diff --git a/litellm/llms/anthropic/messages/responses_adapters/transformation.py b/litellm/llms/anthropic/messages/responses_adapters/transformation.py
index 110e9fc40ff..b4455f1a4e2 100644
--- a/litellm/llms/anthropic/messages/responses_adapters/transformation.py
+++ b/litellm/llms/anthropic/messages/responses_adapters/transformation.py
@@ -584,6 +584,12 @@ class LiteLLMAnthropicToResponsesAPIAdapter:
             if block.get("type") == "text" and isinstance(text := block.get("text"), str) and text
         )
 
+    @staticmethod
+    def _resolve_output_format_strict(strict: bool | None) -> bool:
+        if strict is None:
+            return True
+        return strict
+
     def translate_request(
         self,
         anthropic_request: AnthropicMessagesRequest,
@@ -693,7 +699,7 @@ class LiteLLMAnthropicToResponsesAPIAdapter:
                         "type": "json_schema",
                         "name": "structured_output",
                         "schema": schema,
-                        "strict": output_format.get("strict", False),
+                        "strict": self._resolve_output_format_strict(output_format.get("strict")),
                     }
                 }
 
diff --git a/tests/unit/llms/anthropic/messages/responses_adapters/test_responses_adapters_transformation.py b/tests/unit/llms/anthropic/messages/responses_adapters/test_responses_adapters_transformation.py
index dbb8c925fa0..660f00cd722 100644
--- a/tests/unit/llms/anthropic/messages/responses_adapters/test_responses_adapters_transformation.py
+++ b/tests/unit/llms/anthropic/messages/responses_adapters/test_responses_adapters_transformation.py
@@ -5,7 +5,8 @@ Tests for LiteLLMAnthropicToResponsesAPIAdapter
 
 import json
 import os
-from typing import Any, Dict, Final, List
+from collections.abc import Mapping
+from typing import Any, Dict, Final, List, Literal
 from unittest.mock import MagicMock
 
 import pytest
@@ -24,6 +25,9 @@ from litellm.litellm_core_utils.prompt_templates.common_utils import (
 from litellm.litellm_core_utils.prompt_templates.reasoning_utils import (
     encrypted_reasoning_signature,
 )
+from litellm.llms.anthropic.messages.adapters.transformation import (
+    LiteLLMAnthropicMessagesAdapter,
+)
 from litellm.llms.anthropic.messages.responses_adapters.transformation import (
     LiteLLMAnthropicToResponsesAPIAdapter,
 )
@@ -141,21 +145,23 @@ class TestOutputConfigStructuredOutput:
     }
 
     def test_output_config_format_json_schema_converted(self):
-        """output_config.format.json_schema is converted to OpenAI text.format, defaulting strict to False."""
+        """output_config.format.json_schema becomes strict OpenAI text.format."""
         req = _make_request(output_config={"format": {"type": "json_schema", "schema": self._SCHEMA}})
         kwargs = _ADAPTER.translate_request(req)
         assert "text" in kwargs
         fmt = kwargs["text"]["format"]
         assert fmt["type"] == "json_schema"
         assert fmt["schema"] == self._SCHEMA
-        assert fmt["strict"] is False
+        assert fmt["strict"] is True
         assert fmt["name"] == "structured_output"
 
-    def test_output_config_format_explicit_strict_true_is_preserved(self):
-        """Nested output_config.format with explicit strict=True is preserved."""
-        req = _make_request(output_config={"format": {"type": "json_schema", "schema": self._SCHEMA, "strict": True}})
-        kwargs = _ADAPTER.translate_request(req)
-        assert kwargs["text"]["format"]["strict"] is True
+    @pytest.mark.parametrize("strict", (True, False))
+    def test_output_config_format_explicit_strict_is_preserved(self, strict: bool) -> None:
+        req: Final = _make_request(
+            output_config={"format": {"type": "json_schema", "schema": self._SCHEMA, "strict": strict}}
+        )
+        kwargs: Final = _ADAPTER.translate_request(req)
+        assert kwargs["text"]["format"]["strict"] is strict
 
     def test_output_config_without_format_does_not_set_text(self):
         """output_config with only non-format keys doesn't produce text.format."""
@@ -164,12 +170,12 @@ class TestOutputConfigStructuredOutput:
         assert "text" not in kwargs
 
     def test_output_format_still_works(self):
-        """The original output_format field still takes precedence when present, defaulting strict to False."""
+        """The original output_format field still defaults to strict mode."""
         req = _make_request(output_format={"type": "json_schema", "schema": self._SCHEMA})
         kwargs = _ADAPTER.translate_request(req)
         assert "text" in kwargs
         assert kwargs["text"]["format"]["type"] == "json_schema"
-        assert kwargs["text"]["format"]["strict"] is False
+        assert kwargs["text"]["format"]["strict"] is True
 
     def test_output_format_explicit_strict_false_is_preserved(self):
         """output_format with an explicit strict=False is preserved as False."""
@@ -183,6 +189,23 @@ class TestOutputConfigStructuredOutput:
         kwargs = _ADAPTER.translate_request(req)
         assert kwargs["text"]["format"]["strict"] is True
 
+    @pytest.mark.parametrize("format_field", ("output_format", "output_config"))
+    @pytest.mark.parametrize("strict_options", ({}, {"strict": None}))
+    def test_default_strict_matches_messages_to_chat_adapter(
+        self,
+        format_field: Literal["output_format", "output_config"],
+        strict_options: Mapping[str, None],
+    ) -> None:
+        output_format: Final = {"type": "json_schema", "schema": self._SCHEMA, **strict_options}
+        request: Final = _make_request(
+            **{format_field: output_format if format_field == "output_format" else {"format": output_format}}
+        )
+        chat, _ = LiteLLMAnthropicMessagesAdapter().translate_anthropic_to_openai(request)
+        responses: Final = _ADAPTER.translate_request(request)
+
+        assert chat["response_format"]["json_schema"]["strict"] is True
+        assert responses["text"]["format"]["strict"] is chat["response_format"]["json_schema"]["strict"]
+
     def test_output_format_takes_precedence_over_output_config(self):
         """output_format takes precedence over output_config.format, for both schema and strict."""
         other_schema = {"type": "object", "properties": {"id": {"type": "integer"}}}
diff --git a/tests/unit/llms/openai/responses/count_tokens/test_token_counter.py b/tests/unit/llms/openai/responses/count_tokens/test_token_counter.py
index 29c25c5fdbf..35b544be31f 100644
--- a/tests/unit/llms/openai/responses/count_tokens/test_token_counter.py
+++ b/tests/unit/llms/openai/responses/count_tokens/test_token_counter.py
@@ -301,7 +301,7 @@ async def test_anthropic_format_counts_the_structured_output_schema(monkeypatch:
     assert sent == {
         "model": "gpt-4o",
         "input": [{"type": "message", "role": "user", "content": [{"type": "input_text", "text": "Answer in JSON."}]}],
-        "text": {"format": {"type": "json_schema", "name": "structured_output", "schema": schema, "strict": False}},
+        "text": {"format": {"type": "json_schema", "name": "structured_output", "schema": schema, "strict": True}},
     }