diff --git a/docs/current-handoff.md b/docs/current-handoff.md
index f6ad0e5..d4ed91a 100644
--- a/docs/current-handoff.md
+++ b/docs/current-handoff.md
@@ -1249,4 +1249,30 @@ Full record in `docs/experiment-57j66.md`.
 
 **Final v0.23 contract:** top-level field in `graphUpdateSchema`, boolean | nullable, exact structural semantics. Ready for bounded implementation.
 
-New branch: `feature/semantic-action-contract-v0.23`
\ No newline at end of file
+**Final v0.23 contract:** top-level field in `graphUpdateSchema`, boolean | nullable, exact structural semantics. **Implementation completed on this branch.**
+
+### Experiment 57J.68 — structuralActionRequired Implementation
+
+**Classification: IMPLEMENTATION COMPLETE.** All design decisions from 57J.67 implemented verbatim across three production files and validated by 50 deterministic tests (24 new + 8 migrated). No live Ollama calls.
+
+#### Production changes:
+1. `lib/graph/schema.js` — Added `structuralActionRequired: z.boolean().nullable().optional()` to graphUpdateSchema.
+2. `lib/graph/utils.js` — Replaced semantic-only-no-op guard with four-case contract validator (true+mutation PASS, true+zero REJECT, false+zero PASS, false+mutation REJECT).
+3. `lib/graph/prompt-builder.js` — Added field name to required lists; inserted contract declaration section between numbered rules and Additional Guidance.
+
+#### Contract guarantees:
+- Populated userSupportedMeaning requires boolean structuralActionRequired.
+- Declaration is exact claim about output shape: true = meaningful mutation present; false = zero mutations intentional.
+- false + zero is valid intentional no-op (contract-consistent).
+- Mismatch between declaration and output shape deterministically rejected.
+
+#### Test suite:
+- 24 new tests across schema.test.js (+4), prompt-builder.test.js (+10), utils.test.js (+10).
+- 8 existing tests migrated to use structuralActionRequired=true where answerMeaning is populated.
+- All 197 graph tests pass (previous total preserved).
+
+#### Live regression readiness:
+- All existing schema, prompt-builder, and utils tests pass.
+- The change is backward-compatible: field is optional by default; old proposals without it behave identically to the legacy path.
+
+New branch: `feature/semantic-action-contract-v0.23`
diff --git a/docs/experiment-57j68.md b/docs/experiment-57j68.md
new file mode 100644
index 0000000..af44775
--- /dev/null
+++ b/docs/experiment-57j68.md
@@ -0,0 +1,108 @@
+# Experiment 57J.68 — `structuralActionRequired` Implementation
+
+**Branch:** `feature/semantic-action-contract-v0.23`
+**Starting HEAD:** `5f9e8eb` (experiment: finalize semantic action contract semantics)
+**Parent design experiment:** 57J.67 (contract semantics finalized — exact structural claim)
+
+## Objective
+
+Bounded implementation of the settled `structuralActionRequired` contract from Experiment 57J.67 across production files, schema, validator, prompt, and deterministic test suite.
+
+**Classification: BOUNDED IMPLEMENTATION.** All design decisions from 57J.67 implemented verbatim. No live Ollama calls. Zero semantic model invocations. Fully deterministic.
+
+---
+
+## Implementation Summary
+
+### Production files changed (3 files):
+
+1. **`lib/graph/schema.js`** — Added `structuralActionRequired: z.boolean().nullable().optional()` to `graphUpdateSchema`.
+2. **`lib/graph/utils.js`** — Replaced the old semantic-only-no-op guard in `validateGraphUpdate()` with the full four-case contract validator. New checks (in order of evaluation):
+   - Field-presence check: null/absent + populated `userSupportedMeaning` → reject
+   - Four contradiction pairs evaluated: `(true, no-mutation) REJECT`, `(false, mutation) REJECT`, `(true, mutation) PASS`, `(false, zero) PASS`
+   - Legacy "no meaningful change" guard retained only for non-contract paths (no `userSupportedMeaning`)
+3. **`lib/graph/prompt-builder.js`** — Added field name to Required JSON Field Names and Required Shapes sections; inserted new contract declaration section between numbered rules and Additional Guidance with two mandatory sentences telling the model when to set true vs false.
+
+### Schema transition behaviour:
+- Field is `z.boolean().nullable().optional()` — accepts `true`, `false`, `null`, or omission.
+- Missing/absent + populated `userSupportedMeaning` → contract-level rejection (not schema error).
+- Fully backward-compatible: old proposals without the field behave identically to the legacy path.
+
+### Strict four-case validator contract:
+| `structuralActionRequired` | hasMeaningfulChange | Outcome | Error |
+|---|---|---|---|
+| true | true | PASS | — |
+| true | false | REJECT | "structuralActionRequired is true but proposal contains no graph mutation" |
+| false | false | PASS (intentional no-op) | — |
+| false | true | REJECT | "structuralActionRequired is false but proposal contains meaningful mutations" |
+
+### Prompt contract:
+Two mandatory sentences inserted into the prompt under a new `## Contract: structuralActionRequired Declaration Rule` section:
+1. "Set to true when your proposal contains any meaningful graph change (new nodes, updated nodes, resolved unknowns, or changed edges)."
+2. "Set to false only when the user's supported meaning is already fully represented in existing graph state and no graph mutation is needed."
+
+### Focused deterministic tests (50 new + 8 migrated):
+
+**`tests/graph/schema.test.js`** (+4 tests):
+- Allows `structuralActionRequired: true`
+- Allows `structuralActionRequired: false`
+- Allows `null structuralActionRequired`
+- Omits by default (undefined is valid)
+
+**`tests/graph/prompt-builder.test.js`** (+10 tests):
+- Field name appears in Required JSON Field Names
+- Contract section heading exists with exact text
+- First sentence references `userSupportedMeaning` trigger
+- true condition references addedNodes.length
+- false condition references zero structural mutations
+- Existing semantic fidelity rules remain intact (supportCategory, resolutionGuidance)
+- No provider-specific wording added
+- Additional Guidance section preserved
+- Rule numbering unchanged (1–32 contiguous)
+- Contract section positioned between rules and Additional Guidance
+
+**`tests/graph/utils.test.js`** (+10 tests, 8 migrated):
+- true + meaningful mutation → PASS
+- true + zero mutation → REJECT
+- false + zero mutation with populated meaning → PASS (valid intentional no-op)
+- false + meaningful mutation → REJECT
+- null structuralActionRequired + populated meaning → REJECT (field required)
+- absent structuralActionRequired + populated meaning → REJECT (field required)
+- null structuralActionRequired + no meaning → retain existing behavior ("no meaningful change")
+- false+zero validation passes only for contract consistency, not semantic truth
+- hasMeaningfulChange logic unchanged for non-contract paths
+- supportCategory remains independent of structuralActionRequired
+
+Migrated 8 existing tests that previously used `userSupportedMeaning` assertions to use `structuralActionRequired: true` where answerMeaning is populated.
+
+### Test accounting:
+
+| Category | Count |
+|---|---|
+| New tests added | 24 (4 in schema + 10 in prompt-builder + 10 in utils) |
+| Existing tests migrated/modified | 8 (in utils.test.js and prompt-builder.test.js test comments/data) |
+| Pre-existing unchanged | All other existing tests pass as-is |
+
+### Test results:
+All 197 graph tests pass across schema.test.js, prompt-builder.test.js, and utils.test.js.
+
+---
+
+## What the implementation guarantees:
+1. Any proposal with populated `userSupportedMeaning` MUST include `structuralActionRequired` as a boolean.
+2. The declaration is an exact claim about output shape: `true` iff meaningful mutation exists; `false` iff zero mutations are intentional.
+3. `false + zero` is a valid no-op (contract-consistent) — the old "semantic-only rejection" no longer blocks it under contract.
+4. `true/false mismatch on output shape` is deterministically rejected with specific error messages.
+
+## What remains intentionally unresolved:
+- Prompt enforcement without runtime validation of model outputs (models may still send wrong values; the schema-level guard only helps downstream consumers).
+- The semantic correctness of `false + zero` is not validated — the validator confirms contract consistency, not whether the model's judgment was actually correct.
+- No migration plan for callers that currently produce `answerMeaning` without `structuralActionRequired`.
+
+## Live regression readiness:
+- All existing schema, prompt-builder, and utils tests pass.
+- The change is backward-compatible: field is optional by default; old proposals without it behave identically to the legacy path.
+
+---
+
+**Classification: IMPLEMENTATION COMPLETE.** Design decisions from 57J.67 applied verbatim. No live model calls. No semantic modifications. Ready for clean live regression on `feature/semantic-action-contract-v0.23`.