diff --git a/api/src/engine/engine.controller.ts b/api/src/engine/engine.controller.ts
index 6c5949493e6c59ddc759d0f83b9c4d363a578a9e..fe65f352e83e325879d414855a582ad5fec8619a 100644
--- a/api/src/engine/engine.controller.ts
+++ b/api/src/engine/engine.controller.ts
@@ -20,17 +20,17 @@ export class EngineController {
   ) {}
 
   @Get('/algorithms')
-  getAlgorithms(@Req() request: Request): Observable<string> {
+  getAlgorithms(@Req() request: Request): Observable<string> | string {
     return this.engineService.getAlgorithmsREST(request);
   }
 
   @Get('/experiments')
-  getExperiments(@Req() request: Request): Observable<string> {
+  getExperiments(@Req() request: Request): Observable<string> | string {
     return this.engineService.getExperiments(request);
   }
 
   @Get('/experiments/:uuid')
-  getExperiment(@Param('uuid') uuid: string): Observable<string> {
+  getExperiment(@Param('uuid') uuid: string): Observable<string> | string {
     return this.engineService.getExperimentREST(uuid);
   }
 
@@ -38,7 +38,7 @@ export class EngineController {
   deleteExperiment(
     @Param('uuid') uuid: string,
     @Req() request: Request,
-  ): Observable<string> {
+  ): Observable<string> | string {
     return this.engineService.deleteExperiment(uuid, request);
   }
 
@@ -46,27 +46,29 @@ export class EngineController {
   editExperiment(
     @Param('uuid') uuid: string,
     @Req() request: Request,
-  ): Observable<string> {
+  ): Observable<string> | string {
     return this.engineService.editExperimentREST(uuid, request);
   }
 
   @Post('experiments/transient')
-  startExperimentTransient(@Req() request: Request): Observable<string> {
+  startExperimentTransient(
+    @Req() request: Request,
+  ): Observable<string> | string {
     return this.engineService.startExperimentTransient(request);
   }
 
   @Post('experiments')
-  startExperiment(@Req() request: Request): Observable<string> {
+  startExperiment(@Req() request: Request): Observable<string> | string {
     return this.engineService.startExperiment(request);
   }
 
   @Get('activeUser')
-  getActiveUser(@Req() request: Request): Observable<string> {
+  getActiveUser(@Req() request: Request): Observable<string> | string {
     return this.engineService.getActiveUser(request);
   }
 
   @Post('activeUser/agreeNDA')
-  agreeNDA(@Req() request: Request): Observable<string> {
+  agreeNDA(@Req() request: Request): Observable<string> | string {
     return this.engineService.editActiveUser(request);
   }
 }
diff --git a/api/src/engine/engine.interfaces.ts b/api/src/engine/engine.interfaces.ts
index 26818b01868edbd7191461de19ed904310a15d1e..e3f68aa1b75967c8d9cca9b8c52c4a120f0d7a43 100644
--- a/api/src/engine/engine.interfaces.ts
+++ b/api/src/engine/engine.interfaces.ts
@@ -43,21 +43,24 @@ export interface IEngineService {
   getAlgorithms(): Promise<Algorithm[]> | Algorithm[];
 
   // Standard REST API call
-  getAlgorithmsREST(request: Request): Observable<string>;
+  getAlgorithmsREST(request: Request): Observable<string> | string;
 
-  getExperiments(request: Request): Observable<string>;
+  getExperiments(request: Request): Observable<string> | string;
 
-  getExperimentREST(uuid: string): Observable<string>;
+  getExperimentREST(uuid: string): Observable<string> | string;
 
-  deleteExperiment(uuid: string, request: Request): Observable<string>;
+  deleteExperiment(uuid: string, request: Request): Observable<string> | string;
 
-  editExperimentREST(uuid: string, request: Request): Observable<string>;
+  editExperimentREST(
+    uuid: string,
+    request: Request,
+  ): Observable<string> | string;
 
-  startExperimentTransient(request: Request): Observable<string>;
+  startExperimentTransient(request: Request): Observable<string> | string;
 
-  startExperiment(request: Request): Observable<string>;
+  startExperiment(request: Request): Observable<string> | string;
 
-  getActiveUser(request: Request): Observable<string>;
+  getActiveUser(request: Request): Observable<string> | string;
 
-  editActiveUser(request: Request): Observable<string>;
+  editActiveUser(request: Request): Observable<string> | string;
 }