Back up your Messages

Steps

1. Press “connect” on the Messages card. The panel opens in ~/Library/Messages: press “Select this folder”.
2. Press “select export folder” and choose a folder like ~/Documents/messages. Leave every conversation ticked, or tick the ones you want on People and Groups.
3. Wait for the first pass over your history (“sync now” stays greyed out until it finishes), then press “sync now”.
4. After you unlock Messages, the next sync writes every conversation, the older ones included.
5. Optional, for dated copies: install the command line from its card, grant ~/Backups under “export destinations”, and run exporter export --source messages --dest ~/Backups/messages-$(date +%F).

What you get

~/Documents/messages/
  Anna Kowalski.md
  Cabin crew-57.md
  attachments/

One file per conversation, a heading per day, replies linked to the message they answer, edits with their earlier versions as footnotes, reactions under each message. Photos, video and audio are copied into attachments/, and iPhone HEIC photos are converted to JPEG.

Pitfalls

• On the free allowance the export holds your 10 most recent conversations among the ones you ticked.
• Media that was never downloaded to this Mac is named in the file instead of copied. Open the conversation in Messages so it downloads, then sync again.
• The folder follows Messages: a conversation you delete there leaves the export on the next sync, and Exporter never writes anything back into Messages. Dated copies keep what the mirror drops.

Every field in a conversation file is in the Messages docs, and the export flags in the command line reference.