Say the boundary in operational terms
Name the contact, behavior, topic, access, or commitment you will and will not participate in. For a message about how to support someone living with chronic migraine, use this factual base: use the person's description, note the access barrier or practical need, and avoid guessing about severity.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Ask which accommodation or form of help is useful and follow through without making it a favor. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not demand medical proof in a personal relationship, offer cures, or frame disability as inspiration. Use qualified clinical or emergency care for urgent health concerns and the proper rights process for denied accommodations. You do not need the other person to agree with a boundary for you to follow it consistently.