Crate harness_loop_engine 

harness-loop-engine — loop engineering for harness-rs

Agent = Model + Harness. A harness wraps a single agent call. A loop wraps the harness: it runs that call again and again, on a cadence, with state, verification, budgets, and gates — driving toward a goal over time instead of in one shot. This crate is harness-rs’s loop layer.

“Loop engineering is replacing yourself as the person who prompts the agent. You design the system that does it instead.” — and you stay the engineer responsible for that system.

The building blocks already live elsewhere in harness-rs — scheduling (harness-scheduler), worktrees (harness-sandbox), sub-agents (harness-loop), memory (harness-core), MCP (harness-mcp). What this crate adds is the orchestration discipline that turns those parts into a loop you can trust:

• LoopLevel — maturity levels L1 (report) → L2 (assisted) → L3 (unattended). A loop earns autonomy in stages.
• HumanGate — the proceed-or-escalate decision, tied to the level. Built-ins: AlwaysEscalate, AllowlistGate, CallbackGate.
• TokenBudget — a per-round spend ceiling, because unattended loops spend without bound if you let them.
• LoopSpec — the inert, serializable description of a loop.
• LoopEngine — the runner: recall state → isolate → maker sub-agent → checker sub-agent → gate → record state.
• LoopScheduler — runs loops on their cadence.
• patterns — the seven named production loops (daily triage, PR babysitter, CI sweeper, …), each a ready-made LoopSpec.

The anatomical loop

  schedule (cadence)
       │
       ▼
  recall STATE / memory ──► isolated worktree (sandbox)
       │                          │
       │                          ▼
       │                  maker sub-agent  (proposes)
       │                          │
       │                          ▼
       │                  checker sub-agent  (tests + gates)
       │                          │
       │                          ▼
       │                     human gate? ──┬─ safe/allowlisted ─► proceed
       │                                   └─ risky/ambiguous ──► escalate
       ▼                                            │
  write STATE / memory  ◄────────────────────────── recurse next tick

Two debts to watch

Loop engineering names two failure modes that accrue silently. This crate makes them visible rather than solving them — they are engineering responsibilities, not features:

• Intent debt — the drift between what a loop was meant to do and what it actually does. Antidote: LoopSpec::intent is a required, one-sentence statement of purpose, injected into every maker turn and printed in every report. Review it as the loop evolves.
• Comprehension debt — the gap between what the loop ships and what humans still understand about its behaviour. Antidote: the maker/checker split, the recorded state spine, and rendered reports keep a legible trail of every round.

Safety stance

Verification stays on you — unattended loops make unattended mistakes. Defaults are conservative: L1 makers are strictly read-only, the default gate for every level is AlwaysEscalate, and L3 auto-proceed requires an explicit AllowlistGate. Graduate a loop’s level only as you build trust in it.

Modules

patterns

The production-loop catalogue.

Structs

AllowlistGate

Auto-proceeds only for verified actions whose kind is on the allowlist, and only at L3. Everything else escalates. This is the workhorse gate for unattended loops with a narrow blast radius.

AlwaysEscalate

Never auto-proceeds — every proposal is escalated. This is the correct gate for L1 loops (and a safe default for anything you’re unsure about).

BudgetState

Running tally of spend within a round, checked against a TokenBudget.

CallbackGate

Wraps an arbitrary closure as a gate — for custom policies (budget-aware, time-of-day, denylist, MCP-scope checks, …).

LoopEngine

Binds a LoopSpec to the live pieces it needs to run: a model, the maker/checker tool sets, an isolation sandbox, a gate, and (optionally) memory for the state spine.

LoopScheduler

Ticks registered loops on their cadence.

LoopSpec

Declarative definition of a single loop.

ProposedAction

A change the maker produced and the checker verified, presented to the gate for a proceed-or-escalate decision.

RoundReport

The full record of a round — the maker/checker reports, token spend, the gate decision, and the outcome. Suitable for delivery to a channel and for writing to memory.

StdoutSink

Prints deliverable reports to stdout.

TokenBudget

A declarative spend ceiling for a single round of a loop.

Enums

BudgetLimit

Which ceiling a round crossed.

GateDecision

The gate’s verdict for one proposed action.

LoopLevel

How much autonomy a loop is trusted with.

RoundOutcome

What one round of a loop did.

Traits

HumanGate

Decides what happens to a verified proposal. Implementations encode the human-gate policy; the engine consults this once per round.

LoopSink

Where a finished round’s report goes. Implement this to route reports to Slack, email, a file, a tracker — anywhere. The default is stdout.

Functions

default_gate_for

The gate a level implies when the caller doesn’t specify one.