Evidence-Obligation Pool-Gated Retrieval: Stable Multi-Cycle Retrieval via Evidence Ledger and Warrant Judge

Pool-Gated Retrieval (PGR) established that retrieval in LLM-based agents should be governed by an explicit Warrant gate rather than similarity scores, treating Knowledge Gaps as first-class epistemic facts. However, a stable multi-cycle implementation of PGR has remained elusive. Prior internal SCL-PGR runtime implementations relied on pattern matching and domain-specific dictionaries to control retrieve_knowledge behavior, producing a characteristic failure trajectory: each encountered failure mode prompted the addition of a new guard rule, accumulating implicit domain knowledge in the runtime and reducing generality without converging on correctness. This paper diagnoses the root cause of this instability and presents a restructured implementation that resolves it. We show that the class of failures attributed to retrieval quality in prior implementations are not retrieval failures: they are failures of semantic warrant delegation and evidence obligation lifecycle management. Five structurally distinct failure modes are identified and documented through execution traces from a live debugging record: surface-form synonym collapse, raw chunk promotion as committed fact, cross-entity score promotion, compound query premature finalization, and conditional branch over-retrieval. All five share a common origin — pattern-based control attempts to answer semantic questions using lexical features, a task for which it is structurally inadequate. The proposed architecture introduces two mechanisms. First, a Warrant Judge — a single additional LLM call within retrieve_knowledge — resolves all semantic judgment tasks (synonym equivalence, entity alignment, section-heading-to-value warrant) without domain-specific encoding, replacing the entire class of alias tables, location markers, and entity mismatch heuristics. Second, a Turn-Scoped Evidence Ledger manages retrieve_knowledge as an obligation lifecycle rather than a query-response transaction, tracking each knowledge obligation through states of pending, found, gap, and reused_gap. The Ledger enables structural guarantees that pattern-based guards cannot provide: finalization is permitted if and only if all active obligations are in a terminal state, regardless of query surface form, domain, or language. We demonstrate that these two mechanisms together resolve all five identified failure modes across multi-attribute queries, conditional branch queries, comparative queries, and partial-coverage retrieval — without any domain-specific rules. The result is a PGR runtime in which the boundary between code and LLM responsibility is principled: semantic warrant is delegated to the LLM; obligation state management remains in code. We argue that this separation — not the addition of computational resources — constitutes the core engineering contribution and a necessary condition for stable PGR deployment across domains.