> why would I prefer this to mitmproxy?

Compared to mitmproxy, HTTP Toolkit:

- Has fully automated setup for most browsers, docker containers, Android, all Node.js/Ruby/Python/PHP/Go applications run from intercepted terminal windows, all JVM processes, any Electron apps etc etc. Some of these automated setup steps are very difficult to do manually (e.g. intercepting Android devices, where you can't normally install your own certificates nowadays, or intercepting Node.js, which completely ignores system proxy settings) so this can make a huge difference in non-trivial case.

- Supports targeted interception (intercept just one app/container/browser window) whilst all mitmproxy's manual setup steps are generally focused on helping you intercept your whole machine at once. Intercepting the whole machine means very noisy interception and means that rewriting traffic interferes with all other usage of your machine. Targeted interception means you can do neat things like run two HTTP Toolkit instances independently at the same time, and means you don't need root privileges or permanent configuration settings.

- Has a VPN app for Android, which allows it to capture traffic even if it tries to ignore proxy configuration, means you don't have to manually edit and delete Android proxy settings, and which can automatically tunnel traffic over ADB connections, so you can intercept a device connected via ADB even if its not connectable over the wifi from your computer.

- Has generally friendlier UI & UX (imo). For example, mitmproxy uses a unique custom syntax (https://docs.mitmproxy.org/stable/concepts-filters/) of special characters to define matching & rewriting rules, or requires you to write a full python script. HTTP Toolkit lets you click 'new rule' -> 'GET requests' -> 'match regex <blah>' -> 'then reply with <blah>', and then immediately start injecting automated fake responses. From HTTP Toolkit you can then build named groups or these rules, and import & export them (as JSON) to build libraries you can share with your colleagues.

- Provides lots more background information automatically: e.g. built-in documentation for all standard HTTP headers, body autoformatting for lots more formats, syntax highlighting, code folding, regex searching etc of request & response bodies, plus 'this is how and why this response could be cached' caching explanations, OpenAPI-powered docs for recognized endpoints on 1400+ APIs, etc.

- Includes advanced features to do things like exporting requests as ready-to-use code for various languages & tools, or automatically testing the performance of different compression algorithms on a given response body.

- Is more easily scriptable for automation & end-to-end testing, because all the HTTP-handling internals are usable as a standalone open-source JS library: https://github.com/httptoolkit/mockttp

That said, mitmproxy has been around longer, it's definitely more mature, and it was a big inspiration in many places. It's a great project! It does have some advantages of its own:

- If you strongly prefer a CLI interface, mitmproxy is very focused on that, and HTTP Toolkit is not. HTTP Toolkit could support that too in theory (the backend & frontend are independent) but it definitely doesn't right now, and it's not high on my todo list (contributions welcome though!)

- Mitmproxy is primarily scriptable in Python. You can build automation around HTTP Toolkit's internals using mockttp, but that's JS, and it's mostly usable standalone right now, rather than integrated into normal workflows within the app. If you want very complex scripted rules, mitmproxy has a few more options right now, and lets you do things in python instead of JS, which some people will prefer.

- WebSocket debugging - this is coming for HTTP Toolkit soon, but it's not available today. WebSockets get passed through fine, but they don't appear in the UI, and you can't set up mock rules for them.

> I'd be interested both in why I'd prefer the open source httptoolkit and pro?

There's a list of Pro features at https://httptoolkit.tech/pricing/. Note that it's all open source, even the Pro code, everything.

The general idea is that everything you need to intercept, inspect and manually fiddle with traffic is totally free. Anything optional that most users don't need, but which is helpful for advanced usage or enterprise use cases, requires Pro.