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Deploy docs: reused-process runtimes (Lambda) hit the single-use session manager error, but aren't covered

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Assessment

Difficulty
1/5
Estimated time
1-3 hours
Newbie friendliness
88/100
Issue type
Documentation
Clarity
Clearly specified
Activity status
Active
Tech stack
aws, python
Domain
cloud, documentation

Research direction

Start in docs/troubleshooting.md, where the existing StreamableHTTPSessionManager error cases for Mount and long-running workers are documented. Add the missing AWS Lambda reused-process case, explain that the app should be built fresh inside the handler rather than at module scope, and confirm the entry clearly connects the error to that deployment pattern.

Written by the indexing model from the issue text.

Description

Running our MCP server (Streamable HTTP) on AWS Lambda, every second invocation against a warm container failed outright, on every request, with:

RuntimeError: StreamableHTTPSessionManager .run() can only be called once per instance. Create a new instance if you need to run again.

It happens the first time any Lambda container gets reused, which is the normal, expected behavior in production. Not an edge case. Discovered it testing against real repeated traffic, not a single request. Anyone deploying a Streamable HTTP MCP server to Lambda the natural way (build the app once, at import time, like you would for any long-running server) hits this the moment a container is reused.

Root cause: the app was built once at import time, so its session manager's lifespan ran and finished during the first invocation. The second invocation reuses the same process and tries to enter that same manager's lifespan again, which it refuses.

docs/troubleshooting.md already documents this exact error for two other causes (a Mount swallowing a lifespan, several long-running workers). Neither matches a serverless/reused-process deployment, and nothing pointed us at the actual fix. Had to reproduce the crash directly to find the fix.

Fix: build the app fresh inside the handler, per invocation, instead of at module scope.

Dominant language
Python
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Merged PRs (30d)
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