diff --git a/src/engine/openAi.ts b/src/engine/openAi.ts
index 03148cb..4449877 100644
--- a/src/engine/openAi.ts
+++ b/src/engine/openAi.ts
@@ -45,16 +45,16 @@ export class OpenAiEngine implements AiEngine {
   ): Promise<string | null> => {
     const isReasoningModel = /^(o[1-9]|gpt-5)/.test(this.config.model);
 
-    const params: Record<string, unknown> = {
+    const params = {
       model: this.config.model,
       messages,
       ...(isReasoningModel
         ? { max_completion_tokens: this.config.maxTokensOutput }
         : {
-            temperature: 0,
-            top_p: 0.1,
-            max_tokens: this.config.maxTokensOutput
-          })
+          temperature: 0,
+          top_p: 0.1,
+          max_tokens: this.config.maxTokensOutput
+        })
     };
 
     try {
@@ -68,7 +68,9 @@ export class OpenAiEngine implements AiEngine {
       )
         throw new Error(GenerateCommitMessageErrorEnum.tooMuchTokens);
 
-      const completion = await this.client.chat.completions.create(params);
+      const completion = await this.client.chat.completions.create(
+        params as OpenAI.Chat.Completions.ChatCompletionCreateParamsNonStreaming
+      );
 
       const message = completion.choices[0].message;
       let content = message?.content;
diff --git a/test/unit/openAi.test.ts b/test/unit/openAi.test.ts
new file mode 100644
index 0000000..c1defd5
--- /dev/null
+++ b/test/unit/openAi.test.ts
@@ -0,0 +1,26 @@
+// Test the reasoning model detection regex used in OpenAiEngine.
+// Integration test with the engine is not possible because mistral.ts
+// uses require() which is unavailable in the ESM test environment.
+const REASONING_MODEL_RE = /^(o[1-9]|gpt-5)/;
+
+describe('OpenAiEngine reasoning model detection', () => {
+  it.each([
+    ['o1', true],
+    ['o1-preview', true],
+    ['o1-mini', true],
+    ['o3', true],
+    ['o3-mini', true],
+    ['o4-mini', true],
+    ['gpt-5', true],
+    ['gpt-5-nano', true],
+    ['gpt-4o', false],
+    ['gpt-4o-mini', false],
+    ['gpt-4', false],
+    ['gpt-3.5-turbo', false]
+  ])(
+    'model "%s" isReasoning=%s',
+    (model, expected) => {
+      expect(REASONING_MODEL_RE.test(model)).toBe(expected);
+    }
+  );
+});