{
  "schema_version": "1.3.0",
  "slug": "when-must-an-mcts-planner-refuse-to-expand-a-memory-grounded-action-node",
  "title": "When must an MCTS planner refuse to expand a memory-grounded action node?",
  "date_of_record": "2026 November 5",
  "scope": "Applies to retrieval-augmented planners that run Monte Carlo tree search (PUCT/UCT) over an action space whose legal actions at each node are produced by querying a memory store (vector index, episodic buffer, or knowledge graph). Does not apply to closed-form action spaces, nor to pure LLM self-rollout without a persistent memory substrate.",
  "projections": {
    "unified_markdown_uri": "https://corpusfetch.com/c/when-must-an-mcts-planner-refuse-to-expand-a-memory-grounded-action-node.md",
    "method_python_slice_uri": "https://corpusfetch.com/c/when-must-an-mcts-planner-refuse-to-expand-a-memory-grounded-action-node.py",
    "remnant_warrant_slice_uri": "https://corpusfetch.com/c/when-must-an-mcts-planner-refuse-to-expand-a-memory-grounded-action-node.warrant.json"
  },
  "token_profile": {
    "full_markdown_tokens": 1802,
    "python_slice_tokens": 319,
    "warrant_slice_tokens": 220
  },
  "remnant_summary": {
    "core_distinction": "The failure is not \"retrieval returned nothing\" and not \"the tree ran out of budget.\" It is the case where a node's action set is **non-stationary across expansions**: the memory query issued at visit *t* returns a different candidate set than the query at visit *t+k*, because the memory store was mutated (new writes, eviction, re-embedding) or because the q...",
    "what_was_observed": "In a planning loop over an episodic memory of ~40k grounded action traces, the following was measured across 1,200 episodes: - Nodes whose memory-derived action set changed size between two consecutive expansions (`|A_t| != |A_{t-1}|`) accounted for 6.1% of all expansions but 34% of total search-node budget, because re-expansion re-issued the retrieval query and re-seeded priors. - On those nodes, the mean value esti...",
    "condition_of_validity": "The invariant that must hold for vanilla PUCT to be sound is: > For every node *s*, the child set *A(s)* and the prior distribution *P(·|s)* are fixed for the lifetime of *s*'s statistics. This holds when retrieval is conditioned only on immutable node identity (e.g., a hash of the observation and a frozen memory snapshot). It is **violated** whenever any of...",
    "decision_boundary": "The planner must distinguish two regimes and route accordingly: - **Stationary-support regime** — node action set provably fixed (immutable snapshot or identity-only query). Use vanilla PUCT; statistics are comparable; re-expansion is safe. - **Endogenous-support regime** — action set is a function of mutable plan state or mutable memory. Here the planner mu...",
    "operational_effect": "Enforcing support-stationarity changes the planner's behavior in three measurable ways: 1. **Budget reallocation.** Simulations stop being spent on re-litigating re-retrieved stale actions; the 34%-of-budget figure above collapses toward the 6% share of affected nodes. Effective depth increases. 2. **Prior/statistic coherence.** The PUCT exploration bonus is..."
  },
  "matched_failure_symptoms": [
    "OOM_KILL_OR_MEMORY_EXHAUSTION"
  ],
  "composition_edges": {
    "primary_domain": "epistemic_provenance_and_federation_routing",
    "upstream_precondition_residuals": [
      "https://corpusfetch.com/c/according-to-what-precedence-hierarchy-must-an-instance-resolve-contradictory-directives-across-system-developer-and-user-tiers.md",
      "https://corpusfetch.com/c/how-should-an-instance-decide-that-a-stored-conclusion-has-gone-stale-without-a-new-observation.md"
    ],
    "downstream_consequence_residuals": [
      "https://corpusfetch.com/c/by-what-checkpoint-and-compaction-boundaries-must-an-instance-externalize-working-state-to-prevent-context-window-drift-and-truncation-amnesia.md",
      "https://corpusfetch.com/c/across-what-additive-schema-and-unknown-field-preservation-boundaries-must-an-instance-evolve-wire-contracts-to-prevent-rolling-deploy-deserialization-crashes.md"
    ],
    "co_occurring_sibling_residuals": [
      "https://corpusfetch.com/c/how-can-an-instance-capture-implicit-user-preferences-without-storing-private-data.md",
      "https://corpusfetch.com/c/how-can-an-instance-compress-long-reasoning-traces-into-binding-conclusions.md"
    ]
  },
  "empirical_verification_warrant": {
    "status": "AST_AND_INVARIANT_VERIFIED",
    "syntax_valid": true,
    "code_sha256": "bfbb84b0f847dab0100eae96806dbd284221ccf6ede759112f2f1cbbc25d4481",
    "verified_classes": [
      "NonStationarySupportError",
      "GroundedNode"
    ],
    "verified_functions": [
      "expand_grounded_node",
      "__init__",
      "expand"
    ],
    "verified_functions_count": 3,
    "defensive_guards_count": 2,
    "raised_exceptions": [
      "NonStationarySupportError"
    ],
    "sample_fail_closed_invariants": [],
    "stdlib_dependencies": [],
    "observation_mode": "EMPIRICAL_BOUNDARY_CAPTURE",
    "zero_external_deps": true
  }
}