diff --git a/.github/workflows/test.yaml b/.github/workflows/test.yaml
index 33c0dd2..954b483 100644
--- a/.github/workflows/test.yaml
+++ b/.github/workflows/test.yaml
@@ -5,6 +5,25 @@ on:
   pull_request:
 
 jobs:
+  lint:
+    name: Lint (Python)
+    runs-on: ubuntu-latest
+    timeout-minutes: 5
+    steps:
+      - uses: actions/checkout@v6
+
+      - name: Set up Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: "3.11"
+
+      - name: Install ruff
+        run: pip install ruff
+
+      - name: Lint Python (undefined names, syntax errors)
+        working-directory: libs/openant-core
+        run: ruff check .
+
   python-tests:
     name: Python tests (${{ matrix.os }})
     runs-on: ${{ matrix.os }}
diff --git a/libs/openant-core/core/analyzer.py b/libs/openant-core/core/analyzer.py
index 7fb5966..79a813b 100644
--- a/libs/openant-core/core/analyzer.py
+++ b/libs/openant-core/core/analyzer.py
@@ -390,7 +390,7 @@ def _usage_dict():
 
     # Inject prior usage into tracker so step_report captures the total
     if _summary_input_tokens or _summary_output_tokens:
-        tracker.add_prior_usage(
+        get_global_tracker().add_prior_usage(
             _summary_input_tokens, _summary_output_tokens, _summary_cost_usd)
 
     # Write initial summary
diff --git a/libs/openant-core/pyproject.toml b/libs/openant-core/pyproject.toml
index 266e7db..bf0377a 100644
--- a/libs/openant-core/pyproject.toml
+++ b/libs/openant-core/pyproject.toml
@@ -21,6 +21,7 @@ dependencies = [
 [project.optional-dependencies]
 dev = [
     "pytest>=8.0.0",
+    "ruff>=0.8.0",
 ]
 
 [project.scripts]
@@ -30,6 +31,14 @@ openant = "openant.cli:main"
 requires = ["hatchling"]
 build-backend = "hatchling.build"
 
+[tool.ruff]
+target-version = "py311"
+
+[tool.ruff.lint]
+# Only rules that catch actual bugs (will break at runtime)
+# F821: undefined name, F811: redefined unused name, F823: local var referenced before assignment
+select = ["F821", "F811", "F823"]
+
 [tool.hatch.build.targets.wheel]
 packages = [
     "openant",