diff --git a/docs/current-handoff.md b/docs/current-handoff.md
index 6f4280b..e68fc7e 100644
--- a/docs/current-handoff.md
+++ b/docs/current-handoff.md
@@ -492,6 +492,25 @@ If concurrency instrumentation becomes necessary, prefer metadata only:
 
 Do not log investigation content merely for capacity measurement.
 
+### Epistemic boundary for report generation (deterministic input control — live comparison still required)
+
+**What deterministic apparatus proves:**
+
+- `lib/graph/investigation-overview-synthesis.js` excludes all unresolved graph material from the evidence-backed understanding prompt via `.filter((n) => KNOWN_SUPPORTED_STATUSES.has(n.status))` where `KNOWN_SUPPORTED_STATUSES = new Set(["known", "supported"])`.
+- Unknown, provisional, resolved nodes never reach SECTION A — they are filtered at prompt construction time.
+- **Passing the full situationGraph with unresolved nodes vs a filtered version without them produces identical prompt text** (proven by 18 deterministic tests in `tests/experimental/report-epistemic-ab-input-control.test.js`).
+- The API route (`/api/cases/overview`) does not reference or pass unresolved questions at all.
+- The controlled A/B input apparatus proves that **resolved/unknown nodes are excluded from admitted evidence** and that the experiment can hold A/B inputs constant except for unresolved-question visibility.
+
+**What it does NOT prove:**
+
+- It does **NOT** establish model output behaviour. Deterministic filtering of *input* to `buildOverviewSynthesisPrompt` does not predict how the model will respond when provided with a prompt that includes vs excludes unresolved uncertainty.
+- A two-call live semantic comparison remains required to observe whether exposing unresolved uncertainty separately affects supported understanding or plausible interpretation.
+
+**Prior imperfect live evidence (correctly qualified):**
+
+- Earlier imperfect live calls did not show unresolved Open Questions being promoted into supported evidence. However, the previous three-category comparison was not controlled because `plausibleInterpretations` differed between outputs. Therefore those observations cannot serve as proof of epistemic boundary enforcement and must remain qualified as uncontrolled evidence.
+
 ### Investigation growth
 
 Preserve the existing decision:
diff --git a/tests/experimental/report-epistemic-ab-input-control.test.js b/tests/experimental/report-epistemic-ab-input-control.test.js
index 54a4e72..9610ee7 100644
--- a/tests/experimental/report-epistemic-ab-input-control.test.js
+++ b/tests/experimental/report-epistemic-ab-input-control.test.js
@@ -125,6 +125,39 @@ describe("Controlled A/B report epistemic comparison inputs", () => {
     expect(typeof a.unresolvedQuestions).toBe("object");
   });
 
+  // ── Prompt-builder identity proof (A/B produce identical SECTION A) ────
+
+  it("buildOverviewSynthesisPrompt produces identical prompt text when unknown nodes are present vs removed", async () => {
+    const { buildOverviewSynthesisPrompt } = await import(
+      "@/lib/graph/investigation-overview-synthesis.js"
+    );
+    const aInputs = buildABInputs(fixture, false);
+
+    const baseSg = fixture.situationGraph;
+    const knownSupportedNodes = baseSg.nodes.filter(
+      (n) => ["known", "supported"].includes(n.status)
+    );
+
+    // Version A: filtered graph (unknowns removed)
+    const promptA = buildOverviewSynthesisPrompt(
+      { ...baseSg, nodes: knownSupportedNodes },
+      aInputs.supportedEvidence,
+      aInputs.findings,
+      aInputs.plausibleInterpretations
+    );
+
+    // Version B: full graph (unknowns included) — should be identical because
+    // buildOverviewSynthesisPrompt already filters to known+supported only
+    const promptB = buildOverviewSynthesisPrompt(
+      baseSg,
+      aInputs.supportedEvidence,
+      aInputs.findings,
+      aInputs.plausibleInterpretations
+    );
+
+    expect(promptA).toBe(promptB);
+  });
+
   it("fixed plausible interpretation carries NON-EVIDENCE epistemic status", () => {
     const a = buildABInputs(fixture, false);
     expect(a.plausibleInterpretations).toHaveLength(1);