diff --git a/src/tools/step_manager.py b/src/tools/step_manager.py
index 46ef149..2a56fac 100644
--- a/src/tools/step_manager.py
+++ b/src/tools/step_manager.py
@@ -88,7 +88,7 @@ async def add_body_to_step(
                     request_step_body["body"] = safe_json
                 except json.JSONDecodeError as e:
                     return BaseResult(error=f"Invalid JSON content provided for body_content: {str(e)}")
-                request_headers["Content-Type"] = "application/json"
+                request_headers["Content-Type"] = ["application/json"]
 
             case "xml":
                 try:
@@ -101,7 +101,7 @@ async def add_body_to_step(
                     return BaseResult(error=f"Invalid XML content provided for body_content: {str(e)}")
                 except Exception as e:
                     return BaseResult(error=f"Error processing XML content: {str(e)}")
-                request_headers["Content-Type"] = "application/xml"
+                request_headers["Content-Type"] = ["application/xml"]
 
             case "html":
                 try:
@@ -110,7 +110,7 @@ async def add_body_to_step(
                     request_step_body["body"] = safe_html
                 except Exception as e:
                     return BaseResult(error=f"Error processing HTML content: {str(e)}")
-                request_headers["Content-Type"] = "text/html"
+                request_headers["Content-Type"] = ["text/html"]
 
             case "text":
                 # Remove any null bytes and control characters except newlines/tabs
@@ -121,9 +121,9 @@ async def add_body_to_step(
                     request_step_body["body"] = safe_text
                 except Exception as e:
                     return BaseResult(error=f"Error processing text content: {str(e)}")
-                request_headers["Content-Type"] = "text/plain"
+                request_headers["Content-Type"] = ["text/plain"]
             case _:
-                BaseResult(
+                return BaseResult(
                     error=f"Unsupported body_type {body_type}. Supported types are: json, xml, html, text"
                 )
 
@@ -156,7 +156,9 @@ async def add_assertion_to_step(
     ) -> BaseResult:
         request_result = await self.read(bucket_key, test_id, step_id, result_formatter=None)
         if not request_result or request_result.get("step_type") != "request":
-            return BaseResult(error=f"Step {step_id} is not a request step and cannot have a body added.")
+            return BaseResult(
+                error=f"Step {step_id} is not a request step and cannot have an assertion added."
+            )
         if "assertions" not in request_result or not isinstance(request_result["assertions"], list):
             request_result["assertions"] = []
         new_assertion = {"source": assertion_source, "comparison": assertion_comparison}
diff --git a/tests/test_step_manager.py b/tests/test_step_manager.py
index 2e14d9d..6fe33be 100644
--- a/tests/test_step_manager.py
+++ b/tests/test_step_manager.py
@@ -120,6 +120,9 @@ async def test_add_body_to_step_json(self, mock_token, mock_context):
             )
 
             assert result.error is None
+            # Header values must be arrays per the REST API contract
+            put_payload = mock_api.call_args.kwargs["json"]
+            assert put_payload["headers"]["Content-Type"] == ["application/json"]
 
     async def test_add_body_to_step_xml(self, mock_token, mock_context):
         """Test adding XML body to step"""
@@ -182,6 +185,23 @@ async def test_add_body_to_step_invalid_xml(self, mock_token, mock_context):
         assert result.error is not None
         assert "Invalid XML" in result.error
 
+    async def test_add_body_to_step_unsupported_type(self, mock_token, mock_context):
+        """Test adding body with unsupported body_type returns error without calling the API"""
+        manager = StepManager(mock_token, mock_context)
+
+        with patch("src.tools.step_manager.api_request") as mock_api:
+            result = await manager.add_body_to_step(
+                "bucket_abc",
+                "test_123",
+                "step_123",
+                "yaml",
+                "key: value"
+            )
+
+            assert result.error is not None
+            assert "Unsupported body_type" in result.error
+            mock_api.assert_not_called()
+
     async def test_add_assertion_to_step(self, mock_token, mock_context):
         """Test adding assertion to step"""
         manager = StepManager(mock_token, mock_context)
@@ -213,3 +233,27 @@ async def test_add_assertion_to_step(self, mock_token, mock_context):
 
             assert result.error is None
 
+    async def test_add_assertion_to_non_request_step(self, mock_token, mock_context):
+        """Test adding assertion to a non-request step returns error"""
+        manager = StepManager(mock_token, mock_context)
+
+        with patch.object(manager, 'read') as mock_read:
+            mock_read.return_value = {
+                "id": "step_123",
+                "step_type": "pause",
+                "duration": 5
+            }
+
+            result = await manager.add_assertion_to_step(
+                "bucket_abc",
+                "test_123",
+                "step_123",
+                "response_status",
+                "equals",
+                None,
+                "200"
+            )
+
+            assert result.error is not None
+            assert "cannot have an assertion added" in result.error
+