Tool Design for Agents

Design tools that agents can effectively discover, understand, and use.

Prerequisites

• •Understanding of agent architectures
• •API design experience

Instructions

Core Principle

Tool descriptions are prompt engineering that shapes agent behavior. Poor design creates failure modes no prompt engineering can fix.

Consolidation Principle

If a human engineer cannot definitively say which tool to use, an agent cannot either. Prefer single comprehensive tools over multiple narrow tools.

Instead of: list_users, list_events, create_event Use: schedule_event (handles full workflow)

Description Requirements

Answer four questions:

1. •What does it do? (Clear, specific)
2. •When should it be used? (Triggers and contexts)
3. •What inputs does it accept? (Types, constraints, defaults)
4. •What does it return? (Format, examples, errors)

Architectural Reduction

Production evidence shows removing specialized tools often improves performance.

Vercel d0: 17 tools → 2 (bash + SQL), 80% → 100% success.

When reduction works:

• •Data layer is well-documented
• •Model has sufficient reasoning
• •Tools were constraining, not enabling

Response Format Options

def get_customer(id: str, format: str = "concise"):
    """
    format: "concise" for key fields, "detailed" for complete
    """

Error Message Design

Errors must be actionable for agents:

• •What went wrong
• •How to correct it
• •Retry guidance if applicable

MCP Tool Naming

Always use fully qualified names:

"Use the BigQuery:bigquery_schema tool..."  # Correct
"Use the bigquery_schema tool..."            # May fail

Guidelines

1. •Write descriptions answering what, when, inputs, returns
2. •Use consolidation to reduce ambiguity
3. •Implement response format options
4. •Design errors for agent recovery
5. •Limit to 10-20 tools; use namespacing if more needed
6. •Prefer primitive tools over specialized wrappers
7. •Build minimal architectures that benefit from model improvements

Notes

• •10-20 tools reasonable for most applications
• •Tool description overlap causes model confusion
• •Use agents to optimize their own tool descriptions

Source: muratcankoylan/Agent-Skills-for-Context-Engineering