The default branch in unmarshalResponsesInputItem had two issues:
- It referenced typeField.Type instead of itemType; these differ when the
shorthand role-based format promotes an empty type to "message", meaning
an unhandled type would show the wrong value in the error string.
- It used %s formatting, so an empty type field produced the unhelpful
message "unknown input item type: " with no indication what was missing.
Fix by using itemType (the resolved value) with %q quoting, and add a
dedicated message when itemType is empty (both type and role absent):
"input item missing required 'type' field".
Tests added for the empty-type and missing-type cases.
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
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