MCP Inspector

The MCP Inspector is the reference developer tool for testing and debugging MCP servers. It ships as a single package, @modelcontextprotocol/inspector, providing three clients behind one binary:

Client	Invocation	What it's for
Web	`npx @modelcontextprotocol/inspector`	A full graphical inspector in the browser. The default, and the richest surface.
CLI	`npx @modelcontextprotocol/inspector --cli`	A scriptable, machine-readable client for CI, shell pipelines, and coding agents.
TUI	`npx @modelcontextprotocol/inspector --tui`	An interactive terminal UI, for when a browser isn't available or wanted.

All three are built on the same shared core, so a connection behaves identically across them: the same transports, the same configuration files, the same OAuth state on disk, and the same protocol-era negotiation (legacy vs. modern 2026-07-28).

The package also installs mcpdo, an experimental connection client that connects to a server once and keeps it open across many commands, on the same core.

Quickstart

The Inspector requires Node 22.19.0 or newer and runs directly through npx. No installation is required:

Web

CLI

TUI

# Launch the web UI and connect to a local stdio server
npx @modelcontextprotocol/inspector node path/to/server/index.js

# Or launch with no target and add servers from the UI
npx @modelcontextprotocol/inspector

The command prints a URL carrying a per-launch API token and opens it in your browser (set MCP_AUTO_OPEN_ENABLED=false to stop it opening). See Web client.

# List a server's tools and exit
npx @modelcontextprotocol/inspector --cli node path/to/server/index.js --method tools/list

# Call a tool and pipe the result into jq
npx @modelcontextprotocol/inspector --cli https://api.example.com/mcp --transport http \
  --method tools/call --tool-name get_weather --tool-arg city=Boston --format json | jq .result

See CLI client.

npx @modelcontextprotocol/inspector --tui node path/to/server/index.js

See TUI client.

WebCLITUI

# Launch the web UI and connect to a local stdio server
npx @modelcontextprotocol/inspector node path/to/server/index.js

# Or launch with no target and add servers from the UI
npx @modelcontextprotocol/inspector

The command prints a URL carrying a per-launch API token and opens it in your browser (set MCP_AUTO_OPEN_ENABLED=false to stop it opening). See Web client.

# List a server's tools and exit
npx @modelcontextprotocol/inspector --cli node path/to/server/index.js --method tools/list

# Call a tool and pipe the result into jq
npx @modelcontextprotocol/inspector --cli https://api.example.com/mcp --transport http \
  --method tools/call --tool-name get_weather --tool-arg city=Boston --format json | jq .result

See CLI client.

Inspecting published servers

Pass the command that launches the server as the Inspector's arguments, or point it at a remote server with --server-url:

npm package

PyPI package

Remote HTTP server

npx -y @modelcontextprotocol/inspector npx @modelcontextprotocol/server-filesystem ~/Desktop

npx @modelcontextprotocol/inspector uvx mcp-server-git --repository ~/code/mcp/servers.git

npx @modelcontextprotocol/inspector --server-url https://api.example.com/mcp --transport http

npm packagePyPI packageRemote HTTP server

npx -y @modelcontextprotocol/inspector npx @modelcontextprotocol/server-filesystem ~/Desktop

Always read a server's own README first, since every server requires different commands and arguments.

Warning

The Inspector stores secrets (OAuth tokens, OAuth client secrets, and stdio env: values) in the OS keychain when one is available. On a machine with no keychain (Linux without libsecret or a Secret Service, headless and SSH sessions, Termux, and containers with a mounted volume) they are saved to ~/.mcp-inspector/secrets.json instead, 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.

Launcher flags vs. client flags

mcp-inspector, the binary that npx @modelcontextprotocol/inspector runs, is a thin launcher. It owns only two things:

The mode flag: --web (default), --cli, or --tui. At most one; passing two errors with Specify at most one of --web, --cli, or --tui.
-h / --help.

Everything else (--catalog, --config, --server-url, --transport, --method, the OAuth flags) is defined by the client, not the launcher, and the clients do not all define the same set. The Configuration and flags page is organized that way, by owner.

Note

Mode flags are recognized only at the front of the command line: the first token that isn't --web / --cli / --tui ends launcher parsing, and everything after it is forwarded to the client unchanged. That's what lets a literal --cli appear later as one of your server's own arguments:

mcp-inspector --cli node server.js --cli   # mode is CLI; the trailing --cli goes to server.js