From 46751ad83ed3307fdc0e966d7e850660df2446d9 Mon Sep 17 00:00:00 2001
From: Darien Kindlund <darien@kindlund.com>
Date: Tue, 4 Aug 2026 16:17:01 -0400
Subject: [PATCH] fix(anthropic): coerce explicit additionalProperties to false
 in output_format schema

Anthropic's structured outputs reject any `additionalProperties` value other
than `false` ("output_format.schema: For 'object' type, 'additionalProperties:
true' is not supported. Please set 'additionalProperties' to false")

`filter_anthropic_output_schema` only added the key when it was absent, so an
explicit `true` (or a sub-schema) was copied verbatim into output_format.schema
and 400'd. Coerce it for object schemas instead, at every recursion depth,
matching what the Anthropic Python/TypeScript SDKs do

The permissive tool-use path (map_response_format_to_anthropic_tool, used for
vertex_ai) is deliberately left alone

Fixes #35808
---
 litellm/llms/anthropic/chat/transformation.py |  2 +-
 .../anthropic/test_anthropic_schema_filter.py | 72 ++++++++++++++++++-
 2 files changed, 72 insertions(+), 2 deletions(-)

diff --git a/litellm/llms/anthropic/chat/transformation.py b/litellm/llms/anthropic/chat/transformation.py
index 51b862e79d9..19f5174579b 100644
--- a/litellm/llms/anthropic/chat/transformation.py
+++ b/litellm/llms/anthropic/chat/transformation.py
@@ -597,7 +597,7 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
 
         # Anthropic requires additionalProperties=false for object schemas
         # See: https://docs.anthropic.com/en/docs/build-with-claude/structured-outputs
-        if result.get("type") == "object" and "additionalProperties" not in result:
+        if result.get("type") == "object":
             result["additionalProperties"] = False
 
         return result
diff --git a/tests/litellm/llms/anthropic/test_anthropic_schema_filter.py b/tests/litellm/llms/anthropic/test_anthropic_schema_filter.py
index c10ac5532a0..71c9cfe8f41 100644
--- a/tests/litellm/llms/anthropic/test_anthropic_schema_filter.py
+++ b/tests/litellm/llms/anthropic/test_anthropic_schema_filter.py
@@ -281,7 +281,10 @@ class TestFilterAnthropicOutputSchema:
             "unevaluatedProperties",
         ):
             assert field not in result
-        assert 'properties whose names match each pattern must satisfy: {"^x": {"type": "string"}}' in result["description"]
+        assert (
+            'properties whose names match each pattern must satisfy: {"^x": {"type": "string"}}'
+            in result["description"]
+        )
         assert 'property names must satisfy: {"pattern": "^[a-z]+$"}' in result["description"]
         assert 'dependent required properties: {"first": ["last"]}' in result["description"]
         assert 'dependent schemas: {"first": {"required": ["last"]}}' in result["description"]
@@ -347,3 +350,70 @@ class TestFilterAnthropicOutputSchema:
             "all array items must be unique, minimum number of matching items: 2, "
             "maximum number of matching items: 3."
         )
+
+    def test_coerces_explicit_additional_properties_true(self):
+        """An explicit ``additionalProperties: true`` must be coerced to false.
+
+        Anthropic rejects anything other than false with:
+        "output_format.schema: For 'object' type, 'additionalProperties: true' is
+        not supported".
+        """
+        schema = {
+            "type": "object",
+            "additionalProperties": True,
+            "properties": {"a": {"type": "string"}},
+        }
+
+        result = AnthropicConfig.filter_anthropic_output_schema(schema)
+
+        assert result["additionalProperties"] is False
+
+    def test_coerces_additional_properties_true_when_nested(self):
+        """Nested object schemas are coerced too, at every recursion site."""
+        schema = {
+            "type": "object",
+            "properties": {
+                "obj": {
+                    "type": "object",
+                    "additionalProperties": True,
+                    "properties": {"a": {"type": "string"}},
+                },
+                "rows": {
+                    "type": "array",
+                    "items": {
+                        "type": "object",
+                        "additionalProperties": True,
+                        "properties": {"b": {"type": "string"}},
+                    },
+                },
+            },
+        }
+
+        result = AnthropicConfig.filter_anthropic_output_schema(schema)
+
+        assert result["properties"]["obj"]["additionalProperties"] is False
+        assert result["properties"]["rows"]["items"]["additionalProperties"] is False
+
+    def test_coerces_additional_properties_sub_schema(self):
+        """A sub-schema value (free-form map) is also rejected by Anthropic."""
+        schema = {
+            "type": "object",
+            "additionalProperties": {"type": "string"},
+            "properties": {"a": {"type": "string"}},
+        }
+
+        result = AnthropicConfig.filter_anthropic_output_schema(schema)
+
+        assert result["additionalProperties"] is False
+
+    def test_explicit_additional_properties_false_is_preserved(self):
+        """The already-correct value must survive untouched."""
+        schema = {
+            "type": "object",
+            "additionalProperties": False,
+            "properties": {"a": {"type": "string"}},
+        }
+
+        result = AnthropicConfig.filter_anthropic_output_schema(schema)
+
+        assert result["additionalProperties"] is False