deepseek-harness/packages/subagent/subagent-fork/README.md

# @deepseek-ai/dsh-subagent-fork

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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 computes 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`](../subagent-inprocess/README.md) 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`](../subagent-spawn/README.md) for the run lifecycle, model inheritance, and depth tracking — all shared.

## Model Experience

### Child-agent history and envelope

#### What the model sees

The child receives the parent's balanced completed-turn surface prefix, then the new task content verbatim. A configured persona shadows prompt text in the child's fresh scope; a tool restriction filters its global wire schemas, executable lookup, and Code Mode SDK bindings but not standalone guidance. The parent's tool view and authority are not inherited. An optional structured-output request adds its child-only contract. The parent's current in-flight turn is excluded.

#### Token effect

Forking duplicates retained completed history into separate child requests; the child then accumulates its own tokens independently. Persona changes repeated prompt cost, filtering changes schema or generated SDK cost, and a first-turn fork has no inherited history.

#### KV Cache effect

The child may reuse the inherited byte-identical prefix under the same provider and model. Persona, tool-filter, generated-SDK, or route changes may invalidate reuse before inherited history; later child history is append-only.

### Parent tool result, indirectly

#### What the model sees

The parent receives only the child's own final output through `dsh-tool-subagent`, not the inherited prefix or intermediate work.

#### Token effect

Parent input grows by one data-dependent final result retained until compaction.

#### 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

- **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.