# Substrate Debt

Substrate debt is the accumulated cost of carrying interface assumptions from one kind of user into systems used by another.

UX paid down the constraints of the human substrate: bounded working memory, visual scan limits, attention drift, motor friction, ambiguity tolerance, and social inference. Those choices were correct for humans. When agents inherit them as their only interface, they become debt.

For a human, a form is humane. For an agent, it is serialized schema.

For a human, a button is affordance. For an agent, it is a gesture wrapper around an operation.

For a human, a dashboard is overview. For an agent, it is state converted into pixels and then parsed back into state.

Substrate debt is not a criticism of human-centered design. It is what happens when successful accommodations for one substrate become inherited constraints for another.

## Why It Matters

Agents have different bottlenecks:

- They need explicit state, not visual hints.
- They need typed operations, not UI gestures.
- They need bounded context, not decorative explanation.
- They need recoverable errors, not silent failure.
- They need side-effect clarity before acting.

Paying down substrate debt means designing protocol, schema, memory, error, permission, and handoff surfaces as the experience itself.