[MCP Bundle] tools/list can return empty in debug + FrankenPHP worker mode after successful discovery

[ineersa]

Context

I’m running symfony/mcp-bundle over HTTP transport behind FrankenPHP worker mode with:

• APP_ENV=dev
• APP_DEBUG=1

The MCP endpoint is reachable and authenticated. initialize succeeds, but tools/list can return an empty array
even though MCP tools were discovered at startup.

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Expected behavior

If tools are discovered (e.g. 4 tools), tools/list should return them consistently for subsequent requests in
the same MCP session.

Actual behavior

• initialize succeeds
• tools/list returns:

  {"tools":[]}

At the same time, MCP logs show discovery finding tools and then clearing discovered elements before/around
subsequent request handling.

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Logs / evidence

MCP channel logs show:

• Attribute discovery finished. with context like tools: 4
• followed by: Removed 4 discovered elements from internal registry.

After that, tools/list may return empty.

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Reproduction (HTTP MCP)

1. Configure MCP bundle with HTTP transport and attribute-based tools in src/ (#[McpTool(...)]).
2. Run in FrankenPHP worker mode with debug enabled (APP_DEBUG=1).
3. POST /_mcp initialize:

  {
    "jsonrpc": "2.0",
    "method": "initialize",
    "params": {
      "protocolVersion": "2025-03-26",
      "capabilities": {},
      "clientInfo": { "name": "repro", "version": "1.0" }
    },
    "id": 1
  }

4. Use returned Mcp-Session-Id and call:

  {
    "jsonrpc": "2.0",
    "method": "tools/list",
    "id": 2
  }

5. Observe empty tools in affected runs.

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Additional observations

• Setting APP_DEBUG=0 in the same environment makes tools/list return the discovered tools correctly.
• Behavior appears intermittent / client-flow dependent (request sequence/timing), but logs consistently show
  discovered tools being cleared in debug worker mode.

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Suspected cause

Potential interaction between worker lifecycle + debug registry reset in:

Symfony\AI\McpBundle\Profiler\TraceableRegistry

reset() calls clear(), which seems to remove discovered MCP capabilities across requests in worker mode.

Possibly related to:

• [AI Bundle] add ResetInterface to prevent memory leaks #1533

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Versions

• symfony/mcp-bundle: v0.8.0
• Symfony: 8.0.*
• Runtime: FrankenPHP worker mode
• Transport: HTTP (/_mcp)
• Env where reproduced: APP_ENV=dev, APP_DEBUG=1

[ineersa]

Root Cause Analysis

The chain of events in FrankenPHP worker mode + APP_DEBUG=1:

1. McpBundle::loadExtension() (line 78-81) registers TraceableRegistry as a decorator of mcp.registry when kernel.debug=true, and tags
   it with kernel.reset:

  ->addTag('kernel.reset', ['method' => 'reset'])

2. Builder::build() runs once at worker boot → discovers tools via DiscoveryLoader → populates the real Registry with 4 tools.
3. Symfony's ResetInterface mechanism calls reset() on all kernel.reset-tagged services between HTTP requests in worker mode.
4. TraceableRegistry::reset() delegates to clear(), which delegates to Registry::clear() → removes all discovered (non-manual)
   tools/resources/prompts.
5. Next request hits tools/list → the registry is now empty → returns {"tools":[]}.
6. Tools are never re-discovered because Builder::build() only runs once at container creation (worker boot).

Why APP_DEBUG=0 works: The TraceableRegistry is never registered, so there's no kernel.reset tag, so no clearing between requests.