[Server] `completion/complete` crashes for plain `ref/resource`

[kiryadev]

Description

When a client sends a completion/complete request with ref.type = "ref/resource" pointing to a regular MCP resource, the server triggers a PHP warning/internal error:

Warning: Undefined property: Mcp\Capability\Registry\ResourceReference::$completionProviders

This happens because CompletionCompleteHandler fetches the resource reference and unconditionally reads $reference->completionProviders, but Mcp\Capability\Registry\ResourceReference does not define that property. The property exists on PromptReference and ResourceTemplateReference, but not on plain ResourceReference.

Example request

{
  "jsonrpc": "2.0",
  "id": 16,
  "method": "completion/complete",
  "params": {
    "argument": {
      "name": "tone"
    },
    "ref": {
      "type": "ref/resource",
      "uri": "page-editor://version-1/instruction"
    }
  }
}

Current behavior

The server logs/returns an internal error because PHP raises:

Undefined property: Mcp\Capability\Registry\ResourceReference::$completionProviders

This becomes an ErrorException and produces a JSON-RPC internal error.

Version

Observed on mcp/sdk v0.5.0.
I also checked v0.6.0 and main, and the same access pattern appears to still exist.