diff --git a/components/nimbus/ios/Nimbus/FeatureHolder.swift b/components/nimbus/ios/Nimbus/FeatureHolder.swift
index e5d67b940e..0c67c69568 100644
--- a/components/nimbus/ios/Nimbus/FeatureHolder.swift
+++ b/components/nimbus/ios/Nimbus/FeatureHolder.swift
@@ -12,7 +12,7 @@ public typealias GetSdk = () -> FeaturesInterface?
 ///
 /// There are methods useful for testing, and more advanced uses: these all start with `with`.
 ///
-public class FeatureHolder<T> {
+public class FeatureHolder<T: FMLFeatureInterface> {
     private let lock = NSLock()
     private var cachedValue: T?
 
@@ -56,7 +56,9 @@ public class FeatureHolder<T> {
     /// Send an exposure event for this feature. This should be done when the user is shown the feature, and may change
     /// their behavior because of it.
     public func recordExposure() {
-        getSdk()?.recordExposureEvent(featureId: featureId, experimentSlug: nil)
+        if !value().isModified() {
+            getSdk()?.recordExposureEvent(featureId: featureId, experimentSlug: nil)
+        }
     }
 
     /// Send an exposure event for this feature, in the given experiment.
@@ -69,7 +71,9 @@ public class FeatureHolder<T> {
     ///
     /// - Parameter slug the experiment identifier, likely derived from the {value}.
     public func recordExperimentExposure(slug: String) {
-        getSdk()?.recordExposureEvent(featureId: featureId, experimentSlug: slug)
+        if !value().isModified() {
+            getSdk()?.recordExposureEvent(featureId: featureId, experimentSlug: slug)
+        }
     }
 
     /// Send a malformed feature event for this feature.
@@ -130,3 +134,26 @@ public class FeatureHolder<T> {
         create = initializer
     }
 }
+
+/// A bare-bones interface for the FML generated objects.
+public protocol FMLObjectInterface {}
+
+/// A bare-bones interface for the FML generated features.
+///
+/// App developers should use the generated concrete classes, which
+/// implement this interface.
+///
+public protocol FMLFeatureInterface: FMLObjectInterface {
+    /// A test if the feature configuration has been modified somehow, invalidating any experiment
+    /// that uses it.
+    ///
+    /// This may be `true` if a `pref-key` has been set in the feature manifest and the user has
+    /// set that preference.
+    func isModified() -> Bool
+}
+
+public extension FMLFeatureInterface {
+    func isModified() -> Bool {
+        return false
+    }
+}
diff --git a/components/support/nimbus-fml/src/backends/swift/templates/FeatureTemplate.swift b/components/support/nimbus-fml/src/backends/swift/templates/FeatureTemplate.swift
index 63afe0aa03..42873bcefc 100644
--- a/components/support/nimbus-fml/src/backends/swift/templates/FeatureTemplate.swift
+++ b/components/support/nimbus-fml/src/backends/swift/templates/FeatureTemplate.swift
@@ -1,3 +1,28 @@
 {%- import "macros.swift" as swift %}
 {%- let inner = self.inner() %}
-{% call swift::render_class(inner) %}
\ No newline at end of file
+{%- let class_name = inner.name()|class_name -%}
+{% call swift::render_class(inner) %}
+
+{%- if inner.has_prefs() %}
+
+extension {{ class_name }}: FMLFeatureInterface {
+    public func isModified() -> Bool {
+        guard let prefs = {% call swift::prefs() %} else {
+            return false
+        }
+        let keys = [
+            {%- for p in inner.props() %}
+            {%- if p.has_prefs() %}
+            {{ p.pref_key().unwrap()|quoted }},
+            {%- endif %}
+            {%- endfor %}
+        ]
+        if let _ = keys.first(where: { prefs.object(forKey: $0) != nil }) {
+            return true
+        }
+        return false
+    }
+}
+{%- else %}
+extension {{ class_name }}: FMLFeatureInterface {}
+{%- endif %}
diff --git a/components/support/nimbus-fml/src/backends/swift/templates/ObjectTemplate.swift b/components/support/nimbus-fml/src/backends/swift/templates/ObjectTemplate.swift
index dfb6788393..ced91997e2 100644
--- a/components/support/nimbus-fml/src/backends/swift/templates/ObjectTemplate.swift
+++ b/components/support/nimbus-fml/src/backends/swift/templates/ObjectTemplate.swift
@@ -1,7 +1,7 @@
 {%- import "macros.swift" as swift %}
 {%- let inner = self.inner() %}
 {% call swift::render_class(inner) -%}
-{% let class_name = inner.name()|class_name -%}
+{%- let class_name = inner.name()|class_name %}
 
 public extension {{class_name}} {
     func _mergeWith(_ defaults: {{class_name}}?) -> {{class_name}} {
diff --git a/components/support/nimbus-fml/src/backends/swift/templates/macros.swift b/components/support/nimbus-fml/src/backends/swift/templates/macros.swift
index 92860229b7..4c87c7c909 100644
--- a/components/support/nimbus-fml/src/backends/swift/templates/macros.swift
+++ b/components/support/nimbus-fml/src/backends/swift/templates/macros.swift
@@ -13,7 +13,7 @@
 {% let class_name = inner.name()|class_name -%}
 
 {{ inner.doc()|comment("") }}
-public class {{class_name}} {
+public class {{class_name}}: FMLObjectInterface {
     private let _variables: Variables
     private let _defaults: Defaults
     private let _prefs: UserDefaults?
@@ -71,7 +71,7 @@ public class {{class_name}} {
 
             `has_prefs()` checks if the type can be got from UserDefaults.
             #}
-        if let {{ prefs }} = self._prefs,
+        if let {{ prefs }} = {% call prefs() %},
             let {{ prop_swift }} = {{ prefs }}.object(forKey: {{ key|quoted }}) as? {{ type_swift }} {
             return {{ prop_swift }}
         }
@@ -81,4 +81,6 @@ public class {{class_name}} {
     {% endfor %}
 }
 
-{% endmacro %}}
+{%- endmacro %}}
+
+{% macro prefs() %}self._prefs{% endmacro %}
diff --git a/components/support/nimbus-fml/test/pref_overrides.swift b/components/support/nimbus-fml/test/pref_overrides.swift
index 5e258a2efb..73428b7bb3 100644
--- a/components/support/nimbus-fml/test/pref_overrides.swift
+++ b/components/support/nimbus-fml/test/pref_overrides.swift
@@ -41,6 +41,7 @@ assert(feature0.myBoolean == false)
 assert(feature0.myInt == 0)
 assert(feature0.myString == "from manifest")
 assert(feature0.myText == "from manifest")
+assert(!feature0.isModified())
 
 // Now test that JSON still has an effect.
 let prefs = UserDefaults()
@@ -66,6 +67,7 @@ assert(feature.myBoolean == false)
 assert(feature.myInt == 100)
 assert(feature.myString == "from json")
 assert(feature.myText == "from json")
+assert(!feature.isModified())
 
 // Now set with prefs.
 
@@ -78,3 +80,4 @@ assert(feature.myBoolean == true)
 assert(feature.myInt == 42)
 assert(feature.myString == "from pref")
 assert(feature.myText == "from pref")
+assert(feature.isModified())
diff --git a/components/support/nimbus-fml/test/threadsafe_feature_holder.swift b/components/support/nimbus-fml/test/threadsafe_feature_holder.swift
index d1e391c404..3dc954ad6a 100644
--- a/components/support/nimbus-fml/test/threadsafe_feature_holder.swift
+++ b/components/support/nimbus-fml/test/threadsafe_feature_holder.swift
@@ -5,6 +5,13 @@
 import FeatureManifest
 import Foundation
 
+class Feature: FMLFeatureInterface {
+    let string: String
+    init(_ string: String) {
+        self.string = string
+    }
+}
+
 let queue: OperationQueue = {
     let queue = OperationQueue()
     queue.maxConcurrentOperationCount = 5
@@ -12,7 +19,7 @@ let queue: OperationQueue = {
 }()
 
 let api: FeaturesInterface = HardcodedNimbusFeatures(with: ["test-feature-holder": "{}"])
-let holder = FeatureHolder<String>({ api }, featureId: "test-feature-holder") { _, _ in "NO CRASH" }
+let holder = FeatureHolder<Feature>({ api }, featureId: "test-feature-holder") { _, _ in Feature("NO CRASH") }
 
 for _ in 1 ..< 10000 {
     queue.addOperation {