Function forward_mutated_frames 

async fn forward_mutated_frames(
    parser: &mut ControlStreamParser,
    data: &[u8],
    refused_seen: &mut u64,
    send: &mut SendStream,
    stream_id: u64,
    key: StreamKey,
    pending: &mut PendingQueue,
    deferred: &mut DeferredEffects,
    side: ProxySide,
    ctx: &ForwardCtx,
    report: &Reporter<'_>,
) -> Result<Flow, ProxyError>

Feed bytes into the capturing control parser, then execute the hook’s decision on each completed frame.

This pipe owns the forwarding path: nothing reaches the far side except what this function writes. A frame the decoder refuses is therefore written verbatim rather than skipped — no hook can be consulted about a message that did not decode, but dropping it would remove a control message from a session neither peer knows is missing one.