deepseek-harness/packages/subagent/subagent-fork

@deepseek-ai/dsh-subagent-fork

The fork provider creates an in-process child seeded with the parent's completed conversation turns. It shares all run mechanics with spawn; the session seed is the only behavioral difference.

Seed boundary

The parent's current tool-calling turn is still open when a subagent starts: its log contains the assistant tool call but not the matching tool result or turn/end. Copying that raw log would give the child an invalid, unbalanced session.

Fork therefore uses completedTurnPrefix(parent.session.events): the contiguous prefix ending at the last turn/end. The child sees all completed parent turns and none of the in-flight turn. If the parent has not completed a turn yet, the seed is empty and the child behaves like a fresh spawn.

The seed transfers conversation history only. The child still receives a fresh flat registration scope; it does not inherit the parent's tool restrictions or authority.

Start and capabilities

start(request) passes the completed-turn seed to startInProcessRun and awaits child publication. The shared driver owns cancellation, depth, customization, result reading, and disposal.

Fork advertises { outputSchema: true, depthLimit: true, toolFilter: true, persona: true }, identical to spawn.

Config

Key	Meaning
providerName	Registry name on ctx.subagents (default fork).
See dsh-subagent-spawn for the run lifecycle, model inheritance, and depth tracking — all shared.

Known Limitations and Deferred Work

• Runs expose no sendMessage/resume — the optional runtime capabilities are absent on in-process runs.
• The seed is a one-time snapshot — the child sees the parent's completed turns as of the fork and nothing the parent logs afterwards; there is no live context sharing.