AWS Updates Model Context Protocol (MCP) to Remove Session Affinity Requirements

Stateless MCP Specification

AWS announced updates to the Model Context Protocol (MCP) specification, which removes protocol-level sessions. This change allows requests to be routed to any available server instance, eliminating the protocol requirement for sticky sessions and shared session stores. The updated specification removes the initialize and initialized handshake and the Mcp-Session-Id header, enabling independent request routing behind conventional load balancers.

Simplified AWS Deployments

For AWS deployments, the stateless MCP specification removes the need for infrastructure specifically used to maintain MCP protocol sessions. This allows for replacing session-affine routing with conventional request routing and removing session storage dedicated solely to MCP protocol state. AWS Lambda is now a suitable deployment option due to the protocol's new request-response model, which no longer requires persistent session connections.

New Features and Considerations

The updated MCP specification introduces an optional server/discover operation for clients that need to determine server capabilities before making tool calls. It also includes new Mcp-Method and Mcp-Name headers for gateway routing and throttling, and W3C Trace Context support for distributed tracing. Caching controls are provided via ttlMs and cacheScope. Stream resumability has been removed, requiring clients to implement retry logic for interrupted operations and emphasizing idempotency for tool calls with side effects.

Impact on Agentic AI and Infrastructure

AWS maps these MCP changes to its Well-Architected guidance for agentic AI, covering areas such as monitoring, tracing, security, and tool integration. The shift to a stateless protocol simplifies horizontal scaling and infrastructure management. Community discussions highlight that while the protocol is stateless, applications built on it do not necessarily have to be. Early implementation efforts, such as Apify's MCP server project, are working to integrate stateless support alongside existing sessionful systems.