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MCP Inspector Launcher

The launcher is the package that provides the global mcp-inspector binary (e.g. when users run npx @modelcontextprotocol/inspector). It is not a separate user-facing app—it is the single entrypoint that selects and runs one of the clients (web, CLI, or TUI).

Responsibility

  • Parse mode from a leading prefix of --web (default), --cli, or --tui immediately after the script name.
  • Forward all following arguments unchanged (including tokens that look like mode flags).
  • Dynamically import that app’s runner from clients/{web,cli,tui}/build/index.js (relative to the launcher build output) and call it in-process (no spawn()).

All configuration parsing, config-file loading, and server setup are handled by the app runners and by core; the launcher does not interpret server or client configuration, from files or from env vars. Its one exception is the debug switch described below.

Error reporting. A --cli failure is routed through the CLI's own error sink, so mcp-inspector --cli preserves the CLI exit-code map (1 usage, 2 no-app, 3 auth-required, 4 unreachable, 5 tool-error) and its machine-readable {"error":{…}} stderr envelope — the same as invoking the CLI bin directly. --web / --tui failures print a human-readable Error: <message> and exit 1 (append MCP_DEBUG=1 for the stack). MCP_DEBUG (or DEBUG) is the only variable the launcher reads itself; the rest are listed in Environment variables.

Web server-list flags (--web)

mcp-inspector --web chooses which server list the UI shows and whether it is editable (see specification/v2_catalog_launch_config.md):

Invocation Server list Editable in UI?
mcp-inspector --web Default catalog ~/.mcp-inspector/mcp.json (seeded with the three sample servers if missing) Yes
mcp-inspector --web --catalog <path> (or MCP_CATALOG_PATH=<path>) That file as the active catalog (same seed-if-missing behavior) Yes
mcp-inspector --web --config <path> That file as a read-only session — shown but never written, seeded, or migrated (safe for a foreign config) No
mcp-inspector --web --server-url <url> --transport http --header "Name: Value" (or a positional command) One ad-hoc server held in memory, connectable with the given --headers No

Rules: --catalog and --config are mutually exclusive; neither combines with an ad-hoc target, --header or --protocol-era; --header requires an ad-hoc HTTP/SSE server and is applied to that connection (it is no longer a warn-only no-op). --protocol-era <legacy|auto|modern> requires an ad-hoc server of any transport and sets the era that connection negotiates, so a non-legacy era needs no mcp.json entry.

Seed contents are web-specific. When the web backend creates a missing writable catalog it seeds DEFAULT_SEED_CONFIG (core/mcp/serverList.ts) — a filesystem-server-default scoped to /tmp, the canonical everything-server-default, and example-server-default, the MCP org's remote feature-reference server (Streamable HTTP, no local process and no API key needed) — so a first launch has something to connect to. The CLI and TUI seed an empty catalog instead; see the next section. A read-only --config is never seeded on any surface.

CLI and TUI server-list flags (--cli / --tui)

The CLI and TUI use the same --catalog / --config vocabulary as --web, resolved by the shared core/mcp/node/config.ts helpers:

Invocation Server list source
mcp-inspector --cli / --tui (no source flag, no ad-hoc target) Default writable catalog ~/.mcp-inspector/mcp.json (created/seeded empty if missing)
--catalog <path> (or MCP_CATALOG_PATH=<path>) That file as a writable catalog — seeded empty if missing
--config <path> That file as a read-only session — served as-is, never written or seeded; errors if absent
positional command / --server-url <url> One ad-hoc server

Note the seed contrast with --web above: the CLI and TUI write an empty { "mcpServers": {} } (seedEmptyCatalog in core/mcp/node/config.ts), not the web client's three sample servers — they are non-interactive or list-driven, so sample entries would be noise rather than a starting point.

Rules (shared serverSourceConflict): --catalog and --config are mutually exclusive, and neither combines with an ad-hoc command/URL target. The CLI/TUI do not perform catalog CRUD yet — they are read consumers — so the writable/read-only split currently surfaces only as seed-if-missing (--catalog/default) vs error-if-missing (--config). Full writable persistence is tracked in #1482 / #1432.

Production web build (--web, no --dev)

Prod --web serves static assets from clients/web/dist/, which only exists after a build. In the published package dist/ always ships. In a fresh dev checkout it is absent, so the runner builds it on demand the first time you launch (vite build via npm run build:client, run in clients/web) instead of serving a broken page (#1486). If that build can't run — e.g. dev dependencies are missing — the launcher exits with an actionable error telling you to run npm run build (from clients/web) or relaunch with --dev to use the Vite dev server. --dev never needs dist/; it runs Vite directly.

CI and npm run validate (via the top-level smoke step) exercise this prod path end-to-end with npm run smoke:web (scripts/smoke-web.mjs): it starts mcp-inspector --web against the built dist/ and asserts GET / returns the SPA (HTTP 200) with the injected __INSPECTOR_API_TOKEN__.

CLI and TUI smokes

The top-level smoke step also runs end-to-end smokes for the other two modes through the built launcher artifact (beyond the --help checks in smoke:launcher):

  • npm run smoke:cli (scripts/smoke-cli.mjs) — runs mcp-inspector --cli against the bundled stdio test server via a temp --catalog and asserts tools/list returns the server's tools, plus the --catalog/--config resolution paths (default-catalog seed-on-missing, read-only --config error-without-seed, --catalog/--config conflict).

  • npm run smoke:tui (scripts/smoke-tui.mjs) — launches mcp-inspector --tui --catalog <temp> under a pseudoterminal, asserts the Ink app renders its first frame within a timeout, then waits and asserts it is still running before shutting it down (a shallow boot/render/survival check, not full interaction).

    The second half is the assertion (#2147). Spawned with its stdin on /dev/null, Ink cannot enter raw mode for useInput, so the TUI painted one frame and exited 1 about 40ms later — and this smoke, which settled OK on the first frame, won that race and reported success on every machine. It is local-only (self-skips under CI), which is precisely where a false green goes unnoticed.

Both rebuild test-servers/build on every run — once per process, whether or not it already exists (#2111). Presence is not freshness: a smoke driving a stale fixture reports a product failure rather than a staleness one.

Development

Like the web client, the launcher self-validates from its own folder:

npm run check     # format:check && lint && typecheck && build  (no tests)
npm run validate  # check && test

This has no dependency on the other clients being built — it only checks the launcher's own source. eslint.config.js is a Node-only flat config (the web client's React/Storybook plugins stripped out), and the per-file coverage gate covers parse-launcher-argv.ts (the pure arg-parsing logic); src/index.ts is excluded as binary bootstrap and is instead exercised by the smokes above. The repo-root validate:launcher simply delegates here (cd clients/launcher && npm run validate), and the root local:validate — the first stage of npm run local:gate — runs check instead, so the gate runs the suite once, under coverage:launcher (#2341).

Publishing

The launcher provides the mcp-inspector bin for the single @modelcontextprotocol/inspector tarball. Packaging is a whole-repo concern — how the one-package/single-version tarball is assembled, the "files" allowlist invariants (no source maps, why clients/web/build needs .npmignore, why clients/web/static must ship at that exact path), and the npm run pack:verify publish smoke — is documented in Publishing.

Architecture

For how the launcher fits with the shared config processor and app runners, see the Launcher and config consolidation document in the repo root docs/ folder.