deepseek-harness/packages/subagent/tool-subagent

@deepseek-ai/dsh-tool-subagent

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The model-facing delegation tool over one configured ctx.subagents provider. Changing the provider changes transport without changing the execution contract.

Provider selection and lifecycle

Each plugin instance binds one provider to one toolName; the model receives no provider selector. Load another distinctly named instance to expose another transport. The tool registers only while its provider exists, avoiding sibling load-order and provider-reload dependencies. Its description follows provider.inheritsParentContext: fresh children require standalone prompts, while forked children already see completed parent turns.

A foreground call passes the execution signal through startup and execution, awaits run.result, and always awaits run.dispose() before returning. Only completed returns the canonical { kind: 'foreground', runId, output: JsonValue[] }, rendered as the same final text; abort, refusal, token limit, and other failures become errored tool results without partial output.

With run_in_background: true, the route follows the provider's continuation capability and returns canonical { kind: 'background', taskId, subagentId? }. A resumable provider (spawn, fork) delegates to ctx.subagentControl.startContinuable(), which owns the durable child id, descriptor snapshot, Task registration, and settle-then-dispose ordering; the result includes subagentId, renders as started subagent <childId> as task <taskId>, and accepts follow-up messages through the global send_message tool. A one-shot provider (ACP) keeps the plain parent-owned task, omits subagentId, and renders as started background subagent task <id>. Either way a task-owned signal covers pending startup and the child after the starting call returns; task_kill and owner disposal abort it, settlement awaits startup rollback or child disposal, and completed final text, abort to killed, and other failures to failed map identically. The task has no incremental read; generic task tools own later status, collection, cancellation, and notices. See the background subagent Agent Note and the continuable background subagents Agent Note.

toolFilter changes the child's global tool layer but is not a parent-derived authority ceiling. See the agent-scope security non-goal.

Config

Key	Meaning
provider (required)	Provider name (spawn, fork, acp, ...).
toolName	Model-facing name, default subagent; distinct for every loaded instance.
enableRunInBackground	Exposes background mode, default true; disabling also rejects forced background calls.
agentOptions	Provider-specific child provider, model, and positive maxTokens; the in-process provider treats explicit values as overrides of inherited parent options.
persona	Per-child persona; requires provider persona capability.
toolFilter	Per-child global-tool restriction; requires toolFilter capability.
maxDepth	Absolute delegation-depth cap, default 3 (0 forbids delegation); a numeric cap requires the depthLimit capability and fails the mount without it. 'provider-managed' sends no cap for an out-of-process provider whose budget belongs to the child harness. The tool stays visible at the cap; each attempted start checks the calling agent's current depth and returns an errored tool result when rejected.

Concurrency

Foreground and background calls are exclusive. Children may share the parent's workspace or external resources, and a unary classifier cannot prove that sibling delegations have disjoint effects. See the parallel tool-call Agent Note.

Model Experience

Tool schema

What the model sees

The generated default subagent schema under this instance's configured name while its provider exists. Provider context inheritance changes the tool and prompt descriptions; enabled background mode adds run_in_background.

Token effect

Fixed schema cost per parent request; each provider instance adds one schema.

KV Cache effect

Prefix-stable while provider instances, names, descriptions, and schemas are unchanged. Provider registration lifecycle may invalidate parent reuse from the first changed tool definition.

Foreground result

What the model sees

The call retains the description and prompt. Success contains only the child's final text; other outcomes become Error: <message>. Intermediate child steps stay out of the parent.

Token effect

The prompt and result remain in parent history until compaction; child working context remains in the child.

KV Cache effect

Append-only; newly visible content follows the reusable request prefix and does not invalidate existing KV-cache entries.

Background task result

What the model sees

Start returns exactly started subagent <childId> as task <taskId> on a resumable provider, or started background subagent task <id> on a one-shot provider. The generic task surface provides later status, final output, cancellation responses, and notices; send_message (from dsh-tool-subagent-control) delivers follow-ups to a continuable child.

Token effect

The acknowledgement is retained; final output enters parent history only when collected or injected.

KV Cache effect

Append-only; newly visible content follows the reusable request prefix and does not invalidate existing KV-cache entries.

Known Limitations and Deferred Work

• Background runs expose final output only — intermediate child steps stay in the child session.
• Duplicate names across waiting instances are detected late (TODO(subagent-dup-toolname)) — preventing provider-registration rollback requires a registry of intended names.
• Child policy is fixed per instance — another model, persona, tool filter, or depth cap requires another distinctly named tool.