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.

Model Experience

Context surface	What the model sees	Token effect
Child-agent history	The child receives the parent's balanced completed-turn surface prefix, then the new task. Configured persona and tool restrictions compose only in the child's fresh scope; 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.	Forking duplicates the retained completed history into a separate child's requests; the child then accumulates its own tokens independently. A first-turn fork has no inherited history.
Parent tool result, indirectly	The parent receives only the child's own final output through `dsh-tool-subagent`, not the inherited prefix or intermediate work.	Parent input grows by one data-dependent final result retained until compaction.

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.