Most MCP traffic flows from client to server. But the protocol also lets servers make certain requests back to the client, always under the host's control.
Sampling
With sampling, a server asks the host to run a model completion. For example, a server might need to summarise a document it fetched.
Benefits:
- Servers can use AI without their own model API keys.
- The user's chosen model and host policies apply.
Safeguards:
- The host decides whether to allow the request and may show it to the user.
- The host controls which model is used and what context is included.
Elicitation
With elicitation, a server asks the host to collect information from the user mid-task — such as a missing parameter or a confirmation — through a structured form.
Safeguards:
- The host presents the request and the user can decline.
- Servers shouldn't request sensitive data such as passwords through this route.
Roots
Clients can also tell servers which directories or locations are in scope, helping servers stay within bounds.
Support Varies
Not every host implements every client feature. Servers should check declared capabilities and degrade gracefully.