What is the correct form to express the agnostic quality of something. The usual suffix -ness seems incorrect to me but I could not find a trustworthy reference to either confirm or deny this.

The only derived form I have encountered is the noun agnosticism

To describe my desired use case in an example:

This system comprises a portal and a service
The portal needs to be fully agnostic of the service
The agnosticness/agnosticity of the portal is crucial

Edit: as @sojourner correctly deduced, I should have stated this more clearly, my example is indeed meant to be used in the field of computer science.