feat(gateway): 优化 Antigravity/Gemini 思考块处理

此提交解决了思考块 (thinking blocks) 在转发过程中的兼容性问题。

主要变更：

1. **思考块优化 (Thinking Blocks)**：
   - 在 AntigravityGatewayService 中增加了 sanitizeThinkingBlocks 处理，强制移除思考块中不支持的 cache_control 字段（避免 Anthropic/Vertex AI 报错）
   - 实现历史思考块展平 (Flattening)：将非最后一条消息中的思考块转换为普通文本块，以绕过上游对历史思考块签名的严格校验
   - 增加 cleanCacheControlFromGeminiJSON 作为最后一道防线，确保转换后的 Gemini 请求中不残留非法的 cache_control

2. **GatewayService 缓存控制优化**：
   - 更新缓存控制逻辑，跳过 thinking 块（thinking 块不支持 cache_control 字段）
   - 增加 removeCacheControlFromThinkingBlocks 函数强制清理

关联 Issue: #225

backend/internal/handler/gateway_handler.go

@@ -261,12 +261,11 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 				var failoverErr *service.UpstreamFailoverError
 				if errors.As(err, &failoverErr) {
 					failedAccountIDs[account.ID] = struct{}{}
+					lastFailoverStatus = failoverErr.StatusCode
 					if switchCount >= maxAccountSwitches {
-						lastFailoverStatus = failoverErr.StatusCode
 						h.handleFailoverExhausted(c, lastFailoverStatus, streamStarted)
 						return
 					}
-					lastFailoverStatus = failoverErr.StatusCode
 					switchCount++
 					log.Printf("Account %d: upstream error %d, switching account %d/%d", account.ID, failoverErr.StatusCode, switchCount, maxAccountSwitches)
 					continue
@@ -390,12 +389,11 @@ func (h *GatewayHandler) Messages(c *gin.Context) {
 			var failoverErr *service.UpstreamFailoverError
 			if errors.As(err, &failoverErr) {
 				failedAccountIDs[account.ID] = struct{}{}
+				lastFailoverStatus = failoverErr.StatusCode
 				if switchCount >= maxAccountSwitches {
-					lastFailoverStatus = failoverErr.StatusCode
 					h.handleFailoverExhausted(c, lastFailoverStatus, streamStarted)
 					return
 				}
-				lastFailoverStatus = failoverErr.StatusCode
 				switchCount++
 				log.Printf("Account %d: upstream error %d, switching account %d/%d", account.ID, failoverErr.StatusCode, switchCount, maxAccountSwitches)
 				continue

backend/internal/service/antigravity_gateway_service.go

@@ -523,6 +523,9 @@ func (s *AntigravityGatewayService) Forward(ctx context.Context, c *gin.Context,
 		proxyURL = account.Proxy.URL()
 	}
 
+	// Sanitize thinking blocks (clean cache_control and flatten history thinking)
+	sanitizeThinkingBlocks(&claudeReq)
+
 	// 获取转换选项
 	// Antigravity 上游要求必须包含身份提示词，否则会返回 429
 	transformOpts := s.getClaudeTransformOptions(ctx)
@@ -534,6 +537,9 @@ func (s *AntigravityGatewayService) Forward(ctx context.Context, c *gin.Context,
 		return nil, fmt.Errorf("transform request: %w", err)
 	}
 
+	// Safety net: ensure no cache_control leaked into Gemini request
+	geminiBody = cleanCacheControlFromGeminiJSON(geminiBody)
+
 	// Antigravity 上游只支持流式请求，统一使用 streamGenerateContent
 	// 如果客户端请求非流式，在响应处理阶段会收集完整流式响应后转换返回
 	action := "streamGenerateContent"
@@ -787,6 +793,143 @@ func extractAntigravityErrorMessage(body []byte) string {
 	return ""
 }
 
+// cleanCacheControlFromGeminiJSON removes cache_control from Gemini JSON (emergency fix)
+// This should not be needed if transformation is correct, but serves as a safety net
+func cleanCacheControlFromGeminiJSON(body []byte) []byte {
+	// Try a more robust approach: parse and clean
+	var data map[string]any
+	if err := json.Unmarshal(body, &data); err != nil {
+		log.Printf("[Antigravity] Failed to parse Gemini JSON for cache_control cleaning: %v", err)
+		return body
+	}
+
+	cleaned := removeCacheControlFromAny(data)
+	if !cleaned {
+		return body
+	}
+
+	if result, err := json.Marshal(data); err == nil {
+		log.Printf("[Antigravity] Successfully cleaned cache_control from Gemini JSON")
+		return result
+	}
+
+	return body
+}
+
+// removeCacheControlFromAny recursively removes cache_control fields
+func removeCacheControlFromAny(v any) bool {
+	cleaned := false
+
+	switch val := v.(type) {
+	case map[string]any:
+		for k, child := range val {
+			if k == "cache_control" {
+				delete(val, k)
+				cleaned = true
+			} else if removeCacheControlFromAny(child) {
+				cleaned = true
+			}
+		}
+	case []any:
+		for _, item := range val {
+			if removeCacheControlFromAny(item) {
+				cleaned = true
+			}
+		}
+	}
+
+	return cleaned
+}
+
+// sanitizeThinkingBlocks cleans cache_control and flattens history thinking blocks
+// Thinking blocks do NOT support cache_control field (Anthropic API/Vertex AI requirement)
+// Additionally, history thinking blocks are flattened to text to avoid upstream validation errors
+func sanitizeThinkingBlocks(req *antigravity.ClaudeRequest) {
+	if req == nil {
+		return
+	}
+
+	log.Printf("[Antigravity] sanitizeThinkingBlocks: processing request with %d messages", len(req.Messages))
+
+	// Clean system blocks
+	if len(req.System) > 0 {
+		var systemBlocks []map[string]any
+		if err := json.Unmarshal(req.System, &systemBlocks); err == nil {
+			for i := range systemBlocks {
+				if blockType, _ := systemBlocks[i]["type"].(string); blockType == "thinking" || systemBlocks[i]["thinking"] != nil {
+					if removeCacheControlFromAny(systemBlocks[i]) {
+						log.Printf("[Antigravity] Deep cleaned cache_control from thinking block in system[%d]", i)
+					}
+				}
+			}
+			// Marshal back
+			if cleaned, err := json.Marshal(systemBlocks); err == nil {
+				req.System = cleaned
+			}
+		}
+	}
+
+	// Clean message content blocks and flatten history
+	lastMsgIdx := len(req.Messages) - 1
+	for msgIdx := range req.Messages {
+		raw := req.Messages[msgIdx].Content
+		if len(raw) == 0 {
+			continue
+		}
+
+		// Try to parse as blocks array
+		var blocks []map[string]any
+		if err := json.Unmarshal(raw, &blocks); err != nil {
+			continue
+		}
+
+		cleaned := false
+		for blockIdx := range blocks {
+			blockType, _ := blocks[blockIdx]["type"].(string)
+
+			// Check for thinking blocks (typed or untyped)
+			if blockType == "thinking" || blocks[blockIdx]["thinking"] != nil {
+				// 1. Clean cache_control
+				if removeCacheControlFromAny(blocks[blockIdx]) {
+					log.Printf("[Antigravity] Deep cleaned cache_control from thinking block in messages[%d].content[%d]", msgIdx, blockIdx)
+					cleaned = true
+				}
+
+				// 2. Flatten to text if it's a history message (not the last one)
+				if msgIdx < lastMsgIdx {
+					log.Printf("[Antigravity] Flattening history thinking block to text at messages[%d].content[%d]", msgIdx, blockIdx)
+
+					// Extract thinking content
+					var textContent string
+					if t, ok := blocks[blockIdx]["thinking"].(string); ok {
+						textContent = t
+					} else {
+						// Fallback for non-string content (marshal it)
+						if b, err := json.Marshal(blocks[blockIdx]["thinking"]); err == nil {
+							textContent = string(b)
+						}
+					}
+
+					// Convert to text block
+					blocks[blockIdx]["type"] = "text"
+					blocks[blockIdx]["text"] = textContent
+					delete(blocks[blockIdx], "thinking")
+					delete(blocks[blockIdx], "signature")
+					delete(blocks[blockIdx], "cache_control") // Ensure it's gone
+					cleaned = true
+				}
+			}
+		}
+
+		// Marshal back if modified
+		if cleaned {
+			if marshaled, err := json.Marshal(blocks); err == nil {
+				req.Messages[msgIdx].Content = marshaled
+			}
+		}
+	}
+}
+
 // stripThinkingFromClaudeRequest converts thinking blocks to text blocks in a Claude Messages request.
 // This preserves the thinking content while avoiding signature validation errors.
 // Note: redacted_thinking blocks are removed because they cannot be converted to text.

backend/internal/service/gateway_service.go

@@ -1188,6 +1188,9 @@ func enforceCacheControlLimit(body []byte) []byte {
 		return body
 	}
 
+	// 清理 thinking 块中的非法 cache_control（thinking 块不支持该字段）
+	removeCacheControlFromThinkingBlocks(data)
+
 	// 计算当前 cache_control 块数量
 	count := countCacheControlBlocks(data)
 	if count <= maxCacheControlBlocks {
@@ -1215,6 +1218,7 @@ func enforceCacheControlLimit(body []byte) []byte {
 }
 
 // countCacheControlBlocks 统计 system 和 messages 中的 cache_control 块数量
+// 注意：thinking 块不支持 cache_control，统计时跳过
 func countCacheControlBlocks(data map[string]any) int {
 	count := 0
 
@@ -1222,6 +1226,10 @@ func countCacheControlBlocks(data map[string]any) int {
 	if system, ok := data["system"].([]any); ok {
 		for _, item := range system {
 			if m, ok := item.(map[string]any); ok {
+				// thinking 块不支持 cache_control，跳过
+				if blockType, _ := m["type"].(string); blockType == "thinking" {
+					continue
+				}
 				if _, has := m["cache_control"]; has {
 					count++
 				}
@@ -1236,6 +1244,10 @@ func countCacheControlBlocks(data map[string]any) int {
 				if content, ok := msgMap["content"].([]any); ok {
 					for _, item := range content {
 						if m, ok := item.(map[string]any); ok {
+							// thinking 块不支持 cache_control，跳过
+							if blockType, _ := m["type"].(string); blockType == "thinking" {
+								continue
+							}
 							if _, has := m["cache_control"]; has {
 								count++
 							}
@@ -1251,6 +1263,7 @@ func countCacheControlBlocks(data map[string]any) int {
 
 // removeCacheControlFromMessages 从 messages 中移除一个 cache_control（从头开始）
 // 返回 true 表示成功移除，false 表示没有可移除的
+// 注意：跳过 thinking 块（它不支持 cache_control）
 func removeCacheControlFromMessages(data map[string]any) bool {
 	messages, ok := data["messages"].([]any)
 	if !ok {
@@ -1268,6 +1281,10 @@ func removeCacheControlFromMessages(data map[string]any) bool {
 		}
 		for _, item := range content {
 			if m, ok := item.(map[string]any); ok {
+				// thinking 块不支持 cache_control，跳过
+				if blockType, _ := m["type"].(string); blockType == "thinking" {
+					continue
+				}
 				if _, has := m["cache_control"]; has {
 					delete(m, "cache_control")
 					return true
@@ -1280,6 +1297,7 @@ func removeCacheControlFromMessages(data map[string]any) bool {
 
 // removeCacheControlFromSystem 从 system 中移除一个 cache_control（从尾部开始，保护注入的 prompt）
 // 返回 true 表示成功移除，false 表示没有可移除的
+// 注意：跳过 thinking 块（它不支持 cache_control）
 func removeCacheControlFromSystem(data map[string]any) bool {
 	system, ok := data["system"].([]any)
 	if !ok {
@@ -1289,6 +1307,10 @@ func removeCacheControlFromSystem(data map[string]any) bool {
 	// 从尾部开始移除，保护开头注入的 Claude Code prompt
 	for i := len(system) - 1; i >= 0; i-- {
 		if m, ok := system[i].(map[string]any); ok {
+			// thinking 块不支持 cache_control，跳过
+			if blockType, _ := m["type"].(string); blockType == "thinking" {
+				continue
+			}
 			if _, has := m["cache_control"]; has {
 				delete(m, "cache_control")
 				return true
@@ -1298,6 +1320,44 @@ func removeCacheControlFromSystem(data map[string]any) bool {
 	return false
 }
 
+// removeCacheControlFromThinkingBlocks 强制清理所有 thinking 块中的非法 cache_control
+// thinking 块不支持 cache_control 字段，这个函数确保所有 thinking 块都不含该字段
+func removeCacheControlFromThinkingBlocks(data map[string]any) {
+	// 清理 system 中的 thinking 块
+	if system, ok := data["system"].([]any); ok {
+		for _, item := range system {
+			if m, ok := item.(map[string]any); ok {
+				if blockType, _ := m["type"].(string); blockType == "thinking" {
+					if _, has := m["cache_control"]; has {
+						delete(m, "cache_control")
+						log.Printf("[Warning] Removed illegal cache_control from thinking block in system")
+					}
+				}
+			}
+		}
+	}
+
+	// 清理 messages 中的 thinking 块
+	if messages, ok := data["messages"].([]any); ok {
+		for msgIdx, msg := range messages {
+			if msgMap, ok := msg.(map[string]any); ok {
+				if content, ok := msgMap["content"].([]any); ok {
+					for contentIdx, item := range content {
+						if m, ok := item.(map[string]any); ok {
+							if blockType, _ := m["type"].(string); blockType == "thinking" {
+								if _, has := m["cache_control"]; has {
+									delete(m, "cache_control")
+									log.Printf("[Warning] Removed illegal cache_control from thinking block in messages[%d].content[%d]", msgIdx, contentIdx)
+								}
+							}
+						}
+					}
+				}
+			}
+		}
+	}
+}
+
 // Forward 转发请求到Claude API
 func (s *GatewayService) Forward(ctx context.Context, c *gin.Context, account *Account, parsed *ParsedRequest) (*ForwardResult, error) {
 	startTime := time.Now()