diff --git a/pytest.ini b/pytest.ini
index e7ec383..988ed91 100644
--- a/pytest.ini
+++ b/pytest.ini
@@ -9,3 +9,7 @@ addopts =
     --cov=src/data_analysis_chatbots
     --cov-report=html
     --cov-report=term-missing
+markers =
+    slow: marks tests as slow (deselect with '-m "not slow"')
+    integration: marks tests as integration tests
+    unit: marks tests as unit tests
diff --git a/requirements.txt b/requirements.txt
index 9d9db1d..9f7de88 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -24,6 +24,9 @@ jupyter>=1.0.0,<2.0
 ipykernel>=6.25.0,<7.0
 ipywidgets>=8.1.0,<9.0
 
+# Excel IO (DataLoader.load_excel + pandas.to_excel rely on this)
+openpyxl>=3.1.0,<4.0
+
 # Configuration Management
 pyyaml>=6.0.0,<7.0
 python-dotenv>=1.0.0,<2.0
diff --git a/setup.py b/setup.py
index df6c0f7..58ad14e 100644
--- a/setup.py
+++ b/setup.py
@@ -45,6 +45,10 @@
         "plotly>=5.14.0,<6.0",
         "streamlit>=1.28.0,<2.0",
         "nltk>=3.8.0,<4.0",
+        # Required by DataLoader.load_excel + pandas.to_excel; without it
+        # CI fails with "ModuleNotFoundError: No module named 'openpyxl'"
+        # at the Excel-IO test sites.
+        "openpyxl>=3.1.0,<4.0",
         "pyyaml>=6.0.0,<7.0",
         "python-dotenv>=1.0.0,<2.0",
         "loguru>=0.7.0,<1.0",