proxies who want direct control over HTTP traffic. It is less suitable for teams expecting browser tests to configure themselves, or for users who want a managed mock service instead of a library and its infrastructure responsibilities.

Pros and cons

• Pro: Traffic can be stubbed, verified, captured, inspected, or rewritten, including outside the process under test; that suits both integration testing and proxy workflows.
• Pro: Parallel tests get automatically configured ports, mock URLs, and proxy settings, reducing coordination work across simultaneous runs.
• Pro: It is a free, open-source library that supports multiple protocols and can work with different promise-capable test tools and HTTP clients.
• Con: Browser tests require an external admin server, adding infrastructure and setup to browser-based suites.
• Con: Self-signed HTTPS certificates require careful trust management; system-wide trust or shared private keys can expose secure connections to impersonation.

Alternatives

API Mocking Tools is a broader category list for readers comparing approaches rather than choosing one direct substitute.

• Microcks is another free, open-source option, with support across API, extension, web, and self-hosted platforms; choose it if that wider platform mix matters more than Mockttp’s JavaScript-library focus.
• Mockzilla has a free tier capped at 1K requests per month, 5 requests per second, one simulation, and 10 MB of data transfer. Choose it if a defined request allowance and simulation limit suit your needs.
• Beeceptor offers an Individual plan at $10 a month, with 15,000 requests per month per endpoint, up to 50 mock rules, private endpoints, and 100 CRUD objects. It is worth considering if those endpoint quotas and private endpoints fit your workflow.
• MockServer is free and open source, self-hosted, and requires no SaaS account; choose it if avoiding a SaaS account is the priority.
• WireMock has a free tier with 1,000 calls per month, one user, three mock APIs, and a 10-request-per-second limit. It suits readers who want a hosted-style free allowance with explicit call and user caps.
• Mockfly has a free Starter plan with 500 requests per day, one project, four endpoints, and two responses per endpoint. Consider it if those project and daily-request limits match your needs.
• Knockout API offers a free plan for 30 images per month; it is a narrower alternative for that image-specific allowance.
• MockAPI gives its free plan one project and two resources, while its $5 a month plan includes 20 projects and 50 resources. Choose it if project and resource counts are the main constraint.

Verdict

Choose Mockttp if you are building JavaScript integration tests or custom proxies and want free, programmable control over traffic, especially across parallel runs. Its main advantage is that breadth within a local library; look elsewhere if browser test administration or HTTPS certificate responsibilities outweigh that control.