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The rule is to add the prefix to the front of the word and make a solid word. The prefix “co-” has generally been an exception to this rule and has been hyphenated. I would say that it is “in transition.” That is, it is generally moving toward being made a solid word in many instances.
The problem, as I see it, is that many of the combinations are hard to read.
…He is a codefendant.
…He is a cosponsor.
For the sake of readability, I would continue to hyphenate those that are hard to read.