fix(egress): unblock SSE/chunked streaming through mitmproxy#898
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hittyt merged 2 commits intoMay 18, 2026
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The mitmdump --set stream_large_bodies=1m option (added to prevent OOM on large downloads) buffers any response under 1 MiB before forwarding, which stalls LLM SSE / chunked streams of small chunks until the stream ends or the threshold is hit, producing perceptible stutter downstream. Introduce a bundled system addon that is always loaded by the egress launcher and forces flow.response.stream = True for responses with content-type: text/event-stream or transfer-encoding: chunked. Large- body OOM protection from stream_large_bodies stays intact for everything else. The previous example script add_header.py is renamed to system.py and repurposed as the always-on system addon (wire-transparent: no headers added or altered). User-supplied addons via OPENSANDBOX_EGRESS_MITMPROXY_SCRIPT are still loaded after the system addon and may observe or override its hooks. Refs: https://project.aone.alibaba-inc.com/v2/project/2135082/req/82131871 Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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Normalize content-type and transfer-encoding to lowercase before substring matching in the system addon, so legal mixed-case values like Transfer-Encoding: Chunked or Content-Type: Text/Event-Stream still trigger streaming instead of getting buffered by stream_large_bodies=1m. Also fix the transparent-mode doc, which referenced a non-existent OPENSANDBOX_EGRESS_MITMPROXY_SYSTEM_SCRIPT env var as a way to disable or swap the system addon. The launcher always appends the bundled system addon unconditionally; document that users override behavior via OPENSANDBOX_EGRESS_MITMPROXY_SCRIPT (loaded after the system addon). Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
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Summary
The mitmdump --set stream_large_bodies=1m option (added to prevent OOM on large downloads) buffers any response under 1 MiB before forwarding, which stalls LLM SSE / chunked streams of small chunks until the stream ends or the threshold is hit, producing perceptible stutter downstream.
Introduce a bundled system addon that is always loaded by the egress launcher and forces flow.response.stream = True for responses with content-type: text/event-stream or transfer-encoding: chunked. Large- body OOM protection from stream_large_bodies stays intact for everything else.
The previous example script add_header.py is renamed to system.py and repurposed as the always-on system addon (wire-transparent: no headers added or altered). User-supplied addons via OPENSANDBOX_EGRESS_MITMPROXY_SCRIPT are still loaded after the system addon and may observe or override its hooks.
Testing
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