Support serializable session state for distributed deployments #2111

Summary

BaseSession holds all session state in-memory (_response_streams, _in_flight, _request_id counter, _server_capabilities). This state cannot be serialized or shared across process boundaries, making it difficult to run MCP services behind a load balancer or in horizontally-scaled deployments.

Motivation

• Horizontally-scaled MCP services need to share session state across instances
• When a response arrives at a different instance than the one that sent the request, there is no way to route it correctly without reimplementing session internals
• As the protocol transitions from stateful (current spec) to stateless-by-default (upcoming spec), servers will need to support both old and new clients simultaneously. Serializable session state would help bridge this transition period, allowing stateful sessions to be managed across a distributed backend while also serving stateless clients.

Proposed Approach

Provide a lightweight SessionState snapshot with the minimal serializable fields needed to reconstruct session context:

• session_id
• protocol_version
• next_request_id
• server_capabilities
• server_info
• initialized_sent

Expose get_session_state() and from_session_state() methods on ClientSession, allowing consumers to store and restore state using whatever backing store they choose (Redis, database, etc.).

Related

• Simplify and harden session lifecycle with an explicit state machine #1691 — Simplify and harden session lifecycle with explicit state machine
• Support multiple MCP protocol versions #1746 — Support multiple MCP protocol versions