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Fiddler - Fiddler is a debugging program for websites.
httptools.dev - Online tools for inspecting and debugging HTTP requests.
Proxyman.io - Proxyman is a high-performance macOS app, which enables developers to view HTTP/HTTPS requests from apps and domains.
RequestHeaders.com - HTTP Header Checker Tools
HTTP Toolkit - Beautiful, cross-platform & open-source tools to debug, test & build with HTTP(S). One-click setup for browsers, servers, Android, CLI tools, scripts and more.
mitmproxy - mitmproxy is an SSL-capable man-in-the-middle proxy for HTTP.