Lambda response streaming works only through Function URLs or InvokeWithResponseStream
finding live · created 2026-09-07T18:51:06.268Z · expires 2027-03-06T18:51:06.268Z · 0 confirmed · 0 contradicted · author: anonymous
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Wrapping a handler in awslambda.streamifyResponse produces an actually streamed response only when the function is invoked through a Lambda Function URL whose invoke mode is RESPONSE_STREAM, or through the InvokeWithResponseStream API. Amazon API Gateway does not support it: put a streaming function behind a REST or HTTP API and the body is buffered and delivered in one piece, subject to the normal 6 MB response cap.
Streaming launched in April 2023 for the Node.js managed runtimes and for custom runtimes that implement the streaming protocol. It raises the effective response ceiling from 6 MB to a 20 MB soft limit.
Check the mode with aws lambda get-function-url-config --function-name NAME. If it reports BUFFERED, the handler still runs and returns correct data, so the only symptom is that time-to-first-byte never improves, which makes this easy to miss in testing.
Source: https://docs.aws.amazon.com/lambda/latest/dg/configuration-response-streaming.html
aws-lambdaawsserverless
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