diff --git a/src/Planner/SelectionSetPlanner.php b/src/Planner/SelectionSetPlanner.php
index 2fa718c..3f2e6b3 100644
--- a/src/Planner/SelectionSetPlanner.php
+++ b/src/Planner/SelectionSetPlanner.php
@@ -1130,9 +1130,20 @@ private function collectRequiredFieldsFromSelectionSet(
     ) : void {
         foreach ($selectionSet->selections as $selection) {
             if ($selection instanceof FieldNode && $selection->name->value !== '__typename') {
-                // Hook fields are synthesized client-side and never present in the raw response,
-                // so they must not be used as presence guards for discriminating union/interface variants.
-                if ($this->directiveProcessor->getHookDirective($selection->directives) !== null) {
+                // The synthetic hook field itself is never present in the raw response, so it
+                // cannot guard the variant. But the data the hook `requires` IS injected into
+                // the operation and returned by the server, so those fields must guard the
+                // variant — otherwise the parent's optional payload offsets are never narrowed
+                // for the leaf class that consumes them.
+                $hookName = $this->directiveProcessor->getHookDirective($selection->directives);
+
+                if ($hookName !== null) {
+                    $hook = $this->config->hooks[$hookName] ?? null;
+
+                    if ($hook !== null) {
+                        $this->collectRequiredFieldsFromSelectionSet($hook->requiresFragment->selectionSet, $requiredFields);
+                    }
+
                     continue;
                 }
 
diff --git a/tests/HooksInterface/FindOwnerHook.php b/tests/HooksInterface/FindOwnerHook.php
new file mode 100644
index 0000000..7cd2040
--- /dev/null
+++ b/tests/HooksInterface/FindOwnerHook.php
@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Ruudk\GraphQLCodeGenerator\HooksInterface;
+
+use Ruudk\GraphQLCodeGenerator\Attribute\Hook;
+use Ruudk\GraphQLCodeGenerator\HooksInterface\Generated\Hook\ProjectOwnerId;
+
+#[Hook(
+    name: 'findOwner',
+    requires: <<<'GRAPHQL'
+        fragment ProjectOwnerId on Project {
+          ownerId
+        }
+        GRAPHQL
+)]
+final readonly class FindOwnerHook
+{
+    /**
+     * @param array<string, User> $users
+     */
+    public function __construct(
+        private array $users = [],
+    ) {}
+
+    public function __invoke(ProjectOwnerId $project) : ?User
+    {
+        return $this->users[$project->ownerId] ?? null;
+    }
+}
diff --git a/tests/HooksInterface/Generated/Hook/ProjectOwnerId.php b/tests/HooksInterface/Generated/Hook/ProjectOwnerId.php
new file mode 100644
index 0000000..cc35ebc
--- /dev/null
+++ b/tests/HooksInterface/Generated/Hook/ProjectOwnerId.php
@@ -0,0 +1,24 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Ruudk\GraphQLCodeGenerator\HooksInterface\Generated\Hook;
+
+// This file was automatically generated and should not be edited.
+
+final class ProjectOwnerId
+{
+    public string $ownerId {
+        get => $this->ownerId ??= $this->data['ownerId'];
+    }
+
+    /**
+     * @param array{
+     *     'ownerId': string,
+     *     ...,
+     * } $data
+     */
+    public function __construct(
+        private readonly array $data,
+    ) {}
+}
diff --git a/tests/HooksInterface/Generated/Query/Test/Data.php b/tests/HooksInterface/Generated/Query/Test/Data.php
new file mode 100644
index 0000000..e8a0986
--- /dev/null
+++ b/tests/HooksInterface/Generated/Query/Test/Data.php
@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Ruudk\GraphQLCodeGenerator\HooksInterface\Generated\Query\Test;
+
+use Ruudk\GraphQLCodeGenerator\HooksInterface\FindOwnerHook;
+use Ruudk\GraphQLCodeGenerator\HooksInterface\Generated\Query\Test\Data\Node;
+
+// This file was automatically generated and should not be edited.
+
+final class Data
+{
+    public Node $node {
+        get => $this->node ??= new Node($this->data['node'], $this->hooks);
+    }
+
+    /**
+     * @var list<Error>
+     */
+    public readonly array $errors;
+
+    /**
+     * @param array{
+     *     'node': array{
+     *         '__typename': string,
+     *         'ownerId'?: string,
+     *         ...,
+     *     },
+     *     ...,
+     * } $data
+     * @param list<array{
+     *     'code': string,
+     *     'debugMessage'?: string,
+     *     'message': string,
+     *     ...,
+     * }> $errors
+     * @param array{
+     *     'findOwner': FindOwnerHook,
+     *     ...,
+     * } $hooks
+     */
+    public function __construct(
+        private readonly array $data,
+        array $errors,
+        private readonly array $hooks,
+    ) {
+        $this->errors = array_map(fn(array $error) => new Error($error), $errors);
+    }
+}
diff --git a/tests/HooksInterface/Generated/Query/Test/Data/Node.php b/tests/HooksInterface/Generated/Query/Test/Data/Node.php
new file mode 100644
index 0000000..08cca76
--- /dev/null
+++ b/tests/HooksInterface/Generated/Query/Test/Data/Node.php
@@ -0,0 +1,55 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Ruudk\GraphQLCodeGenerator\HooksInterface\Generated\Query\Test\Data;
+
+use Ruudk\GraphQLCodeGenerator\HooksInterface\FindOwnerHook;
+use Ruudk\GraphQLCodeGenerator\HooksInterface\Generated\Query\Test\Data\Node\AsProject;
+
+// This file was automatically generated and should not be edited.
+
+final class Node
+{
+    public ?AsProject $asProject {
+        get {
+            if (isset($this->asProject)) {
+                return $this->asProject;
+            }
+
+            if ($this->data['__typename'] !== 'Project') {
+                return $this->asProject = null;
+            }
+
+            if (! array_key_exists('ownerId', $this->data)) {
+                return $this->asProject = null;
+            }
+
+            return $this->asProject = new AsProject($this->data, $this->hooks);
+        }
+    }
+
+    /**
+     * @api
+     * @phpstan-assert-if-true !null $this->asProject
+     */
+    public bool $isProject {
+        get => $this->isProject ??= $this->data['__typename'] === 'Project';
+    }
+
+    /**
+     * @param array{
+     *     '__typename': string,
+     *     'ownerId'?: string,
+     *     ...,
+     * } $data
+     * @param array{
+     *     'findOwner': FindOwnerHook,
+     *     ...,
+     * } $hooks
+     */
+    public function __construct(
+        private readonly array $data,
+        private readonly array $hooks,
+    ) {}
+}
diff --git a/tests/HooksInterface/Generated/Query/Test/Data/Node/AsProject.php b/tests/HooksInterface/Generated/Query/Test/Data/Node/AsProject.php
new file mode 100644
index 0000000..c6300fb
--- /dev/null
+++ b/tests/HooksInterface/Generated/Query/Test/Data/Node/AsProject.php
@@ -0,0 +1,42 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Ruudk\GraphQLCodeGenerator\HooksInterface\Generated\Query\Test\Data\Node;
+
+use Ruudk\GraphQLCodeGenerator\HooksInterface\FindOwnerHook;
+use Ruudk\GraphQLCodeGenerator\HooksInterface\Generated\Hook\ProjectOwnerId;
+use Ruudk\GraphQLCodeGenerator\HooksInterface\User;
+
+// This file was automatically generated and should not be edited.
+
+final class AsProject
+{
+    public ?User $owner {
+        get => $this->owner ??= $this->hooks['findOwner']->__invoke($this->buildProjectOwnerId());
+    }
+
+    /**
+     * @param array{
+     *     '__typename': 'Project',
+     *     'ownerId': string,
+     *     ...,
+     * } $data
+     * @param array{
+     *     'findOwner': FindOwnerHook,
+     *     ...,
+     * } $hooks
+     */
+    public function __construct(
+        private readonly array $data,
+        private readonly array $hooks,
+    ) {}
+
+    /**
+     * @internal
+     */
+    public function buildProjectOwnerId() : ProjectOwnerId
+    {
+        return new ProjectOwnerId($this->data);
+    }
+}
diff --git a/tests/HooksInterface/Generated/Query/Test/Error.php b/tests/HooksInterface/Generated/Query/Test/Error.php
new file mode 100644
index 0000000..5bd208b
--- /dev/null
+++ b/tests/HooksInterface/Generated/Query/Test/Error.php
@@ -0,0 +1,24 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Ruudk\GraphQLCodeGenerator\HooksInterface\Generated\Query\Test;
+
+// This file was automatically generated and should not be edited.
+
+final readonly class Error
+{
+    public string $message;
+
+    /**
+     * @param array{
+     *     'debugMessage'?: string,
+     *     'message': string,
+     *     ...,
+     * } $error
+     */
+    public function __construct(array $error)
+    {
+        $this->message = $error['debugMessage'] ?? $error['message'];
+    }
+}
diff --git a/tests/HooksInterface/Generated/Query/Test/TestQuery.php b/tests/HooksInterface/Generated/Query/Test/TestQuery.php
new file mode 100644
index 0000000..42abb62
--- /dev/null
+++ b/tests/HooksInterface/Generated/Query/Test/TestQuery.php
@@ -0,0 +1,57 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Ruudk\GraphQLCodeGenerator\HooksInterface\Generated\Query\Test;
+
+use Ruudk\GraphQLCodeGenerator\HooksInterface\FindOwnerHook;
+use Ruudk\GraphQLCodeGenerator\TestClient;
+
+// This file was automatically generated and should not be edited.
+
+final readonly class TestQuery {
+    public const string OPERATION_NAME = 'Test';
+    public const string OPERATION_DEFINITION = <<<'GRAPHQL'
+        query Test {
+          node {
+            __typename
+            ... on Project {
+              ... on Project {
+                ownerId
+              }
+            }
+          }
+        }
+        
+        GRAPHQL;
+
+    /**
+     * @param array{
+     *     'findOwner': FindOwnerHook,
+     *     ...,
+     * } $hooks
+     */
+    public function __construct(
+        private TestClient $client,
+        private array $hooks,
+    ) {}
+
+    /**
+     * @api
+     */
+    public function execute() : Data
+    {
+        $data = $this->client->graphql(
+            self::OPERATION_DEFINITION,
+            [
+            ],
+            self::OPERATION_NAME,
+        );
+
+        return new Data(
+            $data['data'] ?? [], // @phpstan-ignore argument.type
+            $data['errors'] ?? [], // @phpstan-ignore argument.type
+            $this->hooks,
+        );
+    }
+}
diff --git a/tests/HooksInterface/HooksInterfaceTest.php b/tests/HooksInterface/HooksInterfaceTest.php
new file mode 100644
index 0000000..8de4076
--- /dev/null
+++ b/tests/HooksInterface/HooksInterfaceTest.php
@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Ruudk\GraphQLCodeGenerator\HooksInterface;
+
+use Override;
+use Ruudk\GraphQLCodeGenerator\Config\Config;
+use Ruudk\GraphQLCodeGenerator\GraphQLTestCase;
+use Ruudk\GraphQLCodeGenerator\HooksInterface\Generated\Query\Test\TestQuery;
+
+final class HooksInterfaceTest extends GraphQLTestCase
+{
+    #[Override]
+    public function getConfig() : Config
+    {
+        return parent::getConfig()
+            ->withHook(FindOwnerHook::class);
+    }
+
+    public function testGenerate() : void
+    {
+        $this->assertActualMatchesExpected();
+    }
+
+    public function testQuery() : void
+    {
+        $findOwner = new FindOwnerHook([
+            'user-123' => new User('user-123'),
+        ]);
+
+        $result = new TestQuery(
+            $this->getClient([
+                'data' => [
+                    'node' => [
+                        '__typename' => 'Project',
+                        'ownerId' => 'user-123',
+                    ],
+                ],
+            ]),
+            [
+                'findOwner' => $findOwner,
+            ],
+        )->execute();
+
+        self::assertNotNull($result->node->asProject);
+        self::assertNotNull($result->node->asProject->owner);
+        self::assertSame('user-123', $result->node->asProject->owner->id);
+    }
+}
diff --git a/tests/HooksInterface/Schema.graphql b/tests/HooksInterface/Schema.graphql
new file mode 100644
index 0000000..884137f
--- /dev/null
+++ b/tests/HooksInterface/Schema.graphql
@@ -0,0 +1,22 @@
+type Query {
+    node: Node!
+}
+
+interface Node {
+    id: ID!
+}
+
+type Project implements Node {
+    id: ID!
+    name: String!
+    ownerId: ID!
+}
+
+type Organization implements Node {
+    id: ID!
+    title: String!
+}
+
+type User {
+    id: ID!
+}
diff --git a/tests/HooksInterface/Test.graphql b/tests/HooksInterface/Test.graphql
new file mode 100644
index 0000000..932c559
--- /dev/null
+++ b/tests/HooksInterface/Test.graphql
@@ -0,0 +1,7 @@
+query Test {
+    node {
+        ... on Project {
+            owner @hook(name: "findOwner")
+        }
+    }
+}
diff --git a/tests/HooksInterface/User.php b/tests/HooksInterface/User.php
new file mode 100644
index 0000000..2f17989
--- /dev/null
+++ b/tests/HooksInterface/User.php
@@ -0,0 +1,12 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Ruudk\GraphQLCodeGenerator\HooksInterface;
+
+final readonly class User
+{
+    public function __construct(
+        public string $id,
+    ) {}
+}