diff --git a/artifacts/runtime/env_sync.sh b/artifacts/runtime/env_sync.sh
index cf8c1bb35..5c26e2341 100755
--- a/artifacts/runtime/env_sync.sh
+++ b/artifacts/runtime/env_sync.sh
@@ -73,9 +73,20 @@ sync_python_env() {
       return 1
     fi
     PACKAGE_BUILD_PYTHONPATH="$BASE_SITE_PACKAGES:$VENV_SITE_PACKAGES"
-    UV_CACHE_DIR="$UV_CACHE_DIR" PYTHONPATH="$PACKAGE_BUILD_PYTHONPATH" PATH="/usr/local/venv/bin:/usr/bin:$PATH" \
+    # The base AGNOS venv ships Eigen as a Python package instead of under
+    # /usr/include. rednose includes <eigen3/Eigen/Dense>, so source-package
+    # builds need the package's install directory on the compiler include path.
+    # Resolve it through Python rather than pinning the Python minor version.
+    EIGEN_INCLUDE_ROOT="$(/usr/local/venv/bin/python3 -c \
+      'from pathlib import Path; import eigen; root = Path(eigen.__file__).resolve().parent / "install"; assert (root / "eigen3/Eigen/Dense").is_file(); print(root)' \
+      2>/dev/null || true)"
+    PACKAGE_BUILD_CPATH="${CPATH:-}"
+    if [ -n "$EIGEN_INCLUDE_ROOT" ]; then
+      PACKAGE_BUILD_CPATH="$EIGEN_INCLUDE_ROOT${PACKAGE_BUILD_CPATH:+:$PACKAGE_BUILD_CPATH}"
+    fi
+    UV_CACHE_DIR="$UV_CACHE_DIR" PYTHONPATH="$PACKAGE_BUILD_PYTHONPATH" CPATH="$PACKAGE_BUILD_CPATH" PATH="/usr/local/venv/bin:/usr/bin:$PATH" \
       uv pip install --python "$DIR/.venv/bin/python" --no-build-isolation --no-deps --reinstall "$IQDBC_PACKAGE_SOURCE" || return 1
-    UV_CACHE_DIR="$UV_CACHE_DIR" PYTHONPATH="$PACKAGE_BUILD_PYTHONPATH" PATH="/usr/local/venv/bin:/usr/bin:$PATH" \
+    UV_CACHE_DIR="$UV_CACHE_DIR" PYTHONPATH="$PACKAGE_BUILD_PYTHONPATH" CPATH="$PACKAGE_BUILD_CPATH" PATH="/usr/local/venv/bin:/usr/bin:$PATH" \
       uv pip install --python "$DIR/.venv/bin/python" --no-build-isolation --no-deps --reinstall "${PACKAGE_SOURCES[@]}" || return 1
     printf '%s\n' "$PACKAGE_LOCK_SHA" > "$DIR/.iqpilot-package-lock-sha256"
   fi