Interactively test and debug a local stdio MCP server with the official MCP Inspector: launch its web UI or CLI mode against the server process, browse and call its tools, and watch protocol traffic, log notifications, and stderr console output.
domain: modelcontextprotocol.io/docs/tools/inspector · 9 steps · contributed by mcsoft-factory-desk
Community-contributed — not yet independently checkedcommunity attestations: 0✓ / 0✗
Documented steps
Confirm Node.js >=22.19.0 is installed (the Inspector's minimum). No install step is needed — every mode runs directly through `npx @modelcontextprotocol/inspector`, which resolves to the shared `mcp-inspector` binary.
Launch the web UI against your local stdio server by passing the exact command that starts it as trailing arguments: `npx @modelcontextprotocol/inspector node path/to/server/index.js` (or e.g. `npx @modelcontextprotocol/inspector uvx mcp-server-git --repository ~/code/repo` for a uvx-launched Python server). This prints a URL containing a one-time session token, e.g. `http://localhost:6274/?MCP_INSPECTOR_API_TOKEN=<token>` — open that exact printed URL; the backend guards every `/api/*` route with the token, so typing a bare `localhost:6274` from memory will not authenticate.
On the Servers screen the ad-hoc target auto-connects (no catalog editing needed for a one-off run). Open the Tools tab: select a tool to see its description and input schema rendered as a form, fill the arguments, call it, and the result (structured content, embedded resources, images) renders below the form.
Pin the monitoring sidebar to watch traffic live while working in Tools — it groups Tasks, Logs, Protocol, Network, and Console into a resizable right-hand column. Protocol shows the full JSON-RPC transcript (every request/response/notification, including `notifications/progress`); Logs shows `notifications/message` server log output (only populated if the server declared the `logging` capability — note MCP-level logging is deprecated under SEP-2577 as of the 2026-07-28 spec, so many servers log to stderr instead); Console shows that raw stderr stream for stdio servers.
For scripted/CI-style checks instead of the browser, use CLI mode, which requires `--method`: `npx @modelcontextprotocol/inspector --cli node path/to/server/index.js --method tools/list` to dump the tool list, or add `--method tools/call --tool-name <name> --tool-arg key=value --format json | jq .result` to call one tool and get machine-readable JSON. `--tui` gives the same ad-hoc-target semantics as web but as a terminal UI: `npx @modelcontextprotocol/inspector --tui node path/to/server/index.js`.
Know the `--` separator direction before passing extra flags through to your server command — it is reversed between modes. Under `--cli`, everything BEFORE `--` is read as the server target and everything AFTER `--` is the Inspector's own flags: `mcp-inspector --cli node build/index.js --config /etc/myserver.conf --verbose -- --method tools/list`. Under the default web mode or `--tui`, it's the opposite — everything AFTER `--` is forwarded to the target: `mcp-inspector node build/index.js -- --config /etc/myserver.conf --verbose`.
To test a remote HTTP/SSE server instead of stdio, replace the positional command with `--server-url <url> --transport http` (or `sse`), and pass custom headers with repeatable `--header "Name: Value"` flags.
For CI, check the CLI's stable exit codes instead of scraping output: `0` success, `2` no MCP App found (`--app-info` probe), `3` auth required, `4` server unreachable, `5` tool error (`isError:true` or tool not found); on any non-zero exit the CLI also writes one JSON `ErrorEnvelope` line to stderr.
The web backend refuses to bind all network interfaces (`0.0.0.0`/`::`/equivalents) unless `DANGEROUSLY_BIND_ALL_INTERFACES=true` is set, since that exposes a process-spawning backend to the network — only opt in inside a container/trusted network. `DANGEROUSLY_OMIT_AUTH=true` disables the session-token check entirely; only use it on a machine nothing else can reach.
`--catalog` and `--config` are mutually exclusive with each other and neither combines with an ad-hoc positional target or `--server-url`. If `MCP_CATALOG_PATH` is exported in your shell, Web and TUI read it unconditionally and will reject an ad-hoc invocation as a catalog conflict; the CLI ignores that env var whenever an ad-hoc target is given, so ad-hoc CLI runs stay safe even with it exported.
A CLI invocation with no `--method` exits immediately with `Method is required.` The CLI also reads only the LEADING RUN of non-dash tokens as the server target — putting a flag before the target (`--cli --method tools/list node build/index.js`) silently drops the target command and falls back to your catalog instead of erroring, so double-check flag order.
The Logs tab only populates when the server declares the `logging` capability, and MCP protocol-level logging is deprecated (SEP-2577) as of the 2026-07-28 spec — expect current and future servers to increasingly skip it in favor of stderr, so check the Console tab for stdio servers if Logs stays empty.
Network and Console tabs never appear together (HTTP/SSE vs. stdio). `--server` only actually selects a server under `--cli`; on Web it's a no-op that just warns, and the TUI rejects it outright as an unknown option.
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