diff --git a/docs/ModelUnitTests.md b/docs/ModelUnitTests.md
index 3a553d145..5e6e8a70d 100644
--- a/docs/ModelUnitTests.md
+++ b/docs/ModelUnitTests.md
@@ -1,6 +1,6 @@
 # Model Unit Tests
 Oxidized includes automated unit tests for its models, which require very little
-effort to use. There are three different unit tests for models:
+effort to use. There are three different default unit tests for models:
 - [Device Simulation](ModelUnitTests.md#device-simulation)
 - [Device Prompt](ModelUnitTests.md#device-prompt)
 - [Secrets](ModelUnitTests.md#secrets)
@@ -11,6 +11,9 @@ and the tests will be run automatically with `rake test`. See
 development environment. In the following examples, we use
 [Bundler](https://bundler.io/) and prefix all commands with `bundle exec`.
 
+In addition, you can write [custom unit tests](ModelUnitTests.md#custom-tests)
+for your model
+
 ## Device Simulation
 ### YAML Simulation File
 You need a [YAML simulation file](/docs/DeviceSimulation.md) for your device.
@@ -173,3 +176,11 @@ pass:
   - 'hash-mgmt-user oxidized password hash <secret removed>'
   - 'hash-mgmt-user rocks password hash <secret removed> usertype read-only'
 ```
+
+## Custom tests
+When you write custom tests for your models, please do not use the filenames
+mentioned above, as it will interfere with the standard tests. If you need to
+store a custom simulation file, use `model:description:custom_simulation.yaml`.
+
+The [cumulus test](/spec/model/cumulus_spec.rb) is an example of a custom
+test.
diff --git a/spec/model/cumulus_spec.rb b/spec/model/cumulus_spec.rb
index 04e618c0c..9a84eef1c 100644
--- a/spec/model/cumulus_spec.rb
+++ b/spec/model/cumulus_spec.rb
@@ -12,7 +12,7 @@
                                model: 'cumulus',
                                vars:  { cumulus_use_nvue: true })
 
-    model = YAML.load_file('spec/model/data/cumulus:MSN2010_5.9.2_nvue:simulation.yaml')
+    model = YAML.load_file('spec/model/data/cumulus:MSN2010_5.9.2_nvue:custom_simulation.yaml')
     mockmodel = MockSsh.new(model)
     Net::SSH.stubs(:start).returns mockmodel
 
@@ -32,7 +32,7 @@
                                vars:     { cumulus_routing_daemon: 'frr',
                                            enable:                 true })
 
-    model = YAML.load_file('spec/model/data/cumulus:VX_5.4.0_frr:simulation.yaml')
+    model = YAML.load_file('spec/model/data/cumulus:VX_5.4.0_frr:custom_simulation.yaml')
     mockmodel = MockSsh.new(model)
     Net::SSH.stubs(:start).returns mockmodel
 
diff --git a/spec/model/data/cumulus:MSN2010_5.9.2_nvue:simulation.yaml b/spec/model/data/cumulus:MSN2010_5.9.2_nvue:custom_simulation.yaml
similarity index 100%
rename from spec/model/data/cumulus:MSN2010_5.9.2_nvue:simulation.yaml
rename to spec/model/data/cumulus:MSN2010_5.9.2_nvue:custom_simulation.yaml
diff --git a/spec/model/data/cumulus:VX_5.4.0_frr:simulation.yaml b/spec/model/data/cumulus:VX_5.4.0_frr:custom_simulation.yaml
similarity index 100%
rename from spec/model/data/cumulus:VX_5.4.0_frr:simulation.yaml
rename to spec/model/data/cumulus:VX_5.4.0_frr:custom_simulation.yaml