A2UI sends JSON and MCP Apps sends sandboxed HTML — the least consequential difference between them. One has the agent compose components you own, moving the review into your design system; the other installs an interface someone else wrote, moving it to the server boundary. Sort your surfaces by whether you can enumerate them, then pick.
Four platforms that stand between your agent and a user’s Gmail or Salesforce. The catalog sizes they advertise are counted in four different units and are the part you will outgrow; the token vault none of them market is the part you would hate to build. The decision you cannot retrofit is whose name is on the consent screen.
Five rival vendors agreed on a directory layout on 6 August, and explicitly declined to agree on install, distribution, permissions, sandboxing or provenance. The format makes one bundle of instructions plus credentialed tool access portable across six clients — which is exactly why the compensating controls are now yours.
Every identity vendor now sells “auth for AI agents”, and the phrase covers two opposite problems: letting an agent into your app, and letting your agent out to someone else’s API. Pick on direction first — and notice that a token vault holding a user’s full grant has relocated the credential rather than shrunk it.
The 28 July specification retires the initialize handshake and the Mcp-Session-Id header, and every write-up so far has framed that as plumbing. It is not. Dropping the held-open connection forced Sampling, Roots and Logging onto a twelve-month deprecation clock — and those were the features that made an MCP client a peer rather than a caller. The protocol just settled what it is.
When there is no API, an agent has to drive the browser itself — and four open-source projects disagree on how it should see the page. browser-use reads the DOM, Skyvern looks at pixels, Stagehand lets you dial between code and AI, and Playwright MCP is not an agent at all but the standard browser-tool layer any model can call. Picking one is really two decisions: Python or TypeScript, and a framework or an MCP server.
Two years from Anthropic's launch, MCP isn't a debate — it's a dependency. Every frontier vendor, every major IDE, and one Pinterest team saving 7,000 engineering hours a month all ship against it. The interesting question is no longer *should you use MCP* — it's what fails at this scale and how the 2026 roadmap plans to fix it.