# Experiment 60B.4 — Decision Materiality Rule (prompt-only)

**Branch:** `feature/decision-sufficiency-v0.26`  
**Date:** 2026-08-13  
**Status:** Complete  
**Type:** PROMPT-ONLY — Bounded instruction addition plus deterministic prompt tests. No live model calls.

## Reasoning gap from 60B.3

60B.3 confirmed the engine has no independent decision-sufficiency rule. Rule 20 states:

> "Return selectedQuestion as null only when no consequential unresolved unknown remains."

This defines *when* to return null but does not define what makes an unknown non-consequential. The term "consequential" is undefined at decision level. Live results (60B.1 vs 60B.2) show the model defaults to generic continuation when no explicit closing cue exists — even when option evidence is quantified on both sides.

The gap: **uncertainty remains** is always true during investigation. The prompt does not instruct the model to distinguish this from **remaining uncertainty could materially change which option is preferred**.

## New prompt rule

Added section "Decision Sufficiency Rule" between Proposal Rules and Decision Option Structure Rules in `lib/graph/prompt-builder.js`:

> An unresolved decision between options should not remain open merely because some uncertainty still exists.
>
> Keep a decision context unresolved only when you can identify a specific unresolved factor that could materially change which option is preferred.
>
> If the currently supported evidence is sufficient to distinguish the options and no such material unresolved factor remains, resolve the existing decision context and do not ask a generic continuation question.

## Why this is domain-general

- No financial vocabulary (no £, $, payback, cost comparison)
- No relocation or industry-specific terms
- No numeric thresholds or calculation frameworks
- No keyword-based routing or taxonomic classification
- The three semantics apply to any decision between options with competing evidence:
  1. uncertainty alone is not sufficient reason to continue
  2. continuation requires a specific material factor that could change the preferred option
  3. when no such factor remains, resolve the existing decision context rather than asking a generic question

## Focused test results

**Command:** `npx vitest run tests/graph/prompt-builder.test.js`  
**Result:** PASS (86/86)

New materiality tests verify:
- Positive: uncertainty-alone-is-not-enough, specific-material-factor-required, could-change-preferred-criterion, resolve-when-no-material-factor, generic-continuation-discouraged, Rule-20-preserved, domain-generality
- Negative: no financial thresholds, no currency examples, no relocation examples, no automatic-resolution-when-better-looking, no new schema fields, no new node kinds

## What remains unproven until live regression

1. **Stability** — deterministic prompt tests confirm the instruction text is present and well-formed, but do not verify the model follows it consistently across repeated runs
2. **Cross-domain generalisation** — single-prompt-test coverage does not prove the rule works outside the test cases' structural patterns
3. **Interaction with existing rules** — no regression test confirms the materiality rule does not interfere with Rule 20, option structure rules, or the semantic-to-mutation contract
4. **Edge cases** — decisions where multiple partially-material factors exist; decisions with equal evidence across options; ambiguous factor specificity

## Production changes

| File | Change |
|------|--------|
| `lib/graph/prompt-builder.js` | Added "Decision Sufficiency Rule" section (3 sentences, 3 semantics) |
| No schema changes | |
| No validator changes | |
| No question-selection code changes | |
| No provider integration changes | |

## Commit messages

Production/tests: `feat(reasoning): add decision materiality rule`  
Documentation: `docs: record decision materiality rule`