---
artifact_kind: optimizer-card
artifact_name: mcts-lats
version: 1.0.0
owner: Nous · agreement-search team
last_reviewed: 2026-04-25
next_review: 2026-07-25
optimizer_kind: mcts
related_phase_179_tasks: [179.4.2.3]
---

# Optimizer Card — mcts-lats v1.0.0

## 1. Identity

- **Optimizer name:** mcts-lats
- **Algorithm family:** Monte Carlo Tree Search (UCT, Coulom 2006; Kocsis &
  Szepesvári 2006) extended with LATS (Language Agent Tree Search, Zhou et
  al. 2023) — value-augmented selection.
- **Implementation source:** `libs/nous/agreement-search/src/mcts-lats.ts` with
  shared GA primitives in `ga-operators.ts`.
- **Intended use:** trajectory-interpretable search where the workbench must
  replay the path from root to winning leaf as an explicit clause- edit sequence
  ("this is how we arrived at the agreement"). Also the natural integration
  point for §179.9.4.2 mediator-value models.

## 2. Scoring rule

- **Utility aggregation:** scalar fitness `Q(c)` per node, caller-supplied via
  `FitnessFn`.
- **Selection score:**
  `score(c) = (1 − λ) · Q(c) + λ · V(c) + C · sqrt(ln N(p) / N(c))`.
  `λ = valueWeight`. `λ = 0` recovers plain UCT; `λ = 1` recovers pure
  value-function rollout. `C` is the UCB exploration constant.
- **Fairness metrics reported:** caller selects; full `fairness-metrics.ts` set
  attaches to the run summary.
- **Handling of hard constraints:** every node passes through
  `filterCandidates`; failing nodes are not added to the tree.
- **Handling of uncertainty:** rollout fitness can be posterior mean, LCB, or
  Thompson; the kernel is agnostic.

## 3. Inputs

- **Candidate space:** sandboxed clause-mutator DSL + recombination borrows from
  a reference pool.
- **Initial population / seeds:** root from §179.4.1.1; reference pool from
  §179.4.1.2 decomposition.
- **Time / iteration budget:** iteration count + wall-clock cap + rollout depth.
- **Randomness:** seeded `mulberry32`.

## 4. Outputs

- **Accepted candidate contract:** highest-`Q` leaf along the recorded path from
  root.
- **Pareto-frontier diagnostics:** not the goal; route to NSGA-II / MAP-Elites
  for frontier coverage.
- **Trajectory record:** the path from root to leaf as an ordered list of
  `(action, child-id, Δ-fitness)` tuples — consumed by the workbench audit
  replay.
- **Uncertainty propagation:** leaf carries posterior detail; tree visit-count
  distribution is reported for confidence visualization.

## 5. Evaluation

| Metric                          | Value                                  | Evaluator card                                                      | Date       |
| ------------------------------- | -------------------------------------- | ------------------------------------------------------------------- | ---------- |
| Trajectory faithfulness         | replay byte-identical for given seed   | n/a — `mcts-lats.test.ts` reproducibility assertion                 | 2026-04-25 |
| Diversity preservation          | not the goal here                      | n/a                                                                 | 2026-04-25 |
| Fairness (Nash / KS / max-min)  | reported per leaf; gate runs against   | [fairness-suite](../evaluators/fairness-suite.md)                   | 2026-04-25 |
|                                 | §179.10.6 baseline                     |                                                                     |            |
| Regret vs oracle                | bounded by UCT regret                  | [baseline-benchmark-gate](../evaluators/baseline-benchmark-gate.md) | 2026-04-25 |
|                                 | (O(sqrt(ln N · N)))                    |                                                                     |            |
| Specification-gaming resistance | `spec-gaming-audit.ts` on winning leaf | [fairness-suite](../evaluators/fairness-suite.md)                   | 2026-04-25 |
| Runtime (typical)               | sub-second to seconds for 50 clauses;  | n/a — deterministic synchronous kernel                              | 2026-04-25 |
|                                 | interruptible at any iteration         |                                                                     |            |
| Cost per run                    | $0 LLM-free; with `valueEstimate`      | n/a — orchestrator scoring-cost trace                               | 2026-04-25 |
|                                 | callback the cost is value-model cost  |                                                                     |            |

## 6. Known limitations

- Tree size grows with iterations; memory is `O(nodes × clauses)`. Practical
  cap: ~100k nodes for 50-clause cases.
- Without a value model (`valueWeight = 0`), MCTS falls back to UCT; convergence
  on adversarial landscapes can be slow.
- Rollout depth and exploration constant `C` are caller-supplied;
  misconfiguration yields shallow or biased exploration.

## 7. Guardrails

- **Candidate safety gate:** `filterCandidates` on every node.
- **Clause static validation:** inside `filterCandidates`.
- **Redline separation:** hard constraints filtered before tree insertion.
- **Trajectory audit:** trajectory record is part of the §179.5.3.3
  reproducibility manifest.

## 8. References

- `libs/nous/agreement-search/src/mcts-lats.ts`
- Coulom (2006), "Efficient selectivity and backup operators in Monte-Carlo tree
  search".
- Kocsis, Szepesvári (2006), "Bandit based Monte-Carlo planning".
- Zhou et al. (2023), "Language Agent Tree Search Unifies Reasoning, Acting, and
  Planning in Language Models" (LATS).
- `docs/research/bibliography.md` §tree-search-reasoning