Excel.CustomFunctionProxy: 3 API changes in Office.js (undocumented API)
UndocumentedNot in Microsoft's documentation
Excel.CustomFunctionProxy, not in Microsoft's documentation, implementation changed on the Office.js production CDN in 16.0.20626.15200, first seen 2 Nov 2026, 22:50 UTC. Affects Excel on Mac, web and Windows.
- Production
- implementation changed in 16.0.20626.15200, first seen 2 Nov 2026, 22:50 UTC; rolling out, on 2% of sampled edges.
- Beta
- implementation changed in 16.0.20626.15200, first seen 2 Nov 2026, 22:20 UTC; rolling out, on 1% of sampled edges.
What this means for your add-in
Same name and signature, different implementation: behaviour can differ even though the API surface did not change.
It is not in Microsoft's documentation (neither @types/office-js nor @types/office-js-preview): an internal API that can change in any build. Do not depend on it; if you must, check that it exists before use — if (Excel.CustomFunctionProxy) { … }.
Change history
Changed in the same rollouts
- OfficeCore.RibbonButton implementation
Source
silentship samples every Office.js file on the CDN every 10 minutes and compares the public API surface of each new build with the previous one by reflection. This page lists what changed for Excel.CustomFunctionProxy; it does not include Microsoft's code. The API status comes from Microsoft's published type definitions (@types/office-js and @types/office-js-preview). Excel.CustomFunctionProxy is not in Microsoft's documentation.