per-message dictionary-compressed binary frames matching upstream's encoder: the v1 dictionary (dict id 1612007021, vendored at src/assets/zstd_dictionary, sha-verified against upstream's copy), 128KiB window, content checksum, level 3, one standalone frame per event via session-reset on a per-subscriber compression context. maxMessageSizeBytes still compares the UNCOMPRESSED length (upstream's documented deliberate divergence from original v1). cold replay compresses too. e2e: python zstandard + the dictionary decodes our frames and parses the events — real v1 client compatibility, not just self-consistency. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>