fix(transform schema): preserve const constraints

src/anthropic/lib/_parse/_transform.py

@@ -117,6 +117,10 @@ def transform_schema(
     if is_list(enum):
         strict_schema["enum"] = enum
 
+    # Use a key-presence check because `const: null` is a valid constraint.
+    if "const" in json_schema:
+        strict_schema["const"] = json_schema.pop("const")
+
     description = json_schema.pop("description", None)
     if description is not None:
         strict_schema["description"] = description

tests/lib/_parse/test_transform.py

@@ -73,6 +73,22 @@ def test_enum_schema():
     assert result == snapshot({"type": "string", "enum": ["foo", "bar"]})
 
 
+def test_const_schema():
+    # Pydantic v2 emits `const` for single-value `Literal` fields, e.g.
+    # `Literal["ok"]` -> `{"type": "string", "const": "ok"}`.
+    schema = {"type": "string", "const": "ok"}
+    result = transform_schema(schema)
+    assert result == snapshot({"type": "string", "const": "ok"})
+
+
+def test_const_null_schema():
+    # A present `const: None` constraint must not be confused with an
+    # absent `const` key.
+    schema = {"type": "null", "const": None}
+    result = transform_schema(schema)
+    assert result == snapshot({"type": "null", "const": None})
+
+
 def test_allof():
     schema = {
         "allOf": [