HeaderModifier

HeaderModifier is a free, privacy-friendly tool for developers and testers who need to change HTTP headers. Add a header, override an existing one, append to it, or remove it entirely — then scope it to exactly the sites you choose. Built on Chrome's modern Manifest V3 declarativeNetRequest engine, HeaderModifier modifies headers efficiently at the network layer, with no external servers and no background tracking. ✦ Features • Modify request headers — add new ones or override existing values • Modify response headers — add or override • Set or Append mode — override a value, or append to an existing header (e.g. Cookie, Accept) • Remove headers — just leave the value empty • Profiles — keep multiple independent header sets and switch between them instantly • Colored profiles — color-code, duplicate, or export a single profile • Master pause — suspend every header with one click, then resume when you're ready • Dark mode — automatic (follows your system), or force light or dark • Per-header toggles — enable or disable individual headers with one click • Per-header comments — annotate any header so you remember what it's for • URL filters — apply headers only to certain domains, or exclude specific domains • Import / export — back up or share your configuration as a JSON file • Active-header badge — see at a glance how many headers are live in the current profile ✦ Private by design • No accounts and no sign-up • No tracking, no analytics, no ads • Your configuration is stored locally and never leaves your device • 100% free and open source (MIT licensed) ✦ Common uses • Add an Authorization or custom API header while testing • Override User-Agent, Accept-Language, or Referer • Append to headers like Cookie without overwriting existing values • Remove headers to reproduce edge cases • Simulate different environments with separate profiles, then pause everything with one click ✦ Good to know When you add a custom request header to a cross-origin API call, the browser performs a CORS preflight, and the server must allow your header via its Access-Control-Allow-Headers response for it to be accepted. This is standard browser behavior and applies to every header tool. ✦ What's new in 1.1.0 • Append mode — add to a header instead of overriding it • Master pause switch — suspend all headers instantly, then resume • Dark mode (automatic, light, or dark) • Colored profiles, duplicate a profile, and export a single profile • The toolbar badge now reflects the active profile's live headers • Reliability: edits save as you type, even if you click away ✦ Frequently asked questions Q: How do I add or modify an HTTP request header? A: Open HeaderModifier, click "+ Add" under Request headers, type the header name and value, and it's applied instantly to the sites you target. Q: Can I modify response headers too? A: Yes. Add entries under Response headers to set, override, or remove response headers such as Content-Security-Policy, X-Frame-Options, or CORS headers. Q: How do I remove or delete a header? A: Add the header name and leave its value empty — HeaderModifier will strip that header from the request or response. Q: Can I change my User-Agent or add a Bearer token / Authorization header? A: Yes. Set the User-Agent, Authorization, Cookie, Referer, or any custom header, and scope it to specific domains with URL filters. Q: Does it work in Microsoft Edge and other Chromium browsers? A: Yes. HeaderModifier is a Manifest V3 extension and runs on Chrome, Edge, Brave, and other Chromium-based browsers. Q: Is it really free, and does it collect my data? A: It's 100% free and open source (MIT). It has no accounts, no ads, and no tracking — your configuration stays on your device.