diff --git a/baikai-openai.cabal b/baikai-openai.cabal
--- a/baikai-openai.cabal
+++ b/baikai-openai.cabal
@@ -1,6 +1,6 @@
 cabal-version: 3.4
 name:          baikai-openai
-version:       0.3.0.1
+version:       0.3.0.2
 synopsis:      OpenAI providers for the baikai abstraction
 description:
   Wraps the openai Haskell package as a Baikai Provider for OpenAI's Chat Completions API.
@@ -42,7 +42,7 @@
 
   build-depends:
     , aeson              ^>=2.2
-    , baikai             ^>=0.3.0
+    , baikai             ^>=0.4.0
     , base               >=4.20   && <5
     , base64-bytestring  ^>=1.2
     , bytestring         ^>=0.12
@@ -76,7 +76,7 @@
 
   build-depends:
     , aeson
-    , baikai            ^>=0.3.0
+    , baikai            ^>=0.4.0
     , baikai-openai
     , base              >=4.20   && <5
     , bytestring
diff --git a/src/Baikai/Provider/OpenAI/Interactive.hs b/src/Baikai/Provider/OpenAI/Interactive.hs
--- a/src/Baikai/Provider/OpenAI/Interactive.hs
+++ b/src/Baikai/Provider/OpenAI/Interactive.hs
@@ -29,6 +29,7 @@
   )
 import Baikai.Prelude
 import Baikai.Provider.Cli.Internal qualified as Internal
+import Baikai.ThinkingLevel (renderThinkingLevel)
 import Data.Generics.Labels ()
 import Data.Text qualified as Text
 import System.Process qualified as P
@@ -54,6 +55,7 @@
 codexInteractiveCommand cfg req =
   ( cfg ^. #executable,
     modelArgs req
+      <> effortArgs req
       <> workingDirArgs req
       <> extraDirArgs req
       <> safetyArgs req
@@ -90,6 +92,15 @@
   Nothing -> []
   Just "" -> []
   Just mid -> ["--model", Text.unpack mid]
+
+-- | Codex receives reasoning effort through a config override. The
+-- provider-only @none@ and @ultra@ values remain available through
+-- raw extra arguments.
+effortArgs :: InteractiveLaunchRequest -> [String]
+effortArgs req = case req ^. #effort of
+  Nothing -> []
+  Just lvl ->
+    ["-c", "model_reasoning_effort=" <> Text.unpack (renderThinkingLevel lvl)]
 
 workingDirArgs :: InteractiveLaunchRequest -> [String]
 workingDirArgs req = case req ^. #workingDir of
diff --git a/src/Baikai/Provider/OpenAI/Internal/Request.hs b/src/Baikai/Provider/OpenAI/Internal/Request.hs
--- a/src/Baikai/Provider/OpenAI/Internal/Request.hs
+++ b/src/Baikai/Provider/OpenAI/Internal/Request.hs
@@ -127,6 +127,10 @@
   ThinkingLow -> Chat.ReasoningEffort_Low
   ThinkingMedium -> Chat.ReasoningEffort_Medium
   ThinkingHigh -> Chat.ReasoningEffort_High
+  -- The SDK enum stops at High. Shape.injectThinkingShape restores
+  -- the canonical xhigh/max string in the serialized native OpenAI body.
+  ThinkingXHigh -> Chat.ReasoningEffort_High
+  ThinkingMax -> Chat.ReasoningEffort_High
 
 -- | Map a baikai 'Tool.Tool' into the upstream OpenAI 'Tool_Function'
 -- shape. The compat record's 'supportsStrictMode' flag controls
diff --git a/src/Baikai/Provider/OpenAI/Shape.hs b/src/Baikai/Provider/OpenAI/Shape.hs
--- a/src/Baikai/Provider/OpenAI/Shape.hs
+++ b/src/Baikai/Provider/OpenAI/Shape.hs
@@ -26,7 +26,7 @@
     ThinkingFormat (..),
   )
 import Baikai.Options (Options, cacheRetention, thinking)
-import Baikai.ThinkingLevel (ThinkingLevel (..))
+import Baikai.ThinkingLevel (ThinkingLevel (..), renderThinkingLevel)
 import Data.Aeson (Value (..), (.=))
 import Data.Aeson qualified as Aeson
 import Data.Aeson.Key qualified as AesonKey
@@ -92,15 +92,16 @@
   case thinking opts of
     Nothing -> body
     Just lvl -> case thinkingFormat compat of
-      ThinkingFormatOpenAI -> body
+      ThinkingFormatOpenAI ->
+        insertTop "reasoning_effort" (String (renderThinkingLevel lvl)) body
       ThinkingFormatNone -> body
       ThinkingFormatOpenRouter ->
-        insertTop "reasoning" (Aeson.object ["effort" .= effort lvl]) body
+        insertTop "reasoning" (Aeson.object ["effort" .= compatibleEffort lvl]) body
       ThinkingFormatDeepseek ->
-        insertTop "reasoning_effort" (String (effort lvl)) $
+        insertTop "reasoning_effort" (String (compatibleEffort lvl)) $
           insertTop "thinking" (Aeson.object ["type" .= ("enabled" :: Text)]) body
       ThinkingFormatTogether ->
-        insertTop "reasoning_effort" (String (effort lvl)) $
+        insertTop "reasoning_effort" (String (compatibleEffort lvl)) $
           insertTop "reasoning" (Aeson.object ["enabled" .= True]) body
       ThinkingFormatZai ->
         insertTop "enable_thinking" (Bool True) body
@@ -208,9 +209,13 @@
 key :: Text -> AesonKey.Key
 key = AesonKey.fromText
 
-effort :: ThinkingLevel -> Text
-effort = \case
+-- | Common effort vocabulary supported by the non-native
+-- OpenAI-compatible request shapes above.
+compatibleEffort :: ThinkingLevel -> Text
+compatibleEffort = \case
   ThinkingMinimal -> "low"
   ThinkingLow -> "low"
   ThinkingMedium -> "medium"
   ThinkingHigh -> "high"
+  ThinkingXHigh -> "high"
+  ThinkingMax -> "high"
diff --git a/test/Main.hs b/test/Main.hs
--- a/test/Main.hs
+++ b/test/Main.hs
@@ -44,6 +44,7 @@
     testGroup
       "Baikai.Provider.OpenAI"
       [ commandRenderingTest,
+        effortRenderingTests,
         batchCommandRenderingTest,
         batchSystemPromptTest,
         stderrFloodTest,
@@ -249,6 +250,26 @@
               "System instructions:\nBe precise.\n\nUser request:\ninspect the repo"
             ]
           )
+
+effortRenderingTests :: TestTree
+effortRenderingTests =
+  testGroup
+    "renders interactive reasoning effort"
+    [ testCase name $ do
+        let req = interactiveLaunchRequest "prompt" & #effort .~ Just level
+        codexInteractiveCommand defaultCodexInteractiveConfig req
+          @?= ( "codex",
+                ["-c", "model_reasoning_effort=" <> expected, "--", "prompt"]
+              )
+    | (name, level, expected) <-
+        [ ("minimal", ThinkingMinimal, "minimal"),
+          ("low", ThinkingLow, "low"),
+          ("medium", ThinkingMedium, "medium"),
+          ("high", ThinkingHigh, "high"),
+          ("xhigh", ThinkingXHigh, "xhigh"),
+          ("max", ThinkingMax, "max")
+        ]
+    ]
 
 batchCommandRenderingTest :: TestTree
 batchCommandRenderingTest =
diff --git a/test/ShapeSpec.hs b/test/ShapeSpec.hs
--- a/test/ShapeSpec.hs
+++ b/test/ShapeSpec.hs
@@ -31,6 +31,8 @@
   testGroup
     "ShapeSpec"
     [ deepseekShapeTest,
+      nativeHigherEffortTests,
+      compatibleHigherEffortClampTest,
       openRouterCacheControlTest,
       strictModeGateTest,
       usageStreamingGateTest,
@@ -50,6 +52,33 @@
     lookupTop "max_tokens" value @?= Just (Number 8192)
     lookupTop "thinking" value
       @?= Just (Aeson.object ["type" .= ("enabled" :: Text.Text)])
+    lookupTop "reasoning_effort" value @?= Just (String "high")
+
+nativeHigherEffortTests :: TestTree
+nativeHigherEffortTests =
+  testGroup
+    "native OpenAI higher reasoning effort"
+    [ testCase name $ do
+        value <-
+          shapedBody
+            Models.openai_gpt_5_6_terra
+            (emptyOptions & #thinking .~ Just level)
+            emptyContext
+        lookupTop "reasoning_effort" value @?= Just (String expected)
+    | (name, level, expected) <-
+        [ ("xhigh survives SDK staging", ThinkingXHigh, "xhigh"),
+          ("max survives SDK staging", ThinkingMax, "max")
+        ]
+    ]
+
+compatibleHigherEffortClampTest :: TestTree
+compatibleHigherEffortClampTest =
+  testCase "OpenAI-compatible higher reasoning effort clamps to high" $ do
+    value <-
+      shapedBody
+        Models.deepseek_deepseek_chat
+        (emptyOptions & #thinking .~ Just ThinkingMax)
+        emptyContext
     lookupTop "reasoning_effort" value @?= Just (String "high")
 
 openRouterCacheControlTest :: TestTree