Qualified electric parts suppliers have the toroids you seek.

Don't forget, "ultra precise" requires traceable calibration!

if you know they exist then why not post a link?
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Simple search of ferrite toroids will answer your question

what makes you think that the ferrite is free from hysteresis?

It all depends on the current level you intend to measure, and if AC or DC, and if AC what frequency.

All traditional magnetic materials have some level nonlinear behavior. It all depends on the intricate details of your measurement and your lab setup.

if I asked for a material with magnetic permeability it means that it is AC. it doesn't work in DC.
what I wrote in the first message is clear, "without hysteresis".
I didn't write: with little hysteresis.

you have answered your own question, such materials do not exist.

but what you're saying ?

the answer to your original post: "such materials do not exist."

It's asking for a material to both be influenced by magnetism and not be altered in the slightest by magnetism.

As far as I know liquid oxygen meets that requirement.

Even O2 Analyzers that relied on its atomic magnetic moments.

Arivel: You are looking for a superparamagnetic material. As far as I am aware, the only thing commercially available material are nanoparticles used for magnetic separation. I'm not aware of anyone selling them in bulk form suitable for a current sensor.
Here is a link to one such provider: https://www.sigmaaldrich.com/US/en/search/superpar...

Unfortunately, temperature dependencies, and slight hysterisis may be a problem for the OP.

OP: you like raise tough questions, do not you?

so basically you are looking for perm <10, no hysteresis, under AC? I am not aware of such exact material, but composite ferrite might be close to what you need.
Permeability is a function of B. under AC, it is a function of frequency, and it also consists of real and imaginary parts. hysteresis is a function of frequency too. Normally, the higher frequency, he bigger the hysteresis. lots of factors to take into consideration.

superparamagnetic is nothing more than the competition between thermal energy (kT) and anisotropic energy (which is proportional to volume, KuV). when it goes to nano-scale, decreased volume cannot overcome thermal activity, losing its ferromagnetic. So if the material is bulk with large volume, there does not exist superparamagnetic

This might be a property of a Bose-Einstein condensate. Contact your local university physics department and ask for a couple of kilograms to experiment with. Then you just need to scale up to your volume of production. Easy

For real world ferrites, the OP might consult Lax and Button dealing with ferrimagnetics.