[MCP Bundle] Pattern support decision + ServiceLocator bug

[camilleislasse]

Description

Two issues with MCP tool dependency injection:

1. Design decision needed: Bundle currently only supports invokable pattern, but SDK supports both patterns (invokable + methods).
2. Bug: McpPass::process() passes tag arrays instead of References to ServiceLocatorTagPass

Current Situation

MCP SDK supports two patterns:

1. Invokable: #[McpTool] on class + __invoke() method
2. Methods: #[McpTool] on public methods

MCP Bundle currently:

• Uses registerAttributeForAutoconfiguration()
• Only detects class-level attributes (invokable pattern)
• Method pattern not supported with auto-configuration
• Tools with dependencies crash due to ServiceLocator bug

Pattern Support Decision

Option 1: Keep invokable only

Uses registerAttributeForAutoconfiguration() which only detects class-level attributes. Users need to use #[McpTool] on class with __invoke() method.

Option 2: Support both patterns

Find a way to detect and auto-tag methods with #[McpTool] attributes. Would provide full compatibility with SDK examples and support multiple tools per class.

ServiceLocator Bug (must fix regardless)

Current code in McpPass::process() (line 36):

public function process(ContainerBuilder $container): void
{
    // ... collect services ...

    $allMcpServices = [
        'App\MCP\Tools\WeatherTool' => [['priority' => 0]], // Tag metadata array
        // ...
    ];

    // BUG: Passes tag arrays instead of References
    $serviceLocatorRef = ServiceLocatorTagPass::register($container, $allMcpServices);
}

What ServiceLocatorTagPass::register() expects:

[
    'App\MCP\Tools\WeatherTool' => new Reference('App\MCP\Tools\WeatherTool'),
    // ...
]

Fix (required for invokable pattern to work):

// src/mcp-bundle/src/DependencyInjection/McpPass.php
use Symfony\Component\DependencyInjection\Reference; // Add import

public function process(ContainerBuilder $container): void
{
    if (!$container->hasDefinition('mcp.server.builder')) {
        return;
    }

    $allMcpServices = [];
    $mcpTags = ['mcp.tool', 'mcp.prompt', 'mcp.resource', 'mcp.resource_template'];

    foreach ($mcpTags as $tag) {
        $taggedServices = $container->findTaggedServiceIds($tag);
        $allMcpServices = array_merge($allMcpServices, $taggedServices);
    }

    if ([] === $allMcpServices) {
        return;
    }

    // Transform service IDs to References
    $serviceReferences = [];
    foreach (array_keys($allMcpServices) as $serviceId) {
        $serviceReferences[$serviceId] = new Reference($serviceId);
    }

    $serviceLocatorRef = ServiceLocatorTagPass::register($container, $serviceReferences);

    $container->getDefinition('mcp.server.builder')
        ->addMethodCall('setContainer', [$serviceLocatorRef]);
}

Example: Method Pattern Not Supported

// This does NOT work with current Bundle
namespace App\MCP\Tools;

use App\Service\WeatherService;
use Mcp\Capability\Attribute\McpTool;

class WeatherTool
{
    public function __construct(
        private readonly WeatherService $weatherService,
    ) {
    }

    #[McpTool(name: 'get-weather')]
    public function getWeather(string $city): string
    {
        return $this->weatherService->getCurrentWeather($city);
    }
}

Result:

$ php bin/console debug:container --tag=mcp.tool
No tags found that match "mcp.tool"

Tool is NOT auto-tagged.

Example: Invokable Pattern (Works)

// This DOES work with current Bundle
namespace App\MCP\Tools;

use App\Service\WeatherService;
use Mcp\Capability\Attribute\McpTool;

#[McpTool(name: 'get-weather')]
class WeatherTool
{
    public function __construct(
        private readonly WeatherService $weatherService,
    ) {
    }

    public function __invoke(string $city): string
    {
        $weather = $this->weatherService->getCurrentWeather($city);
        return sprintf('Weather in %s: %s, %d°C',
            $weather['city'],
            $weather['condition'],
            $weather['temp']
        );
    }
}

Result (after ServiceLocator fix):

$ php bin/console debug:container --tag=mcp.tool
App\MCP\Tools\WeatherTool

$ php bin/console debug:container WeatherTool
Arguments: Service(App\Service\WeatherService)

Tool IS auto-tagged, DI works.

Error Without ServiceLocator Fix

Even with correct invokable pattern:

ERROR [mcp] Tool execution failed
"exception" => "Failed to resolve dependency 'App\Service\WeatherService' for 'App\MCP\Tools\WeatherTool'
parameter $weatherService: Failed to resolve dependency 'Psr\Log\LoggerInterface' for 'App\Service\WeatherService'
parameter $logger: Class 'Psr\Log\LoggerInterface' is not instantiable"

Action Items

Must do:

• Fix McpPass::process() to use References instead of tag arrays

Decide:

• Keep invokable-only (document limitation) OR
• Implement method pattern support

Environment

• MCP Bundle version: dev-main
• Symfony version: 7.3+
• PHP version: 8.4

Additional Notes

• This affects all MCP capabilities: #[McpTool], #[McpPrompt], #[McpResource], #[McpResourceTemplate]
• Tools with zero dependencies work by accident (SDK can instantiate with new $className())
• Tools requiring any service dependency crash with current code