From d7c8dc5d42ba2780f772607fd48eddcf2a7b27c9 Mon Sep 17 00:00:00 2001
From: David Davis <daviddavis@users.noreply.github.com>
Date: Fri, 12 Jun 2026 13:23:07 -0400
Subject: [PATCH] Fix missing cookiecutter bootstrap error

Also add a cookiecutter bootstrap test.

Assisted-By: GitHub Copilot (Claude)

Closes #1428
---
 .github/workflows/pr.yml                  | 22 ++++++++++++++++++++++
 CHANGES/1428.bugfix                       |  1 +
 cookiecutter/ci/hooks/post_gen_project.py | 11 ++++++++---
 3 files changed, 31 insertions(+), 3 deletions(-)
 create mode 100644 CHANGES/1428.bugfix

diff --git a/.github/workflows/pr.yml b/.github/workflows/pr.yml
index 742e2457f..3cd588c00 100644
--- a/.github/workflows/pr.yml
+++ b/.github/workflows/pr.yml
@@ -30,6 +30,27 @@ jobs:
     needs:
       - "lint"
     uses: "./.github/workflows/codeql.yml"
+  cookiecutter-bootstrap-check:
+    runs-on: "ubuntu-latest"
+    steps:
+      - name: "Checkout"
+        uses: "actions/checkout@v6"
+        with:
+          fetch-depth: 0
+      - name: "Set up Python"
+        uses: "actions/setup-python@v6"
+        with:
+          python-version: "3.14"
+          allow-prereleases: true
+      - name: "Install uv"
+        uses: "astral-sh/setup-uv@v7"
+        with:
+          enable-cache: true
+      - name: "Bootstrap template sanity check"
+        run: |
+          tmpdir="$(mktemp -d)"
+          pushd "$tmpdir"
+          yes "" | uv run "$GITHUB_WORKSPACE/cookiecutter/apply_templates.py" --bootstrap
   check-commits:
     runs-on: "ubuntu-latest"
     steps:
@@ -69,6 +90,7 @@ jobs:
       - "test"
       - "docs"
       - "codeql"
+      - "cookiecutter-bootstrap-check"
     if: "always()"
     steps:
       - name: "Collect needed jobs results"
diff --git a/CHANGES/1428.bugfix b/CHANGES/1428.bugfix
new file mode 100644
index 000000000..ac4388416
--- /dev/null
+++ b/CHANGES/1428.bugfix
@@ -0,0 +1 @@
+Fixed cookiecutter bootstrap error caused by a missing [tool] section in pyproject.toml.update during template merge.
\ No newline at end of file
diff --git a/cookiecutter/ci/hooks/post_gen_project.py b/cookiecutter/ci/hooks/post_gen_project.py
index 75f845b09..791aaf4ea 100644
--- a/cookiecutter/ci/hooks/post_gen_project.py
+++ b/cookiecutter/ci/hooks/post_gen_project.py
@@ -17,7 +17,11 @@
 
     # TODO How deep is your merge?
     # Is merging on the tool level appropriate?
-    pyproject_toml["tool"].update(pyproject_toml_update["tool"])
+    if "tool" in pyproject_toml_update:
+        if "tool" not in pyproject_toml:
+            pyproject_toml["tool"] = pyproject_toml_update["tool"]
+        else:
+            pyproject_toml["tool"].update(pyproject_toml_update["tool"])
 
     if "dependency-groups" in pyproject_toml_update:
         if "dependency-groups" not in pyproject_toml:
@@ -26,8 +30,9 @@
             pyproject_toml["dependency-groups"].update(pyproject_toml_update["dependency-groups"])
 
     # Remove legacy tools.
-    for tool in ["flake8", "black", "isort"]:
-        pyproject_toml["tool"].pop(tool, None)
+    if "tool" in pyproject_toml:
+        for tool in ["flake8", "black", "isort"]:
+            pyproject_toml["tool"].pop(tool, None)
 
     with open(os.path.join(base_path, "pyproject.toml"), "w") as fp:
         tomlkit.dump(pyproject_toml, fp)