[Bug] /mcp slash command sends empty string for required prompt parameters, causing Zod validation error

Bug Description

When executing an MCP prompt via the /mcp slash command, required parameters (defined with argsSchema) are sent as empty strings, causing Zod validation errors before the prompt handler is reached.

Steps to Reproduce

1. Configure an MCP server that registers a prompt with required parameters using argsSchema:

``typescript
mcpServer.registerPrompt('my_prompt', {
description: 'Example prompt',
argsSchema: {
repository: z.string().regex(/^[\w.\-/]+$/).max(100).describe('Repository name (e.g., "RepoName")'),
},
}, async (params) => { /* handler */ });
``

1. In Claude Code CLI, type /mcp
2. Select the prompt from the dialog
3. The prompt is invoked immediately without prompting the user to input the required repository parameter

Expected Behavior

• Claude Code should prompt the user to input required parameters before invoking the MCP prompt, OR
• Display a user-friendly error message when required parameters are missing

Actual Behavior

The prompt is invoked with an empty string for the required parameter, resulting in a raw Zod validation error:

McpError: MCP error -32602: MCP error -32602: Invalid arguments for prompt my_prompt: [
  {
    "origin": "string",
    "code": "invalid_format",
    "format": "regex",
    "pattern": "/^[\\w.\\-/]+$/",
    "path": ["repository"],
    "message": "Invalid string: must match pattern /^[\\w.\\-/]+$/"
  }
]

Environment

• Claude Code v2.1.74
• macOS (Darwin 24.6.0)

Notes

• This affects any MCP prompt with required parameters that have format validation (regex, min length, etc.)
• MCP tools with required inputSchema parameters work correctly — this issue is specific to prompts with argsSchema
• As a server-side workaround, removing regex validation from the Zod schema and validating manually inside the handler can provide a friendlier error, but this shouldn't be necessary