Agent UX: pm update-many --ids silently drops nonexistent IDs (no unmatched_ids/warning, exit 0) — regresses from single-item item_not_found #596

Summary

pm update-many --ids <a,bad,b> silently drops IDs that don't exist: exit 0, no unmatched_ids, no warnings, and the counts reflect only matched items — so an agent has no structural signal that part of its request was ignored. The single-item path (pm update <bad>) correctly reports item_not_found, so the bulk path regresses from it. Verified under CLI 2026.7.18.

Repro

pm update-many --ids pm-REAL1,pm-zzzz,pm-REAL2 --priority 4 --json
# matched_count: 2, updated_count: 2, skipped_count: 0, failed_count: 0
# no 'unmatched_ids' key, no 'warnings', ids/rows list only the 2 matched, exit 0

pm update pm-zzzz --priority 4 --json
# code: item_not_found   <-- single-item path DOES signal it

Observed vs expected

• Observed: pm-zzzz vanishes. matched_count = 2 (not 3), skipped_count/failed_count = 0, no unmatched_ids, no warning, exit 0. The only way to detect the drop is to diff the requested --ids against the returned ids — a manual cross-reference the envelope never prompts.
• Expected: an unmatched_ids: ["pm-zzzz"] field (or a top-level warnings[]), and/or an unmatched_count, so the drop is machine-detectable without client-side list diffing.

Why it matters for agents

Bulk update-by-ID-list is a common recovery/migration pattern. An agent seeing updated_count: 2, failed_count: 0 reasonably concludes all requested items were processed. A dropped ID may be a typo, a stale ID from a prior session, or a race-deleted item — exactly the cases the agent needs surfaced. Without a structural field, every bulk caller must defensively diff two lists on every call, which is the token-expensive parsing the CLI should absorb. This is the same silent-mutation class the AGENT_UX_CONTRACTS "relationship mutations" section addresses (e.g. ordering_cycle_created), applied to bulk --ids.

Duplicate check

Distinct from previously filed issues. #590 (closed) was cycle-creating --blocked-by succeeding silently; this is --ids silent drop in update-many. Not covered by docs/AGENT_UX_CONTRACTS.md.