deepseek-harness/packages/subagent/subagent-fork/src/index.ts

/**
 * The in-process FORK subagent backend: registers a {@link SubagentProvider} on
 * `ctx.subagents` that runs each child as a child {@link Agent} SEEDED with a prefix of the
 * parent's session log — so the child inherits the parent's conversation context instead of
 * starting fresh. The seed ends at the last `turn/end`: the current tool-call turn is
 * unbalanced and cannot be replayed as a valid child session.
 * @module @deepseek-ai/dsh-subagent-fork
 */

import type { Context } from 'cordis'
import z from 'schemastery'
import type { SessionEvent } from '@deepseek-ai/dsh-session'
import type { Agent } from '@deepseek-ai/dsh-agent'
import type { SubagentCapabilities, SubagentProvider, SubagentStartRequest } from '@deepseek-ai/dsh-subagent'
import { startInProcessRun } from '@deepseek-ai/dsh-subagent-inprocess'

export const name = 'subagent-fork'
// `tools` is deliberately NOT injected — same rationale as subagent-spawn: the
// per-run structured runtime gates its capture-tool registration on `tools`
// itself, so this backend's apply timing (and the delegation tool's position
// in the model-visible tool list) is unchanged by structured output.
export const inject = ['subagents']

/** Config: the registry name to register the provider under. */
export interface Config {
  /** Provider name on `ctx.subagents` (default `fork`). */
  providerName: string
}

export const Config: z<Config> = z.object({
  providerName: z.string().default('fork'),
})

/**
 * The balanced completed-turn prefix of `parent`'s log: every event up to and including the
 * last `turn/end`. The in-flight turn is excluded; before any completed turn the child starts
 * fresh. Because live sequence numbers equal array indexes, the result remains a valid seed
 * beginning at sequence zero.
 * @param parent - the agent whose session log to slice.
 * @returns the seed events, contiguous from seq 0; empty when no turn has completed.
 */
function completedTurnPrefix(parent: Agent): SessionEvent[] {
  const events = parent.session.events
  const lastEnd = events.findLast(e => e.type === 'turn/end')
  if (lastEnd === undefined) return []
  // seq === array index (the append contract), so slice up to and including it.
  return events.slice(0, lastEnd.seq + 1)
}

/**
 * The fork provider. Supports `depthLimit` and `outputSchema` (via the shared
 * in-process structured runtime), plus `toolFilter`/`persona` (scoped
 * restrict() and a scoped shadowing persona section).
 */
class ForkProvider implements SubagentProvider {
  readonly capabilities: SubagentCapabilities = { outputSchema: true, depthLimit: true, toolFilter: true, persona: true }
  // Context contract: a forked child IS seeded with the parent's completed-turn prefix.
  readonly inheritsParentContext = true

  constructor(readonly name: string) {}

  start(request: SubagentStartRequest) {
    const seed = completedTurnPrefix(request.parent)
    return startInProcessRun(request, {
      // Only pass a seed when there's a completed turn to inherit; an empty seed
      // is equivalent to a fresh child, so omit it to keep the session unseeded.
      ...seed.length > 0 ? { seed } : {},
    })
  }
}

export function apply(ctx: Context, config: Config): void {
  ctx.subagents.registerProvider(new ForkProvider(config.providerName))
}