client.writeFrame() is called concurrently from: - pingLoop task (subscriber keepalive) - readLoop auto-pong (server ping response) - close path (connection teardown) without serialization, concurrent writes interleave frame headers/payloads, corrupting the stream and causing GPF in memcpy → Writer.zig → writeAll. adds _write_lock: Io.Mutex to Client struct, acquired around the two writeAll calls in writeFrame(). header construction and payload masking happen outside the lock to minimize hold time. the server-side Conn already has this pattern (lock: Io.Mutex around writeFrame/writeFramed/writeAllIOVec). this brings the client to parity. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>