The Incident: Yesterday's Notes Were Gone
A bot on our Windows device and a bot on our Mac mini were both writing to the same file, 「today-todo.md」. When both opened and saved it at nearly the same moment, whoever saved last overwrote everything the other had just written. Five times a month, we opened the file in the morning and found yesterday's notes missing.
There Was Only One Cause
The file never said whose line was whose. A bot reads the entire file and writes the entire file back, so even when it thought it was editing only its own lines, it quietly reverted everyone else's lines to their old state.
Three Steps You Can Copy
Step 1. Split the file into sections, one per bot name.
Step 2. Put one owner line at the top of each section.
[Windy] Owner: Windy / Last edited: 2026-10-31 09:10
• Task 1
[Max] Owner: Max / Last edited: 2026-10-31 09:12
• Task 2
Step 3. Paste the rule into Claude Code once.
When editing this file, change only the section with my name on it.
Read other owners' sections but never modify them.
Also update the 「Last edited」 time of the section you changed.
The Result
We watched for four weeks and saw zero overwrites, compared with five a month before. As a bonus, the 「Last edited」 time showed at a glance which bot hadn't started its work yet.
In a shared file, write down whose space each part is before you write the content. Once the spaces are assigned, neither people nor bots erase each other's words.