diff --git a/api/core/tools/custom_tool/tool.py b/api/core/tools/custom_tool/tool.py
index 38a13609dbf..0e1da00a3fc 100644
--- a/api/core/tools/custom_tool/tool.py
+++ b/api/core/tools/custom_tool/tool.py
@@ -103,16 +103,17 @@ class ApiTool(Tool):
             elif not isinstance(credentials["api_key_value"], str):
                 raise ToolProviderCredentialValidationError("api_key_value must be a string")
 
+            api_key_value = credentials["api_key_value"]
             if "api_key_header_prefix" in credentials:
                 api_key_header_prefix = credentials["api_key_header_prefix"]
-                if api_key_header_prefix == "basic" and credentials["api_key_value"]:
-                    credentials["api_key_value"] = f"Basic {credentials['api_key_value']}"
-                elif api_key_header_prefix == "bearer" and credentials["api_key_value"]:
-                    credentials["api_key_value"] = f"Bearer {credentials['api_key_value']}"
+                if api_key_header_prefix == "basic" and api_key_value:
+                    api_key_value = f"Basic {api_key_value}"
+                elif api_key_header_prefix == "bearer" and api_key_value:
+                    api_key_value = f"Bearer {api_key_value}"
                 elif api_key_header_prefix == "custom":
                     pass
 
-            headers[api_key_header] = credentials["api_key_value"]
+            headers[api_key_header] = api_key_value
 
         elif credentials["auth_type"] == "api_key_query":
             # For query parameter authentication, we don't add anything to headers
diff --git a/api/tests/unit_tests/core/tools/test_custom_tool.py b/api/tests/unit_tests/core/tools/test_custom_tool.py
index 3f664456bd2..64cd128f18e 100644
--- a/api/tests/unit_tests/core/tools/test_custom_tool.py
+++ b/api/tests/unit_tests/core/tools/test_custom_tool.py
@@ -85,6 +85,10 @@ def test_assembling_request_auth_header_assembly():
     assert headers["Authorization"] == "Bearer abc"
 
     tool.runtime.credentials = {"auth_type": "api_key_header", "api_key_header_prefix": "basic", "api_key_value": "abc"}
+    headers = tool.assembling_request(parameters={})
+    assert headers["Authorization"] == "Basic abc"
+    assert tool.runtime.credentials["api_key_value"] == "abc"
+
     headers = tool.assembling_request(parameters={})
     assert headers["Authorization"] == "Basic abc"