diff --git a/container/codex-runner/src/index.ts b/container/codex-runner/src/index.ts
index f91c878..5331643 100644
--- a/container/codex-runner/src/index.ts
+++ b/container/codex-runner/src/index.ts
@@ -147,7 +147,7 @@ async function runCodexExec(prompt: string, resume: boolean): Promise<{ result:
         '--color', 'never',
       );
       if (codexModel) args.push('-m', codexModel);
-      if (codexEffort) args.push('--effort', codexEffort);
+      if (codexEffort) args.push('-c', `model_reasoning_effort=${codexEffort}`);
       args.push(prompt);
     }