diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8a54395..12be414 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -3,6 +3,17 @@
 All notable changes to this project will be documented in this file. This project adheres to
 [Semantic Versioning](http://semver.org/) and [this changelog format](http://keepachangelog.com/).
 
+## Next Major Release
+
+### Added
+
+- **BREAKING**: The `Identifier` interface now has an `any()` method that returns `true` if any of the given identifiers
+  match the identifier. If you have implemented any custom identifiers, you can add the `IsIdentifier` trait to
+  implement this new method.
+- **BREAKING**: Add `message()` and `messages()` helper methods to the `ErrorList` interface. Although technically
+  breaking, this will not affect most implementations as the concrete error list class provided this package has been
+  updated.
+
 ## Unreleased
 
 ## [4.1.0] - 2025-10-03
diff --git a/docs/guide/upgrade.md b/docs/guide/upgrade.md
index a1f041e..ab13826 100644
--- a/docs/guide/upgrade.md
+++ b/docs/guide/upgrade.md
@@ -1,5 +1,27 @@
 # Upgrade Guide
 
+## 4.x to 5.x
+
+Upgrade using Composer:
+
+```bash
+composer config minimum-stability dev
+composer require cloudcreativity/ddd-modules:^5.0
+```
+
+### Identifiers
+
+A new `any()` method has been added to the `Identifier` interface. This will only affect your implementation if you have
+implemented any custom identifier classes. To solve, add the `IsIdentifier` trait to your class.
+
+### Other Changes
+
+Although this release contains other breaking changes, most implementations can upgrade without making any changes. This
+is because the majority of changes affect classes that are implemented by this package - and it is unlikely you are
+implementing these yourself.
+
+Refer to the changelog for a full list of changes.
+
 ## 3.x to 4.x
 
 Upgrade using Composer:
@@ -8,7 +30,9 @@ Upgrade using Composer:
 composer require cloudcreativity/ddd-modules:^4.0
 ```
 
-Although this release contains breaking changes, most implementations can upgrade without making any changes. This is because the majority of changes affect classes that are implemented by this package - and it is unlikely you are implementing these yourself.
+Although this release contains breaking changes, most implementations can upgrade without making any changes. This is
+because the majority of changes affect classes that are implemented by this package - and it is unlikely you are
+implementing these yourself.
 
 Refer to the changelog for a full list of changes.
 
diff --git a/src/Contracts/Toolkit/Identifiers/Identifier.php b/src/Contracts/Toolkit/Identifiers/Identifier.php
index dd7acd5..3bac5c6 100644
--- a/src/Contracts/Toolkit/Identifiers/Identifier.php
+++ b/src/Contracts/Toolkit/Identifiers/Identifier.php
@@ -22,6 +22,11 @@ interface Identifier extends Stringable, Contextual
      */
     public function is(?self $other): bool;
 
+    /**
+     * Is the identifier any of the provided identifiers?
+     */
+    public function any(?self ...$others): bool;
+
     /**
      * Fluent to-string method.
      */
diff --git a/src/Contracts/Toolkit/Result/ListOfErrors.php b/src/Contracts/Toolkit/Result/ListOfErrors.php
index f69642c..240b9a7 100644
--- a/src/Contracts/Toolkit/Result/ListOfErrors.php
+++ b/src/Contracts/Toolkit/Result/ListOfErrors.php
@@ -38,7 +38,7 @@ public function contains(Closure|UnitEnum $matcher): bool;
     /**
      * Get all the unique error codes in the list.
      *
-     * @return array<UnitEnum>
+     * @return list<UnitEnum>
      */
     public function codes(): array;
 
@@ -47,6 +47,20 @@ public function codes(): array;
      */
     public function code(): ?UnitEnum;
 
+    /**
+     * Get all the unique error messages in the list.
+     *
+     * @return list<non-empty-string>
+     */
+    public function messages(): array;
+
+    /**
+     * Get the first error message in the list.
+     *
+     * @return non-empty-string|null
+     */
+    public function message(): ?string;
+
     /**
      * Return a new instance with the provided error pushed on to the end of the list.
      *
diff --git a/src/Toolkit/Identifiers/Guid.php b/src/Toolkit/Identifiers/Guid.php
index c325872..89c479e 100644
--- a/src/Toolkit/Identifiers/Guid.php
+++ b/src/Toolkit/Identifiers/Guid.php
@@ -23,6 +23,8 @@
 
 final readonly class Guid implements Identifier
 {
+    use IsIdentifier;
+
     public static function from(Identifier $value): self
     {
         if ($value instanceof self) {
diff --git a/src/Toolkit/Identifiers/IntegerId.php b/src/Toolkit/Identifiers/IntegerId.php
index a2bf94a..9dad711 100644
--- a/src/Toolkit/Identifiers/IntegerId.php
+++ b/src/Toolkit/Identifiers/IntegerId.php
@@ -19,6 +19,8 @@
 
 final readonly class IntegerId implements Identifier, JsonSerializable
 {
+    use IsIdentifier;
+
     public static function from(Identifier|int $value): self
     {
         return match(true) {
diff --git a/src/Toolkit/Identifiers/IsIdentifier.php b/src/Toolkit/Identifiers/IsIdentifier.php
new file mode 100644
index 0000000..8f04367
--- /dev/null
+++ b/src/Toolkit/Identifiers/IsIdentifier.php
@@ -0,0 +1,26 @@
+<?php
+
+/*
+ * Copyright 2025 Cloud Creativity Limited
+ *
+ * Use of this source code is governed by an MIT-style
+ * license that can be found in the LICENSE file or at
+ * https://opensource.org/licenses/MIT.
+ */
+
+declare(strict_types=1);
+
+namespace CloudCreativity\Modules\Toolkit\Identifiers;
+
+use CloudCreativity\Modules\Contracts\Toolkit\Identifiers\Identifier;
+
+trait IsIdentifier
+{
+    public function any(?Identifier ...$others): bool
+    {
+        return array_any(
+            $others,
+            fn (?Identifier $other): bool => $this->is($other),
+        );
+    }
+}
diff --git a/src/Toolkit/Identifiers/StringId.php b/src/Toolkit/Identifiers/StringId.php
index 8d128e8..0001ee0 100644
--- a/src/Toolkit/Identifiers/StringId.php
+++ b/src/Toolkit/Identifiers/StringId.php
@@ -19,6 +19,8 @@
 
 final readonly class StringId implements Identifier, JsonSerializable
 {
+    use IsIdentifier;
+
     public static function from(Identifier|string $value): self
     {
         return match(true) {
diff --git a/src/Toolkit/Identifiers/Uuid.php b/src/Toolkit/Identifiers/Uuid.php
index 5e40d39..403c17a 100644
--- a/src/Toolkit/Identifiers/Uuid.php
+++ b/src/Toolkit/Identifiers/Uuid.php
@@ -20,6 +20,8 @@
 
 final class Uuid implements Identifier, JsonSerializable
 {
+    use IsIdentifier;
+
     private static ?IUuidFactory $factory = null;
 
     public static function setFactory(?IUuidFactory $factory): void
diff --git a/src/Toolkit/Result/ListOfErrors.php b/src/Toolkit/Result/ListOfErrors.php
index b75c70b..210ab79 100644
--- a/src/Toolkit/Result/ListOfErrors.php
+++ b/src/Toolkit/Result/ListOfErrors.php
@@ -102,6 +102,34 @@ public function code(): ?UnitEnum
         return null;
     }
 
+    public function messages(): array
+    {
+        $messages = [];
+
+        foreach ($this->stack as $error) {
+            $message = $error->message();
+
+            if (strlen($message) > 0) {
+                $messages[$message] = $message;
+            }
+        }
+
+        return array_values($messages);
+    }
+
+    public function message(): ?string
+    {
+        foreach ($this->stack as $error) {
+            $message = $error->message();
+
+            if (strlen($message) > 0) {
+                return $message;
+            }
+        }
+
+        return null;
+    }
+
     public function push(IError $error): self
     {
         $copy = clone $this;
diff --git a/src/Toolkit/Result/Result.php b/src/Toolkit/Result/Result.php
index 2095261..e8549c7 100644
--- a/src/Toolkit/Result/Result.php
+++ b/src/Toolkit/Result/Result.php
@@ -115,13 +115,7 @@ public function errors(): IListOfErrors
 
     public function error(): ?string
     {
-        foreach ($this->errors as $error) {
-            if ($message = $error->message()) {
-                return $message;
-            }
-        }
-
-        return null;
+        return $this->errors->message();
     }
 
     public function meta(): Meta
diff --git a/tests/Unit/Toolkit/Identifiers/GuidTest.php b/tests/Unit/Toolkit/Identifiers/GuidTest.php
index 5cf3580..b4fff3a 100644
--- a/tests/Unit/Toolkit/Identifiers/GuidTest.php
+++ b/tests/Unit/Toolkit/Identifiers/GuidTest.php
@@ -59,6 +59,12 @@ public function testStringId(string|UnitEnum $type, string $value, string|UnitEn
     {
         $guid = Guid::fromString($type, '123');
 
+        $others = [
+            Guid::fromInteger($type, 123),
+            Guid::fromString($type, '234'),
+            Guid::fromString($other, '123'),
+        ];
+
         $this->assertInstanceOf(\Stringable::class, $guid);
         $this->assertSame($type, $guid->type);
         $this->assertObjectEquals(new StringId('123'), $guid->id);
@@ -67,10 +73,13 @@ public function testStringId(string|UnitEnum $type, string $value, string|UnitEn
         $this->assertTrue($guid->isType($type));
         $this->assertTrue($guid->is($guid));
         $this->assertTrue($guid->is(clone $guid));
-        $this->assertFalse($guid->is(Guid::fromInteger($type, 123)));
-        $this->assertFalse($guid->is(Guid::fromString($type, '234')));
-        $this->assertFalse($guid->is(Guid::fromString($other, '123')));
+        $this->assertTrue($guid->any($others[0], $others[1], clone $guid));
+        $this->assertFalse($guid->is($others[0]));
+        $this->assertFalse($guid->is($others[1]));
+        $this->assertFalse($guid->is($others[2]));
         $this->assertFalse($guid->is(null));
+        $this->assertFalse($guid->any(...$others));
+        $this->assertFalse($guid->any(null, ...$others));
         $this->assertSame(['type' => $value, 'id' => '123'], $guid->context());
         $this->assertEquals($guid, Guid::fromString($type, '123'));
         $this->assertObjectEquals($guid, Guid::fromString($type, '123'));
diff --git a/tests/Unit/Toolkit/Identifiers/IntegerIdTest.php b/tests/Unit/Toolkit/Identifiers/IntegerIdTest.php
index a42a7c8..abb299c 100644
--- a/tests/Unit/Toolkit/Identifiers/IntegerIdTest.php
+++ b/tests/Unit/Toolkit/Identifiers/IntegerIdTest.php
@@ -50,6 +50,7 @@ public function testItIsEquals(): void
         $this->assertObjectEquals($id = new IntegerId(99), $other = IntegerId::from(99));
         $this->assertSame($id, IntegerId::from($id));
         $this->assertTrue($id->is($other));
+        $this->assertTrue($id->any(new IntegerId(1), new IntegerId(2), $other));
     }
 
     public function testItIsNotEqual(): void
@@ -57,6 +58,8 @@ public function testItIsNotEqual(): void
         $id = new IntegerId(99);
         $this->assertFalse($id->equals($other = new IntegerId(100)));
         $this->assertFalse($id->is($other));
+        $this->assertFalse($id->any($other, new IntegerId(200), new IntegerId(300)));
+        $this->assertFalse($id->any());
     }
 
     /**
@@ -77,6 +80,7 @@ public function testIsWithOtherIdentifiers(Identifier $other): void
         $id = new IntegerId(1);
 
         $this->assertFalse($id->is($other));
+        $this->assertFalse($id->any(new IntegerId(2), null, $other));
     }
 
     public function testIsWithNull(): void
diff --git a/tests/Unit/Toolkit/Identifiers/StringIdTest.php b/tests/Unit/Toolkit/Identifiers/StringIdTest.php
index 9564905..77ac42d 100644
--- a/tests/Unit/Toolkit/Identifiers/StringIdTest.php
+++ b/tests/Unit/Toolkit/Identifiers/StringIdTest.php
@@ -56,6 +56,7 @@ public function testItIsEquals(): void
         $this->assertObjectEquals($id = new StringId('99'), $other = StringId::from('99'));
         $this->assertSame($id, StringId::from($id));
         $this->assertTrue($id->is($other));
+        $this->assertTrue($id->any(new StringId('foo'), new StringId('bar'), null, $other));
     }
 
     public function testItIsNotEqual(): void
@@ -63,6 +64,8 @@ public function testItIsNotEqual(): void
         $id = new StringId('99');
         $this->assertFalse($id->equals($other = new StringId('100')));
         $this->assertFalse($id->is($other));
+        $this->assertFalse($id->any(new StringId('foo'), new StringId('bar'), null, $other));
+        $this->assertFalse($id->any());
     }
 
     /**
@@ -83,6 +86,7 @@ public function testIsWithOtherIdentifiers(Identifier $other): void
         $id = new StringId('1');
 
         $this->assertFalse($id->is($other));
+        $this->assertFalse($id->any($other, new StringId('foo'), null));
     }
 
     public function testIsWithNull(): void
diff --git a/tests/Unit/Toolkit/Identifiers/UuidTest.php b/tests/Unit/Toolkit/Identifiers/UuidTest.php
index 47d1365..7f7febc 100644
--- a/tests/Unit/Toolkit/Identifiers/UuidTest.php
+++ b/tests/Unit/Toolkit/Identifiers/UuidTest.php
@@ -55,6 +55,7 @@ public function testItIsEquals(): void
         $this->assertObjectEquals($id = new Uuid($base), $other = Uuid::from($base));
         $this->assertSame($id, Uuid::from($id));
         $this->assertTrue($id->is($other));
+        $this->assertTrue($id->any(null, Uuid::random(), $other));
     }
 
     public function testItIsNotEqual(): void
@@ -64,6 +65,7 @@ public function testItIsNotEqual(): void
             RamseyUuid::fromString('38c7be26-6887-4742-8b6b-7d07b30ca596'),
         )));
         $this->assertFalse($id->is($other));
+        $this->assertFalse($id->any(null, Uuid::random(), $other));
     }
 
     /**
@@ -84,6 +86,8 @@ public function testIsWithOtherIdentifiers(Identifier $other): void
         $id = new Uuid(RamseyUuid::fromString('6dcbad65-ed92-4e60-973b-9ba58a022816'));
 
         $this->assertFalse($id->is($other));
+        $this->assertFalse($id->any(null, Uuid::random(), $other));
+        $this->assertFalse($id->any());
     }
 
     public function testIsWithNull(): void
diff --git a/tests/Unit/Toolkit/Result/ListOfErrorsTest.php b/tests/Unit/Toolkit/Result/ListOfErrorsTest.php
index b52e837..efb83bf 100644
--- a/tests/Unit/Toolkit/Result/ListOfErrorsTest.php
+++ b/tests/Unit/Toolkit/Result/ListOfErrorsTest.php
@@ -152,4 +152,36 @@ public function testCode(): void
         $this->assertSame(TestBackedEnum::Foo, $errors1->code());
         $this->assertNull($errors2->code());
     }
+
+    public function testMessages(): void
+    {
+        $errors = new ListOfErrors(
+            new Error(message: 'Message A'),
+            new Error(message: 'Message B'),
+            new Error(code: TestUnitEnum::Baz),
+            new Error(message: 'Message A'),
+            new Error(message: 'Message C'),
+        );
+
+        $this->assertSame(['Message A', 'Message B', 'Message C'], $errors->messages());
+    }
+
+    public function testMessage(): void
+    {
+        $errors1 = new ListOfErrors(
+            new Error(message: 'Message A'),
+            new Error(message: 'Message B'),
+            new Error(code: TestUnitEnum::Baz),
+            new Error(message: 'Message A'),
+            new Error(message: 'Message C'),
+        );
+
+        $errors2 = new ListOfErrors(
+            new Error(code: TestUnitEnum::Baz),
+            new Error(code: TestBackedEnum::Foo),
+        );
+
+        $this->assertSame('Message A', $errors1->message());
+        $this->assertNull($errors2->message());
+    }
 }