diff --git a/grails-core/src/test/groovy/grails/plugins/DefaultGrailsPluginManagerRegistryContractSpec.groovy b/grails-core/src/test/groovy/grails/plugins/DefaultGrailsPluginManagerRegistryContractSpec.groovy
new file mode 100644
index 00000000000..981b9376547
--- /dev/null
+++ b/grails-core/src/test/groovy/grails/plugins/DefaultGrailsPluginManagerRegistryContractSpec.groovy
@@ -0,0 +1,152 @@
+/*
+ *  Licensed to the Apache Software Foundation (ASF) under one
+ *  or more contributor license agreements.  See the NOTICE file
+ *  distributed with this work for additional information
+ *  regarding copyright ownership.  The ASF licenses this file
+ *  to you under the Apache License, Version 2.0 (the
+ *  "License"); you may not use this file except in compliance
+ *  with the License.  You may obtain a copy of the License at
+ *
+ *    https://www.apache.org/licenses/LICENSE-2.0
+ *
+ *  Unless required by applicable law or agreed to in writing,
+ *  software distributed under the License is distributed on an
+ *  "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ *  KIND, either express or implied.  See the License for the
+ *  specific language governing permissions and limitations
+ *  under the License.
+ */
+package grails.plugins
+
+import grails.core.DefaultGrailsApplication
+import org.apache.grails.core.plugins.DefaultPluginDiscovery
+import org.springframework.core.env.StandardEnvironment
+import spock.lang.Specification
+
+class DefaultGrailsPluginManagerRegistryContractSpec extends Specification {
+
+    void 'the discovery load order consumed by the manager is deterministic for a combined ordering graph'() {
+        given:
+        def pluginClasses = orderingPluginClasses()
+        def expectedLoadOrder = [
+                'registryOrderingRoot',
+                'registryOrderingBefore',
+                'registryOrderingRight',
+                'registryOrderingLeft',
+                'registryOrderingJoin',
+                'registryOrderingTail'
+        ]
+
+        when:
+        def loadOrders = (1..3).collect {
+            def discovery = createDiscovery(pluginClasses)
+            createManager(discovery)
+            discovery.getPluginsInLoadOrder()*.name
+        }
+
+        then:
+        loadOrders == [expectedLoadOrder, expectedLoadOrder, expectedLoadOrder]
+    }
+
+    void 'the loaded registry exposes a deterministic topological order for a combined ordering graph'() {
+        given:
+        def pluginClasses = orderingPluginClasses()
+        def expectedTopologicalOrder = [
+                'registryOrderingBefore',
+                'registryOrderingRoot',
+                'registryOrderingRight',
+                'registryOrderingLeft',
+                'registryOrderingJoin',
+                'registryOrderingTail'
+        ]
+
+        when:
+        def topologicalOrders = (1..3).collect {
+            def discovery = createDiscovery(pluginClasses)
+            createManager(discovery).getAllPlugins()*.name
+        }
+
+        then:
+        topologicalOrders == [expectedTopologicalOrder, expectedTopologicalOrder, expectedTopologicalOrder]
+    }
+
+    void 'the loaded registry de-duplicates plugins and resolves each plugin consistently'() {
+        given:
+        def pluginClasses = orderingPluginClasses()
+        def tailPluginClass = pluginClasses.find { it.name == 'RegistryOrderingTailGrailsPlugin' }
+        def discovery = createDiscovery(pluginClasses + tailPluginClass)
+        def manager = createManager(discovery)
+
+        when:
+        def allPlugins = manager.getAllPlugins()
+        def expectedTailPlugin = allPlugins.find { it.name == 'registryOrderingTail' }
+        def pluginByName = manager.getGrailsPlugin('registryOrderingTail')
+        def pluginByClassName = manager.getGrailsPluginForClassName(tailPluginClass.name)
+        def pluginByVersion = manager.getGrailsPlugin('registryOrderingTail', '1.0.0')
+
+        then:
+        allPlugins.count { it.name == 'registryOrderingTail' } == 1
+        expectedTailPlugin != null
+        manager.hasGrailsPlugin('registryOrderingTail')
+        pluginByName != null
+        pluginByClassName != null
+        pluginByVersion != null
+        pluginByName.is(expectedTailPlugin)
+        pluginByClassName.is(expectedTailPlugin)
+        pluginByVersion.is(expectedTailPlugin)
+    }
+
+    private static DefaultPluginDiscovery createDiscovery(List<Class<?>> pluginClasses) {
+        def discovery = new DefaultPluginDiscovery(pluginClasses as Class<?>[])
+        discovery.loadPluginsFromClasspath = false
+        discovery.init(new StandardEnvironment())
+        discovery
+    }
+
+    private static DefaultGrailsPluginManager createManager(DefaultPluginDiscovery discovery) {
+        def manager = new DefaultGrailsPluginManager(new DefaultGrailsApplication(), discovery)
+        manager.loadPlugins()
+        manager
+    }
+
+    private static List<Class<?>> orderingPluginClasses() {
+        def classLoader = new GroovyClassLoader()
+        [
+                classLoader.parseClass('''
+class RegistryOrderingTailGrailsPlugin {
+    def version = '1.0.0'
+    def loadAfter = ['registryOrderingJoin']
+}
+'''),
+                classLoader.parseClass('''
+class RegistryOrderingJoinGrailsPlugin {
+    def version = '1.0.0'
+    def loadAfter = ['registryOrderingLeft', 'registryOrderingRight']
+}
+'''),
+                classLoader.parseClass('''
+class RegistryOrderingRightGrailsPlugin {
+    def version = '1.0.0'
+    def loadAfter = ['registryOrderingRoot']
+}
+'''),
+                classLoader.parseClass('''
+class RegistryOrderingLeftGrailsPlugin {
+    def version = '1.0.0'
+    def loadAfter = ['registryOrderingRoot']
+}
+'''),
+                classLoader.parseClass('''
+class RegistryOrderingRootGrailsPlugin {
+    def version = '1.0.0'
+}
+'''),
+                classLoader.parseClass('''
+class RegistryOrderingBeforeGrailsPlugin {
+    def version = '1.0.0'
+    def loadBefore = ['registryOrderingRoot']
+}
+''')
+        ]
+    }
+}