TaskOutput tool cannot find completed background tasks — should it be denyable?

Bug: TaskOutput returns "No task found" for completed background tasks

Environment

• Claude Code version: 2.1.81
• OS: macOS (Darwin 25.3.0, arm64)
• Model: claude-opus-4-6 (1M context)

Reproduction steps

1. Launch multiple background commands and agents in parallel:

• 2 background bash commands (run_in_background: true) with sleep 5 / sleep 7
• 2 background agents (run_in_background: true)

1. Wait for all 4 to complete (all <task-notification> messages are received with status: completed)
2. Use TaskOutput tool to retrieve the results of all 4 tasks

Expected behavior

All 4 TaskOutput calls return the task output successfully.

Actual behavior

3 out of 4 TaskOutput calls fail with:

No task found with ID: <task_id>

Only the last-completed task is retrievable. The output files still exist on disk at the expected paths — the task registry has simply lost track of them.

Debug log evidence

10:00:57.478Z [DEBUG] TaskOutput tool validation error: No task found with ID: bw1x8dg3j
10:00:57.975Z [DEBUG] TaskOutput tool validation error: No task found with ID: bkvflxzy4
10:00:58.916Z [DEBUG] TaskOutput tool validation error: No task found with ID: a3569442d23145738

All tasks had previously completed successfully and delivered <task-notification> messages. The output files are confirmed present on disk.

Root cause hypothesis

When a completed task's notification is delivered to the model, the task registry appears to remove/evict the task entry. By the time TaskOutput is called, the tasks are no longer registered — even though their outputs exist on disk.

---

Question: Should TaskOutput be denyable via settings?

In v2.1.81, the model no longer proactively uses TaskOutput — it relies on <task-notification> messages instead, which work correctly. The TaskOutput tool only gets used when explicitly prompted, and when it is used, it hits this bug.

Given that the notification-based flow works well:

1. Is there still a use case for TaskOutput?
2. Can it be added to the deny list in settings so the model doesn't have access to it? Currently it's a deferred tool that gets loaded via ToolSearch, but there's no way to prevent it from being suggested/used.

If TaskOutput is kept, the registry eviction bug needs to be fixed so completed tasks remain retrievable for a reasonable window after completion.