A housing case tool cannot find a case ID. Which error signaling fits?

When a housing case tool cannot find a case ID, honest structured errors let Claude stop, ask for clarification, or search differently instead of continuing on false premises. Returning a typed not-found or equivalent error that echoes the requested ID preserves auditability, supports harness retries, and prevents invented case narratives in resident-facing housing workflows. Returning the nearest matching housing case record with a similarity ranking for manual operator review invites Claude to act on a record that is not the requested case, leading to hallucinated follow-on updates—status changes, notices, or eligibility conclusions—against the wrong record. Returning a cached housing case summary from a previous lookup with a timestamp to preserve active session continuity hands Claude stale resident data instead of a recoverable not-found signal and leaves operators without a clear failure reason in logs. Returning partial housing case fields with a continuation token to resume an interrupted retrieval implies the record exists and the lookup is merely paused, which risks fabricating eligibility conclusions and notice timelines from incomplete data and is especially dangerous for fair-housing documentation. Exam caveat: structured errors should be machine-readable and stable across versions (codes plus messages) so prompt logic and harness retries behave consistently; returning a nearest match with similarity ranking for manual review is not an acceptable substitute for an explicit not-found. Operational check: call the tool with a deliberately invalid case ID, confirm a non-success structured payload, verify Claude does not claim the case was loaded or propose field updates as if retrieval succeeded, and ensure audit logs capture the failed lookup with the identity that requested it.