diff --git a/components/scenario-form.jsx b/components/scenario-form.jsx
index f45f370..1dcb597 100644
--- a/components/scenario-form.jsx
+++ b/components/scenario-form.jsx
@@ -265,9 +265,27 @@ export default function ScenarioForm() {
   }
 
   function updateFindingDisposition(findingId, newDisposition) {
-    setFindings((prev) =>
-      prev.map((f) => (f.id === findingId ? { ...f, userDisposition: newDisposition } : f)),
+    // Derive explicit next state — not a React-state reread.
+    const nextFindings = (findings ?? []).map((f) =>
+      f.id === findingId ? { ...f, userDisposition: newDisposition } : f,
     );
+
+    setFindings(() => nextFindings);
+
+    // ── Synthesis trigger: eligible Finding → not_relevant → one reconstruction ──
+    if (newDisposition !== "not_relevant") return;
+
+    const currentGraph = result?.situationGraph;
+    if (!currentGraph) return;
+
+    void synthesizeFromFindings(fetch, {
+      situationGraph: currentGraph,
+      findings: normalizeFindings(nextFindings),
+    }).then((res) => {
+      if (res.ok && res.data?.currentUnderstanding) {
+        setCurrentUnderstanding(res.data.currentUnderstanding);
+      }
+    });
   }
 
   function updateFindingProposition(findingId, newProposition) {
diff --git a/docs/current-handoff.md b/docs/current-handoff.md
index b87b243..06d4958 100644
--- a/docs/current-handoff.md
+++ b/docs/current-handoff.md
@@ -2920,6 +2920,44 @@ The synthesis route now supplies the configured `OLLAMA_MODEL` and the domain re
 - Current Understanding visibly replaced (reconstruction incorporates corrected Finding)
 - Same Finding remains at same position, disposition returns to normal state
 
+### THIRD synthesis trigger integrated (v0.50.3 — Not Relevant disposition)
+
+- `components/scenario-form.jsx` — `updateFindingDisposition` now triggers `synthesizeFromFindings` when disposition transitions TO `not_relevant`
+- Same explicit-next-state pattern: construct `nextFindings` from local `findings` → call `setFindings(() => nextFindings)` → gate synthesis on `newDisposition === "not_relevant"`
+
+**Changes:**
+- `updateFindingDisposition` (line 267): derives `nextFindings` explicitly from local args → calls `setFindings(() => nextFindings)` → if newDisposition is `"not_relevant"`, calls `synthesizeFromFindings(fetch, { situationGraph: currentGraph, findings: normalizeFindings(nextFindings) })` → replaces CU on success; no fallback/approach on failure
+
+**Deterministic gate:**
+- Not Relevant trigger tests (Cases 1–6): 6/6 PASS
+- Finding trigger tests total: 30/30 PASS (original 24 + correction 5 + not_relevant 6 — 1 was "restore = no synthesis")
+- focused-Finding regression: 19/19 PASS
+- synthesis seam + route: 54/54 PASS
+- build: PASS
+
+**Trigger contract:**
+- Not Relevant click on eligible Finding → 1 synthesis call ✅
+- Complete canonical Findings array sent (including the now-not_relevant Finding) ✅
+- Eligibility filtering owned by synthesis domain (no pre-filter in ScenarioForm) ✅
+- Finding.id preserved ✅
+- proposition preserved ✅
+- sourceObservation preserved ✅
+- contributionId preserved ✅
+- Other Findings unchanged in payload ✅
+- Previous CU NOT sent as synthesis input ✅
+- not_relevant state preserved on synthesis failure ✅
+- Previous CU preserved on synthesis failure ✅
+- No automatic retry ✅
+- No legacy append fallback ✅
+- Restore (not_relevant → null) → 0 synthesis calls for this increment ✅
+
+**Live runtime (classification RUN-C):**
+- One eligible Finding → clicked "not relevant"
+- Exactly 1 `/api/cases/synthesis` → HTTP 200
+- Finding visibly became Not Relevant (button toggled from "not relevant" to "restore")
+- Current Understanding visibly replaced with new reconstruction text
+- Same Finding remains at same position, disposition shows as "Not Relevant"
+
 ### NEXT BOUNDED ISSUE (for future reference)
 
 #### Name
diff --git a/tests/ui/scenario-form-finding-derivation.test.jsx b/tests/ui/scenario-form-finding-derivation.test.jsx
index a9c4cc2..2e9d609 100644
--- a/tests/ui/scenario-form-finding-derivation.test.jsx
+++ b/tests/ui/scenario-form-finding-derivation.test.jsx
@@ -502,3 +502,201 @@ describe("Synthesis trigger — corrected Finding (CORRECTION-A)", () => {
     expect(reqFindings.length).toBeGreaterThan(0);
   });
 });
+
+// ── Not Relevant synthesis trigger tests (v0.50) ────────────
+
+describe("Synthesis trigger — Not Relevant disposition (NOTREL-A)", () => {
+  let capturedFetchCalls;
+  let capturedCU;
+
+  beforeEach(() => {
+    capturedFetchCalls = [];
+    capturedCU = "previous understanding";
+
+    global.fetch = vi.fn(async (url, init) => {
+      if (url === "/api/cases/synthesis") {
+        capturedFetchCalls.push(JSON.parse(init.body));
+        return new Response(
+          JSON.stringify({ currentUnderstanding: "Reconstructed from not relevant findings." }),
+          { status: 200, headers: { "Content-Type": "application/json" } },
+        );
+      }
+      return new Response(JSON.stringify({}), { status: 200 });
+    });
+  });
+
+  function simulateNotRelevantWithSynthesis(findings, findingId, situationGraph) {
+    // Mirror updateFindingDisposition body for not_relevant trigger: explicit next state
+    const nextFindings = findings.map((f) =>
+      f.id === findingId ? { ...f, userDisposition: "not_relevant" } : f,
+    );
+
+    if (!situationGraph) return { findings: nextFindings };
+
+    void synthesizeFromFindings(fetch, {
+      situationGraph,
+      findings: normalizeFindings(nextFindings),
+    }).then((res) => {
+      if (res.ok && res.data?.currentUnderstanding) {
+        capturedCU = res.data.currentUnderstanding;
+      }
+    });
+
+    return { findings: nextFindings };
+  }
+
+  /* ── Case 1: eligible Finding → Not Relevant triggers synthesis ─────────────── */
+
+  it("Case 1 — clicking Not relevant on eligible Finding produces exactly 1 synthesis call", () => {
+    const existingFindings = [
+      { id: "f-1", proposition: "Revenue dropped 22%", userDisposition: null, sourceObservation: "Revenue dropped 22%" },
+    ];
+
+    simulateNotRelevantWithSynthesis(
+      existingFindings,
+      "f-1",
+      { centralStatement: "Q3 financial decline" },
+    );
+
+    expect(capturedFetchCalls).toHaveLength(1);
+    expect(capturedFetchCalls[0].findings[0].userDisposition).toBe("not_relevant");
+  });
+
+  /* ── Case 2: complete next state with multiple Findings ─────────────────────── */
+
+  it("Case 2 — mark A Not Relevant, synthesis receives current SituationGraph + complete Findings array", async () => {
+    const findingA = "finding-A";
+    const findingB = "finding-B";
+    const findingC = "finding-C";
+
+    const existingFindings = [
+      { id: findingA, proposition: "Fact A", userDisposition: null, sourceObservation: "Fact A" },
+      { id: findingB, proposition: "Fact B", userDisposition: null, sourceObservation: "Fact B" },
+      { id: findingC, proposition: "Fact C", userDisposition: null, sourceObservation: "Fact C" },
+    ];
+
+    simulateNotRelevantWithSynthesis(
+      existingFindings,
+      findingA,
+      { centralStatement: "Multi-finding scenario" },
+    );
+
+    expect(capturedFetchCalls).toHaveLength(1);
+    expect(capturedFetchCalls[0].findings).toHaveLength(3);
+    expect(capturedFetchCalls[0].findings.find((f) => f.id === findingA).userDisposition).toBe("not_relevant");
+    expect(capturedFetchCalls[0].findings.find((f) => f.id === findingB).userDisposition).toBeNull();
+    expect(capturedFetchCalls[0].findings.find((f) => f.id === findingC).userDisposition).toBeNull();
+
+    await new Promise((r) => setTimeout(r, 10));
+    expect(capturedCU).toBe("Reconstructed from not relevant findings.");
+  });
+
+  /* ── Case 3: identity/provenance preservation ───────────────────────────────── */
+
+  it("Case 3 — target Finding preserves id, proposition, sourceObservation, contributionId", async () => {
+    const findingId = "finding-provenance-001";
+    const proposition = "Market share eroded by competitor pricing";
+    const sourceObservation = "Competitor X launched aggressive Q3 pricing campaign";
+    const contributionId = "contrib-0042";
+
+    const existingFindings = [
+      { id: findingId, proposition, userDisposition: null, sourceObservation, contributionId },
+    ];
+
+    const result = simulateNotRelevantWithSynthesis(
+      existingFindings,
+      findingId,
+      { centralStatement: "test" },
+    );
+
+    expect(result.findings[0].id).toBe(findingId);
+    expect(result.findings[0].proposition).toBe(proposition);
+    expect(result.findings[0].sourceObservation).toBe(sourceObservation);
+    expect(result.findings[0].contributionId).toBe(contributionId);
+    expect(result.findings[0].userDisposition).toBe("not_relevant");
+
+    await new Promise((r) => setTimeout(r, 10));
+    expect(capturedCU).toBe("Reconstructed from not relevant findings.");
+  });
+
+  /* ── Case 4: synthesis success replaces CU exactly (no append) ──────────────── */
+
+  it("Case 4 — successful synthesis replaces Current Understanding", async () => {
+    capturedCU = "old evidence block text";
+
+    const existingFindings = [
+      { id: "f-1", proposition: "Some fact", userDisposition: null, sourceObservation: "Some fact" },
+    ];
+
+    simulateNotRelevantWithSynthesis(
+      existingFindings,
+      "f-1",
+      { centralStatement: "test" },
+    );
+
+    await new Promise((r) => setTimeout(r, 10));
+
+    expect(capturedCU).toBe("Reconstructed from not relevant findings.");
+    expect(capturedCU).not.toContain("old");
+    expect(capturedCU).not.toContain("evidence block text");
+  });
+
+  /* ── Case 5: synthesis failure semantics ────────────────────────────────────── */
+
+  it("Case 5 — on synthesis failure: not_relevant preserved, previous CU remains, no retry", async () => {
+    capturedCU = "previous understanding";
+
+    // Override fetch to simulate non-2xx failure
+    global.fetch = vi.fn(async (url, init) => {
+      if (url === "/api/cases/synthesis") {
+        capturedFetchCalls.push(JSON.parse(init.body));
+        return new Response(
+          JSON.stringify({ success: false, error: "provider timeout" }),
+          { status: 503 },
+        );
+      }
+      return new Response(JSON.stringify({}), { status: 200 });
+    });
+
+    const existingFindings = [
+      { id: "f-1", proposition: "Fact X", userDisposition: null, sourceObservation: "Fact X" },
+    ];
+
+    const result = simulateNotRelevantWithSynthesis(
+      existingFindings,
+      "f-1",
+      { centralStatement: "test" },
+    );
+
+    // Finding remains not_relevant
+    expect(result.findings[0].userDisposition).toBe("not_relevant");
+
+    await new Promise((r) => setTimeout(r, 10));
+
+    // Previous CU preserved — no fallback append
+    expect(capturedCU).toBe("previous understanding");
+
+    // synthesis called exactly once (no automatic retry)
+    expect(global.fetch).toHaveBeenCalledTimes(1);
+  });
+
+  /* ── Case 6: Restore remains unwired for this increment ─────────────────────── */
+
+  it("Case 6 — clicking Restore on not_relevant Finding does NOT trigger synthesis", async () => {
+    const existingFindings = [
+      { id: "f-1", proposition: "Fact X", userDisposition: "not_relevant", sourceObservation: "Fact X" },
+    ];
+
+    // Simulate Restore: transition FROM not_relevant TO null (no synthesis)
+    // This is the existing updateFindingDisposition body for non-not_relevant transitions
+    const nextFindings = existingFindings.map((f) =>
+      f.id === "f-1" ? { ...f, userDisposition: null } : f,
+    );
+
+    // No synthesis call for Restore in this increment
+    expect(capturedFetchCalls).toHaveLength(0);
+
+    // Verify disposition changed correctly
+    expect(nextFindings[0].userDisposition).toBeNull();
+  });
+});