2.3 — Tool Distribution & Tool Choice

CCAR-F Domain 2 · Task 2.3

What You Need to Know

Tool distribution is the choice of which agent in a multi-agent system holds which tools. The number of tools an agent holds directly affects selection reliability.

The Tool Overload Problem

Giving a single agent ~18 tools degrades selection reliability. Keep each agent's set small and scoped to its role (guide example: 4–5 tools per agent).

Choosing the Right Fix

Situation	Fix
Few tools, two read alike	Sharpen the descriptions (2.1)
Different jobs (query, transform, export)	Split by role, small set each
Variations on one job, sharing a shape	Consolidate into one parameterised tool
Doing more than the agent should	Constrain them (least privilege)

Moving tools onto a second MCP server does not help — the client hands the model every tool from every connected server as one flat list.

Replacing Generic Tools with Constrained Alternatives

Prefer load_document (validates document URLs only) over a generic fetch_url. This is least privilege applied to tool design.

Scoped Cross-Role Tools

For high-frequency simple lookups that would otherwise go through the coordinator (adding 2–3 round trips), give the agent a scoped version of the capability. Complex cases still route through the coordinator.

The tool_choice Configuration

Setting	Behaviour	Use when
"auto" (default)	Model decides whether to call a tool	General conversation
"any"	Model must call a tool, chooses which	Guaranteed structured output from one of multiple schemas
Forced ({"type":"tool","name":"..."})	Model must call one specific tool	Enforce a mandatory first step

Exam Traps

• Routing all simple verification requests through the coordinator when 85% are simple lookups
• Using tool_choice 'auto' when structured output is required
• Giving an agent 18 tools and expecting reliable selection
• Giving a subagent a generic fetch_url tool for loading documents