Harness

harness

The harness is the durable turn loop that wires session-manager, llm-router, and context-manager into an agent. A consumer stays thin: it kicks off a turn, renders the conversation from the session transcript, and reacts to turn boundaries and human-gated calls. The harness streams the assistant message into the session as it generates, so you watch the session, not the harness — there is no agent::events stream, and harness::status is a point-in-time recovery read, not a render feed.

Every invocation is a trigger (iii.trigger({ function_id, payload })); there is no separate "call" verb. Tool dispatch is deny-by-default: a send with no options.functions.allow is a plain chat loop and every model-requested call is refused until you allow globs per send. Sessions are minted by the harness (s_<uuid>) or supplied by you; a send into a running turn folds in as steering (merged: true) instead of erroring, and a repeated idempotency_key returns the original turn without appending.

Prerequisites: session-manager (required — transcript store and change feed) and llm-router (required — generation and the model catalog) must be present. context-manager (token budgeting and compaction) is a soft dependency — absent it, the harness sends raw history. approval-gate (the human-in-the-loop gate) is optional; without it no call is held and every allowed call runs un-gated.

When to Use

• Start or steer an agent turn and return immediately (harness::send).
• Cancel an in-flight turn (harness::stop) or read coarse turn state for recovery and guards (harness::status).
• Chain turns or react to outcomes by binding harness::turn-completed.
• Drive a turn from an arbitrary inbound event (cron tick, webhook, sensor) by translating it into a harness::send.

Boundaries

• Not a transcript feed. Render from session-manager's session::message-added / message-updated / status-changed (reconcile by revision); do not poll harness::status for content.
• Not the approvals engine. The harness ships only the gate mechanics; the policy, decision RPCs (approval::resolve), inbox (approval::list-pending), and prompt triggers live in approval-gate.
• Not a chain guard. options.max_turns bounds a single turn, not a send-completed-send loop; carry your own stop condition.
• Do not trigger the internal functions (below) — they forge call ids and turn progress, so calling them out of band corrupts the turn record.
• An in-run agent cannot start turns: send / run / turn / stop are denied to the model by policy. harness::spawn is the only turn-starter an agent calls directly, and it self-enforces depth, fan-out, and policy subsetting. The other path is event-driven: an agent binds engine::register_trigger straight to harness::spawn (see Reactive triggers), with the spawn spec in the registration metadata, and the engine spawns the sub-agent when the event fires.

Functions

Consumer-facing:

• harness::send — ensure the session, persist the incoming message, and kick off a turn; returns fast or merges into a running turn (steering).
• harness::stop — request cancellation of an in-flight turn; cascades to spawned children.
• harness::status — read the current turn state for a session; null when no turn ever ran. For recovery and guards, not rendering.
• harness::spawn — spawn a sub-agent in a child session. Model-facing (invoked through agent_trigger), not a consumer entry point.

Internal — the harness drives these; never trigger them directly: harness::turn (the durable loop step), harness::function::trigger / harness::function::resolve (dispatch and parked-call settle), harness::sweep-pending (cron expiry), and harness::on-config-change (hot-reload).

Reactive triggers

The harness emits two async turn-boundary trigger types so consumers and siblings react without polling harness::status:

• harness::turn-started — a turn began executing (first loop step).
• harness::turn-completed — a turn reached a terminal status (completed / cancelled / failed), carrying the result or error for chaining, failure toasts, auto-titling, and result delivery.

Bind turn-completed for outcomes and to chain the next hop; bind turn-started only for observability. Delivery is fire-and-forget, at-least-once, and unordered — treat each event as an edge. Nothing replays on reconnect: re-seed with harness::status and approval::list-pending, then rebind. Do not bind these for live transcript rendering — that is session-manager's job.

Binding config filters delivery by session_id, or by parent_session_id to watch the children a turn spawns (in-turn spawns only — a direct harness::spawn call creates no parent link, so filter those by session_id).

Bind an event to a sub-agent with one call — the spawn spec goes in the registration metadata; the fired event is appended to the task:

engine::register_trigger {
  "trigger_type": "harness::turn-completed",
  "config": { "session_id": "<child>" },
  "function_id": "harness::spawn",
  "metadata": { "task": "Summarize the completed run.", "once": true }
}

model defaults to the registering turn's model. A failed upstream turn does not fire the reaction unless continue_on_error: true. Reactive chains stop at depth 8. There is no fan-in join primitive — use the workflow worker to join parallel work.

An in-session registration that omits model also inherits its provider, when the spec pins none; raw engine-side registrations have no turn to inherit from and must pass one, and any supplied model must be a live id from router::models::list (validated at registration and again at fire time). Omit parent_session_id and the child nests under the registering session's root automatically; pin it only to choose a different REAL session (an invented id shows the children as disconnected roots). A trigger-fired spawn has no parent policy to inherit — it gets the harness's read-only default_functions baseline unless options.functions grants more (narrowing only, same as a direct harness::spawn call). Filter turn-completed only by session ids you know exist; registration returns a warning note when the filtered session doesn't. Self-edge drop and the depth cap are the ONLY loop breakers — still design filters so a reaction is not matched by its own subscription. A cycle routed through a state write (or any other hop that starts a fresh turn) re-enters at depth 0 and is NOT throttled: every lap around the cycle spawns another paid turn indefinitely. Design reaction graphs acyclically. This is the in-run agent's chaining path; the registerFunction recipe below is for workers.

How to bind

1. Register a handler: registerFunction('myapp::on-turn-done', handler).
2. Register the trigger:

iii.registerTrigger({
  type: 'harness::turn-completed',
  function_id: 'myapp::on-turn-done',
  config: { session_id: sessionId },
})

For the event payload shape, call get function info on the trigger type.

Hooks (policy siblings only)

The harness also registers five synchronous, in-path hook trigger types: harness::hook::pre-turn, harness::hook::pre-generate, harness::hook::post-generate, harness::hook::pre-trigger, and harness::hook::post-trigger. A bound hook runs in the turn's critical path and the harness acts on its return value (veto / hold / mutate) under a per-binding timeout_ms and on_error policy; pre-trigger / post-trigger bindings take a functions glob list to scope which calls they gate. These are for operator-trusted policy siblings (approval-gate binds pre-trigger); ordinary consumers do not bind hooks.