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 apologize for failing a disability accommodation, use this factual base: name the missed care, privacy breach, exposure, comment, or accommodation and its practical effect.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Correct what is still correctable and ask what support is useful rather than prescribing it. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not debate symptoms, disclose medical details, or make the person reassure you. For urgent symptoms, exposure risk, medication errors, or safeguarding concerns, contact the appropriate clinician or emergency service. You do not need the other person to agree with a boundary for you to follow it consistently.