The game fetches the artwork URL with HTTP/2 cleartext (h2c, prior knowledge), and only from 127.0.0.1. The loopback server spoke HTTP/1.1, so every cover request was answered with an HTTP/1.1 404 and mod playlist/song covers never appeared.
Speak h2c: complete the SETTINGS handshake, read the request header block (HEADERS and CONTINUATION frames), decode :path with a minimal HPACK decoder (static table and Huffman code from RFC 7541 in hpack_tables.h), and serve the matching image as HEADERS plus DATA frames with flow control. Keep the HTTP/1.1 path for other clients. Serve from one stable origin (http://127.0.0.1:47823/music-art/<mod>/<relative>) so the game's HTTP cache is reused, send Cache-Control and an ETag so it stops re-fetching, close gracefully (closing with unread frames queued makes Windows send RST and the client drops the tail), and use a non-blocking listener since several pollers share it.
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
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