# Experiment 57J.78 — Pre-Anchored Update-Only Mode (Committed)

**Branch:** `feature/semantic-action-contract-v0.23`
**Starting HEAD:** `9b7721c` (experiment: audit pre-anchored live apparatus)
**Commit message:** `tooling: add pre-anchored update-only mode to canonical harness`

## Objective

Eliminate the dependency on temporary uncommitted script modifications identified in audit 57J.77, by adding a committed pre-anchored update-only mode to the canonical harness (`scripts/reproduce-multi-turn-investigation.mjs`). This allows any agent session at current HEAD to inject an arbitrary graph into the Update request body without first running Start.

## Changes Made

### 1. scripts/reproduce-multi-turn-investigation.mjs (+177 lines)

Added:
- ESM imports (`fs`, `fileURLToPath`, `path`) for deterministic fixture loading
- `FIXTURE_PATH` constant pointing to `tests/fixtures/pre-anchored-update-savings-realism.json`
- `fixtureMode` env-var selector (default: undefined → normal mode)
- `runUpdateOnlyMode()` async function:
  - Validates ANSWER_2 env-var exists before any live call
  - Loads committed fixture from deterministic path
  - Verifies single savings-realism anchor invariant
  - Deep-copies fixture graph (no mutation of original)
  - Skips Start entirely; sends exactly one Update via `postJson()` through production HTTP route
  - Preserves all hardened capture fields (answerMeaning, updatedProposal, structuralActionRequired, selectedQuestion, persistent graph snapshot)
  - Blocks on missing ANSWER_2 with zero live calls
  - Reports rejection diagnostics identically to normal mode

### 2. tests/reproduce-multi-turn-investigation.harness.test.js (+183 lines)

Added 7 new harness tests:
- Blocked ANSWER_2 → zero calls, correct error message
- Accepted structuralActionRequired=true in capture
- Rejected snapshot preservation with structural linkage errors  
- Exact ANSWER_2 body forwarding verification
- Pre-anchored rejected answerMeaning preservation
- Blocked mode verification (zero fixture load errors)
- Normal-mode isolation proof (accepted/rejected capture unchanged)

Updated `runPreAnchoredSimulation` mock to persist `rejectedProposalSnapshot` on rejection return values. Added `runPreAnchoredSimulationWithBlock()` helper.

## Evidence

| Test Suite | Pre-existing | New | Total | Result |
|------------|-------------|-----|-------|--------|
| Harness harness tests | 39 | 7 | 46 | ALL PASS (19ms) |

- No production code changed
- No Ollama calls made
- No live API calls made
- Normal-mode Start→Update chain preserved under guard
- Syntax validated via `node --check`

## Execution Commands

### Pre-anchored update-only mode:
```bash
FIXTURE_MODE=updateOnly \
  ANSWER_2="I am unsure whether the projected office savings from the relocation are realistic." \
  node scripts/reproduce-multi-turn-investigation.mjs
```

### Normal start→update mode (unchanged):
```bash
node scripts/reproduce-multi-turn-investigation.mjs
```

## Design Decisions

1. **Environment variable over CLI flag:** `FIXTURE_MODE` env-var is simplest, requires no arg parsing, and matches existing pattern (`CONFIDENCE_ENGINE_BASE_URL`).

2. **ANSWER_2 required guard:** Prevents accidental live calls without a clear answer payload. Zero calls made if missing.

3. **ESM imports for path resolution:** `fileURLToPath(import.meta.url)` resolves the fixture path relative to the script location, matching Node.js ESM best practices.

4. **No schema/schema validator changes:** The committed fixture file was already validated per existing schema enums in test 57J.74 (tests on lines 582-624 of the test file).

5. **Normal mode guard:** `fixtureMode !== undefined` check prevents the pre-anchored path from being activated when no env-var is set, preserving all existing start→update behavior.

## Verification

1. All 46 harness tests pass in under 20ms
2. No production code was modified
3. Syntax validated via `node --check`
4. Normal-mode Start→Update chain preserved at its original location (line 63 of the mjs file)
5. Pre-anchored mode explicitly documented with inline JSDoc comments

## Satisfies 57J.77 Recommendation

> "Add committed pre-anchored mode to the canonical harness by adding a single config flag that bypasses Start and reads the fixture into the Update request body, mirroring what runPreAnchoredSimulation() documents as its intended behaviour."

This commit implements exactly that recommendation — `runUpdateOnlyMode()` is the committed implementation of what `runPreAnchoredSimulation()` previously documented only as a test-only mock.