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:       0.3.0.2
+version:       0.4.0.0
 synopsis:      OpenAI providers for the baikai abstraction
 description:
   Wraps the openai Haskell package as a Baikai Provider for OpenAI's Chat Completions API.
diff --git a/src/Baikai/Provider/OpenAI/Cli.hs b/src/Baikai/Provider/OpenAI/Cli.hs
--- a/src/Baikai/Provider/OpenAI/Cli.hs
+++ b/src/Baikai/Provider/OpenAI/Cli.hs
@@ -39,6 +39,7 @@
 import Baikai.Response qualified as Resp
 import Baikai.StopReason (StopReason (..))
 import Baikai.Stream (liftCompleteToStream)
+import Baikai.ThinkingLevel (renderThinkingLevel)
 import Baikai.Usage (zeroUsage)
 import Control.Concurrent (forkIO)
 import Control.Concurrent.MVar (newEmptyMVar, putMVar, takeMVar)
@@ -145,21 +146,30 @@
 -- | Render the executable and arguments for a @codex exec --json@
 -- batch call. The prompt is preceded by @--@ so dash-leading prompts
 -- cannot be parsed as options.
-codexCliCommand :: CodexCliConfig -> Model -> Context -> (FilePath, [String])
-codexCliCommand cfg m ctx =
+codexCliCommand :: CodexCliConfig -> Model -> Context -> Options -> (FilePath, [String])
+codexCliCommand cfg m ctx opts =
   ( cfg ^. #executable,
     ["exec"]
       <> modelArgs m
       <> ["--json"]
       <> ["--skip-git-repo-check" | cfg ^. #skipGitRepoCheck]
       <> ["--ephemeral" | cfg ^. #ephemeral]
+      <> effortArgs opts
       <> fmap Text.unpack (cfg ^. #extraArgs)
       <> ["--", Text.unpack (codexCliPrompt ctx)]
   )
 
+-- | Render Codex's reasoning-effort config override from
+-- 'Options.thinking'. Codex accepts all six Baikai levels verbatim;
+-- when 'thinking' is unset, no override is emitted.
+effortArgs :: Options -> [String]
+effortArgs opts = case opts ^. #thinking of
+  Nothing -> []
+  Just lvl -> ["-c", "model_reasoning_effort=" <> Text.unpack (renderThinkingLevel lvl)]
+
 runCodexCli :: CodexCliConfig -> Model -> Context -> Options -> IO Resp.Response
-runCodexCli cfg m ctx _opts = do
-  let (exe, args) = codexCliCommand cfg m ctx
+runCodexCli cfg m ctx opts = do
+  let (exe, args) = codexCliCommand cfg m ctx opts
       procSpec =
         (P.proc exe args)
           { P.std_in = P.NoStream,
diff --git a/test/Main.hs b/test/Main.hs
--- a/test/Main.hs
+++ b/test/Main.hs
@@ -46,6 +46,7 @@
       [ commandRenderingTest,
         effortRenderingTests,
         batchCommandRenderingTest,
+        batchEffortRenderingTest,
         batchSystemPromptTest,
         stderrFloodTest,
         usageMappingTests,
@@ -280,7 +281,7 @@
             & #api .~ OpenAICompletionsCli
             & #provider .~ "openai"
         ctx = emptyContext & #messages .~ Vector.singleton (user "-begin with a dash")
-    CodexCli.codexCliCommand CodexCli.defaultCodexCliConfig model ctx
+    CodexCli.codexCliCommand CodexCli.defaultCodexCliConfig model ctx emptyOptions
       @?= ( "codex",
             [ "exec",
               "--json",
@@ -291,6 +292,29 @@
             ]
           )
 
+batchEffortRenderingTest :: TestTree
+batchEffortRenderingTest =
+  testCase "codex exec argv renders reasoning effort before the prompt terminator" $ do
+    let model =
+          emptyModel
+            & #modelId .~ ""
+            & #api .~ OpenAICompletionsCli
+            & #provider .~ "openai"
+        ctx = emptyContext & #messages .~ Vector.singleton (user "ping")
+        opts = emptyOptions & #thinking .~ Just ThinkingXHigh
+    CodexCli.codexCliCommand CodexCli.defaultCodexCliConfig model ctx opts
+      @?= ( "codex",
+            [ "exec",
+              "--json",
+              "--skip-git-repo-check",
+              "--ephemeral",
+              "-c",
+              "model_reasoning_effort=xhigh",
+              "--",
+              "ping"
+            ]
+          )
+
 batchSystemPromptTest :: TestTree
 batchSystemPromptTest =
   testCase "codex exec argv carries system prompt in the prompt text" $ do
@@ -303,7 +327,7 @@
           emptyContext
             & #systemPrompt .~ Just "Be terse."
             & #messages .~ Vector.singleton (user "ping")
-    CodexCli.codexCliCommand CodexCli.defaultCodexCliConfig model ctx
+    CodexCli.codexCliCommand CodexCli.defaultCodexCliConfig model ctx emptyOptions
       @?= ( "codex",
             [ "exec",
               "--json",