MCP Inspector

A developer tool for inspecting Model Context Protocol (MCP) servers. It ships as a single package, @modelcontextprotocol/inspector, that provides three ways to inspect a server:

• Web — a Vite + React + Mantine single-page app with a Node backend.
• CLI — a scriptable command-line client for automation, CI, and fast agent feedback loops.
• TUI — an interactive terminal UI built with Ink.

All three run through one global mcp-inspector binary:

npx @modelcontextprotocol/inspector          # web UI (default)
npx @modelcontextprotocol/inspector --cli    # CLI
npx @modelcontextprotocol/inspector --tui    # TUI

[!WARNING] On a machine with no OS keychain, secrets are saved to a plaintext file by default. That covers Linux without libsecret or a Secret Service, headless and SSH sessions, Termux, and containers with a mounted volume. OAuth client secrets and stdio env: values then go to ~/.mcp-inspector/secrets.json, unencrypted unless you supply a key. See Where secrets are stored for how to get a keychain back, encrypt the file, or keep secrets in memory only.

Upgrading from v1? Read the v1 → v2 migration guide — CLI flags, the new --config vs. --catalog split, the Node engine bump, and what no longer ships.

Repo status. This is the v2 line of the Inspector. Active development happens on v2/main (the develop branch — all v2 PRs target it), which is merged into main at milestone releases; main is the default branch and holds the latest released v2, published to the npm latest tag. The legacy v1 line lives on v1/main — security fixes only, published straight from that branch to the npm v1-latest tag (npx @modelcontextprotocol/inspector@v1-latest). See AGENTS.md for branch/board conventions.

Quick start (development)

Requires Node >=22.19.0.

npm install          # at the repo root; postinstall cascades into every client
npm run build        # web → cli → tui → launcher

For day-to-day web iteration, run Vite directly — fast HMR, no launcher build needed:

cd clients/web && npm run dev

The launcher-driven scripts run the built launcher, so build first:

npm run web        # prod web launcher against clients/web/dist
npm run web:dev    # web launcher in --dev mode (Vite)

v2 is not an npm workspace — each client under clients/* keeps its own package.json and node_modules, and shared code lives in core/, consumed via a @inspector/core build-time alias. Every runtime dependency core/ imports is declared once, in the repo-root package.json, and each client declares only what that client alone consumes — its UI stack, its bundler-inlined packages, its dev tooling — which leaves clients/cli and clients/launcher with no runtime dependencies of their own. What that means for adding a dependency (root vs. client, dependencies vs. devDependencies, and the bundler external lists) is in the local-dev skill.

Project layout

inspector/
├── clients/
│   ├── web/          Web client (Vite + React + Mantine). src/ = browser app; server/ = Node backend
│   ├── cli/          CLI client (tsup bundle, @inspector/core alias)
│   ├── tui/          TUI client (Ink + React, tsup bundle)
│   └── launcher/     Shared launcher — provides the `mcp-inspector` bin, dispatches to web/cli/tui
├── core/             Shared code consumed via the `@inspector/core` alias (no package.json)
├── test-servers/     Composable MCP test servers + fixtures used by integration and smoke tests
├── scripts/          Root build/verify tooling (install cascade, smokes, the verify:* guards)
│                     and repo automation run from CI (the dependency, Dependabot-alert and SDK sweeps)
├── docs/             Task-oriented guides — see below
├── specification/    Design/build specifications
├── .claude/skills/   Agent skills: the repo's procedures, invokable by name
├── AGENTS.md         Contribution rules for agents AND humans
└── README.md         You are here

Each client has its own README with client-specific detail: web · cli · tui · launcher.

Documentation

Guide	Covers
Architecture	The @inspector/core shared package, and the web client's "dumb components" + Storybook approach
Testing and the quality gate	What each validate / coverage / smoke / verify:* script covers, the GitHub-CI-vs-local-gate split, and the supported browsers
Writing a skill	How to write a skill description that actually fires, and eval cases that measure it — the case shapes that work, and the tuning loop
Test servers	The composable test servers and the showcase config for every feature — what to run, what to click, and what the broken build did
Publishing	What ships in the tarball, the packaging invariants, and pack:verify
Docker	Running the container image — ports, volumes, and making secrets durable in a container
Where secrets are stored	How the secret store is chosen on every runtime — OS keychain, secrets.json or memory — plus file encryption, locking, and moving back to a keychain
Migrating from v1 to v2	CLI flag mapping, --config vs. --catalog, the Node engine bump, env-var renames
Environment variables	Every variable that changes runtime behavior — auth, ports, storage, the secret store, logging, proxies — plus the Node TLS variables for a self-signed server
MCP server configuration	Which server(s) the Inspector connects to, and the config file format
Reviewing an MCP App	The CLI-first → one-shot-web recipe for automated App-tool review
Smoke-testing an MCP server	The connect → list → call → assert workflow for a shell or CI job: --format json + jq, the exit-code map, and keeping OAuth non-interactive
Launcher and config consolidation	Why the launcher runs a client in-process rather than spawning it
Roadmap, Aug 2026 → Feb 2027	The six-month plan: spec-following work aligned to the published MCP roadmap, official extension support, and the experience work we choose

Testing and the quality gate

Each client self-validates from its own folder; the root scripts chain them. There is no aggregate root test script.

npm run validate     # fast inner loop: format:check + lint + typecheck + build + unit tests
npm run coverage     # the per-file ≥90% gate (lines/statements/functions/branches)
npm run local:gate   # MANDATORY before pushing — every GitHub CI check, plus two local-only ones

npm run local:gate chains every check below, plus the smokes and the Storybook tests. Testing and the quality gate owns the stage list and says what each one covers and why two are local-only; AGENTS.md holds the testing rules themselves.

Contributing — AGENTS.md, CLAUDE.md, and the skills

AGENTS.md is the contract for changing this codebase, and it applies to humans and AI agents alike. It is not agent-only boilerplate — it holds the project's real rules: the version/label conventions, the TypeScript and Mantine/React standards, the testing and coverage requirements, and the mandatory pre-push gate. Read it before making changes, and keep it up to date when you change structure, tooling, or rules.

The repo's procedures — multi-step recipes with commands and live IDs — live in .claude/skills/ instead, one directory per procedure, so they are loaded only when the task calls for them. They are ordinary committed Markdown: an agent that doesn't understand skills can read them, and AGENTS.md carries an index of what exists. Claude Code users invoke one by name (/release, /issue-triage, …).

CLAUDE.md is the entry point Claude Code loads automatically; it includes AGENTS.md, so agents and humans work from the same source of truth. If you use a different agent that reads AGENTS.md, you get the same rules.

A key rule worth surfacing here: all work is issue-driven. Before starting, find or create a tracking issue on the v2 project board; open PRs against v2/main with Closes #<issue>. External contributions are accepted as issues, not pull requests — see CONTRIBUTING.md.

License

See LICENSE. The MCP project is transitioning from the MIT License to Apache-2.0: new code contributions are licensed under Apache-2.0, documentation (excluding specifications) under CC-BY-4.0, and contributions whose authors originally licensed them under MIT and have not granted relicensing consent remain under MIT. The file carries the full Apache-2.0 and MIT texts and links the CC-BY-4.0 legal code.