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Open-Source Testing · PSPDFKit-labs

bypass

Bypass is a lightweight Elixir library that spins up a mock HTTP server for testing. It lets you define prebaked responses to client requests without needing a real server, making it ideal for testing HTTP client behavior in isolation.

Talk to DEV.coDiscuss Implementation
Source: GitHub — github.com/PSPDFKit-labs/bypass
998
GitHub stars
119
Forks
Elixir
Primary language
MIT
License (OSI-approved)

Key facts

Objective fields from the source. Values we can't verify are shown as “Unknown” rather than guessed.

FieldValue
RepositoryPSPDFKit-labs/bypass
OwnerPSPDFKit-labs
Primary languageElixir
LicenseMIT — OSI-approved
Stars998
Forks119
Open issues27
Latest releaseUnknown
Last updated2025-07-10
Sourcehttps://github.com/PSPDFKit-labs/bypass

What bypass is

Bypass is an Elixir/OTP plug-based mock server that intercepts HTTP requests and returns configured responses. It supports concurrent requests, route-specific expectations, dynamic port allocation, and state transitions (down/up) to simulate server failures.

Quickstart

Get the bypass source

Clone the repository and explore it locally.

terminalbash
git clone https://github.com/PSPDFKit-labs/bypass.gitcd bypass# follow the project's README for install & configuration

Need it deployed, integrated, or customized instead? DEV.co ships production installs.

Best use cases

Unit testing HTTP clients

Test how your Elixir HTTP client handles various status codes, error responses, and edge cases without depending on external services.

Integration testing with third-party APIs

Mock external API endpoints (e.g., Twitter, payment processors) to verify your application logic handles success and failure scenarios correctly.

Testing failure scenarios and recovery

Simulate server downtime, network errors, and reconnection logic using Bypass.down() and Bypass.up() to verify client resilience.

Implementation considerations

  • Bypass.open() must not be called in setup_all blocks; use per-test setup to ensure expectation verification works correctly.
  • Supports both ExUnit (default) and ESpec test frameworks; configure :bypass test_framework setting if using ESpec.
  • Multiple concurrent Bypass instances automatically allocate unique ports; pass the port to your HTTP client to connect.
  • Combine expect/expect_once (generic handlers) with expect/stub (route-specific) to handle fallback and specific route logic.
  • Enable :enable_debug_log in config to troubleshoot request routing and expectation matching during development.

When to avoid it — and what to weigh

  • Need production-grade mocking — Bypass is test-focused and not designed for production traffic interception or proxying.
  • Non-Elixir/OTP projects — Bypass requires Elixir 1.10+ and OTP 21+; it is not a language-agnostic mock server tool.
  • Complex HTTPS/TLS testing — Documentation does not clarify TLS/certificate handling; requires review for advanced SSL scenarios.
  • High-performance load testing — Bypass is optimized for test scenarios, not benchmarking or sustained high-concurrency load generation.

License & commercial use

MIT License (Expat): Permissive OSI-approved license. Allows commercial use, modification, and distribution with attribution and no warranty.

MIT License explicitly permits commercial use in derivative works. No copyleft restrictions. Verify you include a copy of the license and retain copyright notices in your project; legal review recommended for embedded/redistributed use.

DEV.co evaluation signals

Editorial assessment — not user reviews. Directional, with an explicit confidence level.

SignalAssessment
MaintenanceActive
DocumentationAdequate
License clarityClear
Deployment complexityLow
DEV.co fitGood
Assessment confidenceHigh
Security considerations

Bypass is test-only and should never run in production. No encryption or certificate validation documented; suitable for local test environments. If testing TLS clients, verify how Bypass handles HTTPS—documentation is unclear on this aspect.

Alternatives to consider

Mox (Elixir)

Lighter-weight mocking library for behavior-driven unit tests; better for isolated unit testing without spinning up a server.

WireMock (JVM, language-agnostic)

HTTP mocking tool that supports multiple languages and provides a REST API for dynamic mock management; heavier but more feature-rich.

VCR (Ruby) / Exvcr (Elixir)

Records and replays HTTP interactions; reduces mock maintenance but introduces test coupling to recorded cassettes.

Software development agency

Build on bypass with DEV.co software developers

Bypass makes it simple to mock HTTP servers in your test suite. Let our Elixir experts help you integrate it into your testing strategy.

Talk to DEV.co

Related open-source tools

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fakermockttpmock-socket

Related on DEV.co

Explore the category and the services that help you build with it.

Open-Source TestingAPI DevelopmentCustom Software DevelopmentTalk to DEV.co

bypass FAQ

Can I use Bypass for production traffic?
No. Bypass is designed for test scenarios only and should never be deployed to production.
How does Bypass handle concurrent requests?
Bypass supports concurrent requests on the same instance and multiple concurrent Bypass instances. Each instance gets a unique dynamically allocated port.
Do I need to manually close the Bypass socket after each test?
No. Bypass automatically registers an on_exit hook with ExUnit to clean up sockets. ESpec users must explicitly call Bypass.verify_expectations! in a finally block.
What versions of Elixir and OTP are supported?
Elixir 1.10+ and OTP 21+. Bypass works with Cowboy 2.

Software development & web development with DEV.co

Pairing bypass with the rest of your stack is where most open-source testing projects stall. DEV.co is a software development agency that does the integration work — plus AI development when models are in scope.

Need help testing your Elixir HTTP clients?

Bypass makes it simple to mock HTTP servers in your test suite. Let our Elixir experts help you integrate it into your testing strategy.

Talk to DEV.coSchedule a Consultation