HeaderLab by Cheon Park

HeaderLab sets, appends and removes HTTP request and response headers on the sites you choose, using the browser's own declarativeNetRequest engine. It holds no access to any site until you grant it.

WHAT IT DOES

• Set, append or remove any header, on the request side or the response side.
• Scope by site. Sites are matched by host, so what the popup shows is what goes on the wire.
• Apply everywhere, as an explicit mode. It costs access to all sites, and the switch does not ask for it — a separate Grant button does.
• Filter by request type. Eight request types, each its own checkbox, main_frame included — which the browser's own default quietly leaves out.
• Pause everything with one switch. The toolbar icon greys out to match, and stays grey across a restart.

DRIVE IT FROM AN AI CODING AGENT

HeaderLab ships an optional command line tool and a skill for Claude Code and Codex, so an agent can read and change your header rules while it works. That bridge is Chrome-only for now: Firefox closes native-messaging ports when an extension's event page goes idle, so this Firefox build does not offer it and asks for no nativeMessaging permission.

WHAT IT DOES NOT DO

• No network calls. No analytics, no telemetry, no remote configuration, no update pings.
• No content scripts. Nothing is injected into any page, and the extension never sees a page's contents.
• No remote code, no CDN, no web fonts, no remote images. Nothing is fetched from outside the package.
• Nothing leaves your machine. Your rules live in the browser's own extension storage.

https://github.com/say8425/headerlab

Open source, Apache-2.0.