[Server] Fix variadic enum parameters declaring their items 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 $category = 'books'`) receiving the default when the client sends `null`.
   * Fix DocBlock types producing wrong schemas, so valid arguments were rejected with `-32602`: PHPStan/Psalm pseudo-types and literal types (`non-empty-string`, `positive-int`, `'asc'|'desc'`, `array<array-key, T>`) get the JSON Schema type of their base type, nullable and union keys of an array shape (`array{nickname: string|null}`) and union elements of a generic (`array<string|null>`) get their own types instead of `object`, and a `null` inside them, or in a class name, no longer makes the parameter nullable.
+  * Fix a variadic enum parameter (e.g. `Status ...$statuses`) declaring its items as `{"type": "object"}`, so every call was rejected with `-32602`: the items list the enum's values, as for a single enum parameter.
   * 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

@@ -366,10 +366,14 @@ private function buildVariadicParameterSchema(array $paramInfo): array
 
         // If no items specified by Schema attribute, infer from type
         if (!isset($paramSchema['items'])) {
+            // Reflection gives a variadic its element type, so an enum constrains each item.
+            $enumItems = $this->applyEnumConstraints([], $paramInfo);
             $itemJsonTypes = $this->mapPhpTypeToJsonSchemaType($paramInfo['type_string']);
             $nonNullItemTypes = array_filter($itemJsonTypes, static fn ($t) => 'null' !== $t);
 
-            if (1 === \count($nonNullItemTypes)) {
+            if ([] !== $enumItems) {
+                $paramSchema['items'] = $enumItems;
+            } elseif (1 === \count($nonNullItemTypes)) {
                 $paramSchema['items'] = ['type' => $nonNullItemTypes[0]];
             }
         }

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

@@ -513,6 +513,14 @@ public function untypedVariadic(...$values): void
     {
     }
 
+    public function variadicBackedEnum(BackedStringEnum ...$options): void
+    {
+    }
+
+    public function variadicUnitEnum(UnitEnum ...$answers): void
+    {
+    }
+
     /**
      * Variadic with Schema constraints.
      *

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

@@ -527,6 +527,16 @@ public function testRepresentsVariadicStringParametersAsArrayOfStrings(): void
         $this->assertArrayNotHasKey('required', $schema);
     }
 
+    public function testRepresentsVariadicEnumParametersAsArrayOfEnumValues(): void
+    {
+        $backed = $this->schemaGenerator->generate(new \ReflectionMethod(SchemaGeneratorFixture::class, 'variadicBackedEnum'));
+        $this->assertEquals(['type' => 'array', 'items' => ['type' => 'string', 'enum' => ['A', 'B']]], $backed['properties']['options']);
+        $this->assertSame([], (new SchemaValidator())->validateAgainstJsonSchema(['options' => ['A', 'B']], $backed));
+
+        $unit = $this->schemaGenerator->generate(new \ReflectionMethod(SchemaGeneratorFixture::class, 'variadicUnitEnum'));
+        $this->assertEquals(['type' => 'array', 'items' => ['type' => 'string', 'enum' => ['Yes', 'No']]], $unit['properties']['answers']);
+    }
+
     public function testAppliesItemConstraintsToVariadicParameters(): void
     {
         $method = new \ReflectionMethod(SchemaGeneratorFixture::class, 'variadicWithConstraints');