There is a special kind of despair reserved for copying a red Next.js overlay into chat, watching the agent invent a fix for Next 14, then realising you are on 16.2 and the docs in its head are fan fiction.

I do not want to be a human clipboard anymore.

Next.js DevTools MCP (and the /_next/mcp endpoint in 16+) finally lets the agent ground itself in the running app instead of my paste buffer.

With next-devtools-mcp wired into your agent config, the coding agent can ask the live dev server questions humans normally answer by squinting:

That last one alone is worth the setup. Training data is sticky. Runtime truth is not.

Boot next dev. Let the MCP server discover the instance. Suddenly the agent is debugging this app, not a composite memory of every app it has ever seen on the internet.

Old loop:

New loop:

You still need taste. You still need tests. But you stop being the lossy transport layer between localhost:3000 and the model.

DevTools MCP tells the agent what Next thinks is wrong. A browser tool tells it what a human would see.

Use both:

Framework truth plus rendered truth. That is the grounding stack.

I had an App Router page that rendered fine in isolation and exploded only when a parallel route mis-matched a slot name. The overlay was noisy. My paste was incomplete. The agent kept "fixing" the wrong file.

Once it could query the running routes and the actual error channel from the dev server, it stopped theorising about useEffect and fixed the slot. Five minutes. No heroic prompting.

That is not magic. That is sensors.

This is a development affordance. Do not expose /_next/mcp to the public internet and call it innovation. Keep it on local/dev instances. Treat it like you treat the React DevTools: powerful, local, not a product surface.

Also: grounded agents can still make bad product decisions. Seeing the error does not grant taste. It just stops them lying about which error they saw.

The best debugging prompt of 2026 is not a better paragraph. It is an agent that can look at the running app without you playing middleman.