The passthrough helper copied the upstream provider's Server: header
(e.g. "cloudflare" from Anthropic) onto the FastAPI response. uvicorn
then added its own Server: header on top, producing two Server: lines
in the wire response. Strict HTTP parsers (aiohttp's, used in CI's
passthrough tests) reject this with "Duplicate 'Server' header found"
and the request fails with a 400.
Same risk for Date, Content-Length, Connection, Keep-Alive: the ASGI
server writes its own copy at serialization time. Forwarding the
upstream's value either duplicates the header or lies about the
re-serialized body length.
Drop these from the forwarded set. Application/business headers
(content-type, x-request-id, anthropic-ratelimit-*, etc.) still pass
through unchanged.
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