---
title: "Subagents"
description: "Delegate work to child agents, either a copy of the agent itself or declared specialists with their own sandbox and skills."
---

A subagent is a child agent that one agent delegates a focused subtask to. Split work into one to run it in parallel, to give the child a narrower set of tools, or to give a specialist its own identity. There are two kinds, the built-in `agent` tool (a copy of the agent itself) and declared subagents (specialists with their own directory).

## The built-in `agent` tool

Every agent gets an `agent` tool by default. The model calls it to delegate a subtask to a copy of itself:

```ts
{
  message: string;       // everything the child needs; it does not see the parent's history
  outputSchema?: object; // when set, the child runs in task mode and returns structured output
}
```

The copy shares the parent's sandbox and tools, and a child's file writes are immediately visible to the parent. That is what makes parallel calls natural: emit multiple `agent` calls in one response to fan out a small, fixed set of independent subtasks. eve runs that batch concurrently and returns every result before the parent continues. Give children non-overlapping write scopes when they work in the shared sandbox. The copy inherits auth and connections, but starts with fresh conversation history and fresh state. If a declared subagent calls `agent`, the child is a copy of _that_ subagent, not the root.

The parent transfers data to the child through the `message` input it gives the subagent. Do not include sensitive data in a subagent request unless that child and its inherited tools, connections, sandbox, and telemetry path are appropriate for that data.

An authored tool at `agent/tools/agent.ts` takes priority over the built-in.

## Declared subagents

A declared subagent lives under `agent/subagents/<id>/` and uses the same `defineAgent` helper as the root. Its location under `subagents/` is the only thing that marks it as a subagent. Declare one when the child needs a clearly different prompt, role, or tool surface.

```ts title="agent/subagents/researcher/agent.ts"
import { defineAgent } from "eve";

export default defineAgent({
  description: "Investigate ambiguous questions before the parent agent responds.",
  model: "anthropic/claude-opus-4.8",
});
```

`description` is required. The parent reads it to decide whether to delegate, so the compiler rejects any subagent whose `agent.ts` leaves it out.

Minimum files:

```text
agent/subagents/researcher/
├── agent.ts            # required (must export a description)
├── instructions.md     # or instructions.ts, optional
├── tools/              # optional, its own tools
├── skills/             # optional, its own skills
├── sandbox/            # optional, its own sandbox + workspace seed
└── subagents/          # optional, nested subagents
```

`schedules/` is not supported inside a declared subagent. Schedules are root-only.

## The isolation boundary

A declared subagent inherits nothing from the root's authored slots. Discovery treats its directory as its own agent root, so it has only the instructions, tools, connections, skills, sandbox, hooks, and nested subagents authored under `agent/subagents/<id>/`. An absent slot falls back to the framework default, not to the root's version.

| Slot         | Built-in `agent` tool         | Declared subagent                      |
| ------------ | ----------------------------- | -------------------------------------- |
| Instructions | Inherited (copy of the agent) | Own `instructions.{md,ts}`, optional   |
| Tools        | Inherited                     | Own `tools/`                           |
| Connections  | Inherited                     | Own `connections/`                     |
| Skills       | Inherited                     | Own `skills/`                          |
| Sandbox      | Shared with parent            | Own `sandbox/`, else framework default |
| Hooks        | Inherited                     | Own `hooks/`                           |
| State        | Fresh                         | Fresh                                  |
| Channels     | Root-only                     | Root-only                              |
| Schedules    | Root-only                     | Root-only                              |

For a declared subagent this means duplicating anything the child needs. When two subagents need the same procedure, copy the markdown under each `skills/` directory, or share typed helpers via `lib/`. The sandbox does not inherit from the parent; it falls back to the framework default unless the subagent authors `subagents/<id>/sandbox.ts` or seeds files via `subagents/<id>/sandbox/workspace/`.

The built-in `agent` tool is the exception. Its children share the parent's sandbox and tools because they are copies of the same agent working on the same files.

`defineState` is never shared, for either kind. Each child starts with fresh durable state.

## What the parent sees

eve lowers every subagent (built-in copy, declared, or [remote](./guides/remote-agents)) into a model-visible tool with the same `{ message, outputSchema? }` shape. The parent packs `message` with everything the child needs, since the child never sees the parent's history. Set `outputSchema` to run the child in task mode, returning structured output as the tool result.

A declared subagent's tool name is the bare path-derived name, with no prefix. `agent/subagents/researcher/` registers as the tool `researcher`. Unlike connection tools (`<connection>__<tool>`), it carries no namespace, so the model, approvals, logs, and evals all reference it by that name. Its input schema is:

```ts
{
  message: string;       // all context the child needs; it never sees the parent's history
  outputSchema?: object; // when set, the child runs in task mode and returns structured output
}
```

Because the name lives in the same runtime tool namespace as authored tools, a subagent named `researcher` collides with a tool named `researcher`. eve rejects the build rather than picking a winner, so keep subagent directory names distinct from tool names.

Do not rely on subagent delegation by itself as an approval boundary. Put sensitive tools behind `approval`, connection approval, route/session authorization, or other controls wherever those tools can be called.

Each delegated subagent spins up its own child session and stream. The parent stream carries only the control-plane events `subagent.called` and `subagent.completed`. To follow the child's full progress, read `subagent.called.data.childSessionId` and subscribe at `GET /eve/v1/session/:childSessionId/stream`.

## When to split

Split out a subagent when the task needs a different prompt or specialist role, a narrower tool surface, or its own runtime context. Don't reach for one when a [skill](./skills) would do. If the agent can keep its identity and needs only an optional procedure, a skill is the lighter choice.

## What to read next

- [Remote agents](./guides/remote-agents): call another eve deployment as a subagent.
