NEVER guess package names → npm info <name> to verify before installing
Validate before adding → npm info → npx -y <pkg> --help → claude mcp add. Caveat: many MCP servers don't implement --help and just wait on stdio — wrap validation in timeout 10 … so it can't hang.
Think UX upfront → headless mode, timeouts, flags — ask user BEFORE installing
Restarts: /mcp can reconnect already-registered servers, but registering a NEW server requires a session restart. Get the config right the first time.

Python servers: claude mcp add <name> -- uvx <pkg> — uvx launches PyPI packages over stdio, no venv management.
Remote servers: claude mcp add --transport http <name> <url> (also --transport sse for legacy SSE servers).
Scopes: project scope lives in .mcp.json at the repo root (checked in); local scope lives in ~/.claude.json under that project's entry (NOT in the repo); user scope is global in ~/.claude.json.
Plugin-bundled servers get tool names mcp__plugin_<plugin>_<server>__<tool> — the full name must stay under the 64-char API limit, so keep plugin/server names short.

"Browser is already in use … use --isolated" = the persistent Chrome profile (~/Library/Caches/ms-playwright-mcp/mcp-chrome-* on macOS, ~/.cache/ms-playwright-mcp/mcp-chrome-* on Linux) is locked by a stale SingletonLock symlink (often pointing at an unrelated regular Chrome pid). The MCP server refuses even after browser_close.
Fix without killing the user's Chrome: if more than one browser MCP server is configured, each has its own profile — switch to the other server rather than killing the pid the lock points at (it may be the user's real browser). Stay with ONE browser server per session; alternating between two hits the lock again.

mcp-setup

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