Using a comma before with "with"

I am trying to document some code and this sentence sounds confusing.

Replaces the existing item associated with the provided key with newItem.

I am tempted to put a comma here but I am not sure if it would be correct.

Replaces the existing item associated with the provided key, with newItem.

If it's incorrect, how can I state my intention clearly?

I would write:

Replaces the existing item, which is associated with the provided key, with newItem

Replaces the existing item, associated with the provided key, with newItem.

And if the key is something that's common between the two items, then you could say something like this:

[With the provided key it] replaces the existing item with newItem.

Or

Replaces the existing item with newItem based on the provided key.

I think you may have been looking at it too long. I see what you mean, but a comma doesn't add anything. My suggestion is to either simply italicize newItem as in:

Replaces the existing item associated with the provided key with newItem.

or change it to something like:

Replaces the provided key's currently associated item with newItem.