Office-JS: Excel Host API no longer available unde

I'm currently running Windows 10 with Microsoft Excel 2016 MSO (16.04266.1001) 64-bit. I'm under the impression that that is the version installed via MSI.

Over the course of the last few months I've implemented a React-based Excel add-in using the Excel Host APIs. Per the Excel JavaScript API requirement sets document available at the Office Dev Center, that build appears to contain the "ExcelApi 1.1, WordApi 1.1, and common API" requirement sets.

As of yesterday, I am no longer able to access the Excel host APIs through JavaScript or through the F12 developer tools (window.Excel === undefined). It appears that all the functionality under that had previously existed under that namespace has become unavailable.

It appears that (among other scripts), the office.js script loaded from the CDN now requests and embeds Excel-15.02.js for the add-in.

I do not know the version of Excel-*.js that was being embedded before yesterday, but after perusing the contents of both Excel-15.02.js, Excel-winrt-16.00.js and other versions available on the CDN, I suspect that it was a later version than 15.02.

For the record, both of the following runtime checks return false - I say both, because "API" is capitalized inconsistently in the documentation available at dev.office.com.

Office.context.requirements.isSetSupported("ExcelApi", 1.1); // false
Office.context.requirements.isSetSupported("ExcelAPI", 1.1); // false

Please advise; best regards.

I am investigating the issue. A few questions:

You mention that the files being loaded include Excel-15*.js files. How are you seeing this -- is it through Fiddler, or through a debugger? If you can send a Fiddler trace, that would be really useful (a Fiddler trace may contain some sensitive info, so you may want to scrub it -- and feel free to send it to me directly rather than posting it here online)
If you browse to https://appsforoffice.microsoft.com/lib/1/hosted/office.js in a browser, what is the version number that you see at the top of the file?
If you are comfortable deleting your IE cache: could you see if the issue continues to repro even after you've cleared the cache?

Feel free to reply here via comments, or as updates to your questions, or to email me directly. My email is my full name (separated by a dot between first and last) at microsoft.com.

I will be updating this answer as I have more info.