The Chromium browsers moved in two directions at once this week. Microsoft Edge committed to removing Manifest V2, following Chrome, which narrows what extensions can do; Edge and Brave both turned on WebMCP, also following Chrome, which adds a new surface for a user's agent to act on a page. In the infrastructure layer, Cloudflare previewed a redesigned SDK for running customer and agent code in isolated sandboxes.
Browser extensions: Edge commits to removing Manifest V2
Microsoft Edge committed to removing Manifest V2, following Chrome onto Manifest V3. The August 7 announcement says the consumer transition away from MV2 begins in August 2026, with a goal to complete it by the end of 2026, and enterprise deprecation following in early 2027. The changes start in the Canary, Dev, and Beta channels and broaden to Stable over the following months, with MV2 extensions turned off by default and users directed to MV3 versions where available. Microsoft says 95% of the top MV2 extensions on the Edge Add-ons site have already moved to MV3, and that of the MV2 extensions with meaningful usage, only three lack a publicly available MV3 version. MV3 removes the request-blocking capability that content-blocking and privacy extensions rely on to stop network requests before they load.
Why it matters: a second major Chromium browser removing MV2 narrows what content-blocking and privacy extensions can do for end users across most of the desktop browser market, leaving Firefox as the remaining major engine that still supports request blocking. The timeline is announced and the rollout is phased, so this is a scheduled removal rather than a completed one.
Standards: WebMCP gains a second and third browser
WebMCP turned on an origin trial in Microsoft Edge and gained experimental support in Brave, extending a Chrome-only experiment to two more Chromium browsers. A cross-browser implementation-status document merged August 12 records an origin trial live in Edge 150, built on Chrome's implementation, alongside the existing Chrome 149 origin trial, plus experimental support wired into Brave's Leo AI chat. WebMCP lets a website expose in-page tools that a user's own agent can call rather than the agent scraping the DOM. It remains a Web Machine Learning Community Group draft, and all three implementations are experimental: an origin trial is a time-boxed, single-vendor test, not stable shipping. Neither Gecko nor WebKit has implemented it; Mozilla's position is under review and Apple's WebKit team has filed concerns.
Why it matters: the API that lets a page hand tools to a user's agent now has three Chromium browsers testing it, which concentrates early momentum on one engine family while the other two engines stay out. The support is experimental across the board, so this is broadening adoption of a draft, not cross-vendor agreement or a stable capability.
Infrastructure: Cloudflare previews a redesigned code-sandbox SDK
Cloudflare shipped a 1.0 preview of its Sandbox SDK, the layer a product uses to run untrusted and agent-driven code in isolated, Container-backed sandboxes. The August 7 preview replaces an older, more complicated design with a simpler, unified way to start a task and stream back its output, and it makes the built-in code interpreter an optional add-on a product includes only when it needs one. Cloudflare says it will ship the finished Sandbox SDK 1.0 once pending Cloudflare Containers features land, so this is a preview rather than a stable release.
Why it matters: this is the isolation layer a product uses to run its customers' or a user's agent's code, so a simpler design lowers the cost of letting users run their own code inside an app. The preview label and the deferred 1.0 mean the design can still change before it is stable.
Also worth knowing
A new MCP proposal drafts per-user identity for a single server connection. The Delegated End-User Context extension, opened August 14 as a draft seeking a sponsor, defines a way to attach each request's end-user identity at three trust levels, anonymous, claimed, and verified, and lets the server set access limits that a client can tighten but not loosen. It targets products that expose MCP to their own end customers.
WASI 0.3.1 broadened what portable WebAssembly plugins can do. The August 11 release lets a single plugin use more than one implementation of the same capability under distinct names, for example a remote store alongside a separate in-memory cache, and adds a built-in dictionary type for key-value data. WebAssembly components are a language-neutral way to package plugins that run in a host application's sandbox.
On the radar
- August 19: WebExtensions Working Group call.
- August 25: WebAssembly Community Group video call.
- August 27: WebExtensions Community Group meeting.
- August 31: Chrome Web Store removes remaining Manifest V2 extension listings.
- End of 2026: Microsoft Edge targets completing the consumer Manifest V2 transition.
- Early 2027: Microsoft Edge enterprise Manifest V2 deprecation.
This Week in Extensibility is curated by Todd Schiller. Research, drafting, and fact checking are AI-assisted.