diff --git a/pyproject.toml b/pyproject.toml
index 871cd2a..8e55d30 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -38,7 +38,13 @@ dev = [
     "mypy>=1.10",
     "pre-commit>=3.7",
     "types-pyyaml>=6.0",
-    "scikit-learn>=1.3",
+    # scikit-learn 1.9.0 changed HistGradientBoostingClassifier defaults in a
+    # way that improves the flat GBM more than the engineered one, making the
+    # headline GBM(eng)−GBM(flat) lift in NB02 go non-positive and breaking
+    # the G13.1 CI gate.  Pin <1.9 while the notebook targets are recalibrated
+    # or the engineered features are updated to restore positive lift on 1.9.
+    # See: https://github.com/leadforge-dev/leadforge/issues/114
+    "scikit-learn>=1.3,<1.9",
     "matplotlib>=3.7",
     # PR 7.2: the preview-page renderers (scripts/preview_{kaggle,hf}_page.py)
     # call into markdown-it-py at test time via render_*_html().  Keeping
@@ -49,7 +55,7 @@ dev = [
     "markdown-it-py>=3.0",
 ]
 scripts = [
-    "scikit-learn>=1.3",
+    "scikit-learn>=1.3,<1.9",  # keep in sync with [dev] pin above
     "matplotlib>=3.7",
 ]
 # Optional dependencies for the platform release packagers.  Installing
@@ -81,7 +87,7 @@ notebooks = [
     # python3`` because the GitHub-hosted runner has no kernelspecs
     # registered out of the box (local dev environments usually do).
     "ipykernel>=6.0",
-    "scikit-learn>=1.3",
+    "scikit-learn>=1.3,<1.9",  # keep in sync with [dev] pin above
     "matplotlib>=3.7",
 ]