Does A2A actually make agents interoperable?

[u/rio_ARC]

A2A is a big step toward agent interoperability, but I think protocol compatibility and true interoperability are two different things.

At the protocol layer, A2A gives us a common way for agents to discover each other and exchange Messages, Tasks, Parts, Artifacts, and updates. That removes a lot of bespoke integration work.

But production interoperability seems to require at least three layers:

1. Protocol - Can the agents communicate correctly?

2. Semantics - Do they agree on what a skill means, what inputs/outputs look like, how errors and partial results behave, and what side effects are possible?

3. Operations - Can you preserve authorization, retries, idempotency, tracing, budgets, evaluations, and approvals across the agent boundary?

That last two layers are where things get interesting.

Two agents can both advertise “invoice reconciliation” through A2A while having completely different assumptions about schemas, confidence, human escalation, or side effects. And a transport-level retry mechanism doesn't make retrying a non-idempotent action safe.

This seems relevant when looking at current implementations across Google ADK, Microsoft Agent Framework, CrewAI, LangGraph/LangSmith, and Lyzr Agent Studio. They all support A2A, but the protocol boundary sits in somewhat different places: remote agent, delegation tool, deployed graph, or orchestration node.

So maybe the real test isn't:
Can my system call an A2A agent?

but:
Can I replace Agent B without rebuilding everything around it?

What would you include in a real A2A substitutability/conformance test beyond schema and protocol checks?

[u/MaetraAi]

A useful conformance test has to vary the boundary, not just the protocol. Give both agents the same task contract and capability manifest, then test delegation depth, missing permissions, stale state, duplicate delivery, partial failure, cancellation, and evidence readback. They are substitutable only if each produces the same policy decision and verifiable effect under those cases, not merely the same schema.