MCP servers

External tools and data sources Claude can call, packaged behind the Model Context Protocol.

What it is

An MCP server exposes tools, resources, and prompts over the Model Context Protocol so Claude can call them. The server runs as a local subprocess (stdio transport) or as a remote HTTP service (streamable HTTP transport). MCP is an open spec, so the same server works with Claude Code, Claude Desktop, the Claude.ai connector directory, and other MCP-aware clients.

You install one MCP server per data source — PubMed, ClinicalTrials.gov, your filesystem, your issue tracker — and Claude picks tools from it as needed.

When to use it

  • You need Claude to query a database or API (PubMed, ChEMBL, your internal services).
  • You want Claude to read or write files on your machine.
  • A vendor publishes a hosted MCP endpoint you want to point Claude at.
  • You’re building a shared team integration committed to a repo via .mcp.json.

How to install / enable

Use claude mcp add from your shell. Flags go before the server name; -- separates stdio commands.

# Remote HTTP server
claude mcp add --transport http pubmed https://pubmed.mcp.claude.com/mcp

# Local stdio server (default transport)
claude mcp add filesystem -- npx -y @modelcontextprotocol/server-filesystem ~/Documents

# Share with your team (commits .mcp.json at repo root)
claude mcp add --transport http paypal --scope project https://mcp.paypal.com/mcp

Inside a session, run /mcp to see status and trigger OAuth login for servers that require it.

Common pitfalls

  • Putting flags after the server name — they’re silently ignored.
  • Forgetting -- before the stdio command and its arguments.
  • Committing ~/.claude.json (it holds local-scope credentials). Use --scope project for .mcp.json instead.
  • Using --transport sse — deprecated; use --transport http.
  • Assuming static API keys work for remote MCP. Remote servers use OAuth 2.1; pass bearer tokens via --header when supported.
  • Trying to expose an MCP server inside a private network. Don’t poke firewall holes — request access to MCP tunnels (Research Preview, May 2026), which run a cloudflared agent outbound from your network. Console-managed only; not yet exposed as claude.ai connectors.
  • Assuming MCP is pull-only. Since v2.1.80 (March 2026), Claude Code supports channels — allowlisted MCP servers that push events into a session (claude --channels discord,telegram,imessage). Useful for CI failures, monitoring alerts, or chat-bridge bots reaching Claude in your terminal. Research preview; claude.ai OAuth only; Team / Enterprise admins must opt in. See the channels cross-cutting note.

See also

Sources