fix(appkit): harden SSE response headers in setupHeaders#331
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MarioCadenas merged 3 commits intomainfrom Apr 30, 2026
Merged
fix(appkit): harden SSE response headers in setupHeaders#331MarioCadenas merged 3 commits intomainfrom
MarioCadenas merged 3 commits intomainfrom
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- Set Content-Type to text/event-stream; charset=utf-8 - Change Cache-Control to no-cache, no-transform (prevents proxies from buffering/transforming the stream) - Add X-Accel-Buffering: no to disable nginx/proxy response buffering (respected by Cloudflare, AWS ALB, GCP, and most corporate proxies) - Remove Connection: keep-alive (forbidden by HTTP/2; Node manages it) - Remove Content-Encoding: none (invalid header value; causes 502/RST in strict intermediaries) - Write a ": ok\n\n" sentinel comment immediately after flushHeaders so any buffering proxy is forced to release the first chunk to the client before the upstream generator yields its first real event. Prevents "Failed to fetch" on cold-start SQL warehouses. Signed-off-by: MarioCadenas <MarioCadenas@users.noreply.github.com>
Writing res.write() synchronously inside setupHeaders fires before the caller attaches error handlers to the response stream. A failed write can emit an async error event on the stream that is not caught by the try/catch, potentially surfacing as a connection reset. The heartbeat already keeps the stream alive during cold starts, so the sentinel is redundant and risky. Signed-off-by: MarioCadenas <MarioCadenas@users.noreply.github.com>
- Update ServingPlugin._handleStream to use the same hardened headers as SSEWriter (charset=utf-8, no-transform, X-Accel-Buffering: no, drop Connection and Content-Encoding) - Update all test assertions to reflect the new header values Signed-off-by: MarioCadenas <MarioCadenas@users.noreply.github.com>
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Nice detective work! 🚀
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What
Improves the SSE response headers set by
SSEWriter.setupHeaders()to be more correct and proxy-friendly.Why
The previous header set had two issues that could cause problems in real deployments:
Connection: keep-aliveis forbidden by the HTTP/2 spec and was being stripped or rejected by HTTP/2 intermediaries anyway. Node.js manages keep-alive internally.Content-Encoding: noneis not a valid header value and could trigger502 Bad GatewayorRST_STREAMerrors in strict proxies (nginx, AWS ALB, GCP LB, Cloudflare).There were also two missing proxy-buffering mitigations:
Cache-Control: no-transformtells proxies not to modify the response body (important for streaming).X-Accel-Buffering: nois the de-facto header for disabling response buffering in nginx and nginx-backed CDNs/proxies (Cloudflare, AWS CloudFront, GCP, and most corporate reverse proxies honour it).Changes
packages/appkit/src/stream/sse-writer.tsConnectionandContent-Encodingremoved;X-Accel-Buffering: noadded