diff --git a/AGENTS.md b/AGENTS.md
index 01e96c86..05ac9bc7 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -19,7 +19,7 @@ The following operations are benign and should be executed without confirmation.
 - Compile or syntax-check Python (`python -c`, `python -m py_compile`)
 - Activate or use the virtual environment (`.venv/bin/python`, `source .venv/bin/activate`)
 - Run pre-commit hooks (`git commit` triggers hooks automatically)
-- Run project scripts (`./scripts/fix-markdown.sh`)
+- Run project scripts (`./scripts/fix-markdown.sh`, `./scripts/new-worktree.sh`)
 - Standard git operations (`git status`, `git diff`, `git log`, `git add`, `git commit`)
 - Package management (`poetry`, `uv`, `pip install`)
 - File inspection (`ls`, `find`, `wc`, `head`, `tail`, `diff`, `cat`)
@@ -64,6 +64,22 @@ those files in the `SpanPanel_Docs/span/docs/` folder in the workspace — not i
 
 Do not commit documentation artifacts to the `span` integration repo itself.
 
+## Worktrees
+
+Create worktrees with `scripts/new-worktree.sh`, never with `git worktree add` or an agent's built-in worktree tool:
+
+```bash
+./scripts/new-worktree.sh <name> [<branch>]    # <branch> defaults to feat/<name>
+```
+
+It creates `../span-<name>` beside the primary checkout from a freshly fetched `origin/main`, prepares it with `setup-hooks.sh`, and links `.env`. From there
+`../../span/span-panel-api` still resolves to the library checkout, so no bridge is needed.
+
+- **Never share a virtual environment between checkouts.** Editable installs record one checkout's source path, so a shared `.venv` imports whichever checkout
+  synced last and tests pass against the wrong code without an error.
+- **To test against a library worktree, install it explicitly.** When a library worktree of the same name exists, the script prints the `uv pip install -e`
+  command rather than running it, because the next `uv sync` silently undoes it. See `developer.md`, "Working against unreleased library code".
+
 ## Service Registration Requirements
 
 All services registered by this integration MUST conform to the following requirements. These align with Home Assistant core's service architecture and ensure
diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 00000000..0010d05b
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1,5 @@
+# Claude Code
+
+<!-- Claude Code reads CLAUDE.md, not AGENTS.md. This imports AGENTS.md so every agent works from the same instructions. -->
+
+@AGENTS.md
diff --git a/developer.md b/developer.md
index c790f5c7..39565b95 100644
--- a/developer.md
+++ b/developer.md
@@ -75,6 +75,18 @@ echo 'dotenv' > .envrc
 direnv allow
 ```
 
+## Worktrees
+
+Create worktrees with the script rather than `git worktree add`:
+
+```bash
+./scripts/new-worktree.sh <name> [<branch>]    # <branch> defaults to feat/<name>
+```
+
+It fetches `origin`, fast-forwards `main` where that is safe, creates `../span-<name>` on a branch that does not track a remote, links `.env` from the primary
+checkout, and runs `setup-hooks.sh` inside it. The worktree gets a virtual environment of its own, which resolves the library from `../../span/span-panel-api`
+like the primary checkout does. When a library worktree of the same name exists, the script ends by printing the command to install it, described below.
+
 ## Working against unreleased library code
 
 `pyproject.toml` installs `span-panel-api` and both adapters from `../../span/span-panel-api` as editable path dependencies. That is a committed, shared file:
diff --git a/scripts/new-worktree.sh b/scripts/new-worktree.sh
new file mode 100755
index 00000000..b287edca
--- /dev/null
+++ b/scripts/new-worktree.sh
@@ -0,0 +1,112 @@
+#!/bin/bash
+#
+# Create a worktree for a branch of work beside the primary checkout, and prepare
+# it with setup-hooks.sh.
+#
+# Usage: scripts/new-worktree.sh <name> [<branch>]
+#
+#   <name>    The worktree is created at ../<repository>-<name>, beside the primary
+#             checkout. From there `../../span/span-panel-api` still resolves to the
+#             library checkout, so no bridge is needed.
+#   <branch>  The branch to create. Defaults to feat/<name>.
+#
+# The branch starts from origin/main as just fetched and does not track it, so a
+# bare `git push` can never land on main. Local main is fast-forwarded as well where
+# that is safe, and the script refuses when local main holds commits origin/main
+# lacks, because branching from origin/main would silently leave them out.
+#
+# The worktree gets a virtual environment of its own rather than a link to the
+# primary checkout's. The environment holds editable installs bound to one checkout's
+# sources, so a shared one imports whichever checkout synced last and tests pass
+# against the wrong code without an error. uv clones packages from its cache, so the
+# second environment costs little. Local configuration that is not bound to a path,
+# `.env`, is linked instead.
+#
+# When a library worktree of the same name exists, the command to test against it is
+# printed rather than run: it is a local override that the next `uv sync` silently
+# undoes, so it should be a step you take knowingly. See developer.md, "Working
+# against unreleased library code".
+
+set -euo pipefail
+
+usage() {
+    echo "Usage: $0 <name> [<branch>]" >&2
+    exit 2
+}
+
+[[ $# -ge 1 && $# -le 2 ]] || usage
+name=$1
+if [[ ! "$name" =~ ^[A-Za-z0-9._-]+$ ]]; then
+    echo "Worktree name '$name' may use only letters, digits, dots, underscores and hyphens." >&2
+    exit 2
+fi
+branch=${2:-feat/$name}
+
+script_dir=$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)
+checkout=$(cd "$script_dir/.." && pwd)
+common_dir=$(git -C "$checkout" rev-parse --path-format=absolute --git-common-dir)
+primary_root=$(dirname "$common_dir")
+target="$(dirname "$primary_root")/$(basename "$primary_root")-$name"
+
+if [[ -e "$target" ]]; then
+    echo "$target already exists." >&2
+    exit 1
+fi
+if git -C "$primary_root" show-ref --verify --quiet "refs/heads/$branch"; then
+    echo "Branch $branch already exists." >&2
+    exit 1
+fi
+
+echo "Fetching origin..."
+git -C "$primary_root" fetch origin
+
+if git -C "$primary_root" show-ref --verify --quiet refs/heads/main; then
+    ahead=$(git -C "$primary_root" rev-list --count origin/main..main)
+    if ((ahead > 0)); then
+        echo "Local main has $ahead commit(s) that origin/main does not. Reconcile main first;" >&2
+        echo "branching from origin/main would silently leave them out." >&2
+        exit 1
+    fi
+
+    # Where main is checked out, move it only if that checkout is clean.
+    main_checkout=$(git -C "$primary_root" worktree list --porcelain |
+        awk '/^worktree /{wt = substr($0, 10)} /^branch refs\/heads\/main$/{print wt}')
+    if [[ -z "$main_checkout" ]]; then
+        # update-ref rather than `branch --force`: that would also rewrite main's
+        # upstream to origin/main, silently re-pointing a main that tracks another
+        # remote. The old value guards against main moving since it was checked.
+        git -C "$primary_root" update-ref -m "new-worktree.sh: fast-forward main to origin/main" \
+            refs/heads/main "$(git -C "$primary_root" rev-parse refs/remotes/origin/main)" \
+            "$(git -C "$primary_root" rev-parse refs/heads/main)"
+    elif [[ -n "$(git -C "$main_checkout" status --porcelain --untracked-files=no)" ]]; then
+        echo "main is checked out with uncommitted changes at $main_checkout; leaving it as it is."
+    elif ! git -C "$main_checkout" merge --ff-only --quiet origin/main; then
+        # Untracked files are not counted as changes above, because a fast-forward
+        # refuses to overwrite one; this is where that refusal lands.
+        echo "Could not fast-forward main at $main_checkout; leaving it as it is."
+    fi
+fi
+
+echo "Creating $target on $branch from origin/main..."
+git -C "$primary_root" worktree add --no-track -b "$branch" "$target" origin/main
+
+if [[ -f "$primary_root/.env" && ! -e "$target/.env" ]]; then
+    ln -s "$primary_root/.env" "$target/.env"
+    echo "Linked .env from the primary checkout."
+fi
+
+echo "Preparing the worktree..."
+(cd "$target" && bash ./setup-hooks.sh)
+
+echo
+echo "Worktree ready: $target"
+echo "Branch:         $branch, from $(git -C "$target" rev-parse --short HEAD), not tracking a remote"
+
+if [[ -d "$target/../../span/span-panel-api-$name" ]]; then
+    echo
+    echo "A library worktree of the same name exists. To test against it, install all three of"
+    echo "its distributions into this worktree's environment; the next \`uv sync\` undoes it:"
+    echo "    uv pip install -e ../../span/span-panel-api-$name \\"
+    echo "                   -e ../../span/span-panel-api-$name/packages/schema-0 \\"
+    echo "                   -e ../../span/span-panel-api-$name/packages/schema-1"
+fi