4.3 — Structured Output with Tool Use

CCAR-F Domain 4 · Task 4.3

What You Need to Know

For guaranteed schema-compliant output: tool_use with JSON schemas (eliminates syntax errors) > prompt-based JSON (no structural guarantees).

tool_choice: Three Modes

Mode	Behaviour	Use when
"auto"	Model decides whether to call a tool	Conversational flexibility needed
"any"	Must call a tool; chooses which	Guaranteed structured output; unknown document type
{"type":"tool","name":"..."}	Must call one specific tool	Enforce a mandatory first step

What tool_use Does NOT Prevent

• Sum discrepancies
• Field placement errors
• Fabrication of values for required fields when source lacks the data

Semantic validation needs additional logic (Task 4.4).

Schema Design for Production

• Optional/nullable fields — primary defence against fabrication
• "unclear" enum value — for genuinely unclear sources
• "other" + detail string — extensible categorisation
• Format normalisation rules in the prompt alongside the schema

Key Concept: tool_use with a JSON schema stops syntax errors, not semantic ones. Optional/nullable fields stop fabrication. tool_choice "any" guarantees structured output for unknown document types.

Exam Traps

• Believing tool_use prevents all extraction errors
• Relying on tool_choice "auto" to guarantee a tool call
• Making all schema fields required to ensure completeness