[BUG] MCP prompts with only optional arguments require input to invoke

Environment

• Claude CLI version: 1.0.73+
• Operating System: macOS (confirmed), likely affects all platforms
• MCP Server: Custom implementation using @modelcontextprotocol/sdk

Bug Description

MCP prompts that have only optional arguments (all marked as required: false) cannot be invoked through Claude's interface without providing at least one character of input, even though all arguments are optional.

Expected Behavior

Prompts with only optional arguments should be invokable with no input, using the default values or proceeding without arguments.

Actual Behavior

The prompt appears in the slash command list but pressing Enter without typing any arguments does nothing. The prompt can only be invoked by typing at least one character (even just ".").

Reproduction Steps

1. Create an MCP server with a prompt that has only optional arguments:

name: example_prompt
description: Example prompt with optional argument
arguments:
  - name: optional_param
    description: Optional parameter
    required: false
    default: "default_value"

1. Configure the MCP server in Claude Code
2. Try to invoke the prompt with /example_prompt and press Enter
3. Notice the prompt doesn't execute
4. Type /example_prompt . and press Enter
5. The prompt now executes

Example Real-World Prompts Affected

From the Simone MCP server (https://github.com/Helmi/claude-simone):

• check_activity - has optional period argument with default "today"
• create_changelog - has optional version_type with default "patch"
• create_commit - has optional issue argument
• work_issue - has optional issue argument (can be gathered interactively)

Workaround

Users must provide at least one character (like ".") to invoke prompts with only optional arguments.

Impact

This affects usability of MCP prompts designed to work with defaults or gather information interactively. It's counterintuitive that optional arguments effectively become required for invocation.

Related Issues

• #4125 discusses MCP prompt argument handling but focuses on multi-word support
• This issue is specifically about optional arguments not being truly optional for invocation