From a4bf59b583d07a3a97cd8704b76870f7bba5c0a7 Mon Sep 17 00:00:00 2001
From: Pravin <pkireri@gmail.com>
Date: Tue, 18 Aug 2026 22:10:20 -0400
Subject: [PATCH] fix(transform schema): preserve const constraints

---
 src/anthropic/lib/_parse/_transform.py |  4 ++++
 tests/lib/_parse/test_transform.py     | 16 ++++++++++++++++
 2 files changed, 20 insertions(+)

diff --git a/src/anthropic/lib/_parse/_transform.py b/src/anthropic/lib/_parse/_transform.py
index ce0c83ac9..a2852f2e9 100644
--- a/src/anthropic/lib/_parse/_transform.py
+++ b/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
diff --git a/tests/lib/_parse/test_transform.py b/tests/lib/_parse/test_transform.py
index 7a2799dce..adfd8e816 100644
--- a/tests/lib/_parse/test_transform.py
+++ b/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": [