When running traffic through mitmproxy, you may encounter 502 Bad Gateway errors that mention HTTP/2 protocol error: *

*Received header value surrounded by whitespace**.

This happens because some upstream servers (including dev.coder.com) send back headers such as Content-Security-Policy

with leading or trailing spaces.

While browsers and many HTTP clients accept these headers, mitmproxy enforces the stricter HTTP/2 and HTTP/1.1 RFCs,

which forbid whitespace around header values.

As a result, mitmproxy rejects the response and surfaces a 502 to the client.

The workaround is to disable HTTP/2 in mitmproxy and force HTTP/1.1 on both the client and upstream sides. This avoids

the strict header validation path and allows

mitmproxy to pass responses through unchanged. You can do this by starting mitmproxy with:

mitmproxy --set http2=false --set upstream_http_version=HTTP/1.1

This ensures coder toolbox http client ↔ mitmproxy ↔ server connections all run over HTTP/1.1, preventing the whitespace

error.