deepseek-harness/packages/subagent/tool-subagent

@deepseek-ai/dsh-tool-subagent

The subagent tool lets the model delegate one self-contained task and collect the child's final output. It is a thin consumer of ctx.subagents; changing the configured provider changes the transport without changing the model-facing execution contract.

Provider selection

Each plugin instance binds to exactly one provider. The model sees { description, prompt }, not a provider selector. To expose multiple transports, load the plugin multiple times with distinct toolName values.

The description is derived from provider.inheritsParentContext: spawn and ACP tell the model to provide a standalone prompt, while fork says the child already sees completed conversation turns. The plugin follows subagent/provider-added and subagent/provider-removed, so concurrent Cordis plugin loading does not create a registration-order dependency.

Lifecycle

execute passes the tool execution's abort signal when present, otherwise supplies an inert signal to satisfy the required SubagentStartRequest.signal. It awaits ctx.subagents.start(...), then awaits run.result inside a try/finally that always calls run.dispose(). The selected signal therefore covers startup and live execution, while disposal guarantees quiescence on success, failure, and abort.

A non-completed stop reason becomes an isError tool result; partial child output is never reported as success. The current tool blocks the parent turn until collection finishes; background and polling modes are deferred.

Config

Key	Meaning
provider	Required ctx.subagents provider name.
toolName	Model-facing tool name (default subagent). Must be unique per plugin instance.
agentOptions	Default child agent options, currently including model.
persona	Per-child persona; requires provider persona capability.
toolFilter	Per-child global-tool restriction; requires provider toolFilter capability.
maxDepth	Absolute delegation-depth cap; requires provider depthLimit capability.

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

Model Experience

Standalone-provider schema

What the model sees: While a fresh-context provider exists, the configured tool uses the generated default subagent schema; the catalog also records how toolName changes the visible name.

Token effect: Fixed schema cost per parent request while mounted. Removing the provider removes the whole schema.

Inherited-context-provider schema

What the model sees: Relative to the generated default subagent schema, a provider that seeds completed turns replaces only the tool and prompt parameter descriptions with the text below; the shape and description parameter stay unchanged.

Token effect: Fixed schema cost per parent request while mounted. Exposing multiple providers adds one independently named schema per load.

Inherited-context-provider tool description

Delegate a task to a subagent that INHERITS this conversation: a child agent seeded with all completed turns so far (it does not see the current in-flight turn), returning only its final result. Use this when the subtask builds on this conversation's context — a follow-up analysis, a review, a continuation — without consuming this conversation's context for the work itself. You receive only its final answer, not its intermediate steps.

Inherited-context-provider prompt description

The task for the subagent. It already sees this conversation's completed turns, so build on them freely and state only what is new.

Tool-call history and result

What the model sees: The task description and full prompt remain in the parent assistant tool call. Success contains only the child's data-dependent final text. Other stop reasons become exactly Error: subagent run was cancelled, Error: subagent run failed, Error: subagent run hit its token limit before finishing, Error: subagent declined the task, or Error: subagent run ended abnormally (<reason>); a call without an owning agent becomes Error: subagent tool requires a calling agent (exec.agent was undefined). Intermediate child steps never enter the parent.

Token effect: Prompt and final output are data-dependent retained tokens. All child working context is paid in the child and omitted from the parent.

Known Limitations and Deferred Work

• Delegation blocks the parent turn — synchronous collect only; background start and poll collection are deferred to the long-running-runtime redesign.
• Duplicate toolName across waiting loads is detected late (TODO(subagent-dup-toolname)) — two loads waiting on providers collide only when a provider arrives, and the throw rolls back the provider's fiber rather than the misconfigured tool's; config-time detection needs a cross-fiber registry of intended names.
• Child policy is fixed per tool registration — model, persona, tool filter, and depth cap come from this plugin load's config, not model-call arguments; exposing another policy requires another distinctly named tool.