diff --git a/docs/current-handoff.md b/docs/current-handoff.md
index 1ce54f5..77421be 100644
--- a/docs/current-handoff.md
+++ b/docs/current-handoff.md
@@ -234,6 +234,14 @@ Experiment 54N tested whether an interpretation disagreement can be judged for m
 - Focused preservation run passed via `npx vitest run tests/graph/apply-proposal.test.js -t "60B.42|60B.11|replaces downstream pricing"`.
 - Broader duplicated post-mutation eligibility checks remain unchanged; this fixes the selector contract only, not the full stale-question lifecycle.
 
+### 60B.43 terminal post-mutation eligibility
+
+- Extended terminal-status exclusion across the remaining post-mutation eligibility checks in `lib/graph/apply-proposal.js` while leaving pre-mutation `validateSelectedQuestion(...)` unchanged.
+- `isSelectableUnresolvedUnknown(...)`, `remainingUnknownExists`, carried-active reuse, unresolved/eligible candidate listing, and pattern-compatible post-mutation selection now all treat `known|resolved|contradicted` as terminal/non-selectable.
+- Deterministic 60B.37-shaped closure regression now passes: proposal applies, `n_enterprise_customer_signing` resolves, `n_product_launch_decision` becomes known, `activeUnknownNodeId` becomes null, and final `selectedQuestion` becomes null.
+- Focused fallback case also passes: when the proposal-selected target becomes known but another real unresolved unknown remains, the terminal target is discarded and the real unresolved node is selected.
+- Focused preservation run passed via `npx vitest run tests/graph/apply-proposal.test.js -t "60B.43|60B.11|replaces downstream pricing"`; live rerun still required to prove the same outcome end-to-end under live conditions.
+
 ### When This Knowledge-Management Phase Is Complete
 
 Provisional criteria for review (all confirmed met by Experiment 38 cold-start test):
diff --git a/docs/experiment-60b43.md b/docs/experiment-60b43.md
new file mode 100644
index 0000000..d6c053a
--- /dev/null
+++ b/docs/experiment-60b43.md
@@ -0,0 +1,123 @@
+# Experiment 60B.43 — Terminal post-mutation eligibility
+
+**Date:** 2026-08-14
+**Branch:** `feature/post-mutation-terminal-eligibility-v0.38`
+
+## Purpose
+
+Extend the 60B.42 selector guard to the remaining post-mutation question-target eligibility checks so terminal-status unknown nodes cannot remain active or become the final selected question after mutation.
+
+## Starting point from 60B.42
+
+60B.42 fixed `selectActiveUnknownCandidate(...)` so it no longer scores or returns unknown-kind nodes whose status is terminal:
+
+- `known`
+- `resolved`
+- `contradicted`
+
+That closed one fallback source, but several independent post-mutation checks in `apply-proposal.js` still used weaker eligibility rules and could keep terminal nodes alive through other paths.
+
+## Remaining post-mutation eligibility changes
+
+This experiment changed post-mutation eligibility only in `lib/graph/apply-proposal.js`.
+
+Updated paths:
+
+- `isSelectableUnresolvedUnknown(...)`
+- `remainingUnknownExists`
+- `carriedActiveUnknownStillUnresolved`
+- `listUnresolvedUnknownCandidates(...)`
+- `listEligibleUnknownCandidates(...)`
+- `selectPatternCompatibleUnknownCandidate(...)`
+
+### Terminal-status rule applied post-mutation
+
+A selectable unresolved post-mutation target now requires:
+
+```text
+kind === unknown
+status NOT IN [known, resolved, contradicted]
+not in resolvedNodeIds
+```
+
+### Important boundary preserved
+
+`validateSelectedQuestion(...)` was **not** changed.
+
+The closure proposal remains valid pre-mutation even when:
+
+- `selectedQuestion.nodeId = n_product_launch_decision`
+- the same proposal updates `n_product_launch_decision -> known`
+
+Only after mutation is that now-terminal target discarded.
+
+## 60B.37 deterministic regression
+
+Added focused regression:
+
+- `60B.43 — terminal post-mutation target is cleared after valid decision closure`
+
+Reproduced the 60B.37-shaped closure:
+
+- existing active unknown: `n_enterprise_customer_signing`
+- proposal resolves `n_enterprise_customer_signing`
+- proposal updates `n_product_launch_decision -> known`
+- proposal still selects `n_product_launch_decision`
+- no added nodes or edges
+
+### Result
+
+- proposal applied successfully
+- customer factor resolved in place
+- decision became known in place
+- both options preserved unchanged
+- `activeUnknownNodeId = null`
+- final `selectedQuestion = null`
+
+## Fallback-to-real-unknown result
+
+Added a second focused case where:
+
+- proposal-selected target becomes `known`
+- another genuine unresolved unknown remains after mutation
+
+Result:
+
+- terminal known target discarded
+- remaining genuine unresolved candidate selected
+
+## Known-only post-mutation result
+
+Added a valid mutated case where the final remaining unknown becomes `known` in the same mutation.
+
+Result:
+
+- `activeUnknownNodeId = null`
+- `selectedQuestion = null`
+
+## Preservation checks
+
+Focused run also preserved:
+
+- 60B.11 preferred-target behaviour
+- pricing prerequisite-first behaviour
+- resolved exclusion
+- contradicted exclusion
+
+## Validation
+
+Command run:
+
+```bash
+npx vitest run tests/graph/apply-proposal.test.js -t "60B.43|60B.11|replaces downstream pricing"
+```
+
+Result:
+
+- PASS — `14 passed | 76 skipped`
+
+## What remains unproven until live rerun
+
+This deterministic bounded fix now clears the exact 60B.37-shaped stale target through the post-mutation production path under test.
+
+What remains unproven until a live rerun is whether the full runtime/orchestration path with the real customer-signing closure answer produces the same null-question closure end state under live conditions.
\ No newline at end of file
diff --git a/lib/graph/apply-proposal.js b/lib/graph/apply-proposal.js
index 5baf149..cc582a6 100644
--- a/lib/graph/apply-proposal.js
+++ b/lib/graph/apply-proposal.js
@@ -1660,7 +1660,7 @@ function isSelectableUnresolvedUnknown(graph, nodeId) {
   return Boolean(
     node &&
     node.kind === "unknown" &&
-    !["resolved", "contradicted"].includes(node.status) &&
+    !["known", "resolved", "contradicted"].includes(node.status) &&
     !(graph.resolvedNodeIds || []).includes(node.id),
   );
 }
@@ -1673,9 +1673,7 @@ function listUnresolvedUnknownCandidates(
 
   return (graph.nodes || []).filter(
     (node) =>
-      node.kind === "unknown" &&
-      !["resolved", "contradicted"].includes(node.status) &&
-      !(graph.resolvedNodeIds || []).includes(node.id) &&
+      isSelectableUnresolvedUnknown(graph, node.id) &&
       !resolvedCurrentTurnSet.has(node.id),
   );
 }
@@ -2183,7 +2181,7 @@ function selectPatternCompatibleUnknownCandidate({
       (node) =>
         node.kind === "unknown" &&
         !excluded.has(node.id) &&
-        !["resolved", "contradicted"].includes(node.status),
+        isSelectableUnresolvedUnknown(graph, node.id),
     )
     .filter(
       (node) =>
@@ -3704,11 +3702,9 @@ export function applyValidatedProposal({
 
   const remainingUnknownExists =
     newActiveUnknownNodeId != null &&
-    updatedSituationGraph.nodes.some(
-      (node) =>
-        node.id === newActiveUnknownNodeId &&
-        node.kind === "unknown" &&
-        !updatedSituationGraph.resolvedNodeIds.includes(node.id),
+    isSelectableUnresolvedUnknown(
+      updatedSituationGraph,
+      newActiveUnknownNodeId,
     );
 
   if (!remainingUnknownExists) {
@@ -3846,9 +3842,8 @@ export function applyValidatedProposal({
     : null;
   const carriedActiveUnknownStillUnresolved = Boolean(
     carriedActiveUnknownNode &&
-    carriedActiveUnknownNode.kind === "unknown" &&
-    !["resolved", "contradicted"].includes(carriedActiveUnknownNode.status) &&
-    !updatedSituationGraph.resolvedNodeIds.includes(
+    isSelectableUnresolvedUnknown(
+      updatedSituationGraph,
       carriedActiveUnknownNode.id,
     ),
   );
diff --git a/tests/graph/apply-proposal.test.js b/tests/graph/apply-proposal.test.js
index 30138b6..4045dc3 100644
--- a/tests/graph/apply-proposal.test.js
+++ b/tests/graph/apply-proposal.test.js
@@ -4384,3 +4384,278 @@ describe("60B.11 — prerequisite-aware question targeting", () => {
     // The node has no depends_on prerequisites, so it passes the guard
   });
 });
+
+describe("60B.43 — terminal post-mutation target is cleared after valid decision closure", () => {
+  function makeProductLaunchClosureFixture({ includeFallbackUnknown = false } = {}) {
+    const productLaunchDecision = makeNode({
+      id: "n_product_launch_decision",
+      label: "Which option leaves us better off overall?",
+      description:
+        "Uncertainty about which of the two product-launch timing options provides superior net value for the organisation.",
+      kind: "unknown",
+      status: "unknown",
+      confidence: "medium",
+    });
+    const launchThisYear = makeNode({
+      id: "opt_launch_this_year",
+      label: "Launch this year",
+      description: "Launch the new software product this year.",
+      kind: "option",
+      status: "known",
+      confidence: "high",
+    });
+    const waitTwelveMonths = makeNode({
+      id: "opt_wait_twelve_months",
+      label: "Wait twelve months",
+      description: "Wait twelve months before launching the product.",
+      kind: "option",
+      status: "known",
+      confidence: "high",
+    });
+    const enterpriseCustomerSigning = makeNode({
+      id: "n_enterprise_customer_signing",
+      label: "Prospective enterprise customer signing status",
+      description:
+        "Unknown whether one prospective enterprise customer will sign if we launch this year, because they account for approximately £700,000 of the £1.2 million expected annual revenue.",
+      kind: "unknown",
+      status: "unknown",
+      confidence: "medium",
+    });
+
+    const nodes = [
+      productLaunchDecision,
+      launchThisYear,
+      waitTwelveMonths,
+      enterpriseCustomerSigning,
+    ];
+    const edges = [
+      makeEdge({
+        id: "e-opt-launch-to-dec",
+        fromNodeId: launchThisYear.id,
+        toNodeId: productLaunchDecision.id,
+        relationship: "contained_in",
+        confidence: "high",
+        description: "Launch this year option is a candidate for the product launch decision.",
+      }),
+      makeEdge({
+        id: "e-opt-wait-to-dec",
+        fromNodeId: waitTwelveMonths.id,
+        toNodeId: productLaunchDecision.id,
+        relationship: "contained_in",
+        confidence: "high",
+        description: "Wait twelve months option is a candidate for the product launch decision.",
+      }),
+      makeEdge({
+        id: "e-customer-signing-to-launch-option",
+        fromNodeId: enterpriseCustomerSigning.id,
+        toNodeId: launchThisYear.id,
+        relationship: "contained_in",
+        confidence: "high",
+        description: "Customer signing status is material to launching this year.",
+      }),
+    ];
+
+    if (includeFallbackUnknown) {
+      const fallbackUnknown = makeNode({
+        id: "n_other_market_evidence",
+        label: "Other market evidence gap",
+        description:
+          "Need other market evidence because the remaining launch case still depends on it.",
+        kind: "unknown",
+        status: "unknown",
+        confidence: "high",
+      });
+      nodes.push(fallbackUnknown);
+      edges.push(
+        makeEdge({
+          id: "e-fallback-to-launch-option",
+          fromNodeId: fallbackUnknown.id,
+          toNodeId: launchThisYear.id,
+          relationship: "may_cause",
+          confidence: "medium",
+          description: "Fallback unresolved evidence remains material to launch timing.",
+        }),
+      );
+    }
+
+    const graph = makeGraph({
+      centralStatement:
+        "We are evaluating two product-launch timing options: launching the new software product this year or waiting twelve months.",
+      nodes,
+      edges,
+      activeUnknownNodeId: enterpriseCustomerSigning.id,
+      resolvedNodeIds: [],
+      currentSummary:
+        "Customer signing is the active material uncertainty in the product-launch decision.",
+    });
+
+    return {
+      graph,
+      ids: {
+        productLaunchDecision: productLaunchDecision.id,
+        launchThisYear: launchThisYear.id,
+        waitTwelveMonths: waitTwelveMonths.id,
+        enterpriseCustomerSigning: enterpriseCustomerSigning.id,
+        fallbackUnknown: includeFallbackUnknown
+          ? "n_other_market_evidence"
+          : null,
+      },
+    };
+  }
+
+  it("accepts the 60B.37-shaped closure proposal, resolves the customer factor, and clears the terminal decision target after mutation", () => {
+    const { graph, ids } = makeProductLaunchClosureFixture();
+
+    const result = applyValidatedProposal({
+      situationGraph: graph,
+      proposal: {
+        updatedNodes: [
+          {
+            nodeId: ids.enterpriseCustomerSigning,
+            previousStatus: "unknown",
+            newStatus: "resolved",
+            previousValue: null,
+            newValue:
+              "Prospective enterprise customer confirmed they will sign if we launch this year.",
+            reason: "The user directly answered the active customer-signing unknown.",
+          },
+          {
+            nodeId: ids.productLaunchDecision,
+            previousStatus: "unknown",
+            newStatus: "known",
+            previousValue: null,
+            newValue:
+              "Launching this year is now known to be the better-supported option.",
+            reason:
+              "With customer signing confirmed and no other material uncertainties remaining, the decision is closed.",
+          },
+        ],
+        addedNodes: [],
+        addedEdges: [],
+        removedEdgeIds: [],
+        resolvedUnknownNodeIds: [ids.enterpriseCustomerSigning],
+        affectedNodeIds: [ids.productLaunchDecision],
+        selectedQuestion: {
+          nodeId: ids.productLaunchDecision,
+          question: "What outcome would demonstrate enough value to justify launching?",
+          reason:
+            "Regression case: proposal still points at the decision node even though it becomes known in the same turn.",
+        },
+      },
+    });
+
+    expect(result.success).toBe(true);
+
+    const resolvedCustomer = result.updatedSituationGraph.nodes.find(
+      (node) => node.id === ids.enterpriseCustomerSigning,
+    );
+    const knownDecision = result.updatedSituationGraph.nodes.find(
+      (node) => node.id === ids.productLaunchDecision,
+    );
+
+    expect(resolvedCustomer?.status).toBe("resolved");
+    expect(knownDecision?.status).toBe("known");
+    expect(result.updatedSituationGraph.nodes.filter((node) => node.id === ids.productLaunchDecision)).toHaveLength(1);
+    expect(result.updatedSituationGraph.nodes.filter((node) => node.id === ids.launchThisYear)).toHaveLength(1);
+    expect(result.updatedSituationGraph.nodes.filter((node) => node.id === ids.waitTwelveMonths)).toHaveLength(1);
+    expect(result.updatedSituationGraph.activeUnknownNodeId).toBeNull();
+    expect(result.selectedQuestion).toBeNull();
+  });
+
+  it("discards a proposal-selected target that becomes known and falls back to another genuine unresolved candidate", () => {
+    const { graph, ids } = makeProductLaunchClosureFixture({
+      includeFallbackUnknown: true,
+    });
+
+    const result = applyValidatedProposal({
+      situationGraph: graph,
+      proposal: {
+        updatedNodes: [
+          {
+            nodeId: ids.enterpriseCustomerSigning,
+            previousStatus: "unknown",
+            newStatus: "resolved",
+            previousValue: null,
+            newValue: "Customer signing confirmed.",
+            reason: "The active customer-signing uncertainty is resolved.",
+          },
+          {
+            nodeId: ids.productLaunchDecision,
+            previousStatus: "unknown",
+            newStatus: "known",
+            previousValue: null,
+            newValue: "The decision node itself becomes known.",
+            reason: "The originally selected target closes during mutation.",
+          },
+        ],
+        addedNodes: [],
+        addedEdges: [],
+        removedEdgeIds: [],
+        resolvedUnknownNodeIds: [ids.enterpriseCustomerSigning],
+        affectedNodeIds: [ids.productLaunchDecision],
+        selectedQuestion: {
+          nodeId: ids.productLaunchDecision,
+          question: "What outcome would demonstrate enough value to justify launching?",
+          reason: "This target becomes terminal and must be discarded post-mutation.",
+        },
+      },
+    });
+
+    expect(result.success).toBe(true);
+    expect(result.updatedSituationGraph.activeUnknownNodeId).toBe(
+      ids.fallbackUnknown,
+    );
+    expect(result.selectedQuestion?.nodeId).toBe(ids.fallbackUnknown);
+    expect(result.selectedQuestion?.nodeId).not.toBe(ids.productLaunchDecision);
+  });
+
+  it("returns no active target and no final question when only terminal unknowns remain after a valid mutation", () => {
+    const graph = makeGraph({
+      centralStatement: "Known-only terminal graph after mutation",
+      nodes: [
+        makeNode({
+          id: "n_known_terminal",
+          label: "Known terminal unknown",
+          description: "This unknown-kind node starts unresolved but becomes known.",
+          kind: "unknown",
+          status: "unknown",
+          confidence: "high",
+        }),
+      ],
+      edges: [],
+      activeUnknownNodeId: "n_known_terminal",
+      resolvedNodeIds: [],
+      currentSummary: "A single unknown remains before mutation.",
+    });
+
+    const result = applyValidatedProposal({
+      situationGraph: graph,
+      proposal: {
+        updatedNodes: [
+          {
+            nodeId: "n_known_terminal",
+            previousStatus: "unknown",
+            newStatus: "known",
+            previousValue: null,
+            newValue: "The previously active unknown is now known.",
+            reason: "The mutation closes the final unknown.",
+          },
+        ],
+        addedNodes: [],
+        addedEdges: [],
+        removedEdgeIds: [],
+        resolvedUnknownNodeIds: [],
+        affectedNodeIds: ["n_known_terminal"],
+        selectedQuestion: {
+          nodeId: "n_known_terminal",
+          question: "What remains to be clarified?",
+          reason: "This target becomes terminal in the same mutation and must be cleared post-mutation.",
+        },
+      },
+    });
+
+    expect(result.success).toBe(true);
+    expect(result.updatedSituationGraph.activeUnknownNodeId).toBeNull();
+    expect(result.selectedQuestion).toBeNull();
+  });
+});