How to Intercept Server-Sent Events in Chrome Extensions (MV3 Guide)

Server-Sent Events (SSE) are everywhere in 2026. ChatGPT, Claude, Gemini, and dozens of AI platforms use SSE streaming to deliver real-time responses. If you're building a Chrome extension that needs to read or analyze these streams, the standard approach doesn't work out of the box. Here's a complete guide to intercepting SSE in Chrome Manifest V3, with real code from building an extension that parses AI platform streams. Why This Is Hard In Manifest V2, you could use webRequest to intercept and read response bodies. MV3 removed that capability. The declarativeNetRequest API can block or redirect requests, but it can't read response content. This means the only way to intercept SSE streams in MV3 is through MAIN world content script injection — overriding window.fetch or XMLHttpRequest before the page's JavaScript loads. The Architecture ┌─────────────────────────────────────┐ │ MAIN World (page context) │ │ ┌─────────────────────────────┐ │ │ │ Override window.fetch │ │ │ │ Clone res