MCP client vs MCP server
Clients drive the agent. Servers expose tools. Mixing those roles creates messy auth and harder audit.
The MCP client is usually your agent framework or IDE side. It lists tools, picks one, and sends a call. The MCP server receives that call and does the work.
Where identity lives
Put agent identity and tool scope on the path into the server. If every tool checks a different key format, operators spend their week reconciling logs.
Keep the agent loop thin. Put policy next to the tools.
Practical rule
Prototype with local servers. Move shared tools (secrets, mail, search, storage, writing) onto a managed MCP surface before you scale the swarm.