What it is

Dev Proxy is an open-source, command-line API simulator maintained under the .NET Foundation and written in C#. It intercepts network requests, which means it works with any type of application and any technology stack rather than being tied to one language or framework. It is free to use and licensed under MIT, with the source living in the developer-tools and API-testing corner of the .NET ecosystem.

The problem it addresses is that the happy path is easy to test and the unhappy path is not. When the APIs an application depends on fail, throttle, or slow down, developers either write throwaway code to provoke those conditions or skip testing them entirely. Dev Proxy removes that choice by simulating API errors, rate limits, and slow responses at the network layer, without any change to the application's own code. Teams can therefore verify behaviour under failure, protect customer data, and stand up mock APIs without writing code they will never ship.

Key capabilities

• Simulates API errors so an application's response to failure can be observed without code changes.
• Reproduces API rate limiting so throttling behaviour and user experience can be verified in advance.
• Simulates slow APIs, allowing developers to build appropriate affordances for latency.
• Stands up mock APIs without writing any code.
• Intercepts network requests, so it works with any application type and tech stack.
• Provides contextual guidance on how APIs are being used, to improve the app.
• Supports OpenAPI-based mocking and Microsoft 365 and Microsoft Graph scenarios, per the project topics.

Who uses it and how

• Developers testing how an application behaves when a dependency returns errors.
• Teams validating throttling and rate-limit handling before those limits are hit in production.
• Engineers exercising latency handling and loading affordances against a deliberately slow API.
• Developers who need a mock API quickly, driven from an OpenAPI definition instead of handwritten stubs.
• Contributors working on chaos-engineering and API-testing workflows, including Hacktoberfest participants.

Getting started

The README directs new users to the getting-started tutorial at https://aka.ms/devproxy/start to install and run Dev Proxy for the first time. The project homepage is https://aka.ms/devproxy.

When to use it — and when not to

Use it when the goal is to test failure, throttling, and latency without modifying application code. The README does not describe any paid product it replaces, and it mentions no database, storage, or mail service that a user must operate, because the tool runs as a local command-line process that intercepts requests. The evident trade-off is that this is a command-line