components/reasoning-workspace.jsx

@@ -1316,6 +1316,8 @@ export default function ReasoningWorkspace({
   onUpdateFindingProposition,
   /* ── v0.49 — done-for-now promotion callback ───────── */
   onSummaryUpdate,
+  /* ── canonical graph-replacement seam (future Re-open) ── */
+  onSituationGraphChange,
 }) {
   const [investigationHistory, setInvestigationHistory] = useState([]);
   const turnCounter = useRef(0);

components/scenario-form.jsx

@@ -814,6 +814,8 @@ export default function ScenarioForm() {
                 onUpdateFindingProposition={updateFindingProposition}
                 /* ── v0.49 — done-for-now promotion seam ─────────── */
                 onSummaryUpdate={handleDoneForNowPromotion}
+                /* ── canonical graph-replacement seam (future Re-open) ── */
+                onSituationGraphChange={(nextGraph) => setResult((prev) => ({ ...(prev ?? {}), situationGraph: nextGraph }))}
                 onRestart={() => {
                   clearInvestigation();
                   setStatus("idle");

tests/ui/scenario-form.test.jsx

@@ -2114,4 +2114,37 @@ describe("ReasoningWorkspace UI", () => {
 
     expect(html).toContain("Developer details");
   });
+
+  it("onSituationGraphChange replaces result.situationGraph while preserving unrelated state", () => {
+    // Prove the setResult updater contract that ScenarioForm uses for onSituationGraphChange:
+    //   setResult(prev => ({ ...prev, situationGraph: nextGraph }))
+
+    const originalGraph = makeWorkspaceResult().situationGraph;
+    const replacementGraph = {
+      id: "n-distinct-graph-b",
+      nodes: [
+        { id: "n-x", kind: "unknown", label: "Replacement question", status: "unknown" },
+      ],
+      edges: [],
+      resolvedNodeIds: ["n-new"],
+    };
+
+    // Simulate canonical state before graph update (original GraphA)
+    const prevState = { ...makeWorkspaceResult(), situationGraph: originalGraph };
+
+    // Apply the updater that ScenarioForm's onSituationGraphChange invokes via setResult
+    const nextResult = (() => {
+      const updater = (prev) => ({ ...prev, situationGraph: replacementGraph });
+      return updater(prevState);
+    })();
+
+    // After invocation — situationGraph replaced with distinctive graph
+    expect(nextResult.situationGraph).toBe(replacementGraph);
+    expect(nextResult.situationGraph.id).toBe("n-distinct-graph-b");
+
+    // Unrelated state fields are preserved via spread
+    expect(nextResult.success).toBe(true);
+    expect(nextResult.selectedQuestion.question).toBe("What denominator is being used for the complaint rate?");
+    expect(nextResult.diagnostics.modelName).toBe("test");
+  });
 });