A log is a timestamped record of what happened inside a system: a job started, a file changed, a request failed, a hook blocked an email send. It captures raw evidence instead of a polished summary, and it stays boring until something breaks.
The sign-in book at the front desk plays the same role. It doesn’t tell the full story of every meeting. It records who arrived, when, and sometimes why, so if something’s unclear later, you have a trail to follow.
Why you care
For client work, “Claude did something weird” isn’t enough to debug. What did it run? What did it edit? What error came back? Without a log, everyone is guessing. Logs also make agent work durable: a session log captures the decisions, errors, and corrections you worked through so a future person or agent can pick up the thread. A log records events, while a trace follows one request through several steps, like following one visitor through the building instead of reading the sign-in book. A good log doubles as a source that ties an output back to what happened.