diff --git a/src/substrait/simple_extension_utils.py b/src/substrait/simple_extension_utils.py
index ad22f5f..e52f68b 100644
--- a/src/substrait/simple_extension_utils.py
+++ b/src/substrait/simple_extension_utils.py
@@ -170,6 +170,7 @@ def build_type_variation(d: dict) -> se.TypeVariation:
 def build_simple_extensions(d: dict) -> se.SimpleExtensions:
     return se.SimpleExtensions(
         urn=d["urn"],
+        description=d.get("description"),
         metadata=d.get("metadata"),
         dependencies=d.get("dependencies"),
         types=[build_type_model(t) for t in d["types"]] if "types" in d else None,
diff --git a/tests/extension_registry/test_registry_lookup.py b/tests/extension_registry/test_registry_lookup.py
index 642262f..ec8c017 100644
--- a/tests/extension_registry/test_registry_lookup.py
+++ b/tests/extension_registry/test_registry_lookup.py
@@ -225,3 +225,15 @@ def test_quantile_binds_return_type_to_distribution(registry):
     match = registry.lookup_function(_ARITHMETIC, "quantile", signature)
     assert match is not None
     assert match[1] == list_(fp64(nullable=False), nullable=True)
+
+
+def test_decimal_avg_returns_nullable_decimal(registry):
+    # Since spec v0.105.0 ``avg`` over a decimal returns ``DECIMAL?<38,S>``: the
+    # average of an empty group is null even when the input is required.
+    match = registry.lookup_function(
+        "extension:io.substrait:functions_arithmetic_decimal",
+        "avg",
+        [decimal(2, 10, nullable=False)],
+    )
+    assert match is not None
+    assert match[1] == decimal(2, 38, nullable=True)
diff --git a/tests/test_simple_extension_utils.py b/tests/test_simple_extension_utils.py
index 9a9ae0a..35094ad 100644
--- a/tests/test_simple_extension_utils.py
+++ b/tests/test_simple_extension_utils.py
@@ -49,3 +49,18 @@ def test_metadata_defaults_to_none_when_absent():
     extensions = build_simple_extensions({"urn": "extension:test:no-metadata"})
 
     assert extensions.metadata is None
+
+
+def test_file_level_description_is_exposed():
+    """Extension files can carry a top-level description (spec v0.105.0)."""
+    extensions = build_simple_extensions(
+        {"urn": "extension:test:described", "description": "Test functions.\n"}
+    )
+
+    assert extensions.description == "Test functions.\n"
+
+
+def test_file_level_description_defaults_to_none_when_absent():
+    extensions = build_simple_extensions({"urn": "extension:test:undescribed"})
+
+    assert extensions.description is None