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The idle-parent trap happens when a parent agent delegates work to a child, then stops its own turn assuming the child’s completion will wake it. That is safe only if the orchestration runtime provides an effective wake-up or durable resume mechanism. Otherwise, the parent can remain idle indefinitely. Keep it in an event-driven wait, or persist the child result and arrange explicit delivery, with defined timeout and recovery behavior.
How the idle-parent trap happens
A parent agent starts or delegates a child task and then relinquishes control while waiting. If the runtime does not schedule the parent again when the child finishes—and does not otherwise provide a timer, event, or durable resume—the child may complete while the parent remains idle. The failure is not delegation itself; it is relying on an unstated wake-up contract.
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Agentproto’s sessions documentation puts the risk plainly: “A supervisor that fans out children should not end its turn to wait for them — nothing wakes an idle parent on a timer.” Its recommended pattern is to keep the supervisor waiting on inbox events, looping until a child reports or no children remain pending: Agentproto sessions documentation.
Choose blocking or non-blocking execution deliberately
The right control flow depends on whether the parent can make useful progress without the child’s result. Helix documents both blocking and non-blocking child execution; they are alternatives with different continuation behavior, not interchangeable meanings of “wait.” See Helix’s subagent documentation for its runtime-specific behavior.
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| Pattern | Use it when | Parent behavior |
|---|---|---|
| Blocking child execution | The next parent decision depends on the child result. | The parent waits for the child outcome before continuing. |
| Non-blocking child execution | The parent has independent work it can do first. | The parent continues, then explicitly checks status or waits for the result. |
Non-blocking execution does not mean the result will arrive automatically at a later point in the parent’s logic. The parent still needs a defined status check, inbox wait, callback, or other supported delivery path.
Make wake-up and recovery part of the design
Before implementing a parent-child workflow, identify what event causes the parent to run again and what state is available when it does. A sound design makes the waiting contract explicit rather than assuming that child completion is enough.
- For event-driven work: Keep the parent in a supported inbox or event wait and continue until the expected child report arrives or the pending set is empty.
- For work that outlives a process: Persist task identity and state, and use a runtime-supported mechanism to deliver the result or resume the parent.
- For bounded waits: Set a timeout and decide whether the parent retries, reports failure, cancels the child, or continues with partial information.
- For every child: Preserve distinct outcomes—running, completed, failed, interrupted, timed out, or suspended—so recovery does not mistake an incomplete task for a successful one.
These are design principles drawn from documented runtime patterns, not a universal API contract. Exact event delivery, persistence, and recovery behavior varies by framework.
Design for restarts and long-running work
A wait that works while a process stays alive may fail across hibernation or restart if it depends on in-memory state. Cloudflare documents durable agent identities and persistent state patterns for these situations. Its documentation distinguishes data that survives hibernation and restarts—such as state, SQL data, schedules, and fiber checkpoints—from in-memory variables, timers, open fetches, and local closures, which do not. It also describes fibers and Workflows for different long-running patterns. See Cloudflare Agents documentation.
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When work must survive a runtime interruption, store enough durable information to reconstruct the relationship between parent and child: task identity, current status, and the result or a reference to it. Then use the runtime’s documented restart-safe mechanism to resume or deliver the outcome. Do not assume a local timer or closure will still exist after a restart.
Compare runtimes by their lifecycle guarantees
“Wait,” “spawn,” and “resume” are framework-specific terms. Compare the guarantees behind them before choosing an orchestration pattern.
| Question | What to verify | Documented example |
|---|---|---|
| What wakes the parent? | Whether the parent waits on an inbox/event, receives an explicit report or callback, or resumes from a durable workflow event. | Agentproto documents inbox waiting; Cloudflare documents schedules, fibers, and Workflows. |
| What survives a restart? | Whether execution is only in memory or whether identity and relevant state are persisted and recoverable. | Cloudflare documents which state persists and which in-memory resources do not. |
| How are incomplete outcomes represented? | Whether running, failed, interrupted, timed out, or suspended states are distinct and actionable. | Helix documents timeout and interruption outcomes, as well as suspension while awaiting children. |
| Can an interrupted child resume? | Whether it resumes with its prior session/context or must start a fresh run. | Helix documents resumable interrupted companions in supported runtimes. |
| Can the parent settle before its children? | Whether the framework tracks ownership and requires children to be disposed of before parent settlement. | UnieAI documents ownership tracking and a parent-settlement constraint: UnieAI documentation. |
These examples describe particular projects and runtimes, not industry-wide standards. Check the version-specific documentation for the framework you deploy; API names and lifecycle behavior can change.
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Debugging a parent that appears stuck
- Check whether the child actually started. Confirm its task identity and current status rather than inferring execution from the parent’s delegation call.
- Inspect the parent’s wait state. Determine whether it is actively waiting on an event, blocked on a child, or simply finished its turn with no configured wake-up.
- Look for a delivered completion signal. Verify the child’s report reached the expected inbox, callback, workflow event, or status store.
- Check timeout and interruption handling. Confirm that the runtime’s outcome is represented distinctly and that the parent has a recovery path for it.
- Test restart behavior if the task is long-running. Verify that required state is durable and that the runtime can resume or notify the parent after hibernation or restart.
A useful diagnostic is to ask, for each transition: “What exact event makes the parent runnable again?” If the answer is only “when the child finishes,” verify that the runtime explicitly guarantees that behavior.
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