Fix list schema rejecting complex objects in batch tools

The default items schema for data_type: 'list' without a ListInputDefinition
was {"type": "string"}, which rejected complex objects like
{"title": "...", "fields": {...}} sent by MCP clients for batch operations.
Changed default to {} (unconstrained) so all batch tools accept complex items.

Fixes #3572001

Reported-by: Guillaume Gérard (guillaumeg)
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: mowens3

src/Mcp/ToolApiSchemaConverter.php

@@ -170,7 +170,7 @@ private function inputDefinitionToSchema(InputDefinitionInterface $definition):
     // Provide best-effort list and map schemas when possible.
     if ($definition instanceof ListInputDefinition || $dataType === 'list' || $definition->isMultiple()) {
       $schema['type'] = 'array';
-      $schema['items'] = ['type' => 'string'];
+      $schema['items'] = new \stdClass();
       if ($definition instanceof ListInputDefinition) {
         $itemDefinition = $definition->getDataDefinition()->getItemDefinition();
         if ($itemDefinition instanceof InputDefinitionInterface) {

tests/src/Unit/Mcp/ToolApiSchemaConverterTest.php

@@ -140,7 +140,7 @@ public function testToolDefinitionToInputSchemaMapsTypesAndConstraints(): void {
     $this->assertStringContainsString('Entity objects should be passed', $properties['ref']['description']);
 
     $this->assertSame('array', $properties['tags']['type']);
-    $this->assertSame('string', $properties['tags']['items']['type']);
+    $this->assertInstanceOf(\stdClass::class, $properties['tags']['items']);
 
     $this->assertSame('object', $properties['meta']['type']);
   }
@@ -165,6 +165,40 @@ public function testToolDefinitionToInputSchemaEncodesEmptyPropertiesAsObject():
     $this->assertSame('{}', json_encode($schema['properties']));
   }
 
+  public function testListWithoutItemDefinitionProducesUnconstrainedItemsSchema(): void {
+    $converter = new ToolApiSchemaConverter();
+
+    $items = $this->mockInputDefinition(
+      dataType: 'list',
+      required: TRUE,
+      description: 'Array of content items',
+      constraints: [],
+    );
+
+    $definition = new ToolDefinition([
+      'id' => 'mcp_tools:batch_test',
+      'provider' => 'mcp_tools',
+      'label' => $this->markup('Batch Test'),
+      'description' => $this->markup('Test batch tool'),
+      'operation' => ToolOperation::Write,
+      'destructive' => FALSE,
+      'input_definitions' => [
+        'items' => $items,
+      ],
+    ]);
+
+    $schema = $converter->toolDefinitionToInputSchema($definition);
+    $properties = $schema['properties'];
+
+    // items must be typed as array.
+    $this->assertSame('array', $properties['items']['type']);
+
+    // Without a ListInputDefinition, items schema must be unconstrained ({})
+    // so complex objects like {"title": "...", "fields": {...}} are accepted.
+    $this->assertInstanceOf(\stdClass::class, $properties['items']['items']);
+    $this->assertSame('{}', json_encode($properties['items']['items']));
+  }
+
   private function mockInputDefinition(
     string $dataType,
     bool $required,

tests/src/Unit/Mcp/ToolInputValidatorTest.php

@@ -202,6 +202,63 @@ public function testValidateReturnsErrorsForInvalidArrayItems(): void {
     $this->assertNotEmpty($result['errors']);
   }
 
+  public function testValidateAcceptsComplexObjectsInUnconstrainedArray(): void {
+    $definition = $this->createMockDefinition();
+
+    // Simulates the schema produced for batch tools where data_type: 'list'
+    // is used without a ListInputDefinition. The items schema must be {}
+    // (unconstrained) so complex objects are accepted.
+    $this->schemaConverter->method('toolDefinitionToInputSchema')
+      ->willReturn([
+        'type' => 'object',
+        'properties' => [
+          'items' => [
+            'type' => 'array',
+            'items' => new \stdClass(),
+          ],
+        ],
+        'required' => ['items'],
+      ]);
+
+    $validator = $this->createValidator();
+    $result = $validator->validate($definition, [
+      'items' => [
+        ['title' => 'Page one', 'fields' => ['body' => 'Content']],
+        ['title' => 'Page two', 'status' => TRUE],
+      ],
+    ]);
+
+    $this->assertTrue($result['valid']);
+    $this->assertEmpty($result['errors']);
+  }
+
+  public function testValidateRejectsComplexObjectsInStringConstrainedArray(): void {
+    $definition = $this->createMockDefinition();
+
+    // When items is constrained to strings, complex objects must be rejected.
+    $this->schemaConverter->method('toolDefinitionToInputSchema')
+      ->willReturn([
+        'type' => 'object',
+        'properties' => [
+          'items' => [
+            'type' => 'array',
+            'items' => ['type' => 'string'],
+          ],
+        ],
+        'required' => ['items'],
+      ]);
+
+    $validator = $this->createValidator();
+    $result = $validator->validate($definition, [
+      'items' => [
+        ['title' => 'Page one', 'fields' => ['body' => 'Content']],
+      ],
+    ]);
+
+    $this->assertFalse($result['valid']);
+    $this->assertNotEmpty($result['errors']);
+  }
+
   public function testValidateReturnsValidForEmptyPropertiesSchema(): void {
     $definition = $this->createMockDefinition();