mountebank

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Introduction

mountebank describes itself as the only open-source service virtualization tool that competes with commercial offerings in protocol diversity, capability and performance. Originally created by Brandon Byars, who maintained it for more than a decade, it is now moving to a collaborative, community-run GitHub organisation; the README notes that the project is in transition while CI/CD integrations are migrated, so pull requests may take time to merge. The latest GitHub release, v2.9.4, was published on 15 April 2026, and the npm package @mbtest/mountebank lists version 2.9.5.

mountebank runs as a Node.js server. You create “imposters”, virtual services listening on a port, by posting JSON to its REST API or loading a configuration file at startup. Each imposter has a protocol (HTTP, HTTPS, TCP or SMTP) and a set of stubs made of predicates (equals, contains, matches, exists, JSONPath and XPath, and more) and responses. Responses can be canned (is), recorded from a real service through a proxy (proxy) for record-and-replay, or computed with JavaScript injection (inject). Behaviours add latency, shell transforms or copy values from requests into responses. Because imposters are language-neutral, the same doubles can serve tests written in any language, and mountebank’s performance has made it popular for virtualising dependencies in load tests.

The source code is at mountebank-testing/mountebank on GitHub under the MIT licence, with about 2,100 stars; the most recent commit was on 2 November 2026.

Features

1. Multi-protocol imposters: HTTP, HTTPS, TCP and SMTP.
2. REST API: Create and inspect imposters from any language.
3. Record and replay: Proxy real services and save responses.
4. Injection and behaviours: JavaScript responses, latency and transforms.

Pros

1. Free to use: Open source under the MIT licence, so the core tool costs nothing to use.
2. Actively maintained: The repository had commits as recent as 2 November 2026.

Cons

1. Project in transition: Community migration may slow merges and releases.
2. JSON-heavy configuration: Large setups need careful organisation.

Usage and Links (Example)

npm install -g @mbtest/mountebank
mb --allowInjection

curl -X POST http://localhost:2525/imposters -H 'Content-Type: application/json' -d '{
  "port": 4545, "protocol": "http",
  "stubs": [{
    "predicates": [{ "equals": { "method": "GET", "path": "/users/1" } }],
    "responses": [{ "is": { "statusCode": 200, "body": { "id": 1, "name": "Ana" } } }]
  }]
}'
curl http://localhost:4545/users/1

Resources:

• mountebank website
• GitHub repository

Pricing

mountebank is free and open source under the MIT licence.

Recommended for?

1. Teams virtualising non-HTTP dependencies such as TCP or SMTP services.
2. Polyglot organisations sharing test doubles across languages and load tests.

Conclusion

mountebank is a capable, language-neutral service virtualization tool; check the community transition status before adopting it for critical pipelines.