fix: only send OpenAI reasoning params for models that support them

src/__tests__/unit/core/openai-codec.test.ts

@@ -0,0 +1,64 @@
+import { describe, expect, it } from 'vitest';
+import { OpenAiCodec } from '@/core/llm/adapters/openai/openai-codec.js';
+import type { ChatMessage } from '@/core/llm/types.js';
+
+const messages: ChatMessage[] = [
+  { role: 'system', content: 'You are helpful.' },
+  { role: 'user', content: 'Hello' },
+];
+
+describe('OpenAiCodec.buildResponsesRequest reasoning parameter', () => {
+  it('omits reasoning for non-reasoning API-key models like gpt-4.1', () => {
+    const request = OpenAiCodec.buildResponsesRequest(messages, {
+      model: 'gpt-4.1',
+      tools: [],
+      oauthMode: false,
+    });
+
+    // gpt-4.1 rejects `reasoning.summary` with a 400; it must not be sent.
+    expect(request.reasoning).toBeUndefined();
+    expect(request.model).toBe('gpt-4.1');
+  });
+
+  it('sends reasoning with the resolved default effort for reasoning API-key models', () => {
+    const request = OpenAiCodec.buildResponsesRequest(messages, {
+      model: 'gpt-5.4',
+      tools: [],
+      oauthMode: false,
+    });
+
+    expect(request.reasoning).toEqual({ summary: 'detailed', effort: 'medium' });
+  });
+
+  it('keeps summary auto in OAuth mode', () => {
+    const request = OpenAiCodec.buildResponsesRequest(messages, {
+      model: 'gpt-5.4',
+      tools: [],
+      oauthMode: true,
+    });
+
+    expect(request.reasoning).toEqual(expect.objectContaining({ summary: 'auto' }));
+  });
+
+  it('omits reasoning for API-key models without a reasoning policy entry', () => {
+    // Codex-family models are OAuth-first and have no API-key reasoning policy;
+    // sending nothing lets the server apply its own default.
+    const request = OpenAiCodec.buildResponsesRequest(messages, {
+      model: 'gpt-5.2-codex',
+      tools: [],
+      oauthMode: false,
+    });
+
+    expect(request.reasoning).toBeUndefined();
+  });
+
+  it('still sends reasoning in OAuth mode for codex models', () => {
+    const request = OpenAiCodec.buildResponsesRequest(messages, {
+      model: 'gpt-5.2-codex',
+      tools: [],
+      oauthMode: true,
+    });
+
+    expect(request.reasoning).toEqual({ summary: 'auto' });
+  });
+});

src/core/llm/adapters/openai/openai-codec.ts

@@ -128,7 +128,7 @@ export class OpenAiCodec {
     input: ResponseInputItem[];
     tools?: FunctionTool[];
     store: boolean;
-    reasoning: { summary: 'auto' | 'detailed'; effort?: OpenAiReasoningEffort };
+    reasoning?: { summary: 'auto' | 'detailed'; effort?: OpenAiReasoningEffort };
     instructions?: string;
   } {
     const systemMessages = options.oauthMode ? messages.filter((message): message is Extract<ChatMessage, { role: 'system' }> => message.role === 'system') : [];
@@ -141,16 +141,26 @@ export class OpenAiCodec {
       model: options.model,
       explicitEffort: options.reasoningEffort,
     });
+    // Non-reasoning API-key models (e.g. gpt-4.1) reject `reasoning.summary`
+    // with a 400, so only send the reasoning block when the model supports it.
+    // OAuth mode keeps its current behavior: the account sign-in allowlist is
+    // reasoning models only, and that path expects `summary: 'auto'`.
+    const includeReasoning =
+      options.oauthMode ||
+      Boolean(reasoningEffort) ||
+      ModelPolicyService.supportsReasoningEffort(options.model);
 
     return {
       model: options.model,
       input: OpenAiCodec.toResponseInput(inputMessages),
       tools: options.tools.length > 0 ? options.tools.map((tool) => OpenAiCodec.toResponseTool(tool)) : undefined,
       store: false,
-      reasoning: {
-        summary: options.oauthMode ? 'auto' : 'detailed',
-        ...(reasoningEffort ? { effort: reasoningEffort } : {}),
-      },
+      ...(includeReasoning ? {
+        reasoning: {
+          summary: options.oauthMode ? 'auto' as const : 'detailed' as const,
+          ...(reasoningEffort ? { effort: reasoningEffort } : {}),
+        },
+      } : {}),
       ...(instructions ? { instructions } : {}),
     };
   }