Legacy SSE Transport in MCP Causes Serverless Bills to Skyrocket
Ranorkk · reddit · 2026-08-04
A developer deploying a remote MCP server on Vercel noticed an unusually high bill. Investigation revealed that the legacy HTTP+SSE transport, kept for backward compatibility, holds long-lived connections in a Serverless environment, causing functions to consume memory while idle waiting for I/O until they are force-killed.
Key Details & Resolution:
- The Anomaly: Provisioned memory costs were 5.5x higher than active CPU costs, with almost 800 timeout errors triggered in a single day.
- Ineffective Fix: Previously lowering memory allocation reduced costs slightly but didn't fix the underlying issue of hanging connections.
- The Solution: After confirming zero real traffic used the SSE path, the developer removed the SSE branch entirely and reduced maxDuration from 300s to 60s.
Takeaway: Long-lived SSE connections are a bad match for Serverless architectures. Developers hosting remote MCP servers on platforms like Vercel or Lambda should check for legacy transports and migrate to stateless request/response models to avoid paying for idle I/O.
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