next-browser

If next-browser is not already on PATH, install @vercel/next-browser globally with the user's package manager, then playwright install chromium.

If next-browser is already installed, it may be outdated. Run next-browser --version and compare against the latest on npm (npm view @vercel/next-browser version). If the installed version is behind, upgrade it (npm install -g @vercel/next-browser@latest or the equivalent for the user's package manager) before proceeding.

Next.js docs awareness

If the project's Next.js version is v16.2.0-canary.37 or later, bundled docs live at node_modules/next/dist/docs/. Before doing PPR work, Cache Components work, or any non-trivial Next.js task, read the relevant doc there — your training data may be outdated. The bundled docs are the source of truth.

See https://nextjs.org/docs/app/guides/ai-agents for background.

When this skill loads

Your first message introduces the tool and asks setup questions. Don't say "ready, what would you like to do?" and don't run speculative commands or auto-discover (port scans, project, config reads).

If the user already provided a URL, cookies, and task in their message, skip the questions — go straight to open and start working. Only ask what's missing.

Otherwise say something like:

This opens a headed browser against your Next.js dev server so I can read the React component tree, see the PPR shell, and check errors the way you would in DevTools. To start:

• What's your dev server URL? (And is it running?)
• Are the pages you're debugging behind a login? If so I'll need your session cookies — easiest is to copy them from your browser's DevTools → Application → Cookies into a JSON file like [{"name":"session","value":"..."}]. If the pages are public, skip this.

Wait for answers. Then open <url> [--cookies-json <file>]. Every other command errors without an open session.

Commands

open <url> [--cookies-json <file>]

Launch browser, navigate to URL. With --cookies-json, sets auth cookies before navigating (domain derived from URL hostname).

$ next-browser open http://localhost:3024/vercel --cookies-json cookies.json
opened → http://localhost:3024/vercel (11 cookies for localhost)

Cookie file format: [{"name":"authorization","value":"Bearer ..."}, ...]

Only name and value are required per cookie — omit domain, path, expires, etc. To create the file, use Bash (echo '[...]' > /tmp/cookies.json) since the Write tool requires a prior Read.

close

Close browser and kill daemon.

goto <url>

Full-page navigation (new document load). Server renders fresh.

$ next-browser goto http://localhost:3024/vercel/~/deployments
→ http://localhost:3024/vercel/~/deployments

push [path]

Client-side navigation via next.router.push(). Without a path, shows an interactive picker of all links (↑/↓, enter).

$ next-browser push /vercel/~/deployments
→ http://localhost:3024/vercel/~/deployments

If push fails silently (URL unchanged), the route wasn't prefetched.

back

Browser back button.

reload

Hard reload current page.

capture-goto [url]

Lock PPR, navigate to the URL, screenshot the shell, unlock, then screenshot frames every ~150ms as the page hydrates and loads data. Stops after 3s of no visual change (hard timeout 30s). Returns a directory of PNGs.

$ next-browser capture-goto http://localhost:3024/vercel/~/deployments
12 frames → /tmp/next-browser-capture-goto-1710000000000

frame-0000.png is the PPR shell. Remaining frames capture hydration → data.

Frame 0 is always the PPR shell (static HTML before hydration). The remaining frames show the transition through hydration and dynamic data loading. Read them with the Read tool to see the visual progression.

Without a URL argument, captures the current page (re-navigates to it).

restart-server

Restart the Next.js dev server (POSTs to /__nextjs_restart_dev). Clears filesystem cache, forces clean recompile. Polls until the server has a new execution ID, then reloads the page.

Last resort. HMR picks up code changes on its own — reach for this only when you have evidence the dev server is wedged (stale output after edits, builds that never finish, errors that don't clear).

restart-server often exits with net::ERR_ABORTED — this is expected (the page detaches during restart). Follow up with goto <url> to re-navigate after the server is back. Don't treat this error as a failure.

ppr lock

Prerequisite: PPR requires cacheComponents to be enabled in next.config. Without it the shell won't have pre-rendered content to show.

Enter PPR instant-navigation mode. Sets the next-instant-navigation-testing cookie. After this:

• goto — server sends raw PPR shell (static HTML with <template> holes). No hydration — what you see is server HTML only.
• push — Next.js router blocks dynamic data writes, shows prefetched shell. Requires the current page to already be hydrated (prefetch is client-side), so lock after you've landed on the origin, not before.

$ next-browser ppr lock
locked

ppr unlock

Exit PPR mode and print the shell analysis. The output can be very large (hundreds of boundaries). Pipe through | head -20 if you only need the summary line and the dynamic holes.

Captures:

1. Locked snapshot — which boundaries are suspended = holes in the shell
2. Releases the lock, waits for boundaries to settle
3. Unlocked snapshot — what blocked each boundary (suspendedBy)
4. Matches them, prints Dynamic holes / Static

$ next-browser ppr unlock
unlocked

# PPR Shell Analysis
# 131 boundaries: 3 dynamic holes, 128 static

## Dynamic holes (suspended in shell)
  Next.Metadata
    rendered by: MetadataWrapper
  TeamDeploymentsLayout at app/(dashboard)/[teamSlug]/.../layout.tsx:37:9
    suspenders unknown: thrown Promise (library using throw instead of use())
  TrackedSuspense at ../../packages/navigation-metrics/.../tracked-suspense.js:6:20
    rendered by: TrackedSuspense > RootLayout > AppLayout
    blocked by:
      - usePathname (SSR): /vercel/~/deployments awaited in <FacePopover>

## Static (pre-rendered in shell)
  GeistProvider at .../geist-provider.tsx:80:9
  TrackedSuspense at ...
  ...

Each hole shows: boundary name + source, rendered by: ownership chain, blocked by: the dynamic calls (hooks, server APIs, scripts, cache, etc.)

errors doesn't report while locked. If the shell looks wrong (empty, bailed to CSR), unlock and goto the page normally, then run errors. Don't debug blind under the lock.

Full bailout (scrollHeight = 0). When PPR bails out completely, unlock returns just "unlocked" with no shell analysis — there are no boundaries to report. In this case, unlock, goto the page normally, then use errors and logs to find the root cause.

tree

Full React component tree (via DevTools flushInitialOperations).

$ next-browser tree
# React component tree
# Columns: depth id parent name [key=...]
# Use `tree <id>` for props/hooks/state. IDs valid until next navigation.

0 38167 - Root
1 38168 38167 HeadManagerContext.Provider
2 38169 38168 Root
...
224 46375 46374 DeploymentsProvider
226 46506 46376 DeploymentsTable

tree <id>

Inspect one component: ancestor path, props, hooks, state, source location (source-mapped to original file).

$ next-browser tree 46375
path: Root > ... > Prerender(TeamDeploymentsPage) > Prerender(FullHeading) > Prerender(TrackedSuspense) > Suspense > DeploymentsProvider
DeploymentsProvider #46375
props:
  children: [<Lazy />, <Lazy />, <span />, <Lazy />, <Lazy />]
hooks:
  IsMobile: undefined (1 sub)
  Router: undefined (2 sub)
  DeploymentListScope: undefined (1 sub)
  User: undefined (4 sub)
  Team: undefined (4 sub)
  ...
  DeploymentsInfinite: undefined (12 sub)
source: app/(dashboard)/[teamSlug]/(team)/~/deployments/_parts/context.tsx:180:10

IDs are valid until navigation. Re-run tree after goto/push.

viewport [WxH]

Show or set the browser viewport size. Useful for testing responsive layouts.

$ next-browser viewport
1440x900

$ next-browser viewport 375x812
viewport set to 375x812

Once set, the viewport stays fixed across navigations. window.resizeTo() via eval is a no-op in Playwright — always use this command to change dimensions.

screenshot

Full-page PNG to a temp file. Returns the path. Read with the Read tool.

$ next-browser screenshot
/var/folders/.../next-browser-1772770369495.png

Don't narrate what the screenshot shows — the user can see the browser. State your conclusion or next action, not a description of the page.

eval <script>

Run JS in page context. Returns the result as JSON.

$ next-browser eval 'document.title'
"Deployments – Vercel"

$ next-browser eval 'document.querySelectorAll("a[href]").length'
47

$ next-browser eval 'document.querySelector("nextjs-portal")?.shadowRoot?.querySelector("[data-nextjs-dialog]")?.textContent'
"Runtime ErrorCall Stack 6..."

Use this to read the Next.js error overlay (it's in shadow DOM).

For multi-statement code that uses return, wrap in an IIFE: next-browser eval '(() => { const els = ...; return els.length; })()'

eval runs synchronously in page context — top-level await is not supported. Wrap in an async IIFE if you need to await: next-browser eval '(async () => { ... })()'.

errors

Build and runtime errors from the Next.js dev server MCP.

$ next-browser errors
{
  "configErrors": [],
  "sessionErrors": [
    {
      "url": "/vercel/~/deployments",
      "buildError": null,
      "runtimeErrors": [
        {
          "type": "console",
          "errorName": "Error",
          "message": "Route \"/[teamSlug]/~/deployments\": Uncached data or `connection()` was accessed outside of `<Suspense>`...",
          "stack": [
            {"file": "app/(dashboard)/.../deployments.tsx", "methodName": "Deployments", "line": 105, "column": 27}
          ]
        }
      ]
    }
  ]
}

buildError is a compile failure. runtimeErrors has type: "runtime" (React errors) and type: "console" (console.error calls).

logs

Recent dev server log output (NDJSON from .next/dev/logs/).

$ next-browser logs
{"timestamp":"00:01:55.381","source":"Server","level":"WARN","message":"[browser] navigation-metrics: skeleton visible was already recorded..."}
{"timestamp":"00:01:55.382","source":"Browser","level":"WARN","message":"navigation-metrics: content visible was already recorded..."}

network

List all network requests since last navigation.

$ next-browser network
# Network requests since last navigation
# Columns: idx status method type ms url [next-action=...]
# Use `network <idx>` for headers and body.

0 200 GET document 508ms http://localhost:3024/vercel
1 200 GET font 0ms http://localhost:3024/_next/static/media/797e433ab948586e.p.d2077940.woff2
2 200 GET stylesheet 6ms http://localhost:3024/_next/static/chunks/_a17e2099._.css
3 200 GET fetch 102ms http://localhost:3024/api/v9/projects next-action=abc123def

Server actions show next-action=<id> suffix.

network <idx>

Full request/response for one entry. Long bodies spill to temp files.

$ next-browser network 0
GET http://localhost:3024/vercel
type: document  508ms

request headers:
  accept: text/html,...
  cookie: authorization=Bearer...; isLoggedIn=1; ...
  user-agent: Mozilla/5.0 ...

response: 200 OK
response headers:
  cache-control: no-cache, must-revalidate
  content-encoding: gzip
  ...

response body:
(8234 bytes written to /tmp/next-browser-12345-0.html)

page

Route segments for the current URL (via Next.js MCP).

$ next-browser page
{
  "sessions": [
    {
      "url": "/vercel/~/deployments",
      "routerType": "app",
      "segments": [
        {"path": "app/(dashboard)/[teamSlug]/(team)/~/deployments/layout.tsx", "type": "layout", ...},
        {"path": "app/(dashboard)/[teamSlug]/(team)/~/deployments/page.tsx", "type": "page", ...},
        {"path": "app/(dashboard)/[teamSlug]/layout.tsx", "type": "layout", ...},
        {"path": "app/(dashboard)/layout.tsx", "type": "layout", ...},
        {"path": "app/layout.tsx", "type": "layout", ...}
      ]
    }
  ]
}

project

Project root and dev server URL.

$ next-browser project
{
  "projectPath": "/Users/judegao/workspace/repo/front/apps/vercel-site",
  "devServerUrl": "http://localhost:3331"
}

routes

All app router routes.

$ next-browser routes
{
  "appRouter": [
    "/[teamSlug]",
    "/[teamSlug]/~/deployments",
    "/[teamSlug]/[project]",
    "/[teamSlug]/[project]/[id]/logs",
    ...
  ]
}

action <id>

Inspect a server action by its ID (from next-action header in network list).

Scenarios

Growing the static shell

The shell is what the user sees the instant they land — before any dynamic data arrives. The measure is the screenshot while locked: does it read as the page itself? A shell can be non-empty and still bad — one Suspense fallback wrapping the whole content area renders something, but it's a monolithic loading state, not the page.

A meaningful shell is the real component tree with small, local fallbacks where data is genuinely pending. Getting there means the composition layer — the layouts and wrappers between those leaf boundaries — can't itself suspend. ppr unlock names what suspended (blocked by:) and where it sits (rendered by:). A suspend high in the tree is what collapses everything beneath it into one fallback.

Work it top-down. For the component that's suspending: can the dynamic access move into a child? If yes, move it — this component becomes sync and rejoins the shell. Follow the access down and ask again.

When you reach a component where it can't move any lower, there are two exits — both are human calls, bring the question to them:

• Wrap it in a Suspense boundary. The fallback UI should resemble what renders inside — design it together, don't assume.
• Cache it so it's available at prerender (Cache Components). Whether this data is safe to cache — staleness, who sees it — is their call, not yours.

Test your hypothesis before proposing a fix. If you suspect a component is the cause, find evidence — check errors, inspect the component with tree, or compare a route where the shell works to one where it doesn't. Don't commit to a root cause or propose changes from a single observation.

There are two shells depending on how the user arrives. They're observed differently and can differ in content — establish which one you're optimizing before touching the browser. If the ask is "make this page load faster" without qualification, ask: cold URL hit, or clicking in from another page (which page)? Don't guess, don't do both.

Direct load — the PPR shell. Server HTML for a cold hit on the URL. Lock first, then goto the target — the lock suppresses hydration so you see exactly what the server sent. Screenshot once the load settles, then unlock.

Client navigation — the prefetched shell. What the router already holds when a link is clicked. The origin page decides this — it's the one doing the prefetching — so goto the origin unlocked and let it fully hydrate. Then lock, push to the target, let the navigation settle, screenshot, unlock. Locking before the origin hydrates means nothing got prefetched and push has nothing to show.

Between iterations: check errors while unlocked.

After making a code change: HMR picks it up — just re-lock, goto the page, and re-test. No need to restart-server.