diff --git a/litellm/main.py b/litellm/main.py
index 1cac723c179..a1b41315b85 100644
--- a/litellm/main.py
+++ b/litellm/main.py
@@ -416,7 +416,8 @@ async def acompletion(
     logprobs: bool | None = None,
     top_logprobs: int | None = None,
     deployment_id=None,
-    reasoning_effort: Literal["none", "minimal", "low", "medium", "high", "xhigh", "max", "ultra", "default"] | None = None,
+    reasoning_effort: Literal["none", "minimal", "low", "medium", "high", "xhigh", "max", "ultra", "default"]
+    | None = None,
     verbosity: Literal["low", "medium", "high"] | None = None,
     safety_identifier: str | None = None,
     service_tier: str | None = None,
@@ -4920,7 +4921,8 @@ def completion(
     logit_bias: dict | None = None,
     user: str | None = None,
     # openai v1.0+ new params
-    reasoning_effort: Literal["none", "minimal", "low", "medium", "high", "xhigh", "max", "ultra", "default"] | None = None,
+    reasoning_effort: Literal["none", "minimal", "low", "medium", "high", "xhigh", "max", "ultra", "default"]
+    | None = None,
     verbosity: Literal["low", "medium", "high"] | None = None,
     response_format: dict | type[BaseModel] | None = None,
     seed: int | None = None,
diff --git a/litellm/router_utils/reasoning_effort_capability.py b/litellm/router_utils/reasoning_effort_capability.py
index f59c53e9287..593b5599703 100644
--- a/litellm/router_utils/reasoning_effort_capability.py
+++ b/litellm/router_utils/reasoning_effort_capability.py
@@ -3,9 +3,15 @@
 The model-map flags carry different polarity per level, mirroring the provider gates
 (gpt_5_transformation.py restricts xhigh to explicit opt-in and treats minimal/low as opt-out;
 anthropic/chat/transformation.py rejects only xhigh/max without an explicit flag): medium and high
-are unconditional for any reasoning model, none/minimal/low are supported unless the map explicitly
+are unconditional for any reasoning model, minimal/low are supported unless the map explicitly
 says false, and xhigh/max require an explicit true. Shipping the resolved list keeps that polarity
 in one place instead of re-encoding it in every consumer.
+
+The none level is the one flag whose polarity is provider-dependent. OpenAI never refuses it on the
+request path (azure/chat/gpt_5_transformation.py is the only caller that does, and it raises
+UnsupportedParamsError unless supports_none_reasoning_effort is explicitly true), so none is opt-out
+everywhere except azure, where it is opt-in. Resolving it the other way for azure would advertise a
+level litellm itself rejects, which is the failure this module exists to prevent.
 """
 
 from collections.abc import Mapping, Sequence
@@ -14,7 +20,6 @@ from typing import Final
 REASONING_EFFORT_CAPABILITY_ORDER: Final = ("none", "minimal", "low", "medium", "high", "xhigh", "max", "ultra")
 
 _OPT_OUT_FLAGS: Final = (
-    ("none", "supports_none_reasoning_effort"),
     ("minimal", "supports_minimal_reasoning_effort"),
     ("low", "supports_low_reasoning_effort"),
 )
@@ -24,6 +29,28 @@ _OPT_IN_FLAGS: Final = (
     ("ultra", "supports_ultra_reasoning_effort"),
 )
 _UNCONDITIONAL_EFFORTS: Final = frozenset(("medium", "high"))
+_NONE_FLAG: Final = "supports_none_reasoning_effort"
+_NONE_OPT_IN_PROVIDERS: Final = frozenset(("azure",))
+
+
+def _supports_none_reasoning_effort(model_info: Mapping[str, object]) -> bool:
+    """Opt-in on azure, whose gpt-5 config raises on reasoning_effort='none' without an explicit
+    true; opt-out elsewhere, where no request path refuses the level. A missing azure flag defers to
+    _supports_factory, the same resolver the azure gate calls, so its bare-model-name fallback
+    (azure/gpt-5.2 inheriting the flag from gpt-5.2) reaches both sides alike."""
+    flag: Final = model_info.get(_NONE_FLAG)
+    if model_info.get("litellm_provider") not in _NONE_OPT_IN_PROVIDERS:
+        return flag is not False
+    if flag is not None:
+        return flag is True
+    model_key: Final = model_info.get("key")
+    if not isinstance(model_key, str):
+        return False
+    from litellm.utils import (
+        _supports_factory,  # pyright: ignore[reportPrivateUsage]  # the resolver the azure gate itself calls; a public wrapper would fork the fallback
+    )
+
+    return _supports_factory(model=model_key, custom_llm_provider=None, key=_NONE_FLAG)
 
 
 def resolve_supported_reasoning_efforts(model_info: Mapping[str, object]) -> tuple[str, ...] | None:
@@ -35,7 +62,8 @@ def resolve_supported_reasoning_efforts(model_info: Mapping[str, object]) -> tup
         return ()
     opt_out: Final = frozenset(effort for effort, flag in _OPT_OUT_FLAGS if model_info.get(flag) is not False)
     opt_in: Final = frozenset(effort for effort, flag in _OPT_IN_FLAGS if model_info.get(flag) is True)
-    allowed: Final = opt_out | _UNCONDITIONAL_EFFORTS | opt_in
+    none_level: Final = frozenset(("none",)) if _supports_none_reasoning_effort(model_info) else frozenset()
+    allowed: Final = opt_out | _UNCONDITIONAL_EFFORTS | opt_in | none_level
     return tuple(effort for effort in REASONING_EFFORT_CAPABILITY_ORDER if effort in allowed)
 
 
diff --git a/tests/test_litellm/router_utils/test_reasoning_effort_capability.py b/tests/test_litellm/router_utils/test_reasoning_effort_capability.py
index ebc7952081c..1c7d5355549 100644
--- a/tests/test_litellm/router_utils/test_reasoning_effort_capability.py
+++ b/tests/test_litellm/router_utils/test_reasoning_effort_capability.py
@@ -1,3 +1,5 @@
+import pytest
+
 from litellm.router_utils.reasoning_effort_capability import (
     intersect_supported_reasoning_efforts,
     resolve_supported_reasoning_efforts,
@@ -64,6 +66,48 @@ class TestResolveSupportedReasoningEfforts:
         assert "xhigh" not in resolved
 
 
+class TestNoneLevelPolarity:
+    def test_none_stays_opt_out_off_azure(self):
+        resolved = resolve_supported_reasoning_efforts(
+            {"supports_reasoning": True, "litellm_provider": "openai", "key": "gpt-5-mini"}
+        )
+        assert resolved is not None and "none" in resolved
+
+    def test_azure_without_the_flag_does_not_advertise_none(self):
+        resolved = resolve_supported_reasoning_efforts(
+            {"supports_reasoning": True, "litellm_provider": "azure", "key": "azure/unmapped-deployment"}
+        )
+        assert resolved == ("minimal", "low", "medium", "high")
+
+    def test_azure_with_the_flag_advertises_none(self):
+        resolved = resolve_supported_reasoning_efforts(
+            {
+                "supports_reasoning": True,
+                "litellm_provider": "azure",
+                "supports_none_reasoning_effort": True,
+                "key": "azure/unmapped-deployment",
+            }
+        )
+        assert resolved is not None and "none" in resolved
+
+    @pytest.mark.parametrize(
+        "model_key",
+        ["azure/gpt-5", "azure/gpt-5-mini", "azure/gpt-5-nano", "azure/gpt-5.2", "azure/gpt-5.6"],
+    )
+    def test_azure_advertisement_matches_the_azure_request_gate(self, model_key):
+        """AzureOpenAIGPT5Config raises UnsupportedParamsError on reasoning_effort='none' for models
+        it does not flag, so advertising the level there would offer routing a 400."""
+        from litellm.llms.azure.chat.gpt_5_transformation import AzureOpenAIGPT5Config
+        from litellm.utils import _get_model_info_helper
+
+        model_info = dict(_get_model_info_helper(model=model_key.split("/", 1)[1], custom_llm_provider="azure"))
+        resolved = resolve_supported_reasoning_efforts(model_info)
+
+        assert resolved is not None
+        gate_accepts_none = AzureOpenAIGPT5Config._supports_reasoning_effort_level(model_key, "none")
+        assert ("none" in resolved) is gate_accepts_none
+
+
 class TestIntersectSupportedReasoningEfforts:
     def test_unknown_never_narrows(self):
         assert intersect_supported_reasoning_efforts(["medium", "high"], None) == ("medium", "high")