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Next.js DevTools MCP

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by vercel
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TypeScript
Hybrid

nextjs_index

Discovers running Next.js development servers and lists their available MCP tools for runtime diagnostics and inspection.

Instructions

Discover all running Next.js development servers and list their available MCP tools.

WHEN TO USE THIS TOOL - Use proactively in these scenarios:

  1. Before implementing ANY changes to the app: When asked to add, modify, or fix anything in the application:

    • "Add a loading state" → Check current component structure and routes first

    • "Fix the navigation" → Inspect existing routes and components

    • "Update the API endpoint" → Query current routes and data flows

    • "Add error handling" → Check runtime errors and component hierarchy

    • "Refactor the auth logic" → Inspect current auth implementation and routes

    • "Optimize performance" → Check runtime diagnostics and component tree Use this to understand where changes should be made and what currently exists.

  2. For diagnostic and investigation questions:

    • "What's happening?" / "What's going on?" / "Why isn't this working?"

    • "Check the errors" / "See what's wrong"

    • "What routes are available?" / "Show me the routes"

    • "Clear the cache" / "Reset everything"

    • Questions about build status, compilation errors, or runtime diagnostics

  3. For agentic codebase search: Use this as FIRST CHOICE for searching the currently running app. If not found, fallback to static codebase search tools.

KEY PRINCIPLE: If the request involves the running Next.js application (whether to investigate OR modify it), query the runtime FIRST to understand current state before proceeding.

REQUIREMENTS:

  • Next.js 16 or later (MCP support was added in v16)

  • If you're on Next.js 15 or earlier, upgrade first by running 'npx @next/codemod@latest upgrade latest'

Next.js 16+ exposes an MCP (Model Context Protocol) endpoint at /_next/mcp automatically when the dev server starts. No configuration needed - MCP is enabled by default in Next.js 16 and later.

This tool discovers all running Next.js servers and returns:

  • Server port, PID, and URL

  • Complete list of available MCP tools for each server

  • Tool descriptions and input schemas

After calling this tool, use 'nextjs_call' to execute specific tools.

[IMPORTANT] If auto-discovery returns no servers:

  1. Ask the user which port their Next.js dev server is running on

  2. Call this tool again with the 'port' parameter set to the user-provided port

If the MCP endpoint is not available:

  1. Ensure you're running Next.js 16 or later (upgrade with 'npx @next/codemod@latest upgrade latest')

  2. Verify the dev server is running (npm run dev)

  3. Check that the dev server started successfully without errors

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
portNo

Tool Definition Quality

A4.6/5.0
Behavior4/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

No annotations, so description carries full burden. It discloses discovery behavior, requirements (Next.js 16+), fallback with port parameter, and what returns. Lacks specifics on error handling or latency but overall transparent.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Well-structured with headings, numbered lists, and bold for key points. Some slight redundancy, but each section earns its place. Content is appropriate for the tool's complexity.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness5/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given no output schema, the description fully explains return values (port, PID, URL, MCP tools list). Also covers prerequisites, fallback procedures, and next steps. Complete for this tool.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters4/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema has one optional parameter 'port' with no description. The description adds meaning by explaining its use as a fallback when auto-discovery fails, and instructs to ask user for port. Compensates for 0% schema coverage well.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the tool discovers running Next.js servers and lists their MCP tools. It distinguishes from sibling nextjs_call by setting up a workflow for execution, and from nextjs_docs which is likely documentation.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Provides explicit scenarios for when to use: before implementing changes, for diagnostic questions, and as first choice for agentic codebase search. Includes fallback instructions and when to use nextjs_call, offering clear decision guidance.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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