Why MCP Deprecated SSE and Went with Streamable HTTP

The Model Context Protocol (MCP) has been evolving rapidly since its introduction, with each iteration improving how AI agents communicate with external services. One of the most significant recent changes is the transition from Server-Sent Events (SSE) to Streamable HTTP as the preferred transport mechanism for remote MCP servers. This architectural shift, introduced in the MCP specification update on March 26, 2025 (version 2025-03-26), represents a fundamental improvement in how AI agents interact with services.

Version Note: The TypeScript SDK version 1.10.0, released on April 17, 2025, was the first to support the new Streamable HTTP transport, which supersedes the SSE transport from protocol version 2024-11-05.

The Original Approach: Server-Sent Events (SSE)

When remote MCP was first introduced, it relied on Server-Sent Events (SSE) as its transport layer. This approach required two separate endpoints:

1. An SSE endpoint (/sse) that established a persistent connection for the client to receive responses
2. A separate messages endpoint (/sse/messages) where clients would send requests

This dual-endpoint architecture worked, but it was like trying to have a conversation using two phones—one for speaking and one for listening. The design introduced several significant challenges:

• Connection management complexity: Maintaining two separate connections increased implementation complexity
• Scalability limitations: Long-lived SSE connections are resource-intensive and challenging to scale
• Connection reliability: If the SSE connection dropped during a long-running operation, responses would be lost
• Implementation overhead: Servers and clients needed logic to coordinate between these separate connections

For developers implementing MCP servers, this meant more complex code, additional error handling, and ensuring proper correlation between requests and responses across different endpoints.

Enter Streamable HTTP: A Simpler, More Powerful Approach

Streamable HTTP offers a more elegant solution by enabling bidirectional communication through a single endpoint. This new transport mechanism was designed to address the limitations of the SSE approach while maintaining backward compatibility.

Here’s how Streamable HTTP transforms the MCP communication model:

1. Single Endpoint Communication

Rather than managing two separate connections, clients now interact with MCP servers through a single endpoint (typically /mcp). This simplifies both implementation and connection management, reducing complexity for both server and client developers.

// Instead of two endpoints:
// - /sse (for receiving)
// - /sse/messages (for sending)


// We now use a single endpoint:
// - /mcp (for both sending and receiving)

2. Dynamic Connection Upgrades

What makes Streamable HTTP particularly powerful is its ability to dynamically adapt based on the interaction needs:

• For simple, quick operations, it behaves like a standard HTTP request/response
• For long-running operations or streaming responses, it can automatically upgrade to an SSE-like streaming connection

This dynamic behavior means that the connection model adapts to the task at hand, optimizing for both simple queries and complex operations.

3. Bidirectional Communication

Unlike traditional HTTP or SSE, Streamable HTTP enables true bidirectional communication. Servers can send notifications or request additional information from clients on the same connection, enabling more sophisticated interaction patterns.

4. Improved Error Handling

The unification of the request and response channels makes error handling more straightforward. All errors, whether from the initial request or during processing, flow through the same channel, simplifying debugging and monitoring.

Backward Compatibility

The MCP TypeScript SDK provides excellent backward compatibility between Streamable HTTP and the older SSE transport. Here’s how you can implement it in your applications:

Client-Side Compatibility

For clients that need to work with both Streamable HTTP and older SSE servers:

import { Client } from "@modelcontextprotocol/sdk/client/index.js";
import { StreamableHTTPClientTransport } from "@modelcontextprotocol/sdk/client/streamableHttp.js";
import { SSEClientTransport } from "@modelcontextprotocol/sdk/client/sse.js";

let client: Client|undefined = undefined;
const baseUrl = new URL(url);

try {
  // First try Streamable HTTP
  client = new Client({
    name: 'streamable-http-client',
    version: '1.0.0'
  });
  const transport = new StreamableHTTPClientTransport(new URL(baseUrl));
  await client.connect(transport);
  console.log("Connected using Streamable HTTP transport");
} catch (error) {
  // If that fails, fall back to SSE
  console.log("Streamable HTTP connection failed, falling back to SSE transport");
  client = new Client({
    name: 'sse-client',
    version: '1.0.0'
  });
  const sseTransport = new SSEClientTransport(baseUrl);
  await client.connect(sseTransport);
  console.log("Connected using SSE transport");
}

Server-Side Compatibility

For servers that need to support both Streamable HTTP and older clients:

import express from "express";
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { StreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/streamableHttp.js";
import { SSEServerTransport } from "@modelcontextprotocol/sdk/server/sse.js";

const server = new McpServer({
  name: "backwards-compatible-server",
  version: "1.0.0"
});

// ... set up server resources, tools, and prompts ...

const app = express();
app.use(express.json());

// Store transports for each session type
const transports = {
  streamable: {},
  sse: {}
};

// Modern Streamable HTTP endpoint
app.all('/mcp', async (req, res) => {
  // Implementation for Streamable HTTP
  const transport = new StreamableHTTPServerTransport(req, res);
  transports.streamable[transport.sessionId] = transport;
  
  res.on("close", () => {
    delete transports.streamable[transport.sessionId];
  });
  
  await server.connect(transport);
});

// Legacy SSE endpoint for older clients
app.get('/sse', async (req, res) => {
  const transport = new SSEServerTransport('/messages', res);
  transports.sse[transport.sessionId] = transport;
  
  res.on("close", () => {
    delete transports.sse[transport.sessionId];
  });
  
  await server.connect(transport);
});

// Legacy message endpoint for older clients
app.post('/messages', async (req, res) => {
  const sessionId = req.query.sessionId as string;
  const transport = transports.sse[sessionId];
  if (transport) {
    await transport.handlePostMessage(req, res, req.body);
  } else {
    res.status(400).send('No transport found for sessionId');
  }
});

app.listen(3000);

Why is SSE Being Deprecated?

While Server-Sent Events (SSE) served the MCP ecosystem well initially, several architectural limitations make it less suitable for complex AI agent interactions:

1. Connection Management Overhead

The two-endpoint model creates unnecessary complexity. Managing connections across different endpoints leads to more code, more potential failure points, and more difficult debugging.

2. Scaling Challenges

SSE connections are persistent and long-lived, which can strain infrastructure, especially as usage scales. Each connection consumes resources for its entire lifespan, even during idle periods.

3. Limited Recovery Options

If an SSE connection drops during a long-running operation, there’s typically no built-in way to resume where it left off. This forces developers to implement custom recovery logic or risk data loss.

4. HTTP/2 and HTTP/3 Compatibility Issues

Some SSE implementations have compatibility challenges with newer HTTP protocols, limiting the ability to leverage performance improvements in modern web infrastructure.

5. Bi-directional Communication Limitations

The one-way nature of SSE means that separate channels are needed for client-to-server communication, creating artificial separation between related operations.

The Road Ahead: Future Improvements

The current implementation of Streamable HTTP already provides feature parity with the SSE transport, but the MCP specification outlines several additional capabilities that will be implemented soon:

1. Resumability

Future versions will enable clients to resume operations exactly where they left off if a connection drops during processing. This eliminates the need to keep connections continuously open and makes MCP more resilient for long-running AI agents.

2. Cancellability

Explicit cancellation mechanisms are being implemented to allow clients to cleanly terminate operations, ensuring better resource management and user experience.

3. Session Management

Secure session handling with unique session IDs will maintain state across multiple connections, enabling more sophisticated agent-to-agent communication patterns that persist beyond individual requests.

Which Will Become the Standard?

Based on the architectural advantages and the direction of the MCP specification, Streamable HTTP is positioned to become the standard transport for remote MCP servers. The simplicity, flexibility, and improved capabilities make it the logical choice for future development.

However, SSE won’t disappear overnight. The MCP ecosystem is committed to backward compatibility, meaning SSE transport will continue to be supported for existing implementations. This dual support ensures a smooth transition period as developers migrate to the new transport mechanism.

For new MCP server implementations, I recommend focusing on Streamable HTTP from the start. For existing servers, adding Streamable HTTP support alongside your current SSE implementation gives you the best of both worlds: backward compatibility and future-readiness.

Recommendations for Developers

As the MCP ecosystem evolves, here are my recommendations for developers:

For New MCP Servers

• Start with Streamable HTTP: Build your implementation using the new transport from the beginning
• Follow the specification closely: The MCP specification is evolving rapidly, so stay up to date with changes
• Implement proper error handling: Design your server to gracefully handle connection issues and client errors

For Existing MCP Servers

• Add dual support: Implement both SSE and Streamable HTTP transport options
• Plan for migration: Consider how you’ll eventually transition to Streamable HTTP only
• Update client libraries: Ensure your client libraries support both transport methods

For MCP Client Developers

• Add Streamable HTTP support: Update your clients to use the new transport when available
• Maintain SSE fallback: Keep SSE support for backward compatibility with existing services
• Leverage new capabilities: Design your client to take advantage of the new features as they become available

Conclusion

The transition from SSE to Streamable HTTP represents a significant architectural improvement for the MCP ecosystem. By simplifying the connection model, enabling bidirectional communication, and laying the groundwork for advanced features like resumability and cancellability, Streamable HTTP addresses many of the limitations of the original SSE implementation.

While both transport mechanisms will coexist in the near term for backward compatibility, Streamable HTTP is clearly the future direction for MCP. Its single-endpoint model aligns better with modern web architecture patterns and provides a more robust foundation for the complex interactions that AI agents require.

For MCP developers, now is the time to embrace this transition and ensure your implementations support both transport mechanisms, with a clear path toward Streamable HTTP as the primary method moving forward.

_{This article was proofread and edited with AI assistance.}