fix(openai): 统一 OAuth instructions 处理逻辑，修复 Codex CLI 400 错误

- 修改 applyCodexOAuthTransform 函数签名，增加 isCodexCLI 参数
- 移除 && !isCodexCLI 条件，对所有 OAuth 请求统一处理
- 新增 applyInstructions/applyCodexCLIInstructions/applyOpenCodeInstructions 辅助函数
- 新增 isInstructionsEmpty 函数检查 instructions 字段是否为空
- 添加 Codex CLI 和非 Codex CLI 场景的测试用例

逻辑说明：
- Codex CLI + 有 instructions: 保持不变
- Codex CLI + 无 instructions: 补充 opencode 指令
- 非 Codex CLI: 使用 opencode 指令覆盖

backend/internal/service/openai_codex_transform.go

@@ -72,7 +72,7 @@ type opencodeCacheMetadata struct {
 	LastChecked int64  `json:"lastChecked"`
 }
 
-func applyCodexOAuthTransform(reqBody map[string]any) codexTransformResult {
+func applyCodexOAuthTransform(reqBody map[string]any, isCodexCLI bool) codexTransformResult {
 	result := codexTransformResult{}
 	// 工具续链需求会影响存储策略与 input 过滤逻辑。
 	needsToolContinuation := NeedsToolContinuation(reqBody)
@@ -118,23 +118,10 @@ func applyCodexOAuthTransform(reqBody map[string]any) codexTransformResult {
 		result.PromptCacheKey = strings.TrimSpace(v)
 	}
 
-	instructions := strings.TrimSpace(getOpenCodeCodexHeader())
-	existingInstructions, _ := reqBody["instructions"].(string)
-	existingInstructions = strings.TrimSpace(existingInstructions)
-
-	if instructions != "" {
-		if existingInstructions != instructions {
-			reqBody["instructions"] = instructions
+	// instructions 处理逻辑：根据是否是 Codex CLI 分别调用不同方法
+	if applyInstructions(reqBody, isCodexCLI) {
 		result.Modified = true
 	}
-	} else if existingInstructions == "" {
-		// 未获取到 opencode 指令时，回退使用 Codex CLI 指令。
-		codexInstructions := strings.TrimSpace(getCodexCLIInstructions())
-		if codexInstructions != "" {
-			reqBody["instructions"] = codexInstructions
-			result.Modified = true
-		}
-	}
 
 	// 续链场景保留 item_reference 与 id，避免 call_id 上下文丢失。
 	if input, ok := reqBody["input"].([]any); ok {
@@ -276,6 +263,72 @@ func GetCodexCLIInstructions() string {
 	return getCodexCLIInstructions()
 }
 
+// applyInstructions 处理 instructions 字段
+// isCodexCLI=true: 仅补充缺失的 instructions（使用 opencode 指令）
+// isCodexCLI=false: 优先使用 opencode 指令覆盖
+func applyInstructions(reqBody map[string]any, isCodexCLI bool) bool {
+	if isCodexCLI {
+		return applyCodexCLIInstructions(reqBody)
+	}
+	return applyOpenCodeInstructions(reqBody)
+}
+
+// applyCodexCLIInstructions 为 Codex CLI 请求补充缺失的 instructions
+// 仅在 instructions 为空时添加 opencode 指令
+func applyCodexCLIInstructions(reqBody map[string]any) bool {
+	if !isInstructionsEmpty(reqBody) {
+		return false // 已有有效 instructions，不修改
+	}
+
+	instructions := strings.TrimSpace(getOpenCodeCodexHeader())
+	if instructions != "" {
+		reqBody["instructions"] = instructions
+		return true
+	}
+
+	return false
+}
+
+// applyOpenCodeInstructions 为非 Codex CLI 请求应用 opencode 指令
+// 优先使用 opencode 指令覆盖
+func applyOpenCodeInstructions(reqBody map[string]any) bool {
+	instructions := strings.TrimSpace(getOpenCodeCodexHeader())
+	existingInstructions, _ := reqBody["instructions"].(string)
+	existingInstructions = strings.TrimSpace(existingInstructions)
+
+	if instructions != "" {
+		if existingInstructions != instructions {
+			reqBody["instructions"] = instructions
+			return true
+		}
+	} else if existingInstructions == "" {
+		codexInstructions := strings.TrimSpace(getCodexCLIInstructions())
+		if codexInstructions != "" {
+			reqBody["instructions"] = codexInstructions
+			return true
+		}
+	}
+
+	return false
+}
+
+// isInstructionsEmpty 检查 instructions 字段是否为空
+// 处理以下情况：字段不存在、nil、空字符串、纯空白字符串
+func isInstructionsEmpty(reqBody map[string]any) bool {
+	val, exists := reqBody["instructions"]
+	if !exists {
+		return true
+	}
+	if val == nil {
+		return true
+	}
+	str, ok := val.(string)
+	if !ok {
+		return true
+	}
+	return strings.TrimSpace(str) == ""
+}
+
 // ReplaceWithCodexInstructions 将请求 instructions 替换为内置 Codex 指令（必要时）。
 func ReplaceWithCodexInstructions(reqBody map[string]any) bool {
 	codexInstructions := strings.TrimSpace(getCodexCLIInstructions())

backend/internal/service/openai_codex_transform_test.go

@@ -23,7 +23,7 @@ func TestApplyCodexOAuthTransform_ToolContinuationPreservesInput(t *testing.T) {
 		"tool_choice": "auto",
 	}
 
-	applyCodexOAuthTransform(reqBody)
+	applyCodexOAuthTransform(reqBody, false)
 
 	// 未显式设置 store=true，默认为 false。
 	store, ok := reqBody["store"].(bool)
@@ -59,7 +59,7 @@ func TestApplyCodexOAuthTransform_ExplicitStoreFalsePreserved(t *testing.T) {
 		"tool_choice": "auto",
 	}
 
-	applyCodexOAuthTransform(reqBody)
+	applyCodexOAuthTransform(reqBody, false)
 
 	store, ok := reqBody["store"].(bool)
 	require.True(t, ok)
@@ -79,7 +79,7 @@ func TestApplyCodexOAuthTransform_ExplicitStoreTrueForcedFalse(t *testing.T) {
 		"tool_choice": "auto",
 	}
 
-	applyCodexOAuthTransform(reqBody)
+	applyCodexOAuthTransform(reqBody, false)
 
 	store, ok := reqBody["store"].(bool)
 	require.True(t, ok)
@@ -97,7 +97,7 @@ func TestApplyCodexOAuthTransform_NonContinuationDefaultsStoreFalseAndStripsIDs(
 		},
 	}
 
-	applyCodexOAuthTransform(reqBody)
+	applyCodexOAuthTransform(reqBody, false)
 
 	store, ok := reqBody["store"].(bool)
 	require.True(t, ok)
@@ -148,7 +148,7 @@ func TestApplyCodexOAuthTransform_NormalizeCodexTools_PreservesResponsesFunction
 		},
 	}
 
-	applyCodexOAuthTransform(reqBody)
+	applyCodexOAuthTransform(reqBody, false)
 
 	tools, ok := reqBody["tools"].([]any)
 	require.True(t, ok)
@@ -169,7 +169,7 @@ func TestApplyCodexOAuthTransform_EmptyInput(t *testing.T) {
 		"input": []any{},
 	}
 
-	applyCodexOAuthTransform(reqBody)
+	applyCodexOAuthTransform(reqBody, false)
 
 	input, ok := reqBody["input"].([]any)
 	require.True(t, ok)
@@ -196,3 +196,77 @@ func setupCodexCache(t *testing.T) {
 	require.NoError(t, err)
 	require.NoError(t, os.WriteFile(filepath.Join(cacheDir, "opencode-codex-header-meta.json"), data, 0o644))
 }
+
+func TestApplyCodexOAuthTransform_CodexCLI_PreservesExistingInstructions(t *testing.T) {
+	// Codex CLI 场景：已有 instructions 时不修改
+	setupCodexCache(t)
+
+	reqBody := map[string]any{
+		"model":        "gpt-5.1",
+		"instructions": "existing instructions",
+	}
+
+	result := applyCodexOAuthTransform(reqBody, true) // isCodexCLI=true
+
+	instructions, ok := reqBody["instructions"].(string)
+	require.True(t, ok)
+	require.Equal(t, "existing instructions", instructions)
+	// Modified 仍可能为 true（因为其他字段被修改），但 instructions 应保持不变
+	_ = result
+}
+
+func TestApplyCodexOAuthTransform_CodexCLI_SuppliesDefaultWhenEmpty(t *testing.T) {
+	// Codex CLI 场景：无 instructions 时补充默认值
+	setupCodexCache(t)
+
+	reqBody := map[string]any{
+		"model": "gpt-5.1",
+		// 没有 instructions 字段
+	}
+
+	result := applyCodexOAuthTransform(reqBody, true) // isCodexCLI=true
+
+	instructions, ok := reqBody["instructions"].(string)
+	require.True(t, ok)
+	require.NotEmpty(t, instructions)
+	require.True(t, result.Modified)
+}
+
+func TestApplyCodexOAuthTransform_NonCodexCLI_OverridesInstructions(t *testing.T) {
+	// 非 Codex CLI 场景：使用 opencode 指令覆盖
+	setupCodexCache(t)
+
+	reqBody := map[string]any{
+		"model":        "gpt-5.1",
+		"instructions": "old instructions",
+	}
+
+	result := applyCodexOAuthTransform(reqBody, false) // isCodexCLI=false
+
+	instructions, ok := reqBody["instructions"].(string)
+	require.True(t, ok)
+	require.NotEqual(t, "old instructions", instructions)
+	require.True(t, result.Modified)
+}
+
+func TestIsInstructionsEmpty(t *testing.T) {
+	tests := []struct {
+		name     string
+		reqBody  map[string]any
+		expected bool
+	}{
+		{"missing field", map[string]any{}, true},
+		{"nil value", map[string]any{"instructions": nil}, true},
+		{"empty string", map[string]any{"instructions": ""}, true},
+		{"whitespace only", map[string]any{"instructions": "   "}, true},
+		{"non-string", map[string]any{"instructions": 123}, true},
+		{"valid string", map[string]any{"instructions": "hello"}, false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			result := isInstructionsEmpty(tt.reqBody)
+			require.Equal(t, tt.expected, result)
+		})
+	}
+}

backend/internal/service/openai_gateway_service.go

@@ -796,8 +796,8 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 		}
 	}
 
-	if account.Type == AccountTypeOAuth && !isCodexCLI {
-		codexResult := applyCodexOAuthTransform(reqBody)
+	if account.Type == AccountTypeOAuth {
+		codexResult := applyCodexOAuthTransform(reqBody, isCodexCLI)
 		if codexResult.Modified {
 			bodyModified = true
 		}