The durable record of what happened: who asked what, what the system did, what it returned, and which versions were involved. You need it before you need it, because the moment you need it is after something has gone wrong.
Technical view
Should capture request and response payloads, identity, timestamps, version identifiers, tool invocations and their results, cost, and any guardrail or moderation decisions, in tamper-evident append-only storage with defined retention. Logs frequently contain personal data and sometimes secrets, so redaction, access control and retention limits are required. For agentic systems, the full trajectory rather than the final output is the meaningful record.
Related
Part of AI Terminology Index, section: Running It in Production.