core/docs/decisions/ADR-0136-statement-layer-corridor.md

ADR-0136 — Statement-Layer Corridor: Graduated GSM8K Admission via Parser Extension

Status: Active Date: 2026-05-23 Author: CORE agents + reviewers Parent: ADR-0131.G Depends on: ADR-0115, ADR-0116, ADR-0117

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Context

ADR-0131.G pinned the GSM8K coverage probe at 0/50 admission and established admitted_wrong == 0 as a standing architectural invariant. The G.x capability axes (G.1–G.5) have landed on main; as of ADR-0131.5 (probe retirement), the GSM8K probe is no longer a per-iteration gate — it activates only when a new iteration claims Δadmission_rate ≥ 0.02.

A taxonomy pass over all 50 refused cases (S.0) produced the following breakdown:

Primary barrier	Count	Notes
context_filler	23	Narrative scene-setters; parser's refusal is correct
compound_statement	5	Two operations in one sentence
rate/capacity/price class	4	Direct targets of S.1
distributive_multiply	1 (+5 secondary)	N bags × M items each
diverse long-tail	17	Age anchors, goal statements, multi-step chains, etc.

The 23 context-filler cases will not be addressed by statement-layer work alone — they need semantic classification of scene-setting sentences, which is a separate architectural concern. The safety rail stands: if a sentence cannot parse into a numeric initial-state candidate, the problem is refused.

The G.x micro-extension approach has been retired. Each G.x axis targeted one regex shape in isolation; the return per axis was tiny and the axes were not composable into a coherent parser narrative. The Statement-Layer Corridor replaces G.x with a phased taxonomy-driven program that:

1. Classifies all refusals before writing any code.
2. Targets the highest-signal unlockable barriers first.
3. Ships each phase as a self-contained unit with a curated axis lane, wrong == 0 gate, and honest GSM8K delta.
4. Never attempts to bypass the context-filler safety rail.

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Decision

Organize parser extension work as a corridor of phases rooted at ADR-0136:

Phase	ADR	Scope	Primary barrier targeted
S.0	(this doc)	Taxonomy — deterministic classification of 50 refused cases	—
S.1	ADR-0136.S.1	Rate/event statements — capacity-rate + earnings-rate shapes	rate class (≤4 cases)
S.2	ADR-0136.S.2	Temporal statements — time anchors, duration expressions	time/age long-tail
S.3	ADR-0136.S.3	Compound statements — two operations in one sentence	compound_statement (5 cases)
S.4	ADR-0136.S.4	Coreference — pronoun + ellipsis resolution across sentences	cross-sentence barriers

Phases S.2–S.4 are deferred; scope and sequencing will be revisited after S.1 lands.

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Taxonomy (S.0)

Stored at: evals/gsm8k_math/train_sample/v1/refusal_taxonomy.json

Schema v1. Each record carries:

• case_id — GSM8K case identifier
• primary_barrier — the single barrier that causes refusal even if all others were resolved
• secondary_barriers — additional barriers present in the problem
• notes — free-text rationale

Key finding. The case with the shallowest barrier is gsm8k-0014:

Bob can shuck 10 oysters in 5 minutes.
How many oysters can he shuck in 2 hours?

This is a single-statement capacity-rate problem with a pronoun question. It is the proof case for S.1: it must admit with answer == 240.0.

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Invariants

These invariants are non-negotiable across all corridor phases:

• admitted_wrong == 0 — no GSM8K case is admitted with a wrong answer.
• Context-filler safety rail — sentences without parseable numeric initial state are refused; no soft-fail or skip-and-continue.
• Honest delta — each phase's PR body states the exact pre/post GSM8K admission count; no rounding, no "approximately".
• No solver/graph/verifier changes — rate-path short-circuits and new extractors live in math_candidate_parser.py and math_candidate_graph.py only.

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Consequences

• The G.x capability axis namespace is closed. New axes use the S.x naming convention.
• Each S.x phase ships its own curated axis lane at evals/math_capability_axes/S<N>_<name>/v1/cases.jsonl.
• The GSM8K probe re-activates when a phase claims ≥1 new admission (Δadmission_rate ≥ 0.02); otherwise it stays retired per ADR-0131.5.
• S.1 is the only phase that can honestly claim unlocking the context-filler-gated cases once sentence-semantic classification lands (out of S.x scope).

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Deferred

• Context-filler gated problems (23 cases) — requires semantic classification of narrative scene-setting sentences; architecturally separate from statement parsing.
• Conditional branching (if she works more than 8 hours) — needs branching semantics in the solver.
• Percentage/interest rates — needs decimal arithmetic extension.
• Multi-statement rate problems (duration in separate sentence from capacity) — needs coreference, addressed in S.4.