The RPC-to-REST parallel is sharp. What made REST stick was not that it was more powerful—it was that the constraints forced teams to think about contracts, versioning, and failure modes up front.

The same pattern is playing out in agent systems right now. Teams that treat tool schemas as "just another prompt" discover six months later that:

1. A schema change in one place breaks three downstream agents nobody remembered existed
2. Debugging a failure requires reconstructing what the agent thought it saw vs. what it actually got
3. Every new agent becomes a custom integration because "handoffs" were never formalized

The practical signal I have seen: teams that version prompts separately from code (and treat them as content) iterate faster than teams that embed prompts in application logic. It is not about governance theater—it is about being able to answer "what changed?" without spelunking through git diffs.

The checklist framing is useful. One addition: Can you replay a failure? If you cannot reconstruct the exact state that led to a bad decision, you are operating blind. That is where durable state and explicit contracts combine—contracts give you the vocabulary to describe what happened, durable state gives you the evidence.