Hybrid Planner

---
name: hybrid-planner
description: Build a hybrid planner — ChatHTN for provably-sound plans, AlphaEvolve for code search with a machine-checkable evaluator — and pick the right one for the problem. Use when you need help with hybrid planner.
license: CC-BY-NC-SA-4.0
phase: 14
lesson: 11
metadata:
  version: 1.0.0
  tags: [planning, htn, chathtn, alphaevolve, evolutionary-search]
---

Given a problem class (policy-bound workflow vs code optimization vs open-ended task), pick a planner and produce a correct scaffold.

Decision:

1. Does the problem have hard preconditions / policy / scheduling constraints? -> HTN (ChatHTN).
2. Does the problem have a deterministic, machine-checkable fitness function? -> Evolutionary (AlphaEvolve).
3. Neither? -> Reach for ReAct (Lesson 01) or ReWOO (Lesson 02) instead.

For HTN, produce:

1. `Operator` type with `preconditions`, `effects_add`, `effects_remove`.
2. `Method` type with `task`, `preconditions`, `subtasks`.
3. A planner that tries methods first, falls back to LLM decomposition, and caches successful LLM decompositions.
4. A validation step that rejects LLM decompositions referencing unknown operators or methods.

For Evolutionary, produce:

1. A seed population of candidate programs.
2. A deterministic evaluator returning a scalar fitness.
3. A mutation operator (LLM-driven or rule-based).
4. A selection loop (keep top-k, mutate, repeat) with early stopping.

Hard rejects:

- ChatHTN where LLM output is applied directly without operator-schema validation. The soundness claim fails.
- AlphaEvolve where the evaluator calls an LLM judge. Fitness must be deterministic; LLM judges introduce stochastic noise the loop cannot recover from.
- Either pattern for open-ended tasks ("write a blog post"). No evaluator, no preconditions -> use ReAct.

Refusal rules:

- If the domain has no clear operator schema, refuse ChatHTN. Suggest ReWOO or plain ReAct.
- If the domain has no machine-checkable fitness, refuse AlphaEvolve. Suggest Self-Refine (Lesson 05).
- If the user wants "planner + LLM makes final call," refuse. The split between symbolic correctness and LLM exploration is load-bearing.

Output: `operators.py`, `methods.py`, `planner.py` (HTN) or `evaluator.py`, `mutator.py`, `loop.py` (evolutionary), plus `README.md` with the decision rationale. End with "what to read next" pointing to Lesson 25 if debate-style verification fits the problem, or Lesson 02 if the task is actually ReWOO-shaped after all.