Test a server with the MCP Inspector

When a client says a server failed, you cannot tell whether the server is broken or the client is misconfigured. The Inspector removes the client: it starts or calls your server directly and shows every message.

Install and run

You do not install it. Run it with npx, pinned to a version:

npx -y @modelcontextprotocol/inspector@2.12.0 --help

Version 2.12.0 declares Node.js 22.19 or later in its package metadata. We tested it on Node 24.21 only.

It has three front ends, chosen with the first argument:

Flag	What you get
--web (default)	A browser UI with a message log
--cli	One command, one result. Good for scripts and CI
--tui	An interactive terminal UI

The mode flag must come before everything else; the rest of the arguments are passed through unchanged.

Pick the protocol era

When you give the Inspector a command or URL directly, it connects with the 2025-11-25 handshake by default (--protocol-era legacy). To test the 2026-07-28 protocol, add --protocol-era auto (try the new protocol, fall back if needed) or --protocol-era modern (new protocol only).

In the terminal

Use the CLI for quick checks. These commands work against any stdio server; replace node src/server.ts with your own command.

TerminalList tools

npx -y @modelcontextprotocol/inspector@2.12.0 --cli \
  node src/server.ts --protocol-era auto --method tools/list

TerminalCall a tool with arguments

npx -y @modelcontextprotocol/inspector@2.12.0 --cli \
  node src/server.ts --protocol-era auto \
  --method tools/call --tool-name add --tool-arg a=2 b=3 --format json

TerminalTest a server over HTTP

npx -y @modelcontextprotocol/inspector@2.12.0 --cli \
  http://127.0.0.1:8931/mcp --protocol-era auto --method tools/list

The transport is detected from the target: a command means stdio, a URL ending in /mcp means Streamable HTTP, and /sse means the old HTTP+SSE transport. Use --transport to override it.

If the call fails, the CLI prints {"error": …} and exits with a non-zero code, so you can use it in scripts.

In the browser

TerminalStart the web UI

npx -y @modelcontextprotocol/inspector@2.12.0 --web \
  --protocol-era auto node src/server.ts

It prints a local address with a one-time token, for example http://127.0.0.1:6274?MCP_INSPECTOR_API_TOKEN=…. Open that exact address, token included. It listens on 127.0.0.1 only, and MCP Apps are rendered in a separate sandbox on port 6275.

Because you started it with a command, the page says Read-only session: the server list cannot be edited. Turn on the switch on the server card to connect.

Inspector 2.12.0 connected to the TypeScript example with –protocol-era auto.

Two things to read in the screenshot:

• The server card shows the negotiated version, here MCP 2026-07-28.
• The message panel is labeled MODERN, and you can see subscriptions/listen, the 2026-07-28 way of receiving change notifications.

Open Tools at the top to call tools from a form.

Which protocol did I get?

• In the CLI output, a result that carries _meta with io.modelcontextprotocol/serverInfo came over 2026-07-28. Without it, the connection used the older handshake.
• If --protocol-era modern fails with Version negotiation failed: the server did not offer pinned protocol version 2026-07-28, the server only supports older versions. In our tests this happens with the Java SDK 2.1.0.

On Linux without a keychain

On a Linux machine with no desktop keychain, the Inspector warns that it stores secrets, such as OAuth tokens, unencrypted in ~/.mcp-inspector/secrets.json with file mode 0600. That is expected on servers and containers. Set MCP_INSPECTOR_SECRET_STORE to choose another store.

Next

• MCP error -32000: Connection closed
• Codex: connect a server
• Checklist for a stdio server that will not start
• Where clients write MCP logs