The Tool Desk
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This guide shows the official TypeScript compatibility pattern, explains host and body-size safeguards, and gives a migration path for legacy clients. The terminology matters: “SSE” here means MCP’s older HTTP+SSE server transport, not a requirement that every modern MCP server keep a server-sent-events connection open.
What “MCP with SSE” means
The MCP TypeScript SDK v1 server guide says: “The older HTTP+SSE transport (protocol version 2024‑11‑05) is supported only for backwards compatibility.” A legacy server opens one SSE response for each client session. The client receives an endpoint event containing a URL such as /messages?sessionId=…, then posts JSON-RPC messages to that URL while reading server responses from the original stream.
Streamable HTTP is the recommended transport for new remote servers. It supports ordinary POST request/response exchanges, optional SSE for server-to-client notifications, optional JSON-only responses, and session management with resumability. Therefore, an application can still use SSE features without adopting the deprecated two-endpoint design.
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Choose the transport before writing code
| Question | Legacy HTTP+SSE | Streamable HTTP |
|---|---|---|
| When to choose it | A required client only supports the 2024-11-05 transport. | Default for a new remote server. |
| Status | Backwards-compatibility bridge; deprecated and planned for removal in v3. | Current recommended remote transport. |
| HTTP shape | Long-lived GET /sse plus POST /messages. |
POST request/response, with optional SSE notifications. |
| Session handling | Your application maps each sessionId to an SSEServerTransport. |
Built-in session-management and resumability patterns. |
| Best fit | Compatibility with an older MCP client. | New clients, simpler deployment, and a migration target. |
Package exports and migration status can change. Check the v1 server guide, the v2 legacy-client guide, and the v2 migration guide against the SDK version you install.
Prerequisites for the compatibility server
- Node.js and TypeScript suitable for the MCP SDK version you select.
- An MCP server implementation containing your tools, resources, or prompts.
- Express (or another HTTP framework) to expose the two routes.
- A client that actually requires the legacy HTTP+SSE transport.
- Explicit host/origin configuration when the process is reachable beyond localhost.
In SDK v2, import the frozen compatibility transport from @modelcontextprotocol/server-legacy/sse. The v2 server itself does not serve HTTP+SSE; this package is a temporary bridge, not a foundation for a greenfield v2 server.
Build the legacy HTTP+SSE server
1. Install dependencies
npm install @modelcontextprotocol/server @modelcontextprotocol/server-legacy express
npm install -D typescript tsx @types/expressUse package versions that are mutually compatible, following the SDK documentation. The exact export surface is version-sensitive.
2. Create the server and session map
The important design is one transport and one server per SSE session. The map lets a later POST find the correct connection.
import express from "express";
import { randomUUID } from "node:crypto";
import { Server } from "@modelcontextprotocol/server";
import { SSEServerTransport } from "@modelcontextprotocol/server-legacy/sse";
const app = express();
// The compatibility example raises this above Express's 100 KB default.
app.use(express.json({ limit: "4mb" }));
const transports = new Map<string, SSEServerTransport>();
function createServer() {
const server = new Server(
{ name: "example-sse-server", version: "1.0.0" },
{ capabilities: { tools: {} } }
);
// Register your tools, resources, and prompts here.
// Example tool registration depends on the SDK version you installed.
return server;
}
app.get("/sse", async (req, res) => {
const transport = new SSEServerTransport("/messages", res);
const sessionId = transport.sessionId;
transports.set(sessionId, transport);
res.on("close", () => {
transports.delete(sessionId);
});
const server = createServer();
await server.connect(transport);
});
app.post("/messages", async (req, res) => {
const sessionId = typeof req.query.sessionId === "string"
? req.query.sessionId
: undefined;
if (!sessionId) {
res.status(400).json({ error: "Missing sessionId" });
return;
}
const transport = transports.get(sessionId);
if (!transport) {
res.status(404).json({ error: "Unknown sessionId" });
return;
}
await transport.handlePostMessage(req, res, req.body);
});
app.listen(3000, "127.0.0.1", () => {
console.log("Legacy MCP SSE server listening on http://127.0.0.1:3000");
});SSEServerTransport('/messages', res) writes the initial SSE response. The transport emits an endpoint event naming /messages?sessionId=…. The client must preserve that session ID on every POST. When the browser or client closes the stream, remove the transport from the map so abandoned sessions do not accumulate.
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3. Register real capabilities
Replace createServer() with your application’s tool, resource, and prompt registrations. Keep registration inside the per-session server factory when each session needs independent state. If state is shared, put the data layer outside the factory and enforce your own authorization and concurrency rules.
4. Test the handshake
- Open
GET /ssewith an MCP client or an SSE-capable HTTP client. - Read the
endpointevent and extract itssessionId. - POST a valid JSON-RPC MCP message to
/messages?sessionId=…. - Read the response and notifications from the original SSE stream.
- Close the stream and confirm the server deletes the session.
Do not treat a successful TCP connection as a successful MCP handshake. The client still has to send protocol messages and negotiate capabilities.
Host validation and remote deployment
When you bind only to localhost, your framework’s defaults provide some protection against DNS-rebinding mistakes. When you bind to 0.0.0.0 or place the server behind a proxy, explicitly allow the hostnames you serve and validate the relevant Host and Origin headers. The v2 example binds beyond localhost while allowing sse.example.com; copy that approach only after replacing it with your real hostname.
app.listen(3000, "0.0.0.0", () => {
console.log("Listening for configured remote hostnames");
});Binding to all interfaces is not an allowlist. Configure the SDK/framework host checks, terminate TLS at a trusted proxy, and ensure the proxy supports long-lived streaming responses. Disable buffering for the SSE route where your proxy requires it, and set idle/read timeouts longer than the expected session lifetime.
Request-size limits, sessions, and reliability
Why the 4 MB setting appears
Express defaults to a 100 KB JSON body limit. The compatibility guidance raises it to 4 MB because the SSE transport accepts messages up to that size. Four megabytes is the documented example’s ceiling, not a universal requirement: choose a lower limit if your tools do not need large payloads, and keep every proxy limit consistent.
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Session lifecycle
- Create the transport only after accepting the SSE request.
- Store it under the exact transport-generated session ID.
- Reject missing, malformed, or unknown IDs before calling
handlePostMessage. - Delete the map entry on
res.closeand during orderly shutdown. - Set operational limits for concurrent sessions and memory.
Failure and retry behavior
A dropped SSE connection invalidates the client’s streaming path. Decide whether the client should establish a new session or whether your application can safely reconnect and restore state. Legacy HTTP+SSE does not provide the resumability model that Streamable HTTP is designed to support. Log session creation, closure, POST status, and transport errors without logging credentials or sensitive tool arguments.
Common errors and fixes
“Cannot find module @modelcontextprotocol/server-legacy/sse”
You are using a package set that does not include the frozen bridge, or an export changed. Install the documented compatibility package for your SDK release and verify the import in the v2 legacy-client guide.
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POST returns 400 Missing sessionId
The client did not use the query string from the SSE endpoint event. Use the complete URL, including ?sessionId=…, and preserve its spelling and encoding.
POST returns 404 Unknown sessionId
The stream closed, the process restarted, or the request reached a different process that has no shared session map. Keep both routes on the same instance or use a shared session architecture; reconnect after a process restart.
413 Payload Too Large
Express or a proxy rejected the body. Compare the client payload with the 4 MB transport example and align limits at every hop rather than raising them blindly.
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The client connects but receives no events
Check proxy buffering, TLS termination, idle timeouts, and whether the response is being flushed as text/event-stream. Confirm that the server connected the MCP instance to the transport after creating the SSE response.
Host or origin validation fails remotely
Add the exact public hostname to the server’s allowlist and send requests with that host. Do not solve this by allowing every hostname.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.When Streamable HTTP is the better implementation
For a new server, follow the SDK’s simpleStreamableHttp.ts starting point, remove features you do not need, and register your capabilities there. Streamable HTTP can return JSON for ordinary calls and open SSE only for notifications, avoiding a permanently open stream when one is unnecessary. It also gives you a current path for session management and resumability.
If you have existing users on HTTP+SSE, expose both transports temporarily: keep the frozen bridge isolated, document the retirement plan, and direct new clients to Streamable HTTP. Test capability negotiation and error handling on both routes. The migration guide confirms that SSEServerTransport was removed from v2’s main package and identifies the legacy copy as temporary.
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Frequently Asked Questions
Does every MCP server need an SSE connection?
No. Streamable HTTP is the recommended remote transport and can use SSE only for server-to-client notifications.
Why does the legacy design have two endpoints?
The client reads the long-lived response from /sse and posts JSON-RPC messages to the session-specific /messages URL announced by the stream.
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Only as a temporary compatibility bridge when an older client requires it. The v2 SDK removed the transport from its main package and directs new work to Streamable HTTP.
The Bottom Line
Use legacy HTTP+SSE only to support clients that require protocol version 2024-11-05. For everything else, build on Streamable HTTP, which can still provide SSE notifications without the deprecated two-route session pattern.
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