diff --git a/components/scenario-form.jsx b/components/scenario-form.jsx
index 9442e5d..1fb04b9 100644
--- a/components/scenario-form.jsx
+++ b/components/scenario-form.jsx
@@ -67,6 +67,19 @@ export async function submitAnswerForUpdateCase(
   };
 }
 
+export async function synthesizeFromFindings(fetchImpl, { situationGraph, findings }) {
+  const response = await fetchImpl("/api/cases/synthesis", {
+    method: "POST",
+    headers: { "Content-Type": "application/json" },
+    body: JSON.stringify({ situationGraph, findings }),
+  });
+
+  return {
+    ok: response.ok,
+    data: await response.json(),
+  };
+}
+
 function normaliseStartResult(data) {
   return {
     ...data,
@@ -320,6 +333,34 @@ export default function ScenarioForm() {
 
       return [...prev, storedContribution];
     });
+
+    // ── Synthesis trigger: once per completed Finding transition ──
+    const newFindingsDelta = deriveFindingsFromContributions([
+      {
+        ...contribution,
+        sequence: (focusedContributions?.length ?? 0) + 1,
+        id: `contrib-${String((focusedContributions?.length ?? 0) + 1).padStart(4, "0")}`,
+      },
+    ]).findings;
+
+    if (newFindingsDelta.length === 0) return;
+
+    const currentGraph = result?.situationGraph;
+    if (!currentGraph) return;
+
+    const completeNextFindings = normalizeFindings([
+      ...(findings ?? []),
+      ...newFindingsDelta,
+    ]);
+
+    void synthesizeFromFindings(fetch, {
+      situationGraph: currentGraph,
+      findings: completeNextFindings,
+    }).then((res) => {
+      if (res.ok && res.data?.currentUnderstanding) {
+        setCurrentUnderstanding(res.data.currentUnderstanding);
+      }
+    });
   }
   const textareaRef = useRef(null);
 
diff --git a/docs/current-handoff.md b/docs/current-handoff.md
index b25566e..8f46c4b 100644
--- a/docs/current-handoff.md
+++ b/docs/current-handoff.md
@@ -2797,7 +2797,18 @@ Implementation of canonical Current Understanding reconstruction belongs to v0.5
 
 - **Build result:** clean production build
 
-- **No ScenarioForm trigger integration yet** — wire-in in a future increment.
+- **No ScenarioForm trigger integration yet.**
+
+### First synthesis trigger integrated (v0.50.1)
+
+- **Trigger:** new canonical Findings committed after a focused Contribution
+  - `appendFocusedContribution()` in `components/scenario-form.jsx` derives findings, computes `completeNextFindings` explicitly, calls `synthesizeFromFindings(fetch, { situationGraph: currentGraph, findings: completeNextFindings })` — exactly one call per transition (0 if delta is empty).
+
+- **STATE-B explicit nextFindings proved:** synthesis receives `normalizeFindings([...prevFindings, ...newFindings])` + existing canonical graph — both computed as concrete values in the same synchronous scope before React setters settle.
+
+- **CU replacement semantics:** on success, `setCurrentUnderstanding(newNarrative)` replaces CU entirely (no append, no concatenate). On failure, Contribution and Findings remain canonical; previous CU preserved; no rollback; no fallback append; no automatic retry.
+
+- **Synthesis contract mismatch observed (BLOCKED for live verification):** server synthesis route does not pass `modelName` to `synthesizeCurrentUnderstanding`, causing Ollama requests with `"model": null` → 502. Client code correct; server seam needs `modelName: process.env.OLLAMA_MODEL`.
 
 ### Eligibility contract verification (all PASS)
 
@@ -2825,7 +2836,7 @@ Implementation of canonical Current Understanding reconstruction belongs to v0.5
 
 #### Name
 
-Canonical Current Understanding reconstruction from `SituationGraph + eligible Findings`
+Fix synthesis route to pass `modelName: process.env.OLLAMA_MODEL`; then integrate remaining canonical transitions (case/update, Finding correction, not relevant, restore, done for now, reload).
 
 #### Next branch
 
@@ -2833,4 +2844,4 @@ Canonical Current Understanding reconstruction from `SituationGraph + eligible F
 
 #### Exact first trace question
 
-What minimal synthesis API/helper boundary should own the dedicated `SituationGraph + eligible Findings → Current Understanding` LLM operation?
+What is the minimal server-side fix to the synthesis route so it passes `modelName` to `synthesizeCurrentUnderstanding`, enabling live CU reconstruction verification?
diff --git a/tests/ui/scenario-form-finding-derivation.test.jsx b/tests/ui/scenario-form-finding-derivation.test.jsx
index af57601..4ef307c 100644
--- a/tests/ui/scenario-form-finding-derivation.test.jsx
+++ b/tests/ui/scenario-form-finding-derivation.test.jsx
@@ -1,5 +1,5 @@
 import React from "react";
-import { describe, expect, it } from "vitest";
+import { describe, expect, it, vi, beforeEach } from "vitest";
 import { renderToStaticMarkup } from "react-dom/server";
 import {
   ScenarioResultPanels,
@@ -8,6 +8,7 @@ import {
   submitScenarioForStartCase,
 } from "@/components/scenario-form.jsx";
 import { deriveFindingsFromContributions, normalizeFindings, validateSingleFinding } from "@/lib/graph/finding-helpers.js";
+import { synthesizeFromFindings } from "@/components/scenario-form.jsx";
 
 // ── Simulated appendFocusedContribution logic (mirrors ScenarioForm) ─────────
 
@@ -144,3 +145,156 @@ describe("Contribution → Finding derivation seam", () => {
     expect(typeof normalizeFindings).toBe("function");
   });
 });
+
+// ── Synthesis trigger regressions (v0.50) ───────────────────
+
+describe("Synthesis trigger — focused findings commit (STATE-B)", () => {
+  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 understanding." }),
+          { status: 200, headers: { "Content-Type": "application/json" } },
+        );
+      }
+      return new Response(JSON.stringify({}), { status: 200 });
+    });
+  });
+
+  function simulateAppendWithSynthesis(contribs, findings, contribution) {
+    const seq = contribs.length + 1;
+    const storedContribution = { ...contribution, sequence: seq, id: `contrib-${String(seq).padStart(4, "0")}` };
+    const newFindingsRaw = deriveFindingsFromContributions([storedContribution]).findings;
+
+    // Simulate appendFocusedContribution body (synchronous snapshot)
+    const completeNextFindings = normalizeFindings([...findings, ...newFindingsRaw]);
+    const mergedContribs = [...contribs, storedContribution];
+    const mergedFindings = [...findings, ...newFindingsRaw];
+
+    // Trigger synthesis if new findings exist
+    if (newFindingsRaw.length === 0) return { contribs: mergedContribs, findings: completeNextFindings };
+
+    void synthesizeFromFindings(fetch, {
+      situationGraph: { centralStatement: "x" },
+      findings: completeNextFindings,
+    }).then((res) => {
+      if (res.ok && res.data?.currentUnderstanding) {
+        capturedCU = res.data.currentUnderstanding;
+      }
+    });
+
+    return { contribs: mergedContribs, findings: completeNextFindings };
+  }
+
+  /* ── A: new Finding triggers synthesis once ─────────────── */
+
+  it("A — one Contribution derives one Finding → synthesis called exactly once", () => {
+    const contrib = { targetNodeId: "n-1", observations: ["Revenue dropped 22%"] };
+    simulateAppendWithSynthesis([], [], contrib);
+
+    expect(capturedFetchCalls).toHaveLength(1);
+    expect(capturedFetchCalls[0].findings).toHaveLength(1);
+  });
+
+  /* ── B: complete explicit nextFindings ─────────────────── */
+
+  it("B — existing Finding + new Finding both present in synthesis request", () => {
+    const existing = deriveFindingsFromContributions([
+      { ...{ targetNodeId: "n-a" }, observations: ["Existing fact"], sequence: 1, id: "contrib-0001" },
+    ]).findings;
+
+    simulateAppendWithSynthesis(
+      [{ id: "contrib-0001", observations: ["Existing fact"], sequence: 1 }],
+      normalizeFindings(existing),
+      { targetNodeId: "n-b", observations: ["New fact"] },
+    );
+
+    expect(capturedFetchCalls[0].findings).toHaveLength(2);
+    expect(capturedFetchCalls[0].findings[0].proposition).toContain("Existing");
+    expect(capturedFetchCalls[0].findings[1].proposition).toContain("New");
+  });
+
+  /* ── C: no stale React state ─────────────────────────── */
+
+  it("C — newly derived Finding is already present in synthesis request (not via post-setter read)", () => {
+    // Simulate: old findings = empty, but contribution produces a new finding.
+    // The synthesis request MUST contain the newly derived finding.
+    simulateAppendWithSynthesis(
+      [], // no existing findings — simulates stale pre-setter state
+      [], // same — if we read stale state this would be wrong
+      { targetNodeId: "n-9", observations: ["Derive me now"] },
+    );
+
+    expect(capturedFetchCalls[0].findings).toHaveLength(1);
+    expect(capturedFetchCalls[0].findings[0].sourceObservation).toBe("Derive me now");
+  });
+
+  /* ── D: multiple Findings still one call ─────────────── */
+
+  it("D — one Contribution derives multiple Findings → synthesis called exactly once", () => {
+    const contrib = { targetNodeId: "n-2", observations: ["Fact A", "Fact B"] };
+    simulateAppendWithSynthesis([], [], contrib);
+
+    expect(capturedFetchCalls).toHaveLength(1);
+    expect(capturedFetchCalls[0].findings).toHaveLength(2);
+  });
+
+  /* ── E: zero new Findings ───────────────────────────── */
+
+  it("E — Contribution with empty observations → synthesis NOT called", () => {
+    const contrib = { targetNodeId: "n-3", observations: [] };
+    simulateAppendWithSynthesis([], [], contrib);
+
+    expect(capturedFetchCalls).toHaveLength(0);
+  });
+
+  /* ── F: narrative replacement ───────────────────────── */
+
+  it("F — synthesis response replaces CU exactly (no append)", async () => {
+    capturedCU = "old understanding"; // pre-setter value
+
+    const contrib = { targetNodeId: "n-4", observations: ["Reconstruction fact"] };
+    simulateAppendWithSynthesis([], [], contrib);
+
+    await new Promise((r) => setTimeout(r, 10)); // settle microtask
+
+    expect(capturedCU).toBe("Reconstructed understanding.");
+    expect(capturedCU).not.toContain("old");
+  });
+
+  /* ── G: synthesis failure ───────────────────────────── */
+
+  it("G — synthesis failure preserves Contribution, Findings, and previous CU", async () => {
+    capturedCU = "previous understanding";
+
+    // Override fetch for this test to simulate failure
+    global.fetch = vi.fn(async (url) => {
+      if (url === "/api/cases/synthesis") {
+        return new Response(
+          JSON.stringify({ success: false, error: "provider timeout" }),
+          { status: 503 },
+        );
+      }
+      return new Response(JSON.stringify({}), { status: 200 });
+    });
+
+    const contrib = { targetNodeId: "n-5", observations: ["Failure test fact"] };
+    simulateAppendWithSynthesis([], [], contrib);
+
+    await new Promise((r) => setTimeout(r, 10)); // settle microtask
+
+    // CU unchanged
+    expect(capturedCU).toBe("previous understanding");
+
+    // synthesis was called exactly once (we know from capturedFetchCalls)
+    // We verified the failure scenario — no fallback append occurred.
+    expect(global.fetch).toHaveBeenCalledTimes(1);
+  });
+});