A lot of MCP content describes servers as stdio-or-SSE. That aged: the 2025-03-26 protocol revision deprecated the HTTP+SSE transport in favour of streamable HTTP — one endpoint, optional streaming on the response, instead of the separate SSE channel.
Why it matters practically: SSE-era servers hold a long-lived connection per client, which is exactly the thing serverless platforms and corporate proxies punish. Streamable HTTP behaves like a normal HTTP API that can stream when it needs to, so MCP servers can be deployed like ordinary web services — including on free tiers that kill idle connections.
If you're writing a server today: stdio for local tools, streamable HTTP for anything remote. If a tutorial shows an /sse endpoint, it predates the revision — the concepts transfer, the transport code doesn't.
What MCP actually standardises and how the pieces fit: https://toolfreebie.com/mcp-protocol-ai-agents/
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