diff --git a/AGENTS.md b/AGENTS.md
index 0eedb7a..a63c66a 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -102,6 +102,10 @@ View (thin data-provider)
 
 This project uses the GSD (Get Shit Done) planning workflow. Planning artifacts live in `.planning/`.
 
+### Worktree-first rule (Non-Negotiable)
+
+Before any GSD workflow or agent edits project files, create or switch into a dedicated git worktree for that GSD task. Discussion, read-only inspection, and status checks may run in the main checkout, but planning artifacts, code changes, docs changes, and workflow state mutations must happen inside the task worktree. If a workflow would edit files and no worktree exists yet, stop and create one first.
+
 ### Key files
 
 | File | Purpose |
diff --git a/CLAUDE.md b/CLAUDE.md
index de607b4..5c68995 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -108,6 +108,10 @@ View (thin data-provider)
 
 This project uses the GSD (Get Shit Done) planning workflow. Planning artifacts live in `.planning/`.
 
+### Worktree-first rule (Non-Negotiable)
+
+Before any GSD workflow or agent edits project files, create or switch into a dedicated git worktree for that GSD task. Discussion, read-only inspection, and status checks may run in the main checkout, but planning artifacts, code changes, docs changes, and workflow state mutations must happen inside the task worktree. If a workflow would edit files and no worktree exists yet, stop and create one first.
+
 ### Key files
 
 | File | Purpose |