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 reconnect with a primary school friend, use this factual base: introduce yourself with one shared place, activity, person, or time period they can recognize.
Define any necessary exception
If an exception is necessary, give it one purpose, one channel, and a clear limit. Offer a low-pressure way to confirm the connection and share a brief reason for reaching out. Make the immediate next step clear enough that the recipient can answer without reconstructing the entire history.
Respond to escalation safely
Do not assume old closeness, demand memories, or ask for personal details before they verify who you are. Respect privacy, identity uncertainty, and any refusal, and avoid contacting a minor without a guardian-appropriate route. You do not need the other person to agree with a boundary for you to follow it consistently.