Discord error 4009: Session timed out

Discord stopped hearing from your client for too long and closed the session. The gateway expects a Heartbeat (op 1) at the interval sent in Hello (heartbeat_interval, typically around 41.25 seconds); it acknowledges each one with Heartbeat ACK (op 11). If heartbeats stop arriving, or the connection goes idle in a way that makes the session stale, Discord closes it with 4009. The session cannot be resumed; you need a new Identify.

On the client side, the most common cause is a blocked event loop. Node.js, Python asyncio and similar runtimes run the heartbeat timer on the same loop as your handlers; a synchronous database call, a CPU-heavy computation, a giant JSON parse or a blocking HTTP request delays the heartbeat past the deadline. Many libraries also detect the missing ACK and proactively close with their own code; either way, the root cause is the same. The second cause is the host sleeping (laptop lid, container pause) or losing network for a long period.

// WebSocket close frame
code: 4009
reason: "Session timed out."

Keep the bot process dedicated to event handling and push heavy work elsewhere. Alert on heartbeat latency spikes, not just on disconnects, so you catch the stall before Discord does.

Session timeouts are usually your process's health telling on itself: late heartbeats today are dropped events tomorrow. Watch heartbeat latency as a first-class metric. The heartbeat interval contract is on the Discord limits page, and a bot that times out repeatedly on a healthy host should be profiled per the checklist in Discord bot not responding.