diff --git a/backend/internal/service/account_test_service.go b/backend/internal/service/account_test_service.go
index 8419c2b4..c17aa3ff 100644
--- a/backend/internal/service/account_test_service.go
+++ b/backend/internal/service/account_test_service.go
@@ -332,7 +332,10 @@ func (s *AccountTestService) testOpenAIAccountConnection(c *gin.Context, account
 		if err != nil {
 			return s.sendErrorAndEnd(c, fmt.Sprintf("Invalid base URL: %s", err.Error()))
 		}
-		apiURL = strings.TrimSuffix(normalizedBaseURL, "/") + "/responses"
+		// Remove /chat/completions suffix if present, then add /responses
+		normalizedBaseURL = strings.TrimSuffix(normalizedBaseURL, "/")
+		normalizedBaseURL = strings.TrimSuffix(normalizedBaseURL, "/chat/completions")
+		apiURL = normalizedBaseURL + "/responses"
 	} else {
 		return s.sendErrorAndEnd(c, fmt.Sprintf("Unsupported account type: %s", account.Type))
 	}
diff --git a/backend/internal/service/openai_gateway_service.go b/backend/internal/service/openai_gateway_service.go
index 214da20a..0e228fb1 100644
--- a/backend/internal/service/openai_gateway_service.go
+++ b/backend/internal/service/openai_gateway_service.go
@@ -740,6 +740,9 @@ func (s *OpenAIGatewayService) buildUpstreamRequest(ctx context.Context, c *gin.
 			if err != nil {
 				return nil, err
 			}
+			// Remove /chat/completions suffix if present, then add /responses
+			validatedURL = strings.TrimSuffix(validatedURL, "/")
+			validatedURL = strings.TrimSuffix(validatedURL, "/chat/completions")
 			targetURL = validatedURL + "/responses"
 		}
 	default: