diff --git a/litellm/llms/anthropic/chat/transformation.py b/litellm/llms/anthropic/chat/transformation.py
index 9938cd7979b..732c13dcb60 100644
--- a/litellm/llms/anthropic/chat/transformation.py
+++ b/litellm/llms/anthropic/chat/transformation.py
@@ -1238,9 +1238,13 @@ def transform_request(
             output_config = optional_params.get("output_config")
             if output_config and isinstance(output_config, dict):
                 effort = output_config.get("effort")
-                if effort and effort not in ["high", "medium", "low"]:
+                if effort and effort not in ["high", "medium", "low", "max"]:
                     raise ValueError(
-                        f"Invalid effort value: {effort}. Must be one of: 'high', 'medium', 'low'"
+                        f"Invalid effort value: {effort}. Must be one of: 'high', 'medium', 'low', 'max'"
+                    )
+                if effort == "max" and not self._is_claude_opus_4_6(model):
+                    raise ValueError(
+                        f"effort='max' is only supported by Claude Opus 4.6. Got model: {model}"
                     )
                 data["output_config"] = output_config
 
diff --git a/tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py b/tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py
index 57e0dd494e0..50e948c1a27 100644
--- a/tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py
+++ b/tests/test_litellm/llms/anthropic/chat/test_anthropic_chat_transformation.py
@@ -1638,6 +1638,41 @@ def test_effort_with_claude_opus_45():
     assert result["model"] == "claude-opus-4-5-20251101"
 
 
+def test_effort_validation_with_opus_46():
+    """Test that all four effort levels are accepted for Claude Opus 4.6."""
+    config = AnthropicConfig()
+
+    messages = [{"role": "user", "content": "Test"}]
+
+    for effort in ["high", "medium", "low", "max"]:
+        optional_params = {"output_config": {"effort": effort}}
+        result = config.transform_request(
+            model="claude-opus-4-6-20260205",
+            messages=messages,
+            optional_params=optional_params,
+            litellm_params={},
+            headers={}
+        )
+        assert result["output_config"]["effort"] == effort
+
+
+def test_max_effort_rejected_for_opus_45():
+    """Test that effort='max' is rejected when using Claude Opus 4.5."""
+    config = AnthropicConfig()
+
+    messages = [{"role": "user", "content": "Test"}]
+
+    with pytest.raises(ValueError, match="effort='max' is only supported by Claude Opus 4.6"):
+        optional_params = {"output_config": {"effort": "max"}}
+        config.transform_request(
+            model="claude-opus-4-5-20251101",
+            messages=messages,
+            optional_params=optional_params,
+            litellm_params={},
+            headers={}
+        )
+
+
 def test_effort_with_other_features():
     """Test effort works alongside other features (thinking, tools)."""
     config = AnthropicConfig()