Why it matters

Migration off SSE is a real operational task for MCP deployments. Understanding both transports helps plan.

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The architecture

Client opens SSE connection to /sse endpoint. Server assigns session ID. Server streams events on this channel.

Client POSTs to /messages?session_id=X for its messages. Server processes and may respond via SSE.

SSE transportClient → SSEget server eventsClient → POSTsend messagesDual channelsession-scopedComplexity of dual channel is why Streamable HTTP replaces it
SSE dual-channel design.
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How it works end to end

Auth: OAuth or API keys via HTTP headers on both channels.

Session state: server keeps state per session ID.

Firewall-friendly: standard HTTP + SSE, works through most proxies.

Migration: Streamable HTTP simpler — single POST endpoint with SSE-style streaming response.