[Server] Fix iterable parameters being declared as objects

CHANGELOG.md

@@ -73,6 +73,7 @@ All notable changes to `mcp/sdk` will be documented in this file.
   * Fix an `int` parameter receiving a wrong value: a number beyond the integer range wrapped around, also past the schema's `minimum` or `maximum`, a numeric string beyond it became `PHP_INT_MAX` or `PHP_INT_MIN`, and `"--5"` became `0`. They are refused now.
   * Fix a nullable parameter with a default (e.g. `?string $category = 'books'`) receiving the default when the client sends `null`.
   * Fix DocBlock types producing wrong schemas, so valid arguments were rejected with `-32602`: PHPStan/Psalm pseudo-types and literal types (`non-empty-string`, `positive-int`, `'asc'|'desc'`, `array<array-key, T>`) get the JSON Schema type of their base type, nullable and union keys of an array shape (`array{nickname: string|null}`) and union elements of a generic (`array<string|null>`) get their own types instead of `object`, and a `null` inside them, or in a class name, no longer makes the parameter nullable.
+  * [BC Break] Generate `{"type": "array"}` for an `iterable` parameter, as for `array` and `iterable<T>`, instead of `{"type": "object"}`, which refused a JSON array with `-32602` although the handler accepts one. A JSON object is refused instead, as for `array`.
   * Fix tool name validation to follow the spec (1 to 128 characters of `A-Z`, `a-z`, `0-9`, `_`, `-` and `.`): names of 65 to 128 characters no longer log a warning, names with a `/` or a trailing newline do.
   * Fix `content`, `messages` and `contents` being sent as a JSON object when the handler returned a non-list array, e.g. from `array_filter()`, and named keys in nested mixed arrays overwriting each other.
   * Fix a scalar `structuredContent` on `2026-07-28` going out as raw text in `content`: `'sunny'` is sent as the JSON `"sunny"`, as SEP-2106 requires.

src/Server/Capability/Discovery/SchemaGenerator.php

@@ -843,7 +843,7 @@ private function mapPhpTypeToJsonSchemaType(string $phpTypeString): array
             'int', 'integer' => ['integer'],
             'float', 'double', 'number' => ['number'],
             'bool', 'boolean' => ['boolean'],
-            'array' => ['array'],
+            'array', 'iterable' => ['array'],
             'object', 'stdclass' => ['object'],
             'null' => ['null'],
             'resource', 'callable' => ['object'],

tests/Unit/Server/Capability/Discovery/SchemaGeneratorFixture.php

@@ -37,6 +37,10 @@ public function typeHintsOnly(string $name, int $age, bool $active, array $tags,
     {
     }
 
+    public function iterableParameters(iterable $items, ?iterable $optional = null): void
+    {
+    }
+
     /**
      * DocBlock types only - no PHP type hints, no Schema attributes.
      *

tests/Unit/Server/Capability/Discovery/SchemaGeneratorTest.php

@@ -51,6 +51,14 @@ public function testInfersBasicTypesFromPhpTypeHints(): void
         $this->assertEqualsCanonicalizing(['name', 'age', 'active', 'tags'], $schema['required']);
     }
 
+    public function testRepresentsIterableParametersAsArrays(): void
+    {
+        $schema = $this->schemaGenerator->generate(new \ReflectionMethod(SchemaGeneratorFixture::class, 'iterableParameters'));
+        $this->assertEquals(['type' => 'array', 'items' => new \stdClass()], $schema['properties']['items']);
+        $this->assertEquals(['type' => ['array', 'null'], 'default' => null, 'items' => new \stdClass()], $schema['properties']['optional']);
+        $this->assertSame([], (new SchemaValidator())->validateAgainstJsonSchema(['items' => [1, 2]], $schema));
+    }
+
     public function testInfersTypesAndDescriptionsFromDocBlockTags(): void
     {
         $method = new \ReflectionMethod(SchemaGeneratorFixture::class, 'docBlockOnly');