From d45c4f238e624b1877ac97df64b63937607fc298 Mon Sep 17 00:00:00 2001
From: "Fusion (runfusion.ai)" <hi@runfusion.ai>
Date: Sun, 17 May 2026 18:09:56 -0700
Subject: [PATCH] fix(FN-4973): finalize executor backstop typing and document
 invariant

Fusion-Task-Id: FN-4973
Fusion-Task-Lineage: 2edb5479-a01a-4775-b121-f4be32792e7a
---
 AGENTS.md                       | 1 +
 packages/engine/src/executor.ts | 3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/AGENTS.md b/AGENTS.md
index 40aa734e6..f79345405 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -175,6 +175,7 @@ Detailed mechanism logs live in `docs/architecture.md` and `docs/design/`. The c
 - **In-review stall deadlock**: identical stalls (same code + reason) repeated past `inReviewStallDeadlockThreshold` (default 3) auto-pause with `pausedReason: "in-review-stall-deadlock"` and `status: "failed"`.
 - **Restart recovery**: `RestartRecoveryCoordinator` classifies interrupted `in-progress` runs. Unusable-worktree session-start failures (`missing`, `incomplete`, `unregistered git worktree`) are recoverable; retries are capped at `MAX_WORKTREE_SESSION_RETRIES=3` before escalating.
 - **Executor pre-session liveness gate (FN-4935)**: the gate now skips for fresh acquisitions (`acquisition.source === "fresh"`), emits structured `not_usable_task_worktree:<classification>` diagnostics (including canonicalized registered-path snapshots) and a `worktree:incomplete-detected` audit event with `source: "executor-liveness-gate"`, while preserving the existing `taskDoneRetryCount` / `MAX_TASK_DONE_REQUEUE_RETRIES` requeue contract. FN-4651 `worktreeSessionRetryCount` remains scoped to the in-review/session-start recovery path.
+- **Stale self-owned active-session reconcile on conflict cleanup (FN-4973)**: when executor worktree-conflict cleanup finds only a same-task stale `activeSessionRegistry` entry and no live in-memory `activeWorktrees` binding for that task/path, it must unregister the stale entry before `removeWorktree` (plus one-shot backstop reconcile on same-task `ActiveSessionWorktreeRemovalError` races). Foreign-task entries remain protected by FN-4811 and must never be reconciled by the requesting task.
 - **Task title/ID drift (FN-4898)**: active and archived title writes normalize foreign embedded `FN-NNN` tokens via `packages/core/src/task-title-id-drift.ts`. Lineage is preserved in `sourceParentTaskId` / description markers, not title embeds.
 - **PR-conflict reclaim wiring (FN-4763)**: GitHub PR refresh now persists normalized `prInfo.mergeable` conflict state and, when conflicting, funnels tasks into self-healing’s existing reclaim machinery (`reclaimPrConflictForTask` / `reclaim-pr-conflicts` stage) so branch-conflict handling stays centralized with existing `inspectBranchConflict` outcomes and unrecoverable pause semantics.
 - **Worktrunk-managed lifecycles**: when `worktrunk.enabled`, self-healing defers prune/idle/worktree-cap sweeps to the worktrunk backend; branch-level reclaim and orphan rescue stay native.
diff --git a/packages/engine/src/executor.ts b/packages/engine/src/executor.ts
index 9b17c7894..2b4cc1e67 100644
--- a/packages/engine/src/executor.ts
+++ b/packages/engine/src/executor.ts
@@ -43,7 +43,8 @@ import { resolveSandboxBackend } from "./sandbox/index.js";
 import type { SandboxBackend } from "./sandbox/types.js";
 import { ModelRegistry, SessionManager, type ToolDefinition, type AgentSession } from "@mariozechner/pi-coding-agent";
 import { PRIORITY_EXECUTE, type AgentSemaphore } from "./concurrency.js";
-import { ActiveSessionWorktreeRemovalError, RemovalReason, classifyTaskWorktree, describeRegisteredWorktrees, getRegisteredWorktreePaths, isGitRepository, isInsideWorktreesDir, isRegisteredGitWorktree, removeWorktree, type WorktreePool } from "./worktree-pool.js";
+import { RemovalReason, classifyTaskWorktree, describeRegisteredWorktrees, getRegisteredWorktreePaths, isGitRepository, isInsideWorktreesDir, isRegisteredGitWorktree, removeWorktree, type WorktreePool } from "./worktree-pool.js";
+import { ActiveSessionWorktreeRemovalError } from "./worktree-backend.js";
 import { activeSessionRegistry, executingTaskLock } from "./active-session-registry.js";
 import {
   StaleWorktreeIndexLockError,