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inspector/clients/launcher/README.md
Cliff Hall cea22d86b8
 feat(web): enforce localhost-only binding; expand loopback origin allow-list (#1796) 
Closes #1795.

Enforce localhost-only binding in code (the "never 0.0.0.0" rule was only a spec note): refuse an all-interfaces HOST — across every spelling the OS binds as the wildcard (inet_aton decimal/octal/hex, IPv6, IPv4-mapped, zone-scoped, IDNA/Unicode) — unless DANGEROUSLY_BIND_ALL_INTERFACES=true (Docker's sanctioned opt-in). Fixes the reported stdio-connect 403 by expanding the default loopback origin allow-list to all three interchangeable forms.

Hardening surfaced across an extended review: closes an IDNA/fullwidth wildcard bypass, a blank-ALLOWED_ORIGINS fail-open, and the CLI/TUI OAuth-callback wildcard bind (loopback-only now, RFC 8252); canonicalizes the origin allow-list the way a browser does; derives frame-ancestors from the real embedder list (IPv6-literal exclusion verified in headless Chromium); strictPort on all three bind points; and preserves the CLI exit-code map + JSON envelope through the mcp-inspector --cli launcher path. Shared host normalization lives in core/node/hostUrl.ts. Docs + tests + smokes throughout.

🤖 Generated with [Claude Code](https://claude.com/claude-code)
2026-07-26 22:15:10 -04:00

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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 config or env vars.

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).

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 Yes
mcp-inspector --web --catalog <path> (or MCP_CATALOG_PATH=<path>) That file as the active catalog (created/seeded if missing) 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 or --header; --header requires an ad-hoc HTTP/SSE server and is applied to that connection (it is no longer a warn-only no-op).

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

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> and asserts the Ink app renders its first frame within a timeout, then shuts it down (a shallow boot/render check, not full interaction).

Both build test-servers/build on demand if it is missing.

Development

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

npm run validate  # format:check && lint && build && test:coverage

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).

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 the cli/tui package.jsons ship), and the npm run pack:verify publish smoke — is documented in the root README.

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.