diff --git a/.github/workflows/tests.yaml b/.github/workflows/tests.yaml
new file mode 100644
index 0000000..d9e60cc
--- /dev/null
+++ b/.github/workflows/tests.yaml
@@ -0,0 +1,37 @@
+name: Unit tests
+
+on:
+  - push
+  - pull_request
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    strategy:
+      fail-fast: false
+      matrix:
+        python-version: ["3.11", "3.12", "3.13"]
+
+    steps:
+    - uses: actions/checkout@v4
+    - uses: mamba-org/setup-micromamba@v2
+      with:
+        environment-file: environment.yml
+        init-shell: >-
+          bash
+        cache-environment: false
+        create-args: python=${{ matrix.python-version }}
+        post-cleanup: 'all'
+    - name: Install xcengine
+      shell: bash -el {0}
+      run: |
+        python --version
+        pip install --no-deps --editable .
+    - name: Run pytest
+      shell: bash -el {0}
+      run: |
+        pytest --cov=xcengine --cov-branch --cov-report=xml
+    - uses: codecov/codecov-action@v5
+      with:
+        verbose: true
diff --git a/README.md b/README.md
index bcef678..02fc0da 100644
--- a/README.md
+++ b/README.md
@@ -1,3 +1,5 @@
+[![Unit tests](https://github.com/xcube-dev/xcengine/actions/workflows/tests.yaml/badge.svg)](https://github.com/xcube-dev/xcengine/actions/workflows/tests.yaml)
+
 # xcengine: turn Jupyter notebooks into Application Packages
 
 xcengine provides tools to convert a Jupyter notebook into one of several
diff --git a/environment.yml b/environment.yml
index 2fa23e1..04b64b8 100644
--- a/environment.yml
+++ b/environment.yml
@@ -3,17 +3,19 @@ channels:
   - conda-forge
 dependencies:
   # Python
-  - python >=3.10
+  - python >=3.11
   # Required
   - click
   - docker-py
   - ipython  # Used by nbconvert to transform IPython syntax to pure Python
   - nbconvert
   - nbformat
+  - pystac
   - pyyaml
   - xcube  # See note below
   # test dependencies
   - pytest
+  - pytest-cov
 
 # Note: xcube is not required for the conversion itself, but is required
 # to run generated scripts outside containers ("create" mode).
diff --git a/xcengine/parameters.py b/xcengine/parameters.py
index 621067f..5a19c75 100644
--- a/xcengine/parameters.py
+++ b/xcengine/parameters.py
@@ -38,6 +38,8 @@ def make_cwl_params(self):
     def from_code(cls, code: str) -> "NotebookParameters":
         # TODO run whole notebook up to params cell, not just the params cell!
         # (Because it might use imports etc. from earlier in the notebook.)
+        # This will need some tweaking of the parameter extraction -- see
+        # comment therein.
         return cls(cls.extract_variables(code))
 
     @classmethod
@@ -60,10 +62,11 @@ def from_yaml_file(cls, path: str | os.PathLike) -> "NotebookParameters":
 
     @classmethod
     def extract_variables(cls, code: str) -> dict[str, tuple[type, Any]]:
-        _old_locals = set(locals().keys())
-        exec(code)
-        newvars = locals().keys() - _old_locals - {"_old_locals"}
-        return {k: cls.make_param_tuple(locals()[k]) for k in newvars}
+        # TODO: just working on a single code block is insufficient:
+        # We should execute everything up to the params cell, but only extract
+        # variables defined in the params cell.
+        exec(code, globals(), locals_ := {})
+        return {k: cls.make_param_tuple(locals_[k]) for k in (locals_.keys())}
 
     @staticmethod
     def make_param_tuple(value: Any) -> tuple[type, Any]:
@@ -98,7 +101,7 @@ def get_cwl_workflow_input(self, var_name: str) -> dict[str, Any]:
 
     def get_cwl_commandline_input(self, var_name: str) -> dict[str, Any]:
         return self.get_cwl_workflow_input(var_name) | {
-            "inputBinding": {"prefix": f"--{var_name.replace("_", "-")}"}
+            "inputBinding": {"prefix": f'--{var_name.replace("_", "-")}'}
         }
 
     def to_yaml(self) -> str: