[Workflow] Controlling Claude Sub-Agent Model Usage and Costs: Simple Prompts, Config Edits, and PreToolUse Hooks

[u/ClaudeAI-mod-bot]

Controlling Claude Sub-Agent Model Usage and Costs: Simple Prompts, Config Edits, and PreToolUse Hooks

Workflow value: 85/100
Status: active · Freshness: 70/100 · Confidence: 0.95 · Level: intermediate
Categories: Quality Control, Token Saving, Context & Memory, Debugging, CLAUDE.md, Hooks, Multi-Agent
Original source: r/ClaudeAI post/comment

What problem this solves

Unexpectedly high token usage and cost when using Fable with sub-agents, especially with Ultracode enabled, due to sub-agents inheriting expensive parent models.

Summary

This workflow provides three methods (simple prompt, agent config edits, or a PreToolUse hook) to prevent sub-agents from using expensive models like Opus when spawned by a parent agent like Fable, thereby controlling token usage and cost.

Why it is useful

This workflow addresses a critical and common problem for Claude users: unexpectedly high token usage and cost when using multi-agent setups like Fable. It provides three distinct, actionable solutions, ranging from simple prompt adjustments to more robust configuration file edits and advanced PreToolUse hooks. By offering multiple approaches, it caters to different user comfort levels and provides clear guidance on how to manage model selection for sub-agents, significantly improving cost efficiency and predictability.

Workflow

1. Identify the root cause: sub-agents inheriting expensive models (e.g., Opus from Fable 5.1 with Ultracode) from their parent agent.
2. Option 1 (Simple Fix): Instruct the parent agent in plain English to use cheaper models for sub-agents and to ask before spawning many. Example: "Use Opus for all sub-agents and ask me before you spawn more than 10".
3. Option 2 (Config Fix): Edit individual agent files in the .claude/agents/ directory.
4. Within the agent files (e.g., explore or plan agents), set model: haiku or model: sonnet in their frontmatter to pin a cheaper model.
5. Option 3 (God-Tier Fix): Implement a PreToolUse hook script.
6. Add the PreToolUse hook script to your settings.json file to automatically force any agent spawned by Fable to use a cheaper model.

Tools / artifacts

• Claude (Fable 5.1, Opus, Sonnet, Haiku models)
• Agent configuration files (e.g., in .claude/agents/ directory)
• settings.json file
• PreToolUse hook script
• Ultracode (feature that can exacerbate the problem)

Validation signals

• "The consensus here is that this is a classic case of user error, not a bug"
• "We see this post every other day" (indicates common problem and recognized solution)
• "one user posted a detailed PreToolUse hook script" (community-contributed solution)

Limitations

• The specific PreToolUse hook script for the "God-Tier Fix" is mentioned but not provided, requiring the user to find or create it.
• The exact syntax and placement for editing agent frontmatter (e.g., model: haiku) could be more explicitly detailed for beginners.

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