What values can ctx.Err() return, and how should you check them?

Err() returns nil while Done() is still open. Once it is closed, Err() returns exactly one of two sentinels:

• context.Canceled: someone called a cancel function, or a parent was canceled.
• context.DeadlineExceeded: a deadline or timeout expired, here or in an ancestor.

Libraries usually wrap these (for example *url.Error from http.Client, or driver errors), so check with errors.Is, not ==. DeadlineExceeded also has a Timeout() bool method that returns true, so code that checks net.Error-style timeouts treats it as a timeout.

resp, err := client.Do(req)
switch {
case errors.Is(err, context.DeadlineExceeded):
	return fmt.Errorf("upstream too slow: %w", err) // e.g. 504
case errors.Is(err, context.Canceled):
	return err // caller went away; usually don't log as an error
case err != nil:
	return fmt.Errorf("upstream: %w", err)
}
defer resp.Body.Close()

Gotcha: tell "client disconnected" (Canceled) apart from "we were too slow" (DeadlineExceeded) in metrics and alerts. Logging every canceled request as a 500 creates noise during deploys and when mobile clients drop.