Delegate to subagents

Two shapes, both real, neither replacing the other.

A roster the author wrote

The child's whole Spec is embedded in the parent's, so one Version hash pins the entire tree. That is load-bearing: the runtime reloads the pinned Version at the top of every Attempt, a reclaiming Worker's included, so a tree that could change between a crash and a reclaim would resume as a different tree.

spec = psych_runtime.AgentSpec(
    name="support",
    model=psych_runtime.ModelRef(model="gpt-4o"),
    subagents=(
        psych_runtime.SubagentRef(
            name="researcher",
            description="Digs through the knowledge base for policy details and cites them",
            spec=research_spec,
        ),
    ),
)

The parent gets a delegate tool listing its subagents by description. Delegation is inline and blocks the parent until the child completes.

A description under 20 characters is refused. Bad routing traces back to vague descriptions more often than to anything else, and this text is exactly what the parent reads to choose.

An envelope permitting composition

What the roster cannot cover is a child whose instructions the model wrote during the Run, which by definition cannot be in the parent's Version. The envelope is the permission that makes that legal, and being on the Spec it joins the Version hash like everything else. "This agent was allowed to write its own subagents" is pinned along with the rest of what it is.

spec = psych_runtime.AgentSpec(
    name="lead",
    model=psych_runtime.ModelRef(model="gpt-4o"),
    spawn=psych_runtime.SpawnEnvelope(
        tools=("search_*", "read_doc"),  # ceiling, not a grant
        models=("gpt-4o-mini",),  # empty means the parent's own model only
        max_depth=2,
        max_alive=3,
        may_message=True,
    ),
)

The parent gets spawn_subagent, check_subagent and message_subagent (the last only when may_message is true).

The composed child Spec is published as a Version like any other, and the spawn record holds its hash. So crash recovery is unchanged: the child Run pins a hash, and replaying it re-reads that Spec rather than re-composing one from a prompt the model would write differently the second time.

The model must write a real brief

Three floors, and each catches a different failure:

These are floors low enough that no legitimate brief hits them and high enough to catch "do the research".

Access narrows down the tree

SpawnEnvelope.tools is a ceiling, not a grant. What a child actually gets is the intersection of what the model asked for, this list, and what the parent itself holds, computed by the same narrow the validator and the per-turn resolver call. A parent cannot write itself a child with more access than it has.

Empty tools means everything the parent holds, matching the empty-means-all rule everywhere else. Inverting it here alone would make spawn silently useless rather than obviously wrong, and the fix people reach for is *.

Empty models means the parent's own model and nothing else. A model the author never mentioned is not a model they priced.

Depth and how many at once

• Limits.max_delegation_depth (default 3) bounds a tree an author wrote and can read.
• SpawnEnvelope.max_depth (default 2) bounds a tree nobody has read yet. Separate numbers because they answer different questions.
• Limits.max_fanout_per_turn (default 4) counts delegations started in one turn, which is right for a blocking delegation that finishes inside the turn.
• SpawnEnvelope.max_alive (default 3) counts background children still running. A per-turn cap would let a parent hold twenty children open by spawning four a turn for five turns. Alive is the number that costs money.

A parent with nothing left to do

It suspends on SuspendReason.CHILDREN rather than holding its lease. A fan-out of four children holding four Workers doing nothing, while those children's own children queue behind them, is exactly what suspension exists to prevent.

A finished child tells its parent with a Record, not a callback, so a parent that is suspended, on another machine, or not running at all still gets the news. A parent about to suspend reconciles its children first and does not suspend at all if they have already finished.

The whole tree is reconstructable from the log: a child's spawn, every message its parent sent it, and its ending are all Records. Nothing about a running tree lives in a process.

Reading the tree

report = await psych_runtime.report(store, run_id, child_depth=2)
report.totals  # this Run alone
report.subtree  # this Run plus every descendant

child_depth is bounded rather than unlimited so a deep tree cannot read the whole database by accident. Both totals exist because "what did this agent cost" and "what did this request cost" stop being the same question the moment a tree exists.

Gotchas

• A child does not inherit the parent's conversation. Everything it needs is in the task.
• may_message=False makes spawning fire-and-forget. Right for a fan-out of independent jobs, wrong for anything a person is watching.
• tools and models are sorted sets. Two authors writing the same envelope in a different order have written the same agent.
• A workflow's AgentStep is a third thing. It runs a nested agent as a deterministic step; see psych-workflows.