[Capability] An array shape with a nullable or union key rejects valid tool arguments #612

Describe the bug

A tool whose array parameter is documented with an array shape that has a nullable key, such as @param array{name: string, nickname: string|null} $user, rejects every call with -32602. SchemaGenerator looks for null anywhere in the type string to decide whether a parameter is nullable, so the null of the nickname key makes the whole parameter array|null and the schema loses the shape.

A ?string key does not hit that check, but the shape conversion maps ?string and union keys such as int|string to object, so those keys reject their values instead.

To Reproduce

final class Users
{
    /**
     * @param array{name: string, nickname: string|null} $user
     */
    public function greet(array $user): string
    {
        return 'Hello '.($user['nickname'] ?? $user['name']);
    }
}

Server::builder()
    ->setServerInfo('repro', '1.0.0')
    ->addTool([Users::class, 'greet'], 'greet')
    ->build()
    ->run(new StdioTransport());

tools/list gives user this schema:

{"type": ["array", "null"], "items": {}}

and tools/call with {"user": {"name": "Ada", "nickname": null}} returns:

-32602 Invalid parameters for tool 'greet': Property '/user': Invalid type. Expected `array|null`, but received `object`.

With array{name: string, age: ?int} or array{id: int|string}, the schema keeps the shape but types age and id as object, so "age": 5 and "id": 7 fail with "Expected object, but received integer".

Expected behavior

The schema keeps the shape and types each key from its own union, for example:

{"type": "object", "properties": {"name": {"type": "string"}, "nickname": {"type": ["string", "null"]}}, "required": ["name", "nickname"]}

and the call succeeds. Only a top-level |null, as in array{id: int}|null, makes the parameter nullable.

Environment

mcp/sdk v0.9.0 and main (57b45eb), PHP 8.5.

I have a fix with a test and will open a PR.