Merge pull request #1428 from YanzheL/fix/openai-gateway-content-session-hash-fallback

fix(gateway): add content-based session hash fallback for non-Codex clients

backend/internal/service/openai_gateway_service.go

@@ -1121,6 +1121,7 @@ func (s *OpenAIGatewayService) ExtractSessionID(c *gin.Context, body []byte) str
 //  1. Header: session_id
 //  2. Header: conversation_id
 //  3. Body:   prompt_cache_key (opencode)
+//  4. Body:   content-based fallback (model + system + tools + first user message)
 func (s *OpenAIGatewayService) GenerateSessionHash(c *gin.Context, body []byte) string {
 	if c == nil {
 		return ""
@@ -1133,6 +1134,9 @@ func (s *OpenAIGatewayService) GenerateSessionHash(c *gin.Context, body []byte)
 	if sessionID == "" && len(body) > 0 {
 		sessionID = strings.TrimSpace(gjson.GetBytes(body, "prompt_cache_key").String())
 	}
+	if sessionID == "" && len(body) > 0 {
+		sessionID = deriveOpenAIContentSessionSeed(body)
+	}
 	if sessionID == "" {
 		return ""
 	}