diff --git a/packages/opencode/src/provider/transform.ts b/packages/opencode/src/provider/transform.ts
index 481d56a31e0..4d24bb0e1f0 100644
--- a/packages/opencode/src/provider/transform.ts
+++ b/packages/opencode/src/provider/transform.ts
@@ -482,6 +482,12 @@ export namespace ProviderTransform {
         }
         // GPT models via Kilo need encrypted reasoning content to avoid org_id mismatch
         if (!model.id.includes("gpt") && !model.id.includes("gemini-3")) return {}
+        // kilocode_change - Codex models use object-based reasoning format for OpenRouter
+        // OpenRouter expects { reasoning: { effort: "high" } } format
+        // See: https://openrouter.ai/docs/api/api-reference/chat/send-chat-completion-request#request.body.reasoning
+        if (model.id.includes("codex")) {
+          return Object.fromEntries(OPENAI_EFFORTS.map((effort) => [effort, { reasoning: { effort } }]))
+        }
         return Object.fromEntries(
           OPENAI_EFFORTS.map((effort) => [
             effort,
diff --git a/packages/opencode/test/provider/transform.test.ts b/packages/opencode/test/provider/transform.test.ts
index a00cc5dd461..30a7dfc80d1 100644
--- a/packages/opencode/test/provider/transform.test.ts
+++ b/packages/opencode/test/provider/transform.test.ts
@@ -1669,6 +1669,23 @@ describe("ProviderTransform.variants", () => {
       expect(result.low).toEqual({ reasoning: { effort: "low" } })
       expect(result.high).toEqual({ reasoning: { effort: "high" } })
     })
+
+    test("codex models return OPENAI_EFFORTS with object-based reasoning format", () => {
+      const model = createMockModel({
+        id: "kilo/openai/gpt-5.2-codex",
+        providerID: "kilo",
+        api: {
+          id: "openai/gpt-5.2-codex",
+          url: "https://gateway.kilo.ai",
+          npm: "@kilocode/kilo-gateway",
+        },
+      })
+      const result = ProviderTransform.variants(model)
+      expect(Object.keys(result)).toEqual(["none", "minimal", "low", "medium", "high", "xhigh"])
+      expect(result.low).toEqual({ reasoning: { effort: "low" } })
+      expect(result.high).toEqual({ reasoning: { effort: "high" } })
+      expect(result.xhigh).toEqual({ reasoning: { effort: "xhigh" } })
+    })
   })
   // kilocode_change end