Agent UX: pm merge reconcile over-reports discarded_authors/discarded_events on the driver union path (false data-loss alarm; complements #603) #646

Summary

In the merge-driver happy path — where the field-aware driver (pm merge install) has already union-merged an append-only field like notes — pm merge reconcile reports discarded_events, discarded_authors, and reverted_fields (and emits history_repair_discarded_authors:<author> / history_repair_reconcile_discards_events:N warnings) even though nothing is actually lost: the on-disk item keeps every author's content and attribution, and the history log keeps every author's event. The warning describes an internal replay operation, not a user-visible outcome, so it reads as cross-author data loss when there is none.

This is the complementary case to the (correctly closed) #603: that issue fixed under-reporting of genuine loss; this issue is over-reporting of loss that did not happen. In a multi-agent workflow the false alarm is expensive — an agent that sees discarded_authors:agent-b will follow the recovery_hint and burn context re-inspecting/re-applying a note that is already present and correctly attributed. project management = context management, so a misleading warning is a real cost.

Environment

• @unbrained/pm-cli 2026.7.22 (installed from npm)
• Field-aware merge driver active (pm merge install)

Reproduction

git init -q && git config user.email a@b.c && git config user.name t
pm init mrg
pm merge install                      # field-aware driver (unions notes, resolves scalars)
git add -A && git commit -qm init
ID=$(pm --json create issue "shared item" | jq -r '.id')
git add -A && git commit -qm seed
BASE=$(git branch --show-current)

git checkout -qb agent-a
pm --author agent-a notes  "$ID" --add "note from agent A"
pm --author agent-a update "$ID" --priority 1
git add -A && git commit -qm a

git checkout -q "$BASE" && git checkout -qb agent-b
pm --author agent-b notes  "$ID" --add "note from agent B"
pm --author agent-b update "$ID" --assignee agent-b
git add -A && git commit -qm b

git merge agent-a -m "merge a into b"   # driver: 0 conflicts, union_fields:[notes], fields_from_theirs:[priority]
pm merge reconcile --dry-run            # <-- see output below

pm merge reconcile output (dry-run and apply identical)

reconciliation:
  reverted_field_count: 1
  reverted_fields: [ "notes" ]
  discarded_events:
    - index: 4
      author: "agent-b"
      op: "note_add"
      fields: [ "notes" ]
  discarded_authors: [ "agent-b" ]
  recovery_hint: "Reconciliation keeps the on-disk item and overwrites these
                  replayed mutations. To recover one, inspect it with
                  pm history <id> --diff --field <field>, then re-apply ..."
warnings:
  - "history_repair_discarded_authors:agent-b"
  - "history_repair_reconcile_discards_events:1"

Actual end state after pm merge reconcile (nothing was lost)

pm show "$ID" --depth full   # notes = [ {A, author:agent-a}, {B, author:agent-b} ]  ← both, correct authors
                             # priority: 1 (agent-a)   assignee: agent-b (agent-b)
pm history --verify "$ID"    # ok:true, current_matches_latest:true
pm validate                  # no *_hash_mismatch (only benign metadata warnings)

The history log also still contains agent-b's note_add event (plus a new history_repair event) — it was not removed. So discarded_events:[agent-b note_add], discarded_authors:[agent-b], and reverted_fields:[notes] are all false for the union-merged field: content preserved, attribution preserved, history event preserved.

Expected

When the reconciled field is an append-only/union field whose on-disk value already contains the "discarded" author's contribution, reconcile should classify that discard as net-lossless and NOT emit a data-loss warning for it. Concretely:

• Distinguish net-lossy discards (the discarded event's value is absent from the final on-disk item — the pm history-repair cements cross-author data loss after a lossy merge; pm validate then reports fully green (no revert warning / replay check) #603 case, keep the loud warning) from net-lossless replayed-event discards (value still present — suppress or downgrade to info).
• discarded_authors / history_repair_discarded_authors should list only authors whose content is actually gone from the item, so the signal stays trustworthy.
• The recovery_hint should fire only for net-lossy discards.

Why it matters

The #603 fix (surfacing real loss) is correct and should stay. But over-firing it on the driver union path trains agents to distrust the warning or to waste a recovery loop on phantom loss — undermining the exact multi-agent "merge → reconcile → validate" workflow that GH-623 was built to make safe. A precise, outcome-based warning keeps pm merge reconcile trustworthy for autonomous multi-agent use.

Filed from fleet verification of 2026.7.22 (companion tracker item pm-cli-website-qbpf sibling); repro is fully self-contained above.