diff --git a/src/Mappers/RecursiveTypeMapper.php b/src/Mappers/RecursiveTypeMapper.php
index 4df79548a6..bb65537b30 100644
--- a/src/Mappers/RecursiveTypeMapper.php
+++ b/src/Mappers/RecursiveTypeMapper.php
@@ -308,15 +308,19 @@ public function mapClassToType(string $className, OutputType|null $subType): Mut
 
         // In the event this type was already part of cache, let's not extend it.
         if ($this->typeRegistry->hasType($type->name)) {
+            // Already-registered interface: return its concrete companion (as the non-cached path
+            // does below), else getMutableObjectType() throws on it.
+            if ($type instanceof MutableInterfaceType) {
+                return $this->classToTypeCache[$cacheKey]
+                    ??= $this->getGeneratedObjectTypeFromInterfaceType($type);
+            }
+
             $cachedType = $this->typeRegistry->getMutableObjectType($type->name);
             if ($cachedType !== $type) {
                 throw new RuntimeException('Cached type in registry is not the type returned by type mapper.');
             }
 
-            //if ($cachedType->getStatus() === MutableObjectType::STATUS_FROZEN) {
-                return $type;
-
-            //}
+            return $type;
         }
 
         $this->typeRegistry->registerType($type);
diff --git a/tests/Fixtures/InterfaceAsOutputType/Controllers/ProductController.php b/tests/Fixtures/InterfaceAsOutputType/Controllers/ProductController.php
new file mode 100644
index 0000000000..49ccc6d724
--- /dev/null
+++ b/tests/Fixtures/InterfaceAsOutputType/Controllers/ProductController.php
@@ -0,0 +1,35 @@
+<?php
+
+declare(strict_types=1);
+
+namespace TheCodingMachine\GraphQLite\Fixtures\InterfaceAsOutputType\Controllers;
+
+use TheCodingMachine\GraphQLite\Annotations\Mutation;
+use TheCodingMachine\GraphQLite\Annotations\Query;
+use TheCodingMachine\GraphQLite\Fixtures\InterfaceAsOutputType\Types\Book;
+use TheCodingMachine\GraphQLite\Fixtures\InterfaceAsOutputType\Types\CreateProductInput;
+use TheCodingMachine\GraphQLite\Fixtures\InterfaceAsOutputType\Types\ProductInterface;
+use TheCodingMachine\GraphQLite\Fixtures\InterfaceAsOutputType\Types\ProductItems;
+
+class ProductController
+{
+    #[Query]
+    public function product(): ProductInterface
+    {
+        return new Book('The Trial');
+    }
+
+    #[Query]
+    public function products(): ProductItems
+    {
+        return new ProductItems([new Book('The Trial'), new Book('The Castle')]);
+    }
+
+    // The input type forces GraphQLite to materialise output types mid-way through input field
+    // building, which is the ordering that triggered the regression.
+    #[Mutation]
+    public function createProduct(CreateProductInput $input): ProductInterface
+    {
+        return new Book($input->name);
+    }
+}
diff --git a/tests/Fixtures/InterfaceAsOutputType/Types/Book.php b/tests/Fixtures/InterfaceAsOutputType/Types/Book.php
new file mode 100644
index 0000000000..575da154d4
--- /dev/null
+++ b/tests/Fixtures/InterfaceAsOutputType/Types/Book.php
@@ -0,0 +1,27 @@
+<?php
+
+declare(strict_types=1);
+
+namespace TheCodingMachine\GraphQLite\Fixtures\InterfaceAsOutputType\Types;
+
+use TheCodingMachine\GraphQLite\Annotations\Field;
+use TheCodingMachine\GraphQLite\Annotations\Type;
+
+#[Type]
+class Book implements ProductInterface
+{
+    public function __construct(private string $name)
+    {
+    }
+
+    public function getName(): string
+    {
+        return $this->name;
+    }
+
+    #[Field]
+    public function getAuthor(): string
+    {
+        return 'Franz Kafka';
+    }
+}
diff --git a/tests/Fixtures/InterfaceAsOutputType/Types/CreateProductInput.php b/tests/Fixtures/InterfaceAsOutputType/Types/CreateProductInput.php
new file mode 100644
index 0000000000..389e0d2d55
--- /dev/null
+++ b/tests/Fixtures/InterfaceAsOutputType/Types/CreateProductInput.php
@@ -0,0 +1,18 @@
+<?php
+
+declare(strict_types=1);
+
+namespace TheCodingMachine\GraphQLite\Fixtures\InterfaceAsOutputType\Types;
+
+use TheCodingMachine\GraphQLite\Annotations\Field;
+use TheCodingMachine\GraphQLite\Annotations\Input;
+
+#[Input]
+class CreateProductInput
+{
+    public function __construct(
+        #[Field]
+        public string $name,
+    ) {
+    }
+}
diff --git a/tests/Fixtures/InterfaceAsOutputType/Types/ProductInterface.php b/tests/Fixtures/InterfaceAsOutputType/Types/ProductInterface.php
new file mode 100644
index 0000000000..06723c3e33
--- /dev/null
+++ b/tests/Fixtures/InterfaceAsOutputType/Types/ProductInterface.php
@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace TheCodingMachine\GraphQLite\Fixtures\InterfaceAsOutputType\Types;
+
+use TheCodingMachine\GraphQLite\Annotations\Field;
+use TheCodingMachine\GraphQLite\Annotations\Type;
+
+#[Type(name: 'Product')]
+interface ProductInterface
+{
+    #[Field]
+    public function getName(): string;
+}
diff --git a/tests/Fixtures/InterfaceAsOutputType/Types/ProductItems.php b/tests/Fixtures/InterfaceAsOutputType/Types/ProductItems.php
new file mode 100644
index 0000000000..5944a320aa
--- /dev/null
+++ b/tests/Fixtures/InterfaceAsOutputType/Types/ProductItems.php
@@ -0,0 +1,25 @@
+<?php
+
+declare(strict_types=1);
+
+namespace TheCodingMachine\GraphQLite\Fixtures\InterfaceAsOutputType\Types;
+
+use TheCodingMachine\GraphQLite\Annotations\Field;
+use TheCodingMachine\GraphQLite\Annotations\Type;
+
+#[Type]
+class ProductItems
+{
+    /** @param list<Book> $products */
+    public function __construct(private array $products)
+    {
+    }
+
+    // Referencing the interface by name via outputType resolves it early, which is what left the
+    // interface "already registered" by the time getOutputTypes() reached its class.
+    #[Field(outputType: '[Product]')]
+    public function getItems(): array
+    {
+        return $this->products;
+    }
+}
diff --git a/tests/Integration/InterfaceAsOutputTypeTest.php b/tests/Integration/InterfaceAsOutputTypeTest.php
new file mode 100644
index 0000000000..0a92d80d99
--- /dev/null
+++ b/tests/Integration/InterfaceAsOutputTypeTest.php
@@ -0,0 +1,76 @@
+<?php
+
+declare(strict_types=1);
+
+namespace TheCodingMachine\GraphQLite\Integration;
+
+use GraphQL\Error\DebugFlag;
+use GraphQL\GraphQL;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\Cache\Adapter\ArrayAdapter;
+use Symfony\Component\Cache\Psr16Cache;
+use TheCodingMachine\GraphQLite\Containers\BasicAutoWiringContainer;
+use TheCodingMachine\GraphQLite\Containers\EmptyContainer;
+use TheCodingMachine\GraphQLite\Schema;
+use TheCodingMachine\GraphQLite\SchemaFactory;
+
+/**
+ * A #[Type] on a PHP interface (with a custom name and a single concrete implementer) that is used
+ * as an output type used to break schema building: getOutputTypes() called the object-only
+ * mapClassToType() on the interface class once it was already registered, throwing
+ * "Expected GraphQL type ... to be an MutableObjectType".
+ */
+class InterfaceAsOutputTypeTest extends TestCase
+{
+    private Schema $schema;
+
+    public function setUp(): void
+    {
+        $container = new BasicAutoWiringContainer(new EmptyContainer());
+
+        $schemaFactory = new SchemaFactory(new Psr16Cache(new ArrayAdapter()), $container);
+        $schemaFactory->addNamespace('TheCodingMachine\\GraphQLite\\Fixtures\\InterfaceAsOutputType');
+
+        $this->schema = $schemaFactory->createSchema();
+    }
+
+    public function testSchemaBuildsWhenInterfaceIsUsedAsOutputType(): void
+    {
+        // Resolve the interface by name first (as a name-based outputType reference does), so it is
+        // already registered by the time the full type map / getOutputTypes() runs.
+        $this->schema->getType('Product');
+
+        $this->schema->assertValid();
+
+        $this->assertNotNull($this->schema->getType('Product'));
+        $this->assertNotNull($this->schema->getType('Book'));
+    }
+
+    public function testQueryReturningInterfaceResolvesConcreteType(): void
+    {
+        $result = GraphQL::executeQuery(
+            $this->schema,
+            '
+            query {
+                product {
+                    name
+                    ... on Book {
+                        author
+                    }
+                }
+            }
+            ',
+        );
+
+        $this->assertSame(
+            [
+                'product' => [
+                    'name' => 'The Trial',
+                    'author' => 'Franz Kafka',
+                ],
+            ],
+            $result->toArray(DebugFlag::RETHROW_INTERNAL_EXCEPTIONS)['data']
+                ?? $result->toArray(DebugFlag::RETHROW_INTERNAL_EXCEPTIONS)['errors'],
+        );
+    }
+}