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Launcher and config consolidation

This document explains the architecture of the mcp-inspector application entrypoints and configuration processing.

How things used to work (and the challenges)

Previously, the project suffered from split config logic and an unnecessary process boundary for the web server:

  1. Two processes for web and CLI: The main mcp-inspector entrypoint would parse arguments and then spawn() a child process for the actual web server (either Vite for dev, or node dist/server.js for prod).
  2. Config via environment variables: To pass config (like server command, transport, auth token, etc.) from the launcher to the child process, the runner serialized everything into an unwieldy list of environment variables (MCP_INITIAL_*, MCP_ENV_VARS, etc.).
  3. Doesn't scale: Multi-server or complex config required more environment variables and ad-hoc encoding (e.g. JSON in env). This was fragile and easy to get out of sync.
  4. Config logic split: Config parsing was duplicated across the web runner, the CLI runner, and the TUI runner.
  5. Direct launch was inconsistent: Running npm run dev or calling a client's binary directly skipped parts of the launcher logic, leading to inconsistent behavior.

How things work now (Current Design)

The architecture is now consolidated into a single-process model with a shared configuration processor.

1. Dedicated Launcher

A dedicated package under clients/launcher/ (src/index.ts → build/index.js) serves as the global mcp-inspector binary.

  • Responsibility: Its only job is to choose which app to run (--web, --cli, or --tui) and to forward process.argv.
  • No spawn: It dynamically imports the chosen app's runner and calls it in-process.

2. Shared Config Processor

All configuration parsing and merging rules live in core (core/mcp/node/config.ts).

  • Input: Parsed argument options (file path, server name, env vars, transport, headers) and a mode (single or multi).
  • Output: A list of MCPServerConfig objects.
  • Benefits: Web, CLI, and TUI all share the exact same rules for loading config files, applying command-line overrides, and resolving environment variables.

3. App Runners

Each app (Web, CLI, TUI) exposes a runner function (runWeb(argv), runCli(argv), runTui(argv)).

  • The runner uses Commander to parse the arguments.
  • It calls the shared core config processor with the relevant server-config subset.
  • It receives the config list and starts the application logic.
  • Direct launch (e.g. node clients/cli/build/index.js, or a client's own npm run dev) just imports the runner and passes process.argv. This guarantees identical behavior whether invoked via the launcher or directly. v2 ships a single mcp-inspector bin; there are no longer per-client binaries.

4. Web Server In-Process (Config as Object)

The web app no longer uses spawn() or environment variable handoffs. The runner process is the web server.

  • WebServerConfig Object: The runner builds a typed WebServerConfig object (containing port, initial MCP config, auth, etc.) and passes it directly to the server.
  • Vite Dev: startViteDevServer(config) uses Vite's Node API (createServer from vite) in the same process. It passes the config directly to the Hono Vite plugin via honoMiddlewarePlugin(config).
  • Hono Prod: startHonoServer(config) starts the production server in the same process.
  • Benefits:
    • Simpler & Scalable: No env var encoding/decoding. Multi-server config is easily passed as a standard JS object.
    • Easier debugging: A single Node process means no spawn/kill plumbing and clear shutdown logic.

5. Summary of Execution Flow

Component Responsibility
Launcher Detects --web/--cli/--tui and calls the app runner in-process.
Runner Parses argv using Commander and calls the core config processor.
Core Config Applies file/cli merging rules, returns MCPServerConfig object(s).
App Execution Runner uses the config object to directly start Vite API (dev), Hono API (prod), CLI method, or TUI app.

v2 notes

This document was written for v1.5 and describes an architecture that still holds on v2/main; only the paths moved. Two things worth knowing when reading it against the current tree:

  • The clients live under clients/* (clients/web, clients/cli, clients/tui, clients/launcher). v2 is not an npm workspace — each client keeps its own package.json and node_modules, and shared code in core/ is consumed through the @inspector/core build-time alias rather than as a dependency. See the root README.
  • The shared config processor gained the --catalog (writable) vs. --config (read-only) split, which the launcher's argument tables and MCP server configuration describe.