sanitize_tool_data() injects non-standard "type": "action" field, causing Claude Code to silently drop tools

[brigleb]

Summary

The ToolsHandler::sanitize_tool_data() method adds a "type": "action" field to every tool in the tools/list response. This field is not part of the MCP specification for Tools. Claude Code's strict schema validation silently drops tools with non-standard fields, resulting in only 1 of 3 built-in tools being available.

Current Behavior

In includes/Handlers/Tools/ToolsHandler.php line 223–227:

$safe_tool = array(
    'name'        => $tool['name'] ?? '',
    'description' => $tool['description'] ?? '',
    'type'        => $tool['type'] ?? 'action',  // ← not in MCP spec
);

This produces tool definitions like:

{
  "name": "mcp-adapter-execute-ability",
  "description": "Execute a WordPress ability...",
  "type": "action",
  ...
}

The MCP spec's Tool object only defines: name, description, inputSchema, title, outputSchema, annotations, and _meta. The type field is not among them.

Impact

Claude Code v2.0.21+ performs strict schema validation on MCP tool definitions before passing them to the Anthropic API. Tools with non-standard fields are silently dropped. In practice, this means 2 of 3 built-in tools (mcp-adapter-execute-ability and mcp-adapter-get-ability-info) are unavailable in Claude Code, while mcp-adapter-discover-abilities works (likely because its simpler schema passes validation despite the extra field).

This effectively makes the MCP adapter unusable from Claude Code — you can discover abilities but cannot execute them or get their info.

Related Claude Code issues:

• #10858 — non-standard "strict" field caused identical silent tool dropping
• #10606 — strict MCP schema validation introduced in v2.0.21

Steps to Reproduce

1. Install WordPress 6.9 + MCP Adapter plugin + ACF Pro 6.8 (with AI enabled)
2. Configure .mcp.json per the README for stdio transport
3. Connect Claude Code to the MCP server
4. Run mcp-adapter-discover-abilities — works
5. Try to call mcp-adapter-execute-ability or mcp-adapter-get-ability-info — tool not found

Verify the server does register all 3 tools by querying directly:

echo '{"jsonrpc":"2.0","id":1,"method":"tools/list","params":{}}' | wp mcp-adapter serve --server=mcp-adapter-default-server --user=admin

Suggested Fix

Remove the 'type' line from sanitize_tool_data():

$safe_tool = array(
    'name'        => $tool['name'] ?? '',
    'description' => $tool['description'] ?? '',
);

Note that McpTool::to_array() (line 321) correctly omits type — the non-standard field is only introduced during sanitization.

Environment

• MCP Adapter: 0.4.1
• WordPress: 6.9 (beta)
• ACF Pro: 6.8.0-beta2
• Claude Code: 2.1.62
• PHP: 8.x
• macOS (Laravel Herd)