why does runId exist in the events, but in none of the messages?

[sccolbert]

RunAgentInput allows a runId to be specified, which is echo'd in RunStartedEvent and RunFinishedEvent, but it is not present in any other event or message. What's the purpose/intent of this id? If it were to be generally useful to associate messages/events with a particular run, it would need to be echo'd in all events/messages contained within a run.

[pmeier]

not present in any other event

It doesn't have to be. As all events that you get back in one response will belong to the same run for which the client has generated the run ID. As the events are just temporary, it would be a waste of bandwidth to transmit the run ID, thread ID, and parent run ID over and over again.

not present in any other [...] message

The run ID is present on the run itself rather than on the messages. Since runs and messages roughly have a 1:n connection that means we only need to store / transmit the run ID once rather than n times.

[sccolbert]

My point is that if the runId only corresponds to a single call to runAgent and it's not present anywhere else, then it's not needed at all, since it's isolated to the single request. If it's not meant for a broader use case, it doesn't need to be there. Otherwise, it needs to be everywhere.

At some point, I can imagine wanting to kick off multiple runs simultaneously from one or more agents in a single thread, in which case the runId will become important for tracing messages back to their run origin.

[fjgbue]

I think there are two different identities here that are easy to conflate:

• threadId is the durable conversation identity.
• runId is the execution identity for one invocation of the agent inside that thread.

That makes runId useful on RUN_STARTED and RUN_FINISHED because those events define the run envelope. Messages are more durable artifacts of the thread. A message can be restored from history, included in a MESSAGES_SNAPSHOT, compacted, or replayed later without necessarily caring which execution originally produced it.

So for the current model, I would not treat message objects as having a required foreign key back to a run. I would store the event stream append-only if I need provenance: RUN_STARTED(runId) followed by the streamed message/tool/state events, then RUN_FINISHED(runId). That preserves the relationship without repeating runId on every event or message.

Where I agree with your concern is concurrent or nested runs. Once multiple runs can interleave in one stream, consumers need a reliable correlation boundary. The newer parentRunId / lineage idea helps with branching, but interleaved concurrent streams would likely need either non-interleaving guarantees or event-level correlation. Until then, I would assume a single event stream response belongs to one run, and use the stored event envelope for traceability.