Mechanism
The vendor's underlying model changes behaviour between versions without proportionate notice. A tool that produced one set of outputs in January produces materially different outputs in March on the same inputs. Variants: vendor-deliberate upgrade, vendor-undeclared training refresh, downstream third-party model change.
Evidence (what the Evidence Register holds)
Vendor change-log subscription; per-upgrade impact assessment record; methodology versioned against the version of the model in use; gold-set re-run record on each model upgrade.
Mitigation
Vendor change-log SLAs in contracts; model upgrade notices with customer-impact assessment; gold-set re-runs on every model upgrade; methodology version pinning to model version.
Editorial Framing
Model drift is the silent class. Functions discover it when outputs that worked in pilot stop working in production, or when a tool the team has trusted for nine months starts producing materially different results. The contractual mitigation (change-log SLAs + impact-assessment requirements) is what most vendor contracts in 2024–2025 missed.
Indicative Examples
- Vendor-deliberate upgrade with material behaviour change
- Vendor-undeclared training refresh
- Downstream third-party model change (the vendor uses a third-party model that changes)