diff --git a/README.md b/README.md
index 8b99713..ea51886 100644
--- a/README.md
+++ b/README.md
@@ -17,3 +17,8 @@ Alternatively the project can be build and run with the following commands:
 
 Now open [http://localhost:9090/v1.0/ui/](http://localhost:9090/v1.0/ui/) in a browser to explore the API through Swagger UI
 
+# Running tests
+
+Unit tetsts are located in the "tests" folder and can be run witht the following command
+
+    python -m pytest
diff --git a/optimizerapi/openapi/specification.yml b/optimizerapi/openapi/specification.yml
index c7406f8..7123d2c 100644
--- a/optimizerapi/openapi/specification.yml
+++ b/optimizerapi/openapi/specification.yml
@@ -14,6 +14,11 @@ paths:
       responses:
         '200':
           description: Result of running the optimizer with the specified parameters
+          content:
+            text/plain:
+              schema:
+                type: string
+                example: hello
       parameters:
         - name: params
           in: query
@@ -21,4 +26,25 @@ paths:
           required: false
           schema:
             type: string
-            example: "test"
+            example: "[1,2,3]"
+        - name: Xi
+          in: query
+          description: "Xi value to be passed to ProcessOptimizer"
+          required: false
+          schema:
+            type: number
+            example: 0.01
+        - name: yi
+          in: query
+          description: "yi value to be passed to ProcessOptimizer"
+          required: false
+          schema:
+            type: number
+            example: 1.0
+        - name: kappa
+          in: query
+          description: "Kappa value to be passed to ProcessOptimizer"
+          required: false
+          schema:
+            type: number
+            example: 1.96
diff --git a/optimizerapi/optimizer.py b/optimizerapi/optimizer.py
index 62c23a9..1638adc 100644
--- a/optimizerapi/optimizer.py
+++ b/optimizerapi/optimizer.py
@@ -20,7 +20,7 @@ def run(params: str = None, Xi: [float] = [0.01], yi: [Number] = [1], kappa: flo
     }
     optimizer = Optimizer(space, **hyperparams)
     # TODO call optimizer with proper Xi and Yi values
-    result = optimizer.tell(Xi, yi)
+    result = optimizer.tell([Xi], yi)
     if params:
         return 'Run with {params} = {result}'.format(params=params, result=result)
     else:
diff --git a/tests/test_optimizer.py b/tests/test_optimizer.py
index 8023451..29b7696 100644
--- a/tests/test_optimizer.py
+++ b/tests/test_optimizer.py
@@ -3,6 +3,6 @@
 def test_first_sample():
     Xi = [0.01]
     yi = 1
-    result = optimizer.run(params=None, Xi=Xi, yi=yi)
+    result = optimizer.run(params="{}", Xi=Xi, yi=yi)
     print(result)
     assert "Run with" in result
\ No newline at end of file