Change on the Agent Surface

About this paper

Semantic versioning, the changelog, the roadmap, the Sunset header, the twelve-month runway — every part of the apparatus for managing API change was built to communicate with a person who reads. When the consumer is an agent, most of it keeps working and one part quietly stops, and that part is the one almost nobody is watching.

This paper maps the three layers that govern change in the Model Context Protocol and shows how the discipline degrades in exact proportion to how close you get to the thing the consumer depends on. The specification governs itself well, with a twelve-month deprecation window and a mandatory migration path. The registry governs identity. The tool surface an agent actually binds to — which tools exist, what they are named, what their descriptions say — is governed by nobody. Grounded in three independent measurements, including a full walk of the public MCP registry covering 66,954 publication records across 20,359 servers, where 96.9% of version strings parse as valid semantic versions: this is not a population that ignores versioning guidance, it is one that has only been given guidance about identity. You get the mechanism behind the class of change no existing tool can detect, a table of which changes actually break an agent and which they shrug off, eight rules you can apply without waiting for a new specification, and a self-assessment you can run this afternoon.

What's inside

1. The three layers, and what governs each
2. What the evidence shows
3. The description is not documentation, it is input
4. Which changes actually break an agent
5. The practice — eight rules for the agent surface
6. Anti-patterns worth watching for
7. Provider self-assessment
8. Where this is going
9. Appendix A — Evidence and method
10. Appendix B — Sources and further reading

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