Tool Use Pattern

Tool use (function calling) lets models invoke external capabilities with structured arguments and observe results.

Design good tools

• Small and sharp — one clear job
• Strict schemas — types, enums, required fields
• Actionable errors — “order_id not found” beats “error”
• Idempotent writes when possible
• Document side effects in the description

Selection quality

Overlapping tools confuse models. Prefer:

• Distinct names
• Non-overlapping responsibilities
• Examples in descriptions (short)

Execution loop

model proposes tool call → validate schema → policy check → execute →
return observation → model continues

Never skip validation/policy because the model “sounded sure.”

Verification

For side-effecting tools, verify postconditions in code before reporting success to users.

MCP and portability

Standard interfaces like MCP help expose tools across hosts, but security remains your job.