If your MCP integration still uses the old HTTP+SSE transport, it's on borrowed time. SSE was deprecated back in the 2025-03-26 spec when Streamable HTTP replaced it, and through 2026 the vendors are actually pulling the plug: Keboola removed it on 1 April 2026, and Atlassian's Rovo MCP server cuts HTTP+SSE on 30 June 2026. Miss the date and the connection simply stops working. Here's how to get ahead of it.
Why SSE is going away
The two-way Server-Sent Events transport looked clever but aged badly. It held one TCP connection open per client, which is expensive and fragile: a network blip drops the stream and you get the classic failures — No connection established for request ID, SSE error: Non-200 status code, sessions that never recover. Worse, two identical SSE servers behind a load balancer produced split-brain state, because tool state stored on server A was invisible to server B. Streamable HTTP fixes this by collapsing the old two-endpoint design into a single MCP endpoint that handles both POST and GET, with no persistent socket to babysit — and no long-lived backchannel to authenticate and monitor.
What "migrate" means on the server
The mechanical change is smaller than it sounds. Swap SSEServerTransport for StreamableHTTPServerTransport. Move from two endpoints (one for the SSE stream, one for messages) to a single MCP endpoint. Add proper POST and GET handlers, return the correct Accept and Content-Type behaviour, and validate Origin headers to block cross-site abuse. If you're on a current SDK, much of this is a transport-class substitution plus a route change rather than a rewrite. Pair it with the 2026 stateless migration and you land on infrastructure that scales on plain HTTP.
What it means if you're just a user
You may not run the server at all — you might just have a connector configured in Claude or Cursor that points at a vendor's SSE URL. In that case the fix is to update the connector to the vendor's new Streamable HTTP endpoint before their deadline. Atlassian, for instance, published a new Rovo MCP URL; the old SSE one stops responding on 30 June. Check each remote connector you've added and confirm it's on the HTTP endpoint, not a /sse path. If a previously-working connector suddenly throws connection-closed errors, a silent SSE shutdown is the first thing to suspect — see MCP server connection closed error.
Don't confuse this with stdio
Local servers launched by your client over stdio are unaffected — this deprecation is purely about the remote HTTP transport. If your mcpServers config spawns a command with npx or uvx, nothing here applies. The deadline only bites for remote, URL-based connectors: when in doubt, check whether the entry has a url (remote, possibly affected) or a command (local stdio, fine). See MCP config file location.
Going further
This is one half of the 2026 transport story; the other is the stateless server migration. For how remote servers are converging on hosted OAuth endpoints, see enterprise MCP gateway deployment and private MCP server hosting. Browse the developer-tools category or grab a ready loadout.