deepseek-harness/docs/core-data-structures/subagent.md

Subagent

The subagent seam — an agent delegating work to a child agent. Like bash it is one optional capability, not part of the agent-loop spine, so its vocabulary lives here rather than in core.md. But it differs from every other seam on one axis: multiple provider implementations coexist in one context, registered by name (ctx.subagents), where bash allows only one executor. The registry shape mirrors the LLM adapter registry, not the single-service bash executor.

Interface: dsh-subagent (ctx.subagents + the vocabulary below). Implementations are sibling packages (dsh-subagent-spawn, -fork, -acp); the model-facing consumer is dsh-tool-subagent. The proposal and rationale: the subagent RFC.

Source: packages/subagent/subagent/src/types.ts

Two kinds of capability, discovered two ways

A provider advertises its start-time features on a static descriptor the service checks BEFORE a run exists; a request that needs one the provider lacks is rejected loud (SubagentError('UNSUPPORTED_CAPABILITY')), never accepted-then-ignored. Runtime features (steering, resume) are instead optional methods on SubagentRun — the method's presence IS the capability, and TS narrowing is the discovery mechanism.

interface SubagentCapabilities {
  outputSchema: boolean
  depthLimit: boolean
  toolFilter: boolean
}

The start request

What a caller asks for when starting a subagent. The tool layer builds this from the model's { description, prompt } plus its own config; the service validates the start-time capabilities against the named provider, then passes it to provider.start. parent is REQUIRED — in-process backends read parent.session.header for the working directory, the parentSession lineage, and the delegation depth. The three optional fields (outputSchema, maxDepth, toolFilter) each gate on the matching SubagentCapabilities flag.

interface SubagentStartRequest {
  prompt: ContentBlock[]
  parent: Agent
  signal?: AbortSignal
  agentOptions?: AgentOptions
  outputSchema?: SchemaSpec
  maxDepth?: number
  toolFilter?: { allow?: string[]; deny?: string[] }
}

The terminal result: SubagentResult

The outcome of a run, resolved by SubagentRun.result. structured is present iff the request carried an outputSchema AND the provider honored it. A non-completed stopReason means output may be partial — the consumer maps it to an isError tool result rather than reporting partial output as success.

interface SubagentResult {
  output: ContentBlock[]
  structured?: unknown
  stopReason: SubagentStopReason
}

SubagentStopReason is a merge-extensible derived union — a backend may add variants, so consumers branch on the known cases and treat an unknown terminal reason as a failure:

interface SubagentStopReasonMap {
  completed: 'completed'
  aborted: 'aborted'
  error: 'error'
  'max-tokens': 'max-tokens'
  refusal: 'refusal'
}

A live run: SubagentRun

The handle the consumer holds while a child executes. The consumer awaits result, may cancel mid-flight, and MUST dispose on every path to reach child quiescence (no leaked idle child / session). result does NOT reject on a child-level failure — a model/transport failure resolves with stopReason: 'error' — so the consumer maps a non-completed reason to an isError result; it rejects only on an infrastructure fault the seam cannot represent. sendMessage and resume are OPTIONAL: a provider that supports the runtime capability defines the method; one that doesn't omits it.

interface SubagentRun {
  readonly id: AgentId
  readonly result: Promise<SubagentResult>
  cancel(reason?: string): void
  dispose(): Promise<void>
  sendMessage?(content: ContentBlock[]): void
  resume?(content: ContentBlock[]): SubagentRun
}

The provider seam: SubagentProvider

One transport for running a child agent. Implementations register under a unique name via SubagentService.registerProvider; multiple coexist in one context. The service validates every requested start-time capability before calling start, so an implementation may assume e.g. request.maxDepth is honorable when present.

interface SubagentProvider {
  readonly name: string
  readonly capabilities: SubagentCapabilities
  start(request: SubagentStartRequest): SubagentRun
}

The service (ctx.subagents) emits subagent/start when a run begins and subagent/end when it settles (see the events catalog). subagent/end carries lastAssistantMessage (the child's final output) on the settle path, so an observer sees WHAT the subagent produced without holding the run (absent when the run rejected at the infrastructure level — no result was produced). These are observe-only events: both are plain emits (the subagent/end fires from a detached .then after the result settles and awaits no listener), so a subscriber observes but cannot change the run. Both emits contain a thrown listener per listener (logged, never propagated): one bad subscriber can neither strand a live run, surface as an unhandled rejection on the detached settle hook, nor starve the listeners registered after it.

In-process backends: depth and seed

The two in-process backends (dsh-subagent-spawn fresh, dsh-subagent-fork seeded) run the child as a child Agent on the same context via ctx.agents.create. Two pieces of vocabulary ride on the existing agent/session types rather than new core types:

• Delegation depth is a merge-extensible AgentOptions.subagentDepth field (0 for a top-level agent, parent + 1 for a child). The seam owns it — the loop neither sets nor reads it — so a nested spawn reads its parent's depth from parent.options.subagentDepth and the depthLimit capability caps the tree by refusing a child whose depth would exceed request.maxDepth.
• Fork seeding uses CreateAgentOptions.seed (a SessionEvent[] prefix threaded through AgentLoop.createAgent → ctx.sessions.prepare({ seed }), the same primitive resume uses). The fork backend passes a balanced completed-turn prefix of the parent's log — the parent's events up to and including its last turn/end — so the seed is contiguous-from-0 and the invariants replay accepts it (the in-flight, unbalanced turn is excluded).