diff --git a/.github/workflows/main.yml b/.github/workflows/main.yml
new file mode 100644
index 00000000..44621a16
--- /dev/null
+++ b/.github/workflows/main.yml
@@ -0,0 +1,81 @@
+name: CI
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+    branches: [main]
+    paths:
+      - .github/workflows/main.yml
+
+permissions:
+  contents: read
+
+concurrency:
+  group: ci-${{ github.ref }}
+  cancel-in-progress: false
+
+jobs:
+  test:
+    name: Default branch checks
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v6
+      - name: Set up Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: '3.13'
+      - name: Install uv
+        uses: astral-sh/setup-uv@v8.1.0
+        with:
+          version: latest
+      - name: Install dependencies
+        run: uv pip install -e ".[dev]" --system
+      - name: Lint
+        run: make lint
+      - name: Run tests
+        run: make test
+
+  deploy-docs:
+    name: Build and deploy documentation
+    needs: test
+    runs-on: ubuntu-latest
+    if: github.repository == 'PolicyEngine/microdf'
+    permissions:
+      contents: write
+    steps:
+      - uses: actions/checkout@v6
+      - name: Set up Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: '3.13'
+      - name: Set up Node.js
+        uses: actions/setup-node@v6
+        with:
+          node-version: '24'
+      - name: Install uv
+        uses: astral-sh/setup-uv@v8.1.0
+        with:
+          version: latest
+      - name: Install Python dependencies
+        run: uv pip install -e ".[dev]" --system
+      - name: Install MyST
+        run: npm install -g mystmd
+      - name: Build documentation with MyST
+        working-directory: docs
+        env:
+          BASE_URL: /microdf
+        run: |
+          myst build --html
+          touch _build/html/.nojekyll
+      - name: Check documentation output
+        run: |
+          test -s docs/_build/html/index.html
+          test -f docs/_build/html/.nojekyll
+      - name: Deploy
+        if: github.event_name == 'push' && github.ref == 'refs/heads/main'
+        uses: JamesIves/github-pages-deploy-action@v4
+        with:
+          token: ${{ secrets.GITHUB_TOKEN }}
+          branch: gh-pages
+          folder: docs/_build/html
diff --git a/.github/workflows/master.yml b/.github/workflows/master.yml
deleted file mode 100644
index 72d0a756..00000000
--- a/.github/workflows/master.yml
+++ /dev/null
@@ -1,66 +0,0 @@
-name: CI
-
-on:
-  push:
-    branches: [ master ]
-
-jobs:
-  test:
-    name: Test
-    runs-on: ubuntu-latest
-    steps:
-      - uses: actions/checkout@v6
-      
-      - name: Set up Python
-        uses: actions/setup-python@v6
-        with:
-          python-version: '3.13'
-      - name: Install uv
-        uses: astral-sh/setup-uv@v8.1.0
-        with:
-          version: "latest" 
-      - name: Install dependencies
-        run: |
-          uv pip install -e ".[dev]" --system
-      - name: Run tests
-        run: make test
-
-  deploy-docs:
-    name: Build and deploy documentation
-    needs: test
-    runs-on: ubuntu-latest
-    if: github.repository == 'policyengine/microdf'
-    
-    steps:
-      - uses: actions/checkout@v6
-      
-      - name: Set up Python
-        uses: actions/setup-python@v6
-        with:
-          python-version: '3.13'
-      - name: Install uv
-        uses: astral-sh/setup-uv@v8.1.0
-        with:
-          version: "latest"
-      - name: Install dependencies
-        run: |
-          uv pip install -e ".[dev,docs]" --system
-      - name: Build Jupyter Book
-        shell: bash -l {0}
-        run: |
-          jb build docs/.
-      - name: Check documentation build
-        run: |
-          for notebook in $(find docs/_build/jupyter_execute -name "*.ipynb"); do
-            if grep -q '"output_type": "error"' "$notebook"; then
-              echo "Error found in $notebook"
-              cat "$notebook"
-              exit 1
-            fi
-          done
-      - name: Deploy
-        uses: JamesIves/github-pages-deploy-action@v4
-        with:
-          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
-          BRANCH: gh-pages  # The branch the action should deploy to.
-          FOLDER: docs/_build/html  # The folder the action should deploy.
diff --git a/changelog.d/default-branch-ci.fixed.md b/changelog.d/default-branch-ci.fixed.md
new file mode 100644
index 00000000..efcb49a8
--- /dev/null
+++ b/changelog.d/default-branch-ci.fixed.md
@@ -0,0 +1 @@
+Restore tests and lint on pushes to main and build the deployed documentation with MyST instead of the retired Jupyter Book command. Include and validate the `.nojekyll` marker so GitHub Pages can publish the prebuilt site.