fusion/docs/triage-duplicate-detection-postmortem.md

Triage Duplicate Detection Post-Mortem (FN-4774)

Summary

Between 14:29 and 16:05 on 2026-05-16, three independent tasks were filed and executed for the same defect: rebase-merge done-task diff truncation in packages/dashboard/src/routes/register-session-diff-routes.ts (with related tests in packages/dashboard/src/__tests__/routes-diff.test.ts). This duplicated implementation and review effort.

Timeline

• FN-4726
  • Created: 14:29
  • Merged: 14:52
  • GitHub tracking: #400
  • Scope touched: register-session-diff-routes.ts, routes-diff.test.ts
• FN-4734
  • Created: 14:54
  • Finalization: 15:33 (auto no-op finalize)
  • GitHub tracking: #414
  • Scope touched: same target files
• FN-4741
  • Created: 15:04
  • Merged: 16:05
  • GitHub tracking: #424
  • Scope touched: same target files

Root cause

1. Triage duplicate scan blind spot: fn_task_list intentionally excludes done tasks (tasks.filter((t) => t.column !== "done") in packages/engine/src/triage.ts), so recently completed fixes were invisible to the duplicate check.
2. No triage keyword search tool: Triage had no exposed keyword search over full task history (including done/archived), despite TaskStore.searchTasks(...) already existing and backed by FTS5.
3. GitHub issue duplication gap: Tracking issue creation did not include a check for existing closed issues covering the same files/keywords.

Decision

1. Add fn_task_search triage tool that wraps store.searchTasks(...) with defaults that include historical tasks (includeArchived: true, done tasks included by default).
2. Strengthen duplicate-check prompt guidance so triage must run both list and keyword search (multiple phrases from title/description/file paths/symptoms/symbol names), then inspect likely matches before filing.
3. Record GitHub-side dedup as follow-up work (new task) rather than expanding this fix’s scope.

Acceptance-criteria mapping

1. Document what happened and why: covered by timeline + root-cause analysis above.
2. Close triage duplicate-detection gap: addressed by adding fn_task_search and prompt instructions to query done/archived history.
3. Preserve scope discipline: GitHub-side dedup is explicitly deferred into a follow-up task instead of being implemented here.

Incident closure (FN-4866)

FN-4866 was filed as a follow-up investigation request for the same FN-4726/FN-4734 duplicate-merge incident and is now closed against previously shipped safeguards.

• FN-4774 — Added the fn_task_search triage tool over store.searchTasks(...) (FTS5) with historical coverage defaults (includeArchived: true, done tasks included by default) and updated duplicate-check prompt guidance in packages/engine/src/triage.ts.
• FN-4815 — Added regression coverage in packages/engine/src/__tests__/triage-duplicate-search-regression.test.ts for the tool contract, duplicate-check prompt guidance, and end-to-end duplicate discovery before task creation.
• FN-4829 — Added create-time duplicate gating (POST /tasks/duplicate-check, acknowledgedDuplicates / bypassDuplicateCheck on POST /tasks, DuplicateWarningModal, and task:duplicate-warning-overridden activity event).

FN-4866 acceptance-criteria mapping: root-cause analysis ✓ (see Root cause above); safeguard recommendation with linked implementation tasks ✓ (FN-4774 and FN-4829); regression test plan ✓ (FN-4815, plus explicit named-case coverage added under this closure task).