Agent tool schema drift can fail silently when registrations lag behind code
hannune · reddit · 2026-07-22
A Reddit post describes a common agent failure mode: the underlying function changes, but the tool registration does not.
If a tool like searchentities gains a new required parameter while its schema/description stays stale, the model will keep calling the old interface. Depending on dispatch behavior, that causes either a hard error or a silent drift in results. The author argues that output-quality evals often miss this, and recommends response-side validation, versioning tool names when interfaces break, and canary evals that exercise the full call-response cycle. The broader point is that tool descriptions are part of the contract and need change control, not just code review.
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