RESTler

At a glance

• Editor score
  7.1 / 10
• Pricing
  Free plan
• Best for
  Security-focused teams fuzzing OpenAPI REST services
• Free plan
  Yes
• Paid from
  Free
• Facts checked
  21 Sep 2026

• Where it wins

  • Compiles OpenAPI specifications and infers request dependencies
  • Finds resource leaks, hierarchy violations, and access-control issues
  • Supports smoke tests, stateful fuzzing, replay logs, and CI/CD pipelines

• Where it doesn't

  • Supports REST APIs rather than GraphQL or SOAP services
  • Does not provide a hosted SaaS interface or scheduled runs
  • Lacks mock-server and contract-testing capabilities

Our verdict on RESTler

RESTler is an open-source API testing tool from Microsoft Research for developers and security testers validating REST APIs. It accepts OpenAPI or Swagger specifications, compiles them into fuzzing grammars, and analyzes dependencies between request types. Teams can run it locally or in Docker against a live cloud service, making it a fit for security-focused engineering groups that need deeper testing of stateful API behavior rather than basic request checks.

Its strongest area is the depth of its fuzzing workflow. RESTler supports compilation, endpoint smoke testing, specification coverage reporting, fuzz-lean runs, deeper stateful fuzzing, and replay. Configurable dictionaries and payload examples help tailor test inputs, while custom authentication token refresh commands support APIs with expiring credentials. Security checkers cover resource leaks, hierarchy violations, and unauthorized resource access. Bug buckets and replay logs provide a structured path from discovery to reproduction, which suits teams investigating failures in CI/CD pipelines.

RESTler is deliberately narrow. Its published capabilities center on REST services described by OpenAPI or Swagger, with no GraphQL or SOAP support, mock server, contract testing, or scheduled runs. It is also a locally built or Docker-run tool rather than a hosted SaaS product, so teams should be prepared to manage execution within their own development or security workflow. Choose RESTler when stateful fuzzing and security findings matter more than broad protocol coverage or a managed interface. Teams seeking API mocking, contract-focused validation, scheduled testing, or multi-protocol coverage should consider an alternative.

Your product? Our editors can test RESTler in depth and publish a review, labelled as paid · from $149/mo →

RESTler pricing

RESTler fact sheet

Alternatives to RESTler

• PostmanA mature API test platform with broad protocol support and a wide feature set.9.3
• ApidogA broad API toolkit for teams that need design, testing, automation, and documentation together.9.2
• SoapUIA broad, free API testing desktop tool for functional, contract, performance, and load work.9.0

See all RESTler alternatives →

Featured on iTechGuides

RESTler is listed in our API Testing Tools directory. Add the badge to your site — it links back to this page.

<a href="https://www.itechguides.com/products/restler/"><img src="https://www.itechguides.com/best/badge/restler.svg" alt="Featured on iTechGuides" width="230" height="46"></a>

Reviewed by iTechGuides Editors · Editorial team · Updated Oct 2026

Advertiser disclosure: iTechGuides is reader-supported. Vendors can pay for top positions in our rankings and for a place on other products' pages, and we may earn a commission when you click some links. Bought places stand at the number bought; this page and our disclosure say so. Paying vendors may get a fuller, more detailed review from our editors, and that review can change their score. How we rank.

Last updated 7 Oct 2026 · How we research and update