diff --git a/ci_cd/generate_model_prices_schema.py b/ci_cd/generate_model_prices_schema.py
index 5e1c4b0dcd9..57cc742d5c4 100644
--- a/ci_cd/generate_model_prices_schema.py
+++ b/ci_cd/generate_model_prices_schema.py
@@ -220,6 +220,15 @@ def string_key_schemas(modes: tuple) -> dict[str, JsonSchema]:
             "description": "Highest reasoning effort the Bedrock output_config accepts for this model.",
             "enum": ["low", "medium", "high", "max", "xhigh"],
         },
+        "default_reasoning_effort": {
+            "type": "string",
+            "description": (
+                "Reasoning effort the provider applies when the request omits reasoning_effort. "
+                "Gates whether a non-default temperature or the top_p/logprobs sampling params are "
+                "accepted, which hold only when the effort resolves to 'none'."
+            ),
+            "enum": ["none", "minimal", "low", "medium", "high", "xhigh"],
+        },
         "comment": STRING,
         "audio_transcription_config": STRING,
     }
diff --git a/litellm/llms/azure/chat/gpt_5_transformation.py b/litellm/llms/azure/chat/gpt_5_transformation.py
index d7584083327..6fdd277a04f 100644
--- a/litellm/llms/azure/chat/gpt_5_transformation.py
+++ b/litellm/llms/azure/chat/gpt_5_transformation.py
@@ -19,19 +19,19 @@ class AzureOpenAIGPT5Config(AzureOpenAIConfig, OpenAIGPT5Config):
     GPT5_SERIES_ROUTE = "gpt5_series/"
 
     @classmethod
-    def _supports_reasoning_effort_level(cls, model: str, level: str) -> bool:
-        """Override to handle gpt5_series/ prefix used for Azure routing.
+    def _model_map_lookup_name(cls, model: str) -> str:
+        """Normalise an Azure routing name to its cost-map key.
 
-        The parent class calls ``_supports_factory(model, custom_llm_provider=None)``
-        which fails to resolve ``gpt5_series/gpt-5.1`` to the correct Azure model
-        entry. Strip the prefix and prepend ``azure/`` so the lookup finds
-        ``azure/gpt-5.1`` in model_prices_and_context_window.json.
+        Neither ``gpt5_series/gpt-5.1`` nor a bare ``gpt-5.1`` is a key in
+        model_prices_and_context_window.json; ``azure/gpt-5.1`` is. Overriding the shared
+        resolver rather than one lookup means the supports, explicitly-disabled and
+        default-effort answers all read the same entry.
         """
         if model.startswith(cls.GPT5_SERIES_ROUTE):
-            model = "azure/" + model[len(cls.GPT5_SERIES_ROUTE) :]
-        elif not model.startswith("azure/"):
-            model = "azure/" + model
-        return super()._supports_reasoning_effort_level(model, level)
+            return "azure/" + model[len(cls.GPT5_SERIES_ROUTE) :]
+        if model.startswith("azure/"):
+            return model
+        return "azure/" + model
 
     @classmethod
     def is_model_gpt_5_model(cls, model: str) -> bool:
diff --git a/litellm/llms/openai/chat/gpt_5_transformation.py b/litellm/llms/openai/chat/gpt_5_transformation.py
index 3a65e4a9426..0223be300b0 100644
--- a/litellm/llms/openai/chat/gpt_5_transformation.py
+++ b/litellm/llms/openai/chat/gpt_5_transformation.py
@@ -6,11 +6,28 @@ import litellm
 from litellm.utils import (
     _is_explicitly_disabled_factory,
     _supports_factory,
+    declared_value_factory,
 )
 
 from .gpt_transformation import OpenAIGPTConfig
 
 
+def _catalogue_declares_default_effort() -> bool:
+    """Whether the loaded cost map carries default_reasoning_effort for ANY entry.
+
+    The map is fetched from the published branch at import time, so it can be OLDER than the
+    code reading it. On such a map every model looks undeclared, and treating that as "reasoning
+    is active" would silently strip temperature from the gpt-5.1/5.2/5.4 deployments that accept
+    it - a regression caused purely by data lag rather than by anything about the model.
+
+    So the absence of the key is only meaningful once the catalogue is known to carry it at all.
+    A map that has never heard of the key predates the feature, and the honest answer there is
+    the one litellm gave before it existed. Scanning costs ~80us on the largest published map and
+    only on the fallback path, which is noise beside the request it precedes.
+    """
+    return any(isinstance(entry, dict) and "default_reasoning_effort" in entry for entry in litellm.model_cost.values())
+
+
 def _normalize_reasoning_effort_for_chat_completion(
     value: str | dict | None,
 ) -> str | None:
@@ -114,6 +131,17 @@ class OpenAIGPT5Config(OpenAIGPTConfig):
         except (ValueError, IndexError):
             return False
 
+    @classmethod
+    def _model_map_lookup_name(cls, model: str) -> str:
+        """The name this model is looked up by in the cost map.
+
+        Identity here, because an OpenAI model name is already its map key. Azure overrides
+        it: its routing prefixes are not map keys, so every capability lookup has to
+        normalise the name the same way, and doing that in ONE place is what keeps the
+        supports/disabled/default answers from disagreeing about which entry they read.
+        """
+        return model
+
     @classmethod
     def _supports_reasoning_effort_level(cls, model: str, level: str) -> bool:
         """Check if the model supports a specific reasoning_effort level.
@@ -123,11 +151,40 @@ class OpenAIGPT5Config(OpenAIGPTConfig):
         Returns False for unknown models (safe fallback).
         """
         return _supports_factory(
-            model=model,
+            model=cls._model_map_lookup_name(model),
             custom_llm_provider=None,
             key=f"supports_{level}_reasoning_effort",
         )
 
+    @classmethod
+    def effort_resolves_to_none(cls, model: str, effective_effort: str | None) -> bool:
+        """Whether this request's reasoning effort ends up as "none", which is the single
+        condition under which the provider accepts a non-default temperature or the
+        top_p/logprobs sampling params.
+
+        An explicit reasoning_effort answers outright. When the request omits it the answer
+        is the model's DEFAULT effort, which only the map can state: supporting "none" is a
+        different fact from defaulting to it, and reading the former as the latter is what
+        forwarded temperature=0 to every gpt-5.5/5.6 deployment.
+
+        An undeclared default resolves to False. The map not saying is not the model
+        saying no, so the gate takes the conservative branch: a param the provider would
+        have rejected gets dropped or refused with an actionable error, and a model
+        released before its map entry declares a default needs no code change to be safe.
+        """
+        if effective_effort is not None:
+            return effective_effort == "none"
+        declared: Final = declared_value_factory(
+            model=cls._model_map_lookup_name(model),
+            custom_llm_provider=None,
+            key="default_reasoning_effort",
+        )
+        if declared is not None:
+            return declared == "none"
+        if not _catalogue_declares_default_effort():
+            return cls._supports_reasoning_effort_level(model, "none")
+        return False
+
     @classmethod
     def _is_reasoning_effort_level_explicitly_disabled(cls, model: str, level: str) -> bool:
         """Return True only when the model map explicitly sets the capability to False.
@@ -140,7 +197,7 @@ class OpenAIGPT5Config(OpenAIGPTConfig):
         Use this for opt-out checks where unknown models should be allowed through.
         """
         return _is_explicitly_disabled_factory(
-            model=model,
+            model=cls._model_map_lookup_name(model),
             custom_llm_provider=None,
             key=f"supports_{level}_reasoning_effort",
         )
@@ -260,15 +317,16 @@ class OpenAIGPT5Config(OpenAIGPTConfig):
         if supports_none:
             sampling_params: Final = ["logprobs", "top_logprobs", "top_p"]
             has_sampling: Final = any(p in non_default_params for p in sampling_params)
-            if has_sampling and effective_effort not in (None, "none"):
+            if has_sampling and not self.effort_resolves_to_none(model, effective_effort):
                 if litellm.drop_params or drop_params:
                     for p in sampling_params:
                         non_default_params.pop(p, None)
                 else:
                     raise litellm.utils.UnsupportedParamsError(
                         message=(
-                            "gpt-5.1/5.2/5.4 only support logprobs, top_p, top_logprobs when "
-                            f"reasoning_effort='none'. Current reasoning_effort='{effective_effort}'. "
+                            f"{model} only supports logprobs, top_p, top_logprobs when reasoning_effort "
+                            "resolves to 'none', either set explicitly on the request or declared as the "
+                            f"model's default_reasoning_effort. Current reasoning_effort={effective_effort!r}. "
                             "To drop unsupported params set `litellm.drop_params = True`"
                         ),
                         status_code=400,
@@ -277,17 +335,19 @@ class OpenAIGPT5Config(OpenAIGPTConfig):
         if "temperature" in non_default_params:
             temperature_value: Final[float | None] = non_default_params.pop("temperature")
             if temperature_value is not None:
-                # models supporting reasoning_effort="none" also support flexible temperature
-                if supports_none and (effective_effort == "none" or effective_effort is None) or temperature_value == 1:
+                # a non-default temperature rides on the effort resolving to "none", not on
+                # the model merely supporting it
+                if (supports_none and self.effort_resolves_to_none(model, effective_effort)) or temperature_value == 1:
                     optional_params["temperature"] = temperature_value
                 elif litellm.drop_params or drop_params:
                     pass
                 else:
                     raise litellm.utils.UnsupportedParamsError(
                         message=(
-                            f"gpt-5 models (including gpt-5-codex) don't support temperature={temperature_value}. "
-                            "Only temperature=1 is supported. "
-                            "For gpt-5.1, temperature is supported when reasoning_effort='none' (or not specified, as it defaults to 'none'). "
+                            f"{model} doesn't support temperature={temperature_value} while reasoning is "
+                            "active. Only temperature=1 is supported unless reasoning_effort resolves to "
+                            "'none', either set explicitly on the request or declared as the model's "
+                            "default_reasoning_effort. "
                             "To drop unsupported params set `litellm.drop_params = True`"
                         ),
                         status_code=400,
diff --git a/litellm/llms/openai/responses/transformation.py b/litellm/llms/openai/responses/transformation.py
index b2a69564908..2fa44cfc2e3 100644
--- a/litellm/llms/openai/responses/transformation.py
+++ b/litellm/llms/openai/responses/transformation.py
@@ -61,6 +61,20 @@ class OpenAIResponsesAPIConfig(BaseResponsesAPIConfig):
             key="supports_none_reasoning_effort",
         )
 
+    @staticmethod
+    def _effort_resolves_to_none(model: str, effort: str | None) -> bool:
+        """Whether this request's reasoning effort ends up as "none", the one condition
+        under which a non-default temperature is accepted.
+
+        Delegates to the chat-completions gpt-5 config so both surfaces answer from one
+        rule: the Responses API reaches the same models over a different wire, and a second
+        copy of the rule here is what let this surface keep forwarding temperature after the
+        chat surface stopped.
+        """
+        from litellm.llms.openai.chat.gpt_5_transformation import OpenAIGPT5Config
+
+        return OpenAIGPT5Config.effort_resolves_to_none(model, effort)
+
     @staticmethod
     def _enforce_min_max_output_tokens(max_output_tokens: "int | None") -> "int | None":
         """Raise sub-minimum max_output_tokens up to the OpenAI Responses API minimum.
@@ -116,17 +130,17 @@ class OpenAIResponsesAPIConfig(BaseResponsesAPIConfig):
                 reasoning: Final = params.get("reasoning") or {}
                 effort: Final = reasoning.get("effort") if isinstance(reasoning, dict) else None
                 supports_none: Final = self._supports_reasoning_effort_none(model=model)
-                if supports_none and (effort == "none" or effort is None):
+                if supports_none and self._effort_resolves_to_none(model, effort):
                     pass  # flexible temperature allowed
                 elif drop_params or litellm.drop_params:
                     params.pop("temperature", None)
                 else:
                     raise litellm.UnsupportedParamsError(
                         message=(
-                            f"gpt-5 models don't support temperature={temperature}. "
-                            "Only temperature=1 is supported. "
-                            "For models like gpt-5.1/5.4, temperature is supported "
-                            "when reasoning.effort='none' (or not specified). "
+                            f"{model} doesn't support temperature={temperature} while reasoning is "
+                            "active. Only temperature=1 is supported unless reasoning.effort resolves "
+                            "to 'none', either set explicitly on the request or declared as the "
+                            "model's default_reasoning_effort. "
                             "To drop unsupported params set `litellm.drop_params = True`"
                         ),
                         status_code=400,
diff --git a/litellm/model_prices_and_context_window_backup.json b/litellm/model_prices_and_context_window_backup.json
index 77572f69b8b..895cc6c884f 100644
--- a/litellm/model_prices_and_context_window_backup.json
+++ b/litellm/model_prices_and_context_window_backup.json
@@ -3409,6 +3409,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -3456,6 +3457,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -3589,6 +3591,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
@@ -3630,6 +3633,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
@@ -3671,6 +3675,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
@@ -3712,6 +3717,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
@@ -3937,7 +3943,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/eu/gpt-5.1-chat": {
         "cache_read_input_token_cost": 1.4e-07,
@@ -3972,7 +3979,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/eu/gpt-5.1-codex": {
         "deprecation_date": "2027-05-15",
@@ -4247,7 +4255,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/global/gpt-5.1-chat": {
         "cache_read_input_token_cost": 1.25e-07,
@@ -4282,7 +4291,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/global/gpt-5.1-codex": {
         "deprecation_date": "2027-05-15",
@@ -5367,6 +5377,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_minimal_reasoning_effort": true
     },
     "azure/gpt-5.1-chat-2025-11-13": {
@@ -5404,7 +5415,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": false,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/gpt-5.1-codex-2025-11-13": {
         "cache_read_input_token_cost": 1.25e-07,
@@ -5833,7 +5845,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/gpt-5.1-chat": {
         "cache_read_input_token_cost": 1.25e-07,
@@ -5868,7 +5881,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/gpt-5.1-codex": {
         "deprecation_date": "2027-05-15",
@@ -6315,6 +6329,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -6354,6 +6369,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -6393,6 +6409,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -6438,6 +6455,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -6477,6 +6495,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -6516,6 +6535,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -7663,6 +7683,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true
     },
     "azure/gpt-5.4-mini-2026-03-17": {
@@ -7704,6 +7725,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true
     },
     "azure/gpt-5.4-nano": {
@@ -7745,6 +7767,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true
     },
     "azure/gpt-5.4-nano-2026-03-17": {
@@ -7786,6 +7809,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true
     },
     "azure/gpt-image-1": {
@@ -8856,7 +8880,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/us/gpt-5.1-chat": {
         "cache_read_input_token_cost": 1.4e-07,
@@ -8891,7 +8916,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/us/gpt-5.1-codex": {
         "deprecation_date": "2027-05-15",
@@ -26292,6 +26318,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": false,
         "supports_minimal_reasoning_effort": true
     },
@@ -26336,6 +26363,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": false,
         "supports_minimal_reasoning_effort": true
     },
@@ -26381,6 +26409,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": false,
         "supports_minimal_reasoning_effort": true
     },
@@ -26426,6 +26455,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -26471,6 +26501,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -27295,6 +27326,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -27343,6 +27375,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -27492,6 +27525,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
@@ -27543,6 +27577,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
@@ -27591,6 +27626,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
@@ -27639,6 +27675,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
diff --git a/litellm/types/utils.py b/litellm/types/utils.py
index 95429e899c9..9b2f20c2259 100644
--- a/litellm/types/utils.py
+++ b/litellm/types/utils.py
@@ -165,6 +165,7 @@ class ProviderSpecificModelInfo(TypedDict, total=False):
     supports_xhigh_reasoning_effort: bool | None
     supports_max_reasoning_effort: bool | None
     reasoning_effort_levels: ReadOnly[Sequence[str] | None]
+    default_reasoning_effort: ReadOnly[Literal["none", "minimal", "low", "medium", "high", "xhigh"] | None]
     supports_output_config: bool | None
     supports_image_size: bool | None
     bedrock_output_config_effort_ceiling: Literal["low", "medium", "high", "max", "xhigh"] | None
diff --git a/litellm/utils.py b/litellm/utils.py
index 520c40f67c0..37c7098813e 100644
--- a/litellm/utils.py
+++ b/litellm/utils.py
@@ -2596,6 +2596,46 @@ def _supports_factory(model: str, custom_llm_provider: str | None, key: str) ->
         return False
 
 
+def declared_value_factory(model: str, custom_llm_provider: str | None, key: str) -> str | None:
+    """Return a string value the model map declares for *key*, or ``None`` when it says nothing.
+
+    The string-valued sibling of :func:`_supports_factory` and
+    :func:`_is_explicitly_disabled_factory`, public where those two are not because it is read
+    from the provider configs rather than from this module, sharing their
+    ``get_llm_provider`` -> ``_get_model_info_helper`` chain and their unprefixed-twin
+    fallback (#20885), so a provider-prefixed entry that omits the key still answers
+    from the bare entry that carries it.
+
+    ``None`` means "the map does not say", never "the map says no" - callers decide what
+    an unknown declaration implies, and for a capability gate that decision must be the
+    conservative one.
+    """
+    try:
+        resolved: Final = litellm.get_llm_provider(model=model, custom_llm_provider=custom_llm_provider)
+        resolved_model: Final = resolved[0]
+        resolved_provider: Final = resolved[1]
+        model_info: Final = _get_model_info_helper(model=resolved_model, custom_llm_provider=resolved_provider)
+        declared: Final = model_info.get(key)
+        if isinstance(declared, str):
+            return declared
+        bare_model_key: Final = _get_model_cost_key(resolved_model)
+        bare_entry: Final = litellm.model_cost.get(bare_model_key) if bare_model_key is not None else None
+        if isinstance(bare_entry, dict):
+            bare_declared: Final = bare_entry.get(key)
+            if isinstance(bare_declared, str):
+                return bare_declared
+        return None
+    except Exception as e:  # noqa: BLE001  # an unreadable map entry means "not declared", never a failed call
+        verbose_logger.debug(
+            "Model not found or error in reading %s. You passed model=%s, custom_llm_provider=%s. Error: %s",
+            key,
+            model,
+            custom_llm_provider,
+            e,
+        )
+        return None
+
+
 def _is_explicitly_disabled_factory(model: str, custom_llm_provider: str | None, key: str) -> bool:
     """Return True only when the model map explicitly sets *key* to ``False``.
 
@@ -5890,6 +5930,7 @@ def _get_model_info_helper(
                 supports_xhigh_reasoning_effort=_model_info.get("supports_xhigh_reasoning_effort", None),
                 supports_max_reasoning_effort=_model_info.get("supports_max_reasoning_effort", None),
                 reasoning_effort_levels=_model_info.get("reasoning_effort_levels", None),
+                default_reasoning_effort=_model_info.get("default_reasoning_effort", None),
                 bedrock_output_config_effort_ceiling=_model_info.get("bedrock_output_config_effort_ceiling", None),
                 bedrock_converse_supports_strict_tools=_model_info.get("bedrock_converse_supports_strict_tools", None),
                 supports_computer_use=_model_info.get("supports_computer_use", None),
diff --git a/model_prices_and_context_window.json b/model_prices_and_context_window.json
index 77572f69b8b..895cc6c884f 100644
--- a/model_prices_and_context_window.json
+++ b/model_prices_and_context_window.json
@@ -3409,6 +3409,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -3456,6 +3457,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -3589,6 +3591,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
@@ -3630,6 +3633,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
@@ -3671,6 +3675,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
@@ -3712,6 +3717,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
@@ -3937,7 +3943,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/eu/gpt-5.1-chat": {
         "cache_read_input_token_cost": 1.4e-07,
@@ -3972,7 +3979,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/eu/gpt-5.1-codex": {
         "deprecation_date": "2027-05-15",
@@ -4247,7 +4255,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/global/gpt-5.1-chat": {
         "cache_read_input_token_cost": 1.25e-07,
@@ -4282,7 +4291,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/global/gpt-5.1-codex": {
         "deprecation_date": "2027-05-15",
@@ -5367,6 +5377,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_minimal_reasoning_effort": true
     },
     "azure/gpt-5.1-chat-2025-11-13": {
@@ -5404,7 +5415,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": false,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/gpt-5.1-codex-2025-11-13": {
         "cache_read_input_token_cost": 1.25e-07,
@@ -5833,7 +5845,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/gpt-5.1-chat": {
         "cache_read_input_token_cost": 1.25e-07,
@@ -5868,7 +5881,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/gpt-5.1-codex": {
         "deprecation_date": "2027-05-15",
@@ -6315,6 +6329,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -6354,6 +6369,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -6393,6 +6409,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -6438,6 +6455,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -6477,6 +6495,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -6516,6 +6535,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -7663,6 +7683,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true
     },
     "azure/gpt-5.4-mini-2026-03-17": {
@@ -7704,6 +7725,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true
     },
     "azure/gpt-5.4-nano": {
@@ -7745,6 +7767,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true
     },
     "azure/gpt-5.4-nano-2026-03-17": {
@@ -7786,6 +7809,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true
     },
     "azure/gpt-image-1": {
@@ -8856,7 +8880,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/us/gpt-5.1-chat": {
         "cache_read_input_token_cost": 1.4e-07,
@@ -8891,7 +8916,8 @@
         "supports_system_messages": true,
         "supports_tool_choice": true,
         "supports_vision": true,
-        "supports_none_reasoning_effort": true
+        "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none"
     },
     "azure/us/gpt-5.1-codex": {
         "deprecation_date": "2027-05-15",
@@ -26292,6 +26318,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": false,
         "supports_minimal_reasoning_effort": true
     },
@@ -26336,6 +26363,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": false,
         "supports_minimal_reasoning_effort": true
     },
@@ -26381,6 +26409,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": false,
         "supports_minimal_reasoning_effort": true
     },
@@ -26426,6 +26455,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -26471,6 +26501,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -27295,6 +27326,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -27343,6 +27375,7 @@
         "supports_tool_choice": true,
         "supports_vision": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": true
     },
@@ -27492,6 +27525,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
@@ -27543,6 +27577,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
@@ -27591,6 +27626,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
@@ -27639,6 +27675,7 @@
         "supports_vision": true,
         "supports_web_search": true,
         "supports_none_reasoning_effort": true,
+        "default_reasoning_effort": "none",
         "supports_xhigh_reasoning_effort": true,
         "supports_minimal_reasoning_effort": false
     },
diff --git a/model_prices_and_context_window.schema.json b/model_prices_and_context_window.schema.json
index 6e837354c60..3f6d3b4f910 100644
--- a/model_prices_and_context_window.schema.json
+++ b/model_prices_and_context_window.schema.json
@@ -179,6 +179,18 @@
         "comment": {
           "type": "string"
         },
+        "default_reasoning_effort": {
+          "type": "string",
+          "description": "Reasoning effort the provider applies when the request omits reasoning_effort. Gates whether a non-default temperature or the top_p/logprobs sampling params are accepted, which hold only when the effort resolves to 'none'.",
+          "enum": [
+            "none",
+            "minimal",
+            "low",
+            "medium",
+            "high",
+            "xhigh"
+          ]
+        },
         "deprecation_date": {
           "type": "string",
           "description": "Date the provider deprecates the model, YYYY-MM-DD.",
diff --git a/tests/test_litellm/llms/azure/chat/test_azure_gpt5_transformation.py b/tests/test_litellm/llms/azure/chat/test_azure_gpt5_transformation.py
index 83562331b9a..e06cae97283 100644
--- a/tests/test_litellm/llms/azure/chat/test_azure_gpt5_transformation.py
+++ b/tests/test_litellm/llms/azure/chat/test_azure_gpt5_transformation.py
@@ -1,6 +1,7 @@
 import pytest
 
 import litellm
+from litellm.litellm_core_utils.get_model_cost_map import get_model_cost_map
 from litellm.llms.azure.chat.gpt_5_transformation import AzureOpenAIGPT5Config
 
 
@@ -9,6 +10,15 @@ def config() -> AzureOpenAIGPT5Config:
     return AzureOpenAIGPT5Config()
 
 
+@pytest.fixture(autouse=True)
+def use_local_model_cost_map(monkeypatch: pytest.MonkeyPatch):
+    """Pin the bundled cost map: these gates read model-map capability keys, and the default
+    import path fetches the published map, which lags a key added in this repo."""
+    monkeypatch.setenv("LITELLM_LOCAL_MODEL_COST_MAP", "True")
+    monkeypatch.setattr(litellm, "model_cost", get_model_cost_map(url=litellm.model_cost_map_url))
+    litellm.add_known_models(model_cost_map=litellm.model_cost)
+
+
 def test_azure_gpt5_supports_reasoning_effort(config: AzureOpenAIGPT5Config):
     assert "reasoning_effort" in config.get_supported_openai_params(model="gpt-5")
     assert "reasoning_effort" in config.get_supported_openai_params(
@@ -299,3 +309,30 @@ def test_azure_gpt5_1_does_not_support_logprobs(config: AzureOpenAIGPT5Config):
     supported_params = config.get_supported_openai_params(model="gpt-5.1")
     assert "logprobs" not in supported_params
     assert "top_logprobs" not in supported_params
+
+
+class TestAzureResolvesTheDeclaredDefaultEffort:
+    """Azure reaches the same models under names that are not cost-map keys. Every capability
+    lookup therefore has to normalise the name identically, which is why the normalisation is
+    one overridden resolver rather than a rewrite inside a single lookup.
+    """
+
+    @pytest.mark.parametrize(
+        "model, temperature_survives",
+        [
+            ("azure/gpt-5.1", True),
+            ("gpt5_series/gpt-5.1", True),
+            ("gpt-5.1", True),
+            ("azure/gpt-5.6-terra", False),
+            ("gpt5_series/gpt-5.6-terra", False),
+            ("azure/gpt-5.5", False),
+        ],
+    )
+    def test_every_azure_name_shape_reads_the_same_entry(self, config, model, temperature_survives):
+        mapped = config.map_openai_params(
+            non_default_params={"temperature": 0},
+            optional_params={},
+            model=model,
+            drop_params=True,
+        )
+        assert ("temperature" in mapped) is temperature_survives
diff --git a/tests/test_litellm/llms/openai/responses/test_openai_responses_transformation.py b/tests/test_litellm/llms/openai/responses/test_openai_responses_transformation.py
index c03c632363d..e314b94444b 100644
--- a/tests/test_litellm/llms/openai/responses/test_openai_responses_transformation.py
+++ b/tests/test_litellm/llms/openai/responses/test_openai_responses_transformation.py
@@ -1593,3 +1593,36 @@ class TestPromptCacheOptionsOnResponsesPath:
             "text": "hi",
             "prompt_cache_breakpoint": {"mode": "explicit"},
         }
+
+
+class TestResponsesSurfaceSharesTheEffortRule:
+    """The Responses API reaches the same gpt-5 models over a different wire, and the default
+    /v1/messages bridge for openai models routes through it. It carried its own copy of the
+    temperature rule, so fixing chat completions alone left this surface still forwarding
+    temperature to a model that rejects it.
+    """
+
+    @pytest.mark.parametrize(
+        "model, effort, temperature_survives",
+        [
+            ("gpt-5.1", None, True),
+            ("gpt-5.4", None, True),
+            ("gpt-5.5", None, False),
+            ("gpt-5.6-terra", None, False),
+            ("gpt-5.6-sol", None, False),
+            ("gpt-5.6-terra", "none", True),
+            ("gpt-5.6-terra", "medium", False),
+        ],
+    )
+    def test_temperature_follows_the_resolved_effort(
+        self, local_model_cost_map, model, effort, temperature_survives
+    ):
+        params = {"temperature": 0}
+        if effort is not None:
+            params["reasoning"] = {"effort": effort}
+        mapped = OpenAIResponsesAPIConfig().map_openai_params(
+            response_api_optional_params=params,
+            model=model,
+            drop_params=True,
+        )
+        assert ("temperature" in mapped) is temperature_survives
diff --git a/tests/test_litellm/llms/openai/test_gpt5_transformation.py b/tests/test_litellm/llms/openai/test_gpt5_transformation.py
index a35b75a6106..9c5bd34d59a 100644
--- a/tests/test_litellm/llms/openai/test_gpt5_transformation.py
+++ b/tests/test_litellm/llms/openai/test_gpt5_transformation.py
@@ -1,3 +1,5 @@
+import re
+
 import pytest
 
 import litellm
@@ -137,7 +139,7 @@ def test_gpt5_codex_temperature_error(config: OpenAIConfig):
     """Test that GPT-5-Codex raises error for unsupported temperature when drop_params=False."""
     with pytest.raises(
         litellm.utils.UnsupportedParamsError,
-        match="gpt-5 models \\(including gpt-5-codex\\)",
+        match=re.escape("gpt-5-codex doesn't support temperature=0.7 while reasoning is active"),
     ):
         config.map_openai_params(
             non_default_params={"temperature": 0.7},
@@ -1385,3 +1387,121 @@ def test_gpt5_drops_xhigh_when_requested(config: OpenAIConfig):
         drop_params=True,
     )
     assert "reasoning_effort" not in params
+
+
+class TestDefaultReasoningEffortGatesSamplingParams:
+    """A non-default temperature rides on the effort RESOLVING to "none", which for a request
+    that omits reasoning_effort is the model's declared default_reasoning_effort - not on the
+    model merely supporting "none". gpt-5.5 and gpt-5.6 support it and do not default to it,
+    so reading one fact as the other forwarded temperature=0 and the provider rejected it.
+
+    Every expectation below was measured against the live provider before being pinned here.
+    """
+
+    @pytest.mark.parametrize(
+        "model, effort, temperature_survives",
+        [
+            # declares default_reasoning_effort="none": reasoning is off, sampling is free
+            ("gpt-5.1", None, True),
+            ("gpt-5.2", None, True),
+            ("gpt-5.4", None, True),
+            ("gpt-5.4-nano", None, True),
+            # declares no default: reasoning is active, so the provider takes only temperature=1
+            ("gpt-5.5", None, False),
+            ("gpt-5.6", None, False),
+            ("gpt-5.6-terra", None, False),
+            ("gpt-5.6-sol", None, False),
+            # an explicit effort always wins over the declared default, both ways
+            ("gpt-5.6-terra", "none", True),
+            ("gpt-5.6-terra", "medium", False),
+            ("gpt-5.1", "medium", False),
+        ],
+    )
+    def test_temperature_follows_the_resolved_effort(self, model, effort, temperature_survives):
+        params = {"temperature": 0} if effort is None else {"temperature": 0, "reasoning_effort": effort}
+        mapped = OpenAIGPT5Config().map_openai_params(
+            non_default_params=params,
+            optional_params={},
+            model=model,
+            drop_params=True,
+        )
+        assert ("temperature" in mapped) is temperature_survives
+
+    @pytest.mark.parametrize("model, top_p_survives", [("gpt-5.1", True), ("gpt-5.6-terra", False)])
+    def test_the_same_rule_gates_top_p(self, model, top_p_survives):
+        """top_p/logprobs are gated by the identical condition, so they were identically wrong."""
+        mapped = OpenAIGPT5Config().map_openai_params(
+            non_default_params={"top_p": 0.5},
+            optional_params={},
+            model=model,
+            drop_params=True,
+        )
+        assert ("top_p" in mapped) is top_p_survives
+
+    def test_an_undeclared_model_is_refused_rather_than_forwarded(self):
+        """Without drop_params the caller gets an actionable 400 naming the remedy, instead of
+        the provider's own rejection arriving from an upstream it did not address."""
+        with pytest.raises(litellm.utils.UnsupportedParamsError, match="default_reasoning_effort"):
+            OpenAIGPT5Config().map_openai_params(
+                non_default_params={"temperature": 0},
+                optional_params={},
+                model="gpt-5.6-terra",
+                drop_params=False,
+            )
+
+
+class TestACatalogueOlderThanTheCodeDoesNotStripTemperature:
+    """The cost map is fetched from the published branch at import time, so it can be OLDER than
+    the code reading it. On such a map every model looks undeclared, and reading that as
+    "reasoning is active" silently stripped temperature from the gpt-5.1/5.2/5.4 deployments that
+    accept it - a regression caused by data lag rather than by anything about the model.
+
+    Absence of the key only means something once the catalogue is known to carry it at all.
+    """
+
+    @staticmethod
+    def _map_without_the_key(monkeypatch: pytest.MonkeyPatch) -> None:
+        stripped = {
+            name: {k: v for k, v in entry.items() if k != "default_reasoning_effort"}
+            if isinstance(entry, dict)
+            else entry
+            for name, entry in litellm.model_cost.items()
+        }
+        monkeypatch.setattr(litellm, "model_cost", stripped)
+
+    @pytest.mark.parametrize("model", ["gpt-5.1", "gpt-5.2", "gpt-5.4", "gpt-5.4-nano"])
+    def test_a_pre_feature_catalogue_keeps_the_answer_it_gave_before(self, monkeypatch, model):
+        """These models accept temperature=0, verified against the provider. On a map that predates
+        the key they must keep it, exactly as they did before this feature existed."""
+        self._map_without_the_key(monkeypatch)
+
+        mapped = OpenAIGPT5Config().map_openai_params(
+            non_default_params={"temperature": 0},
+            optional_params={},
+            model=model,
+            drop_params=True,
+        )
+        assert mapped.get("temperature") == 0
+
+    @pytest.mark.parametrize("model", ["gpt-5.1", "gpt-5.4"])
+    def test_the_same_holds_for_the_sampling_params(self, monkeypatch, model):
+        self._map_without_the_key(monkeypatch)
+
+        mapped = OpenAIGPT5Config().map_openai_params(
+            non_default_params={"top_p": 0.5},
+            optional_params={},
+            model=model,
+            drop_params=True,
+        )
+        assert mapped.get("top_p") == 0.5
+
+    def test_once_the_catalogue_declares_the_key_the_conservative_answer_returns(self):
+        """The bundled map DOES carry the key, so an undeclared model there is a real statement
+        that its default is not none, and temperature is dropped."""
+        mapped = OpenAIGPT5Config().map_openai_params(
+            non_default_params={"temperature": 0},
+            optional_params={},
+            model="gpt-5.6-terra",
+            drop_params=True,
+        )
+        assert "temperature" not in mapped
diff --git a/tests/test_litellm/test_utils.py b/tests/test_litellm/test_utils.py
index e9b2851f717..362e55680a3 100644
--- a/tests/test_litellm/test_utils.py
+++ b/tests/test_litellm/test_utils.py
@@ -1018,6 +1018,10 @@ def test_aaamodel_prices_and_context_window_json_is_valid():
                     "type": "array",
                     "items": {"type": "string", "enum": ["none", "minimal", "low", "medium", "high", "xhigh", "max"]},
                 },
+                "default_reasoning_effort": {
+                    "type": "string",
+                    "enum": ["none", "minimal", "low", "medium", "high", "xhigh"],
+                },
                 "supports_adaptive_thinking": {"type": "boolean"},
                 "supports_legacy_thinking": {"type": "boolean"},
                 "thinking_always_on": {"type": "boolean"},
@@ -5639,3 +5643,27 @@ def test_snapshot_exception_for_hook_preserves_suppress_context_flag() -> None:
     snapshot = _snapshot_exception_for_hook(e)
     assert snapshot.__suppress_context__ is False
     assert snapshot.__context__ is e.__context__
+
+
+class TestDefaultReasoningEffortHydration:
+    """`get_model_info` is the public shape every other capability key is readable through, so
+    the declared default has to survive hydration too, not only the raw-map fallback the
+    request-path gate happens to reach it by.
+    """
+
+    @pytest.mark.parametrize(
+        "model, provider",
+        [("gpt-5.1", "openai"), ("gpt-5.4", "openai"), ("azure/gpt-5.1", "azure")],
+    )
+    def test_the_declared_default_survives_model_info_hydration(self, local_model_cost_map, model, provider):
+        from litellm.utils import _get_model_info_helper
+
+        model_info = dict(_get_model_info_helper(model=model, custom_llm_provider=provider))
+        assert model_info["default_reasoning_effort"] == "none"
+
+    def test_a_model_that_declares_nothing_hydrates_to_none(self, local_model_cost_map):
+        """Absent means "the map does not say", which the gate reads as reasoning being active."""
+        from litellm.utils import _get_model_info_helper
+
+        model_info = dict(_get_model_info_helper(model="gpt-5.6-terra", custom_llm_provider="openai"))
+        assert model_info.get("default_reasoning_effort") is None