Observation
Optional fields marked as nullable in JSON tool schemas are filled with plausible-sounding but fabricated values at a rate 4× higher than required fields.
Evidence
Analyzed 2,800 tool call responses across 14 tools with optional nullable fields. Optional fields received hallucinated values 23% of the time vs 5.8% for required fields. Models interpret optionality as permission to fill in gaps, rather than omit them.
Implications
Remove optional fields from tool schemas unless the tool genuinely requires them. If optional fields must exist, add explicit instruction: "Leave null if not directly stated in user input." Consider two-schema approach: minimal schema for first call, richer schema for enrichment call when source data is available.
