diff --git a/CLAUDE.md b/CLAUDE.md
index fcae41d02..73b2ec4b6 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -147,6 +147,29 @@ worktree's source into the worktree's own environment.
 
 **Full documentation**: `docs/developer/guides/branching-strategy.md` (Git Worktrees section)
 
+#### Worktree branch policy
+
+**Worktrees must NEVER be on `development` or `main` directly.**
+
+In this repo, `main` is the *release* branch (tagged quarterly, essentially
+read-only history) and `development` is the integration trunk where active
+work converges. The default repository checkout (`~/+Underworld/underworld3-pixi`)
+should usually sit on `development` — that's where you read the current
+working state and pull updates. All work — even work intended to land
+on `development` — happens on a side branch (`feature/...`, `bugfix/...`,
+`docs/...`) in a worktree, then merges to `development` via PR.
+
+`./uw worktree create` enforces this for new worktrees (always on a
+side branch, reset to `origin/development`). It's on you not to break
+it manually:
+
+- Never `git checkout development` (or `main`) inside a worktree
+- Never `git worktree add ... development` to put a worktree on
+  `development` directly
+- If you find a worktree on `development` (e.g. from older tooling),
+  branch off immediately (`git switch -c bugfix/whatever`) before
+  committing
+
 #### Creating and using a worktree
 
 ```bash
diff --git a/docs/developer/guides/branching-strategy.md b/docs/developer/guides/branching-strategy.md
index f4f8f3be0..e538b4bd5 100644
--- a/docs/developer/guides/branching-strategy.md
+++ b/docs/developer/guides/branching-strategy.md
@@ -155,6 +155,35 @@ one set of dependencies and one (expensive) PETSc compilation.
   half-finished work.
 - **Clean PRs**: Each worktree has its own branch, so commits stay focused.
 
+### Branch policy: worktrees are always on a side branch
+
+**Worktrees must never be on `development` or `main` directly.**
+
+In this repo, `main` is the *release* branch (tagged quarterly, essentially
+read-only history) and `development` is the integration trunk where active
+work converges. The default repository checkout
+(`~/+Underworld/underworld3-pixi`) should usually sit on `development` — that's
+where you read the current working state and pull updates.
+
+All work — including work intended to land on `development` — happens on a
+side branch (`feature/...`, `bugfix/...`, `docs/...`) in a worktree, then
+merges to `development` via PR.
+
+`./uw worktree create` enforces this for new worktrees: it creates the
+worktree on `<prefix>/<name>` and resets to `origin/development`, never
+checking out `development` itself in the new worktree.
+
+**Don't break it manually:**
+
+- Never `git checkout development` (or `main`) inside a worktree
+- Never `git worktree add ... development` to put a worktree directly on
+  `development`
+- If you find a worktree on `development` (e.g. from older tooling), branch
+  off immediately (`git switch -c bugfix/whatever`) before committing
+
+The default repo checkout is the *only* place that should be on
+`development`. Worktrees are *always* on side branches.
+
 ### Lifecycle
 
 ```bash
diff --git a/uw b/uw
index fcedc68ac..bbba37e14 100755
--- a/uw
+++ b/uw
@@ -1151,6 +1151,29 @@ worktree_create() {
         exit 1
     fi
 
+    # Policy: worktrees must NEVER be on `development` or `main` directly.
+    # `main` is the release branch (tagged history); `development` is the
+    # integration trunk and lives in the default repo checkout. All work
+    # happens on side branches. Refuse misleading prefixes.
+    if [ "$branch_prefix" = "development" ] || [ "$branch_prefix" = "main" ]; then
+        local main_repo
+        main_repo=$(get_main_repo)
+        echo -e "${YELLOW}Refusing to use '$branch_prefix' as a branch prefix.${NC}"
+        echo ""
+        echo "Worktrees in this repo are always on a side branch:"
+        echo "  feature/<name>   bugfix/<name>   docs/<name>   ..."
+        echo ""
+        echo "If you wanted a checkout of '$branch_prefix' to read code from,"
+        echo "use the main repo at:"
+        echo "  $main_repo"
+        echo ""
+        echo "If you wanted to do work that will merge to development,"
+        echo "use a regular prefix:"
+        echo "  ./uw worktree create $name           # → feature/$name"
+        echo "  ./uw worktree create $name bugfix    # → bugfix/$name"
+        exit 1
+    fi
+
     local main_repo=$(get_main_repo)
     local wt_path="$main_repo/.claude/worktrees/$name"
     local branch_name="${branch_prefix}/${name}"
@@ -1324,7 +1347,13 @@ worktree_list() {
             local dirty=""
             [ "$status" -gt 0 ] && dirty="  ${YELLOW}($status modified)${NC}"
 
-            echo -e "  ${GREEN}${wt_name}${NC}  [$branch]  ($linked)$dirty"
+            # Policy advisory: worktrees should never be on development or main
+            local policy=""
+            if [ "$branch" = "development" ] || [ "$branch" = "main" ]; then
+                policy="  ${YELLOW}⚠ on '$branch' — branch off to a side branch${NC}"
+            fi
+
+            echo -e "  ${GREEN}${wt_name}${NC}  [$branch]  ($linked)$dirty$policy"
             echo "    $d"
         else
             echo -e "  ${YELLOW}${wt_name}${NC}  (not a git worktree)"