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@@ -546,3 +546,21 @@ Exact implementation boundary:
 
 ---
 
+### Experiment 57J.39 — Semantic-to-Mutation Contract Implementation (Option B)
+
+**Classification: E — IMPLEMENTATION COMPLETE.**
+
+Implemented the bounded Option B from 57J.38 with ownership correction: prompt owns structural materialization obligation; validator owns only the structural fact that `answerMeaning` alone is not graph progress.
+
+**Production changes:**
+- **prompt-builder.js** rule #6 replaced: added explicit MUST structural-materialization rule. userSupportedMeaning must be expressed through graph structure — update/refine existing, resolve unknown, new unknown, or justified relationship. answerMeaning alone is insufficient.
+- **utils.js** validateGraphUpdate(): added specific diagnostic "answerMeaning.userSupportedMeaning is populated, but the proposal contains no graph mutation" inside the `!hasMeaningfulChange` path, ordered before generic no-op error for exact same proposal.
+
+**Tests:** 15 new focused tests (8 in utils.test.js for all contract cases; 7 in prompt-builder.test.js for MUST rule verification). All 68 + 15 = 83 focused tests pass. No regressions in update-route (13), harness (8), or rejected-proposal-snapshot (7) tests. One pre-existing orchestrator failure unrelated to this work.
+
+**Not changed:** hasMeaningfulChange definition, schema, node/edge semantics, provenance, answerability, decomposition, reasoning taxonomy, semantic classifiers, provider integration, Behaviour Selection. possibleInference alone does not trigger the new diagnostic.
+
+Ollama calls: 0. Dev server disturbed: NO. Branch: feature/semantic-to-mutation-contract-v0.17 from HEAD 3b868b2.
+
+---
+
diff --git a/docs/experiment-57j39.md b/docs/experiment-57j39.md
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+### Experiment 57J.39 — Semantic-to-Mutation Contract Implementation (Option B)
+
+**Objective:** Implement the agreed Option B from 57J.38 with ownership correction: prompt owns structural materialization obligation, validator owns only the structural fact that `answerMeaning` alone is not graph progress.
+
+**Implementation boundary (strict):**
+1. One MUST rule in prompt Additional Guidance (replaced rule #6 in prompt-builder.js)
+2. One deterministic check in `validateGraphUpdate()` after `hasMeaningfulChange` (utils.js)
+3. Focused tests proving each contract case
+
+**Changes to production code:**
+
+#### Prompt contract (lib/graph/prompt-builder.js)
+Replaced ambiguous rule #6 ("Then inspect the answer for newly introduced consequential uncertainty.") with explicit MUST:
+
+> "If answerMeaning.userSupportedMeaning contains consequential information or unresolved uncertainty that is not already represented in the graph, you MUST express its effect through structural mutation. This may be an update/refinement of existing structure, resolution of an existing unknown, a genuinely new unknown, or a justified relationship. answerMeaning alone is not sufficient for a successful proposal."
+
+#### Validator contract (lib/graph/utils.js)
+Added specific diagnostic inside the existing `!hasMeaningfulChange` rejection path:
+
+> "answerMeaning.userSupportedMeaning is populated, but the proposal contains no graph mutation. answerMeaning alone does not constitute graph progress."
+
+This diagnostic fires BEFORE the generic "Update contains no meaningful change" only when `userSupportedMeaning` is populated AND there is zero structural mutation. The generic error remains for all other structurally empty proposals.
+
+**Not changed:**
+- `hasMeaningfulChange` definition (variable still computes the same structural fields)
+- Schema
+- Graph node/edge semantics
+- Provenance, answerability, decomposition, reasoning taxonomy
+- Semantic overlap rules or classifiers
+- Provider integration or Behaviour Selection
+- `possibleInference` handling
+
+**Tests added:**
+
+*utils.test.js — semantic-to-mutation contract (8 tests):*
+1. semantic-only no-op → REJECT with specific error (not generic)
+2. ordinary no-op (answerMeaning null) → REJECT with "no meaningful change"
+3. possibleInference only → does NOT trigger new error, generic no-op applies
+4. update existing structure (status change) → ACCEPT past guard
+5. resolve existing unknown → counts as structural progress
+6. add new structure (new unknown) → counts as structural progress
+7. duplicate avoidance preserved with populated userSupportedMeaning
+8. value-only change → counts as structural progress
+
+*prompt-builder.test.js — MUST rule verification (7 tests):*
+9-15. Verify prompt contains MUST rule, permits update/resolve/new unknown, states answerMeaning alone insufficient, does not force new node, references userSupportedMeaning not possibleInference
+
+**Test results:**
+- utils.test.js: 68 passed (0 failed)
+- prompt-builder.test.js: 15 passed (0 failed)
+- cases-update-route.test.js: 13 passed (0 failed)
+- harness tests: 8 passed (0 failed)
+- rejected-proposal-snapshot.test.js: 7 passed (0 failed)
+- orchestrator.test.js: 31 passed, 1 pre-existing failure (unrelated)
+
+**What this implementation now guarantees:**
+- A proposal with populated `userSupportedMeaning` and zero structural mutation receives a specific, actionable rejection error — not the generic no-op message
+- The prompt explicitly instructs the model that meaningful user-supported meaning must be expressed through graph structure, not just stated in answerMeaning
+- No new semantic classifier, schema change, or provider-specific logic is introduced
+- possibleInference alone does not trigger the specific diagnostic
+- Duplicate avoidance and all existing validation behavior is preserved
+
+**What it intentionally does NOT guarantee:**
+- That `userSupportedMeaning` contains truly consequential meaning (validator doesn't judge that)
+- That the LLM will comply with the MUST rule in live use (that requires empirical verification)
+- Resolution of cold-start variance or other downstream defects
+
+**Classification: E — IMPLEMENTATION COMPLETE**
+Configured Ollama: none used. Production code changed: prompt-builder.js, utils.js. Tests permanently changed: utils.test.js (+8), prompt-builder.test.js (+7). Dev server disturbed: NO. Ollama calls: 0.
+
+---