diff --git a/.claude/skills/implementer/SKILL.md b/.claude/skills/implementer/SKILL.md
index 41f9df5..364d7d2 100644
--- a/.claude/skills/implementer/SKILL.md
+++ b/.claude/skills/implementer/SKILL.md
@@ -39,6 +39,19 @@ a worker made all 59 of its edits in the primary's tree and never noticed.
 test "$(git rev-parse --show-toplevel)" = "$PWD" || cd "$(git rev-parse --show-toplevel)"
 ```
 
+**Never run `git stash` (or `git stash pop`/`apply`/`drop`).** Your worktree is isolated, but the
+stash is **not**: `refs/stash` is one stack shared by the primary's checkout and every other
+worker's worktree of this repo. Measured on 2026-09-04 — `git stash list` from a worker's worktree
+and from the primary's tree returned byte-identical output. So a `git stash` you run can be popped
+into someone else's tree, and a `git stash pop` you run can drop **another worker's** uncommitted
+edits on top of yours. This has already happened here: two workers were running in parallel and one
+of them had its in-progress edit silently overwritten by the other's stash.
+
+The branch is your isolation, so use it instead. To set work aside, commit it on your own branch
+(`git commit -m "wip: ..."`) and carry on; to try something and back out, use
+`git diff > /tmp/<your-branch>.patch` then `git checkout -- <file>`. Both stay inside your worktree.
+If you find a stash entry you did not create, leave it alone and say so in your report.
+
 ## 2. Implement
 
 - Implement exactly the scope the lead named. Keep the diff focused; note anything out of scope