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The Model Context Protocol (MCP) has evolved with the introduction of the Streamable HTTP transport in its 2025-03-26 specification, replacing the previous HTTP+SSE (Server-Sent Events) transport. This new approach addresses key limitations while maintaining the advantages of the previous implementation, making it a more robust solution for AI agent infrastructure.

What is Streamable HTTP?

Streamable HTTP is a transport mechanism for MCP that enables servers to operate as independent processes handling multiple client connections through HTTP POST and GET requests. It can optionally utilize Server-Sent Events (SSE) for streaming multiple server messages, accommodating both basic MCP servers and feature-rich servers that support streaming and server-to-client notifications and requests.

Key Advantages Over Traditional SSE

1. Unified Endpoint Architecture

Unlike the previous HTTP+SSE transport which required separate endpoints, Streamable HTTP simplifies the architecture by providing a single HTTP endpoint that supports both POST and GET methods. For example, this could be a URL like https://example.com/mcp. This consolidation eliminates the complexity of managing multiple endpoints and streamlines implementation for developers.

2. Flexible Communication Patterns

Streamable HTTP offers more versatile communication patterns:

• Client to Server: Every JSON-RPC message sent from the client is a new HTTP POST request to the MCP endpoint.

• Server to Client: The server can respond with either application/json for simple responses or text/event-stream to initiate an SSE stream for more complex interactions.

• Proactive Server Communication: Clients can issue an HTTP GET to the MCP endpoint to open an SSE stream, allowing the server to communicate to the client without the client first sending data via HTTP POST.

3. Robust Session Management

The Streamable HTTP transport introduces a comprehensive session management system. Servers can assign a session ID at initialization time by including it in an Mcp-Session-Id header. This ID should be globally unique and cryptographically secure.

Clients must include this session ID in all subsequent requests, allowing servers to maintain state across multiple interactions. Servers can terminate sessions when needed, and clients can explicitly end sessions they no longer need.

4. Improved Resumability and Fault Tolerance

To support resuming broken connections and redelivering potentially lost messages, servers can attach an id field to their SSE events. If a client needs to resume after a connection break, it can issue an HTTP GET with a Last-Event-ID header to indicate the last event received.

The server can use this information to replay messages that would have been sent after the last event ID on the disconnected stream and resume from that point. This ensures greater reliability in unstable network conditions.

5. Multiple Concurrent Connections

Clients can maintain multiple SSE stream connections simultaneously. The server sends each JSON-RPC message on only one of the connected streams, avoiding redundant broadcasts across multiple channels. This enables more complex interaction patterns and better resource utilization.

Implementation in New MCP Servers

New MCP servers implementing Streamable HTTP should:

1. Provide a Unified Endpoint: Create a single HTTP endpoint that handles both POST and GET methods for all MCP communications.

2. Support Both Communication Modes: Implement handlers for both synchronous JSON responses and asynchronous SSE streams.

3. Implement Session Management: Develop a system for generating, tracking, and validating session IDs to maintain state across client interactions.

4. Enable Resumability: Assign unique IDs to SSE events and implement logic to replay messages when clients reconnect with a Last-Event-ID header.

5. Ensure Security: Validate Origin headers on all incoming connections to prevent DNS rebinding attacks, bind only to localhost when running locally, and implement proper authentication for all connections.

6. Support Backwards Compatibility: For servers that need to support older clients, continue hosting both the SSE and POST endpoints of the old transport alongside the new MCP endpoint.

Conclusion

The Streamable HTTP transport represents a significant improvement over the previous HTTP+SSE approach, offering more flexibility, better state management, improved reliability, and a simplified architecture. For AI agent infrastructure developers, this new transport mechanism provides a more robust foundation for building complex, stateful interactions between clients, servers, and language models.

By consolidating endpoints, supporting flexible communication patterns, and implementing robust session management and resumability features, Streamable HTTP addresses the key limitations of the previous transport while maintaining its advantages. New MCP servers should adopt this approach to leverage these benefits and provide a more resilient and feature-rich experience for their users.