diff --git a/relay/taskchannel/ali/adaptor.go b/relay/taskchannel/ali/adaptor.go
index b64f27d..4c89fe2 100644
--- a/relay/taskchannel/ali/adaptor.go
+++ b/relay/taskchannel/ali/adaptor.go
@@ -44,6 +44,19 @@ type dashScopeVideoParams struct {
 	Seed         *int   `json:"seed,omitempty"`
 }
 
+// aliMetadata Ali DashScope metadata 参数结构体（用于 UnmarshalMetadata 映射）
+type aliMetadata struct {
+	NegativePrompt string `json:"negative_prompt,omitempty"`
+	AudioURL       string `json:"audio_url,omitempty"`
+	Resolution     string `json:"resolution,omitempty"`
+	Ratio          string `json:"ratio,omitempty"`
+	Size           string `json:"size,omitempty"`
+	Duration       *int   `json:"duration,omitempty"`
+	PromptExtend   *bool  `json:"prompt_extend,omitempty"`
+	Watermark      *bool  `json:"watermark,omitempty"`
+	Seed           *int   `json:"seed,omitempty"`
+}
+
 // dashScopeImageRequest DashScope 图片生成请求
 type dashScopeImageRequest struct {
 	Model      string              `json:"model"`
@@ -130,10 +143,12 @@ func (a *AliAdaptor) EstimateBilling(_ context.Context, _ *common.RelayInfo, bod
 
 	if a.isVideo {
 		// duration 影响计费
-		if metadata, ok := req["metadata"].(map[string]any); ok {
-			if v, ok := metadata["duration"].(float64); ok && v > 0 {
-				ratios["duration"] = v
-			}
+		metadata := taskchannel.ExtractMetadata(req)
+		var meta struct {
+			Duration *int `json:"duration"`
+		}
+		if taskchannel.UnmarshalMetadata(metadata, &meta) == nil && meta.Duration != nil && *meta.Duration > 0 {
+			ratios["duration"] = float64(*meta.Duration)
 		}
 		if seconds, ok := req["seconds"].(string); ok {
 			if d, err := parseInt(seconds); err == nil && d > 0 {
@@ -202,55 +217,33 @@ func (a *AliAdaptor) buildVideoRequest(info *common.RelayInfo, req map[string]an
 	}
 
 	params := &dashScopeVideoParams{}
-	hasParams := false
 
-	if metadata, ok := req["metadata"].(map[string]any); ok {
-		if v, ok := metadata["negative_prompt"].(string); ok {
-			dsReq.Input.NegativePrompt = v
-		}
-		if v, ok := metadata["audio_url"].(string); ok {
-			dsReq.Input.AudioURL = v
-		}
-		if v, ok := metadata["resolution"].(string); ok {
-			params.Resolution = v
-			hasParams = true
-		}
-		if v, ok := metadata["ratio"].(string); ok {
-			params.Ratio = v
-			hasParams = true
-		}
-		if v, ok := metadata["size"].(string); ok {
-			params.Size = strings.ReplaceAll(v, "x", "*")
-			hasParams = true
-		}
-		if v, ok := metadata["duration"].(float64); ok && v > 0 {
-			d := int(v)
-			params.Duration = &d
-			hasParams = true
-		}
-		if v, ok := metadata["prompt_extend"].(bool); ok {
-			params.PromptExtend = &v
-			hasParams = true
-		}
-		if v, ok := metadata["watermark"].(bool); ok {
-			params.Watermark = &v
-			hasParams = true
-		}
-		if v, ok := metadata["seed"].(float64); ok {
-			s := int(v)
-			params.Seed = &s
-			hasParams = true
-		}
+	// 从 metadata 提取参数
+	metadata := taskchannel.ExtractMetadata(req)
+	var meta aliMetadata
+	if err := taskchannel.UnmarshalMetadata(metadata, &meta); err != nil {
+		return nil, fmt.Errorf("parse ali metadata: %w", err)
 	}
+	dsReq.Input.NegativePrompt = meta.NegativePrompt
+	dsReq.Input.AudioURL = meta.AudioURL
+	params.Resolution = meta.Resolution
+	params.Ratio = meta.Ratio
+	if meta.Size != "" {
+		params.Size = strings.ReplaceAll(meta.Size, "x", "*")
+	}
+	params.Duration = meta.Duration
+	params.PromptExtend = meta.PromptExtend
+	params.Watermark = meta.Watermark
+	params.Seed = meta.Seed
 
 	if seconds, ok := req["seconds"].(string); ok {
 		if d, err := parseInt(seconds); err == nil && d > 0 {
 			params.Duration = &d
-			hasParams = true
 		}
 	}
 
-	if hasParams {
+	if params.Resolution != "" || params.Ratio != "" || params.Size != "" ||
+		params.Duration != nil || params.PromptExtend != nil || params.Watermark != nil || params.Seed != nil {
 		dsReq.Parameters = params
 	}
 
diff --git a/relay/taskchannel/helpers.go b/relay/taskchannel/helpers.go
new file mode 100644
index 0000000..c7f0225
--- /dev/null
+++ b/relay/taskchannel/helpers.go
@@ -0,0 +1,52 @@
+package taskchannel
+
+import (
+	"encoding/json"
+	"fmt"
+)
+
+// ExtractMetadata 从已解析的请求 map 中提取 metadata 子 map。
+// 同时删除 "model" 字段防止通过 metadata 覆盖计费模型。
+func ExtractMetadata(req map[string]any) map[string]any {
+	if req == nil {
+		return nil
+	}
+	meta, ok := req["metadata"].(map[string]any)
+	if !ok {
+		return nil
+	}
+	delete(meta, "model")
+	return meta
+}
+
+// UnmarshalMetadata 将 metadata map 通过 JSON 往返反序列化到目标结构体。
+// metadata 为 nil 时直接返回 nil，不修改 target。
+func UnmarshalMetadata(metadata map[string]any, target any) error {
+	if len(metadata) == 0 {
+		return nil
+	}
+	metaBytes, err := json.Marshal(metadata)
+	if err != nil {
+		return fmt.Errorf("marshal metadata failed: %w", err)
+	}
+	if err := json.Unmarshal(metaBytes, target); err != nil {
+		return fmt.Errorf("unmarshal metadata failed: %w", err)
+	}
+	return nil
+}
+
+// DefaultString 返回 val（非空时），否则返回 fallback。
+func DefaultString(val, fallback string) string {
+	if val == "" {
+		return fallback
+	}
+	return val
+}
+
+// DefaultInt 返回 val（非零时），否则返回 fallback。
+func DefaultInt(val, fallback int) int {
+	if val == 0 {
+		return fallback
+	}
+	return val
+}
diff --git a/relay/taskchannel/kling/adaptor.go b/relay/taskchannel/kling/adaptor.go
index e57e241..1f86878 100644
--- a/relay/taskchannel/kling/adaptor.go
+++ b/relay/taskchannel/kling/adaptor.go
@@ -51,6 +51,19 @@ type klingRequest struct {
 	CameraControl  map[string]any `json:"camera_control,omitempty"`
 }
 
+// klingMetadata 可灵 metadata 参数结构体（用于 UnmarshalMetadata 映射）
+type klingMetadata struct {
+	NegativePrompt string         `json:"negative_prompt,omitempty"`
+	Mode           string         `json:"mode,omitempty"`
+	Duration       any            `json:"duration,omitempty"` // string 或 number
+	AspectRatio    string         `json:"aspect_ratio,omitempty"`
+	CfgScale       *float64       `json:"cfg_scale,omitempty"`
+	Image          string         `json:"image,omitempty"`
+	ImageTail      string         `json:"image_tail,omitempty"`
+	Sound          string         `json:"sound,omitempty"`
+	CameraControl  map[string]any `json:"camera_control,omitempty"`
+}
+
 // klingSubmitResponse 可灵任务提交响应
 type klingSubmitResponse struct {
 	Code    int    `json:"code"`
@@ -116,10 +129,12 @@ func (a *KlingAdaptor) EstimateBilling(_ context.Context, _ *common.RelayInfo, b
 	}
 
 	// 时长加价：10s = 2x
-	if metadata, ok := req["metadata"].(map[string]any); ok {
-		if v, ok := metadata["duration"].(float64); ok && v >= 10 {
-			ratios["duration_multiplier"] = 2.0
-		}
+	metadata := taskchannel.ExtractMetadata(req)
+	var meta struct {
+		Duration float64 `json:"duration"`
+	}
+	if taskchannel.UnmarshalMetadata(metadata, &meta) == nil && meta.Duration >= 10 {
+		ratios["duration_multiplier"] = 2.0
 	}
 	if seconds, ok := req["seconds"].(string); ok {
 		if d, err := parseInt(seconds); err == nil && d >= 10 {
@@ -143,10 +158,9 @@ func determineTaskType(req map[string]any) string {
 		}
 	}
 	// 有图片输入则 image2video
-	if metadata, ok := req["metadata"].(map[string]any); ok {
-		if _, hasImg := metadata["image"]; hasImg {
-			return "image2video"
-		}
+	metadata := taskchannel.ExtractMetadata(req)
+	if _, hasImg := metadata["image"]; hasImg {
+		return "image2video"
 	}
 	if _, hasImg := req["images"].([]any); hasImg {
 		return "image2video"
@@ -204,36 +218,25 @@ func (a *KlingAdaptor) BuildRequestBody(_ context.Context, info *common.RelayInf
 	}
 
 	// 从 metadata 提取参数
-	if metadata, ok := req["metadata"].(map[string]any); ok {
-		if v, ok := metadata["negative_prompt"].(string); ok {
-			kReq.NegativePrompt = v
-		}
-		if v, ok := metadata["mode"].(string); ok {
-			kReq.Mode = v
-		}
-		if v, ok := metadata["duration"].(string); ok {
-			kReq.Duration = v
-		} else if v, ok := metadata["duration"].(float64); ok {
-			kReq.Duration = fmt.Sprintf("%d", int(v))
-		}
-		if v, ok := metadata["aspect_ratio"].(string); ok {
-			kReq.AspectRatio = v
-		}
-		if v, ok := metadata["cfg_scale"].(float64); ok {
-			kReq.CfgScale = &v
-		}
-		if v, ok := metadata["image"].(string); ok {
-			kReq.Image = v
-		}
-		if v, ok := metadata["image_tail"].(string); ok {
-			kReq.ImageTail = v
-		}
-		if v, ok := metadata["sound"].(string); ok {
-			kReq.Sound = v
-		}
-		if v, ok := metadata["camera_control"].(map[string]any); ok {
-			kReq.CameraControl = v
-		}
+	metadata := taskchannel.ExtractMetadata(req)
+	var meta klingMetadata
+	if err := taskchannel.UnmarshalMetadata(metadata, &meta); err != nil {
+		return nil, fmt.Errorf("parse kling metadata: %w", err)
+	}
+	kReq.NegativePrompt = meta.NegativePrompt
+	kReq.Mode = meta.Mode
+	kReq.AspectRatio = meta.AspectRatio
+	kReq.CfgScale = meta.CfgScale
+	kReq.Image = meta.Image
+	kReq.ImageTail = meta.ImageTail
+	kReq.Sound = meta.Sound
+	kReq.CameraControl = meta.CameraControl
+	// duration 支持 string 或 number 两种类型
+	switch v := meta.Duration.(type) {
+	case string:
+		kReq.Duration = v
+	case float64:
+		kReq.Duration = fmt.Sprintf("%d", int(v))
 	}
 
 	// seconds 字段映射到 duration
diff --git a/relay/taskchannel/volcengine/adaptor.go b/relay/taskchannel/volcengine/adaptor.go
index dbbedfd..5f59df1 100644
--- a/relay/taskchannel/volcengine/adaptor.go
+++ b/relay/taskchannel/volcengine/adaptor.go
@@ -48,6 +48,18 @@ type submitRequest struct {
 	ReturnLastFrame *bool         `json:"return_last_frame,omitempty"`
 }
 
+// volcengineMetadata 火山引擎 metadata 参数结构体（用于 UnmarshalMetadata 映射）
+type volcengineMetadata struct {
+	Content         []contentItem `json:"content,omitempty"`
+	Resolution      string        `json:"resolution,omitempty"`
+	Ratio           string        `json:"ratio,omitempty"`
+	Duration        *int          `json:"duration,omitempty"`
+	Seed            *int          `json:"seed,omitempty"`
+	Watermark       *bool         `json:"watermark,omitempty"`
+	GenerateAudio   *bool         `json:"generate_audio,omitempty"`
+	ReturnLastFrame *bool         `json:"return_last_frame,omitempty"`
+}
+
 // submitResponse 提交任务响应
 type submitResponse struct {
 	ID string `json:"id"`
@@ -186,65 +198,20 @@ func (a *VolcengineVideoAdaptor) BuildRequestBody(_ context.Context, info *commo
 	}
 
 	// 从 metadata 中提取额外参数
-	if metadata, ok := req["metadata"].(map[string]any); ok {
-		// content 数组中的额外项（如 video_url、audio_url）
-		if contentRaw, ok := metadata["content"].([]any); ok {
-			for _, item := range contentRaw {
-				if itemMap, ok := item.(map[string]any); ok {
-					ci := contentItem{}
-					if t, ok := itemMap["type"].(string); ok {
-						ci.Type = t
-					}
-					if t, ok := itemMap["text"].(string); ok {
-						ci.Text = t
-					}
-					if imgURL, ok := itemMap["image_url"].(map[string]any); ok {
-						if u, ok := imgURL["url"].(string); ok {
-							ci.ImageURL = &mediaURL{URL: u}
-						}
-					}
-					if vidURL, ok := itemMap["video_url"].(map[string]any); ok {
-						if u, ok := vidURL["url"].(string); ok {
-							ci.VideoURL = &mediaURL{URL: u}
-						}
-					}
-					if audURL, ok := itemMap["audio_url"].(map[string]any); ok {
-						if u, ok := audURL["url"].(string); ok {
-							ci.AudioURL = &mediaURL{URL: u}
-						}
-					}
-					if ci.Type != "" || ci.Text != "" {
-						volcReq.Content = append(volcReq.Content, ci)
-					}
-				}
-			}
-		}
-
-		// 其他参数
-		if v, ok := metadata["resolution"].(string); ok {
-			volcReq.Resolution = v
-		}
-		if v, ok := metadata["ratio"].(string); ok {
-			volcReq.Ratio = v
-		}
-		if v, ok := metadata["duration"].(float64); ok && v > 0 {
-			d := int(v)
-			volcReq.Duration = &d
-		}
-		if v, ok := metadata["seed"].(float64); ok {
-			s := int(v)
-			volcReq.Seed = &s
-		}
-		if v, ok := metadata["watermark"].(bool); ok {
-			volcReq.Watermark = &v
-		}
-		if v, ok := metadata["generate_audio"].(bool); ok {
-			volcReq.GenerateAudio = &v
-		}
-		if v, ok := metadata["return_last_frame"].(bool); ok {
-			volcReq.ReturnLastFrame = &v
-		}
+	metadata := taskchannel.ExtractMetadata(req)
+	var meta volcengineMetadata
+	if err := taskchannel.UnmarshalMetadata(metadata, &meta); err != nil {
+		return nil, fmt.Errorf("parse volcengine metadata: %w", err)
 	}
+	// content 数组中的额外项（如 video_url、audio_url）
+	volcReq.Content = append(volcReq.Content, meta.Content...)
+	volcReq.Resolution = meta.Resolution
+	volcReq.Ratio = meta.Ratio
+	volcReq.Duration = meta.Duration
+	volcReq.Seed = meta.Seed
+	volcReq.Watermark = meta.Watermark
+	volcReq.GenerateAudio = meta.GenerateAudio
+	volcReq.ReturnLastFrame = meta.ReturnLastFrame
 
 	// 从根层级提取 resolution（支持 input.resolution 格式）
 	if v, ok := req["resolution"].(string); ok && v != "" && volcReq.Resolution == "" {
@@ -410,20 +377,19 @@ func (a *VolcengineVideoAdaptor) GetChannelName() string {
 
 // hasVideoInput 检测请求是否包含视频输入
 func hasVideoInput(req map[string]any) bool {
-	// 检查 metadata.content 中是否有 video_url 类型
-	metadata, _ := req["metadata"].(map[string]any)
-	if metadata == nil {
+	metadata := taskchannel.ExtractMetadata(req)
+	if len(metadata) == 0 {
 		return false
 	}
-	content, _ := metadata["content"].([]any)
-	for _, item := range content {
-		if m, ok := item.(map[string]any); ok {
-			if m["type"] == "video_url" {
-				return true
-			}
-			if _, has := m["video_url"]; has {
-				return true
-			}
+	var meta struct {
+		Content []contentItem `json:"content"`
+	}
+	if taskchannel.UnmarshalMetadata(metadata, &meta) != nil {
+		return false
+	}
+	for _, c := range meta.Content {
+		if c.Type == "video_url" || c.VideoURL != nil {
+			return true
 		}
 	}
 	return false
@@ -449,10 +415,12 @@ func extractDuration(req map[string]any) int {
 		}
 	}
 	// 从 metadata 提取
-	if metadata, ok := req["metadata"].(map[string]any); ok {
-		if d, ok := metadata["duration"].(float64); ok && d > 0 {
-			return int(d)
-		}
+	metadata := taskchannel.ExtractMetadata(req)
+	var meta struct {
+		Duration *int `json:"duration"`
+	}
+	if taskchannel.UnmarshalMetadata(metadata, &meta) == nil && meta.Duration != nil && *meta.Duration > 0 {
+		return *meta.Duration
 	}
 	if s, ok := req["seconds"].(string); ok {
 		if d, err := parseInt(s); err == nil && d > 0 {
@@ -469,10 +437,12 @@ func extractResolution(req map[string]any) string {
 		return r
 	}
 	// 从 metadata 提取
-	if metadata, ok := req["metadata"].(map[string]any); ok {
-		if r, ok := metadata["resolution"].(string); ok && r != "" {
-			return r
-		}
+	metadata := taskchannel.ExtractMetadata(req)
+	var meta struct {
+		Resolution string `json:"resolution"`
+	}
+	if taskchannel.UnmarshalMetadata(metadata, &meta) == nil && meta.Resolution != "" {
+		return meta.Resolution
 	}
 	return "720p"
 }