In a post conviction writ, an applicant is claiming that the State failed to disclose a plea offer with the codefendant to testify. Is there a bright line rule of when the State must disclose it has been in negotiations with a co-actor to testify? Any good case or cases on this?

Facts are an offer for a plea from murder to agg assault was made and then rejected by codefendant. That is certain. After that there is a conflict. Prosecutor says no other offers were made until after trial, although the prosecutor did in her own mind think it was appropriate to plea the co-actor to some lesser charge. Codefendant's attorney says she "understood" her client would get a better deal if he testified, but then waivers. In one instance she claims it was just based on her experience as an attorney that if I convince my client to do the right thing and testify the prosecutor won't turn around and try to jack up my client, and then in another instance said the prosecutor specifically said "I will consider his testimony in coming up with a plea offer."

What do the courts look for? Just representation made by the State? Or do they also look at what the defendant and his attorney thought. (Seems weird we can't be mind readers). Any help would be appreciated.