diff --git a/backend/internal/errors/codes.go b/backend/internal/errors/codes.go
new file mode 100644
index 0000000..739cfdb
--- /dev/null
+++ b/backend/internal/errors/codes.go
@@ -0,0 +1,33 @@
+package errors
+
+import "github.com/hhs/camtalk/internal/models"
+
+// 错误码常量，与 docs/03-接口文档.md 保持一致。
+const (
+	CodeInvalidMessage  = "INVALID_MESSAGE"
+	CodeSessionNotFound = "SESSION_NOT_FOUND"
+	CodeRateLimited     = "RATE_LIMITED"
+	CodeImageTooLarge   = "IMAGE_TOO_LARGE"
+	CodeAudioTooShort   = "AUDIO_TOO_SHORT"
+	CodeLLMTimeout      = "LLM_TIMEOUT"
+	CodeLLMError        = "LLM_ERROR"
+	CodeSTTError        = "STT_ERROR"
+	CodeTTSError        = "TTS_ERROR"
+	CodeInternalError   = "INTERNAL_ERROR"
+)
+
+// Sender 定义发送 WS 错误消息的接口，便于测试 mock。
+type Sender interface {
+	SendError(code, requestID, message string)
+}
+
+// SendWSError 向客户端发送 error 消息。
+// sender 是一个具有 sendJSON 方法的对象，这里用接口抽象。
+func SendWSError(sender interface{ SendJSON(v any) error }, code, requestID string, err error) {
+	_ = sender.SendJSON(models.WsError{
+		Type:      "error",
+		Code:      code,
+		RequestID: requestID,
+		Message:   err.Error(),
+	})
+}