diff --git a/litellm/llms/anthropic/common_utils.py b/litellm/llms/anthropic/common_utils.py
index 3297aa95715..9461e40cf2e 100644
--- a/litellm/llms/anthropic/common_utils.py
+++ b/litellm/llms/anthropic/common_utils.py
@@ -440,6 +440,16 @@ class AnthropicModelInfo(BaseLLMModelInfo):
         """
         return AnthropicModelInfo._supports_model_capability(model, "thinking_always_on", custom_llm_provider)
 
+    @staticmethod
+    def _supports_legacy_thinking(model: str, custom_llm_provider: str) -> bool:
+        """Whether ``model`` is an adaptive-thinking model that still accepts legacy
+        ``thinking.type=enabled`` with ``budget_tokens`` (the Claude 4.6 family).
+        The model cost map is authoritative: an explicit ``supports_legacy_thinking``
+        entry resolved under ``custom_llm_provider``, or a ``fallback_generalizations``
+        rule for unmapped 4.6 ids. Absent flag means the model rejects the legacy shape.
+        """
+        return AnthropicModelInfo._supports_model_capability(model, "supports_legacy_thinking", custom_llm_provider)
+
     @staticmethod
     def maybe_drop_disabled_thinking(
         model: str,
diff --git a/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py b/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py
index adabfa2d62d..032bf0130ce 100644
--- a/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py
+++ b/litellm/llms/anthropic/experimental_pass_through/messages/transformation.py
@@ -379,13 +379,19 @@ class AnthropicMessagesConfig(BaseAnthropicMessagesConfig):
     def _translate_legacy_thinking_for_adaptive_model(
         model: str, optional_params: dict, custom_llm_provider: str
     ) -> None:
-        """Translate legacy ``thinking.type=enabled`` to adaptive for 4.6/4.7.
-        Caller-provided ``output_config.effort`` is never overridden.
+        """Translate legacy ``thinking.type=enabled`` to adaptive for the
+        adaptive-thinking models that reject it (4.7+ and the 5 families).
+        Models flagged ``supports_legacy_thinking`` (the 4.6 family) accept the
+        legacy shape natively, so it is forwarded verbatim and the caller's
+        ``budget_tokens`` cap keeps applying. Caller-provided
+        ``output_config.effort`` is never overridden.
         """
         from litellm.llms.anthropic.chat.transformation import AnthropicConfig
 
         if not AnthropicModelInfo._is_adaptive_thinking_model(model, custom_llm_provider):
             return
+        if AnthropicModelInfo._supports_legacy_thinking(model, custom_llm_provider):
+            return
         thinking: Final = optional_params.get("thinking")
         if not isinstance(thinking, dict) or thinking.get("type") != "enabled":
             return
diff --git a/litellm/model_prices_and_context_window_backup.json b/litellm/model_prices_and_context_window_backup.json
index 3af7d9e5019..dda327bf9f6 100644
--- a/litellm/model_prices_and_context_window_backup.json
+++ b/litellm/model_prices_and_context_window_backup.json
@@ -1019,6 +1019,7 @@
     },
     "anthropic.claude-opus-4-6-v1": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_creation_input_token_cost_above_1hr": 1e-05,
         "cache_read_input_token_cost": 5e-07,
@@ -1053,6 +1054,7 @@
     },
     "global.anthropic.claude-opus-4-6-v1": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_creation_input_token_cost_above_1hr": 1e-05,
         "cache_read_input_token_cost": 5e-07,
@@ -1087,6 +1089,7 @@
     },
     "us.anthropic.claude-opus-4-6-v1": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.875e-06,
         "cache_creation_input_token_cost_above_1hr": 1.1e-05,
         "cache_read_input_token_cost": 5.5e-07,
@@ -1121,6 +1124,7 @@
     },
     "eu.anthropic.claude-opus-4-6-v1": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.875e-06,
         "cache_creation_input_token_cost_above_1hr": 1.1e-05,
         "cache_read_input_token_cost": 5.5e-07,
@@ -1155,6 +1159,7 @@
     },
     "au.anthropic.claude-opus-4-6-v1": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.875e-06,
         "cache_creation_input_token_cost_above_1hr": 1.1e-05,
         "cache_read_input_token_cost": 5.5e-07,
@@ -2233,6 +2238,7 @@
     },
     "anthropic.claude-sonnet-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 3.75e-06,
         "cache_creation_input_token_cost_above_1hr": 6e-06,
         "cache_read_input_token_cost": 3e-07,
@@ -2266,6 +2272,7 @@
     },
     "global.anthropic.claude-sonnet-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 3.75e-06,
         "cache_creation_input_token_cost_above_1hr": 6e-06,
         "cache_read_input_token_cost": 3e-07,
@@ -2299,6 +2306,7 @@
     },
     "us.anthropic.claude-sonnet-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 4.125e-06,
         "cache_creation_input_token_cost_above_1hr": 6.6e-06,
         "cache_read_input_token_cost": 3.3e-07,
@@ -2332,6 +2340,7 @@
     },
     "eu.anthropic.claude-sonnet-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 4.125e-06,
         "cache_creation_input_token_cost_above_1hr": 6.6e-06,
         "cache_read_input_token_cost": 3.3e-07,
@@ -2365,6 +2374,7 @@
     },
     "au.anthropic.claude-sonnet-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 4.125e-06,
         "cache_creation_input_token_cost_above_1hr": 6.6e-06,
         "cache_read_input_token_cost": 3.3e-07,
@@ -2398,6 +2408,7 @@
     },
     "jp.anthropic.claude-sonnet-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 4.125e-06,
         "cache_creation_input_token_cost_above_1hr": 6.6e-06,
         "cache_read_input_token_cost": 3.3e-07,
@@ -2950,6 +2961,7 @@
     "azure_ai/claude-opus-4-6": {
         "deprecation_date": "2027-02-02",
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "input_cost_per_token": 5e-06,
         "output_cost_per_token": 2.5e-05,
         "litellm_provider": "azure_ai",
@@ -3181,6 +3193,7 @@
     "azure_ai/claude-sonnet-4-6": {
         "deprecation_date": "2027-02-10",
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 3.75e-06,
         "cache_creation_input_token_cost_above_1hr": 6e-06,
         "cache_read_input_token_cost": 3e-07,
@@ -12489,6 +12502,7 @@
             "search_context_size_medium": 0.01
         },
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "supports_assistant_prefill": true,
         "supports_computer_use": true,
         "supports_function_calling": true,
@@ -12698,6 +12712,7 @@
             "search_context_size_medium": 0.01
         },
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "supports_assistant_prefill": false,
         "supports_computer_use": true,
         "supports_function_calling": true,
@@ -12735,6 +12750,7 @@
             "search_context_size_medium": 0.01
         },
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "supports_assistant_prefill": false,
         "supports_computer_use": true,
         "supports_function_calling": true,
@@ -14677,6 +14693,7 @@
         "source": "https://www.databricks.com/product/pricing/proprietary-foundation-model-serving",
         "supports_assistant_prefill": true,
         "supports_function_calling": true,
+        "supports_legacy_thinking": true,
         "supports_reasoning": true,
         "supports_tool_choice": true
     },
@@ -14753,6 +14770,7 @@
         "source": "https://www.databricks.com/product/pricing/proprietary-foundation-model-serving",
         "supports_assistant_prefill": true,
         "supports_function_calling": true,
+        "supports_legacy_thinking": true,
         "supports_reasoning": true,
         "supports_tool_choice": true
     },
@@ -23180,6 +23198,7 @@
     },
     "github_copilot/claude-opus-4.6-fast": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "litellm_provider": "github_copilot",
         "max_input_tokens": 128000,
         "max_output_tokens": 16000,
@@ -33563,6 +33582,7 @@
     },
     "openrouter/anthropic/claude-sonnet-4.6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 3.75e-06,
         "cache_creation_input_token_cost_above_200k_tokens": 7.5e-06,
         "cache_read_input_token_cost": 3e-07,
@@ -33607,6 +33627,7 @@
     },
     "openrouter/anthropic/claude-opus-4.6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_read_input_token_cost": 5e-07,
         "input_cost_per_token": 5e-06,
@@ -35681,6 +35702,7 @@
     },
     "perplexity/anthropic/claude-opus-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "litellm_provider": "perplexity",
         "mode": "responses",
         "supports_web_search": true,
@@ -39056,6 +39078,7 @@
     },
     "vercel_ai_gateway/anthropic/claude-opus-4.6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_read_input_token_cost": 5e-07,
         "input_cost_per_token": 5e-06,
@@ -40315,6 +40338,7 @@
         "deprecation_date": "2027-02-05",
         "regional_endpoint_uplift_multiplier": 1.1,
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_creation_input_token_cost_above_1hr": 1e-05,
         "cache_read_input_token_cost": 5e-07,
@@ -40347,6 +40371,7 @@
         "deprecation_date": "2027-02-05",
         "regional_endpoint_uplift_multiplier": 1.1,
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_creation_input_token_cost_above_1hr": 1e-05,
         "cache_read_input_token_cost": 5e-07,
@@ -40712,6 +40737,7 @@
     "vertex_ai/claude-sonnet-4-6": {
         "regional_endpoint_uplift_multiplier": 1.1,
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 3.75e-06,
         "cache_creation_input_token_cost_above_1hr": 6e-06,
         "cache_read_input_token_cost": 3e-07,
@@ -48481,6 +48507,7 @@
     "vertex_ai/claude-sonnet-4-6@default": {
         "regional_endpoint_uplift_multiplier": 1.1,
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 3.75e-06,
         "cache_creation_input_token_cost_above_1hr": 6e-06,
         "cache_read_input_token_cost": 3e-07,
@@ -49265,6 +49292,7 @@
     },
     "snowflake/claude-sonnet-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "max_tokens": 16384,
         "max_input_tokens": 200000,
         "max_output_tokens": 16384,
@@ -50255,6 +50283,14 @@
                     "supports_adaptive_thinking": true
                 }
             },
+            {
+                "name": "claude-legacy-thinking",
+                "pattern": "claude-[a-z]+-4[-._]6(?!\\d)",
+                "description": "Claude at version 4.6 exactly, in any id shape that contains claude-<family>-4-6 (dotted and underscored minors included, dated releases such as claude-sonnet-4-6-20260219 too). The 4.6 family is adaptive-thinking yet still accepts legacy thinking.type=enabled with budget_tokens, so the caller's hard budget cap is forwarded verbatim instead of being rewritten to an uncapped output_config.effort. The lookahead keeps two-digit minors such as 4-60 from matching. 4.7+ and 5+ majors reject the legacy shape and stay on the adaptive translation.",
+                "model_info": {
+                    "supports_legacy_thinking": true
+                }
+            },
             {
                 "name": "claude-always-on-thinking",
                 "pattern": "claude-(?:fable|mythos)-",
diff --git a/litellm/types/utils.py b/litellm/types/utils.py
index 94526de0757..e8e8edb85b3 100644
--- a/litellm/types/utils.py
+++ b/litellm/types/utils.py
@@ -154,6 +154,7 @@ class ProviderSpecificModelInfo(TypedDict, total=False):
     supports_web_search: bool | None
     supports_reasoning: bool | None
     supports_adaptive_thinking: bool | None
+    supports_legacy_thinking: ReadOnly[bool | None]
     thinking_always_on: ReadOnly[bool | None]
     supports_tool_search: bool | None
     supports_mid_conversation_system: bool | None
diff --git a/litellm/utils.py b/litellm/utils.py
index e5ce7157e77..c0e26ef19d4 100644
--- a/litellm/utils.py
+++ b/litellm/utils.py
@@ -5753,6 +5753,7 @@ def _get_model_info_helper(
                 supports_url_context=_model_info.get("supports_url_context", None),
                 supports_reasoning=_model_info.get("supports_reasoning", None),
                 supports_adaptive_thinking=_model_info.get("supports_adaptive_thinking", None),
+                supports_legacy_thinking=_model_info.get("supports_legacy_thinking", None),
                 thinking_always_on=_model_info.get("thinking_always_on", None),
                 supports_tool_search=_model_info.get("supports_tool_search", None),
                 supports_mid_conversation_system=_model_info.get("supports_mid_conversation_system", None),
diff --git a/model_prices_and_context_window.json b/model_prices_and_context_window.json
index 3af7d9e5019..dda327bf9f6 100644
--- a/model_prices_and_context_window.json
+++ b/model_prices_and_context_window.json
@@ -1019,6 +1019,7 @@
     },
     "anthropic.claude-opus-4-6-v1": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_creation_input_token_cost_above_1hr": 1e-05,
         "cache_read_input_token_cost": 5e-07,
@@ -1053,6 +1054,7 @@
     },
     "global.anthropic.claude-opus-4-6-v1": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_creation_input_token_cost_above_1hr": 1e-05,
         "cache_read_input_token_cost": 5e-07,
@@ -1087,6 +1089,7 @@
     },
     "us.anthropic.claude-opus-4-6-v1": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.875e-06,
         "cache_creation_input_token_cost_above_1hr": 1.1e-05,
         "cache_read_input_token_cost": 5.5e-07,
@@ -1121,6 +1124,7 @@
     },
     "eu.anthropic.claude-opus-4-6-v1": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.875e-06,
         "cache_creation_input_token_cost_above_1hr": 1.1e-05,
         "cache_read_input_token_cost": 5.5e-07,
@@ -1155,6 +1159,7 @@
     },
     "au.anthropic.claude-opus-4-6-v1": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.875e-06,
         "cache_creation_input_token_cost_above_1hr": 1.1e-05,
         "cache_read_input_token_cost": 5.5e-07,
@@ -2233,6 +2238,7 @@
     },
     "anthropic.claude-sonnet-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 3.75e-06,
         "cache_creation_input_token_cost_above_1hr": 6e-06,
         "cache_read_input_token_cost": 3e-07,
@@ -2266,6 +2272,7 @@
     },
     "global.anthropic.claude-sonnet-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 3.75e-06,
         "cache_creation_input_token_cost_above_1hr": 6e-06,
         "cache_read_input_token_cost": 3e-07,
@@ -2299,6 +2306,7 @@
     },
     "us.anthropic.claude-sonnet-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 4.125e-06,
         "cache_creation_input_token_cost_above_1hr": 6.6e-06,
         "cache_read_input_token_cost": 3.3e-07,
@@ -2332,6 +2340,7 @@
     },
     "eu.anthropic.claude-sonnet-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 4.125e-06,
         "cache_creation_input_token_cost_above_1hr": 6.6e-06,
         "cache_read_input_token_cost": 3.3e-07,
@@ -2365,6 +2374,7 @@
     },
     "au.anthropic.claude-sonnet-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 4.125e-06,
         "cache_creation_input_token_cost_above_1hr": 6.6e-06,
         "cache_read_input_token_cost": 3.3e-07,
@@ -2398,6 +2408,7 @@
     },
     "jp.anthropic.claude-sonnet-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 4.125e-06,
         "cache_creation_input_token_cost_above_1hr": 6.6e-06,
         "cache_read_input_token_cost": 3.3e-07,
@@ -2950,6 +2961,7 @@
     "azure_ai/claude-opus-4-6": {
         "deprecation_date": "2027-02-02",
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "input_cost_per_token": 5e-06,
         "output_cost_per_token": 2.5e-05,
         "litellm_provider": "azure_ai",
@@ -3181,6 +3193,7 @@
     "azure_ai/claude-sonnet-4-6": {
         "deprecation_date": "2027-02-10",
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 3.75e-06,
         "cache_creation_input_token_cost_above_1hr": 6e-06,
         "cache_read_input_token_cost": 3e-07,
@@ -12489,6 +12502,7 @@
             "search_context_size_medium": 0.01
         },
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "supports_assistant_prefill": true,
         "supports_computer_use": true,
         "supports_function_calling": true,
@@ -12698,6 +12712,7 @@
             "search_context_size_medium": 0.01
         },
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "supports_assistant_prefill": false,
         "supports_computer_use": true,
         "supports_function_calling": true,
@@ -12735,6 +12750,7 @@
             "search_context_size_medium": 0.01
         },
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "supports_assistant_prefill": false,
         "supports_computer_use": true,
         "supports_function_calling": true,
@@ -14677,6 +14693,7 @@
         "source": "https://www.databricks.com/product/pricing/proprietary-foundation-model-serving",
         "supports_assistant_prefill": true,
         "supports_function_calling": true,
+        "supports_legacy_thinking": true,
         "supports_reasoning": true,
         "supports_tool_choice": true
     },
@@ -14753,6 +14770,7 @@
         "source": "https://www.databricks.com/product/pricing/proprietary-foundation-model-serving",
         "supports_assistant_prefill": true,
         "supports_function_calling": true,
+        "supports_legacy_thinking": true,
         "supports_reasoning": true,
         "supports_tool_choice": true
     },
@@ -23180,6 +23198,7 @@
     },
     "github_copilot/claude-opus-4.6-fast": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "litellm_provider": "github_copilot",
         "max_input_tokens": 128000,
         "max_output_tokens": 16000,
@@ -33563,6 +33582,7 @@
     },
     "openrouter/anthropic/claude-sonnet-4.6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 3.75e-06,
         "cache_creation_input_token_cost_above_200k_tokens": 7.5e-06,
         "cache_read_input_token_cost": 3e-07,
@@ -33607,6 +33627,7 @@
     },
     "openrouter/anthropic/claude-opus-4.6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_read_input_token_cost": 5e-07,
         "input_cost_per_token": 5e-06,
@@ -35681,6 +35702,7 @@
     },
     "perplexity/anthropic/claude-opus-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "litellm_provider": "perplexity",
         "mode": "responses",
         "supports_web_search": true,
@@ -39056,6 +39078,7 @@
     },
     "vercel_ai_gateway/anthropic/claude-opus-4.6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_read_input_token_cost": 5e-07,
         "input_cost_per_token": 5e-06,
@@ -40315,6 +40338,7 @@
         "deprecation_date": "2027-02-05",
         "regional_endpoint_uplift_multiplier": 1.1,
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_creation_input_token_cost_above_1hr": 1e-05,
         "cache_read_input_token_cost": 5e-07,
@@ -40347,6 +40371,7 @@
         "deprecation_date": "2027-02-05",
         "regional_endpoint_uplift_multiplier": 1.1,
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 6.25e-06,
         "cache_creation_input_token_cost_above_1hr": 1e-05,
         "cache_read_input_token_cost": 5e-07,
@@ -40712,6 +40737,7 @@
     "vertex_ai/claude-sonnet-4-6": {
         "regional_endpoint_uplift_multiplier": 1.1,
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 3.75e-06,
         "cache_creation_input_token_cost_above_1hr": 6e-06,
         "cache_read_input_token_cost": 3e-07,
@@ -48481,6 +48507,7 @@
     "vertex_ai/claude-sonnet-4-6@default": {
         "regional_endpoint_uplift_multiplier": 1.1,
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "cache_creation_input_token_cost": 3.75e-06,
         "cache_creation_input_token_cost_above_1hr": 6e-06,
         "cache_read_input_token_cost": 3e-07,
@@ -49265,6 +49292,7 @@
     },
     "snowflake/claude-sonnet-4-6": {
         "supports_adaptive_thinking": true,
+        "supports_legacy_thinking": true,
         "max_tokens": 16384,
         "max_input_tokens": 200000,
         "max_output_tokens": 16384,
@@ -50255,6 +50283,14 @@
                     "supports_adaptive_thinking": true
                 }
             },
+            {
+                "name": "claude-legacy-thinking",
+                "pattern": "claude-[a-z]+-4[-._]6(?!\\d)",
+                "description": "Claude at version 4.6 exactly, in any id shape that contains claude-<family>-4-6 (dotted and underscored minors included, dated releases such as claude-sonnet-4-6-20260219 too). The 4.6 family is adaptive-thinking yet still accepts legacy thinking.type=enabled with budget_tokens, so the caller's hard budget cap is forwarded verbatim instead of being rewritten to an uncapped output_config.effort. The lookahead keeps two-digit minors such as 4-60 from matching. 4.7+ and 5+ majors reject the legacy shape and stay on the adaptive translation.",
+                "model_info": {
+                    "supports_legacy_thinking": true
+                }
+            },
             {
                 "name": "claude-always-on-thinking",
                 "pattern": "claude-(?:fable|mythos)-",
diff --git a/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_reasoning_effort_translation.py b/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_reasoning_effort_translation.py
index f393a7b50b1..12ab536ed45 100644
--- a/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_reasoning_effort_translation.py
+++ b/tests/test_litellm/llms/anthropic/experimental_pass_through/messages/test_reasoning_effort_translation.py
@@ -2,7 +2,6 @@
 
 import pytest
 
-import litellm
 from litellm.constants import (
     DEFAULT_REASONING_EFFORT_HIGH_THINKING_BUDGET,
     DEFAULT_REASONING_EFFORT_MEDIUM_THINKING_BUDGET,
@@ -17,7 +16,6 @@ from litellm.llms.bedrock.messages.invoke_transformations.anthropic_claude3_tran
 )
 
 
-
 @pytest.mark.parametrize(
     "reasoning_effort,expected_effort",
     [
@@ -258,19 +256,22 @@ def test_reasoning_effort_in_supported_params():
     "model",
     [
         "claude-sonnet-4-6",
-        "bedrock/invoke/us.anthropic.claude-sonnet-4-6",
-        "vertex_ai/claude-sonnet-4-6",
         "claude-opus-4-6",
+        "claude-sonnet-4-6-20260219",
+        "bedrock/invoke/us.anthropic.claude-sonnet-4-6",
         "bedrock/invoke/us.anthropic.claude-opus-4-6-v1:0",
+        "vertex_ai/claude-sonnet-4-6",
         "vertex_ai/claude-opus-4-6",
+        "azure_ai/claude-sonnet-4-6",
     ],
 )
-def test_legacy_thinking_high_budget_clamps_to_high_when_xhigh_unsupported(
-    local_model_cost_map, model
-):
-    """Claude Code sends ``thinking.budget_tokens=31999``; Sonnet 4.6 and Opus 4.6
-    have no ``xhigh`` tier, so the translator must emit ``high`` rather than the
-    provider-invalid ``xhigh`` (regression for issue #29282)."""
+def test_legacy_thinking_budget_preserved_verbatim_on_46(local_model_cost_map, model):
+    """Regression for the passthrough silently dropping a caller's hard thinking
+    budget: the 4.6 family accepts ``thinking.type=enabled`` with ``budget_tokens``
+    natively, so rewriting it to ``thinking.type=adaptive`` + ``output_config.effort``
+    (which carries no ceiling) let reasoning run past the requested cap. The legacy
+    shape must be forwarded verbatim, in every 4.6 id shape including unmapped dated
+    releases resolved by the ``claude-legacy-thinking`` fallback rule."""
     config = AnthropicMessagesConfig()
     optional_params = {
         "max_tokens": 1024,
@@ -285,8 +286,8 @@ def test_legacy_thinking_high_budget_clamps_to_high_when_xhigh_unsupported(
         headers={},
     )
 
-    assert result.get("thinking") == {"type": "adaptive"}
-    assert result.get("output_config") == {"effort": "high"}
+    assert result.get("thinking") == {"type": "enabled", "budget_tokens": 31999}
+    assert "output_config" not in result
 
 
 def test_legacy_thinking_high_budget_keeps_xhigh_when_supported():
@@ -343,11 +344,44 @@ def test_legacy_thinking_translates_to_adaptive_for_opus_48(
     assert result.get("output_config") == {"effort": "xhigh"}
 
 
+@pytest.mark.parametrize(
+    "model,expected_effort",
+    [
+        ("claude-sonnet-5", "xhigh"),
+        ("claude-opus-5", "xhigh"),
+        ("claude-newfamily-6", "high"),
+    ],
+)
+def test_legacy_thinking_translates_to_adaptive_for_5_and_future_models(
+    local_model_cost_map, model, expected_effort
+):
+    """The 5 families reject ``thinking.type=enabled``, so the adaptive translation
+    stays the safe default for every adaptive model not flagged
+    ``supports_legacy_thinking``, unmapped future ids included. An unmapped id
+    cannot prove ``xhigh`` support, so its high-budget bucket clamps to ``high``."""
+    config = AnthropicMessagesConfig()
+    optional_params = {
+        "max_tokens": 1024,
+        "thinking": {"type": "enabled", "budget_tokens": 31999},
+    }
+
+    result = config.transform_anthropic_messages_request(
+        model=model,
+        messages=[{"role": "user", "content": "Hello"}],
+        anthropic_messages_optional_request_params=optional_params,
+        litellm_params={},
+        headers={},
+    )
+
+    assert result.get("thinking") == {"type": "adaptive"}
+    assert result.get("output_config") == {"effort": expected_effort}
+
+
 @pytest.mark.parametrize(
     "budget_tokens,expected_effort",
     [
-        (DEFAULT_REASONING_EFFORT_XHIGH_THINKING_BUDGET * 2, "high"),
-        (DEFAULT_REASONING_EFFORT_XHIGH_THINKING_BUDGET, "high"),
+        (DEFAULT_REASONING_EFFORT_XHIGH_THINKING_BUDGET * 2, "xhigh"),
+        (DEFAULT_REASONING_EFFORT_XHIGH_THINKING_BUDGET, "xhigh"),
         (DEFAULT_REASONING_EFFORT_HIGH_THINKING_BUDGET, "high"),
         (DEFAULT_REASONING_EFFORT_HIGH_THINKING_BUDGET - 1, "medium"),
         (DEFAULT_REASONING_EFFORT_MEDIUM_THINKING_BUDGET, "medium"),
@@ -355,7 +389,9 @@ def test_legacy_thinking_translates_to_adaptive_for_opus_48(
         (1, "low"),
     ],
 )
-def test_legacy_thinking_budget_buckets_on_sonnet_46(budget_tokens, expected_effort):
+def test_legacy_thinking_budget_buckets_on_opus_48(
+    local_model_cost_map, budget_tokens, expected_effort
+):
     config = AnthropicMessagesConfig()
     optional_params = {
         "max_tokens": 1024,
@@ -363,7 +399,7 @@ def test_legacy_thinking_budget_buckets_on_sonnet_46(budget_tokens, expected_eff
     }
 
     result = config.transform_anthropic_messages_request(
-        model="claude-sonnet-4-6",
+        model="claude-opus-4-8",
         messages=[{"role": "user", "content": "Hello"}],
         anthropic_messages_optional_request_params=optional_params,
         litellm_params={},
@@ -373,7 +409,29 @@ def test_legacy_thinking_budget_buckets_on_sonnet_46(budget_tokens, expected_eff
     assert result.get("output_config") == {"effort": expected_effort}
 
 
-def test_legacy_thinking_does_not_override_explicit_output_config():
+def test_legacy_thinking_does_not_override_explicit_output_config(local_model_cost_map):
+    config = AnthropicMessagesConfig()
+    optional_params = {
+        "max_tokens": 1024,
+        "thinking": {"type": "enabled", "budget_tokens": 31999},
+        "output_config": {"effort": "low"},
+    }
+
+    result = config.transform_anthropic_messages_request(
+        model="claude-opus-4-8",
+        messages=[{"role": "user", "content": "Hello"}],
+        anthropic_messages_optional_request_params=optional_params,
+        litellm_params={},
+        headers={},
+    )
+
+    assert result.get("thinking") == {"type": "adaptive"}
+    assert result.get("output_config") == {"effort": "low"}
+
+
+def test_legacy_thinking_with_explicit_output_config_untouched_on_46(
+    local_model_cost_map,
+):
     config = AnthropicMessagesConfig()
     optional_params = {
         "max_tokens": 1024,
@@ -389,6 +447,7 @@ def test_legacy_thinking_does_not_override_explicit_output_config():
         headers={},
     )
 
+    assert result.get("thinking") == {"type": "enabled", "budget_tokens": 31999}
     assert result.get("output_config") == {"effort": "low"}