[FEATURE] Support features such as loops, conditionals, variable assignments, etc. in custom slash command files

Preflight Checklist

• [x] I have searched existing requests and this feature hasn't been requested yet
• [x] This is a single feature request (not multiple features)

Problem Statement

I sometimes wish I could use something similar to a Mustache template in my custom slash command Markdown files:

---
description: Simplified example
---

Perform these tasks:
{{#ARGUMENTS}}
- {{.}}
{{/ARGUMENTS}}

So /my-slash-command Foo Bar Baz would expand to the following document before getting passed to the API as a prompt, etc.:

---
description: Simplified example
---

Perform these tasks:
- Foo
- Bar
- Baz

Proposed Solution

Solution 1:

• Adopt a popular and universal templating language such as mustache.js and let us use it in our custom slash command .md files

Solution 2:

• Let us write custom slash commands in TypeScript (a quick and dirty sketch):

// saved as .claude/commands/brainstorm.ts
interface SlashCommandInput {
  description: string;
  allowedTools?: string[] | undefined | null;
  arguments: Record<string, unknown>;
}

function slashCommand(input: SlashCommandInput): string {
  const { description, allowedTools, arguments: args } = input;
  
  // Hard-coded task template
  const taskTemplate: string = 'Brainstorm $3 solutions for problem $2 in $1';
  
  // Process allowed tools - handle empty, undefined, or null cases
  const toolsList: string = allowedTools && allowedTools.length > 0 
    ? allowedTools.join(', ') 
    : '';
  
  // Generate argument hint from the keys of the arguments object
  const argKeys: string[] = Object.keys(args);
  const argumentHint: string = argKeys
    .map((key: string) => `[${key}]`)
    .join(' ');
  
  // Construct and return the formatted string
  return `---
description: ${description}
allowed-tools: ${toolsList}
argument-hint: ${argumentHint}
---
_Task:_
${taskTemplate}`;
}

export { slashCommand };

Alternative Solutions

_No response_

Priority

Medium - Would be very helpful

Feature Category

Interactive mode (TUI)

Use Case Example

_No response_

Additional Context

_No response_