diff --git a/lib/graph/prompt-builder.js b/lib/graph/prompt-builder.js
index 7d7d53e..92dd294 100644
--- a/lib/graph/prompt-builder.js
+++ b/lib/graph/prompt-builder.js
@@ -123,6 +123,7 @@ The JSON object must contain exactly these top-level fields:
 
 ## Additional Guidance
 - If the answer only clarifies an existing unknown, prefer updatedNodes and resolvedUnknownNodeIds over creating duplicate nodes.
+- When rule #6 applies to explicitly unresolved uncertainty: first check whether an existing unresolved node already represents the same uncertainty; if so, update/refine that existing structure rather than adding a duplicate; if no such node exists, add a new unknown that directly represents the unresolved uncertainty; do not use an edge alone to represent a previously unrepresented uncertainty.
 - When an answer resolves an existing unknown, include that existing node ID in resolvedUnknownNodeIds and update that node rather than creating only a parallel observation.
 - If the answer creates a more specific decision situation, add the smallest set of new nodes and edges needed to represent that situation and only its most consequential unknowns.
 - If you add a new unknown, do not leave it floating: connect it with an added edge to the relevant decision/context node created or updated from the answer.
diff --git a/tests/graph/prompt-builder.test.js b/tests/graph/prompt-builder.test.js
index fcd9064..a102562 100644
--- a/tests/graph/prompt-builder.test.js
+++ b/tests/graph/prompt-builder.test.js
@@ -281,3 +281,155 @@ describe("buildGraphUpdatePrompt — semantic-to-mutation MUST rule", () => {
     expect(fullPrompt).not.toContain("must add a new node");
   });
 });
+
+// ── Experiment 57J.46 — existing-first uncertainty fallback ───
+
+describe("buildGraphUpdatePrompt — 57J.46 existing-first uncertainty fallback", () => {
+  function getAdditionalGuidance(prompt) {
+    return prompt.split("## Additional Guidance")[1];
+  }
+
+  // Test 1 — existing-first ordering exists
+  it("test 1: assembled prompt explicitly says to check for an equivalent unresolved unknown first", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    const guidance = getAdditionalGuidance(prompt);
+    expect(guidance).toContain("first check whether an existing unresolved node");
+    expect(guidance).toContain("represents the same uncertainty");
+  });
+
+  // Test 2 — reuse path explicit
+  it("test 2: prompt says update/refine existing structure rather than add a duplicate when equivalent exists", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    const guidance = getAdditionalGuidance(prompt);
+    expect(guidance).toContain("if so, update/refine that existing structure");
+    expect(guidance).toContain("rather than adding a duplicate");
+  });
+
+  // Test 3 — fallback-to-add explicit
+  it("test 3: prompt explicitly requires adding a new unknown when no equivalent unresolved unknown exists", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    const guidance = getAdditionalGuidance(prompt);
+    expect(guidance).toContain("if no such node exists");
+    expect(guidance).toContain("add a new unknown that directly represents the unresolved uncertainty");
+  });
+
+  // Test 4 — ordered fallback means: existing first, otherwise add
+  it("test 4: full ordered fallback is present in the assembled prompt", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    const guidance = getAdditionalGuidance(prompt);
+    // The entire rule must be present as a single coherent instruction:
+    expect(guidance).toContain(
+      "When rule #6 applies to explicitly unresolved uncertainty: first check whether an existing unresolved node already represents the same uncertainty; if so, update/refine that existing structure rather than adding a duplicate; if no such node exists, add a new unknown that directly represents the unresolved uncertainty; do not use an edge alone to represent a previously unrepresented uncertainty.",
+    );
+  });
+
+  // Test 5 — merely related node is insufficient
+  it("test 5: prompt does not imply a general related state/cost node counts as representing the same uncertainty", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    const guidance = getAdditionalGuidance(prompt);
+    // The rule requires "same uncertainty" — not "related" or "similar":
+    expect(guidance).toContain("same uncertainty");
+    // Must not use weaker criteria:
+    expect(guidance).not.toContain("similar");
+    expect(guidance).not.toContain("related to");
+  });
+
+  // Test 6 — edge-only insufficient
+  it("test 6: prompt explicitly prevents using an edge alone to represent previously unrepresented uncertainty", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    const guidance = getAdditionalGuidance(prompt);
+    expect(guidance).toContain(
+      "do not use an edge alone to represent a previously unrepresented uncertainty",
+    );
+  });
+
+  // Test 7 — possibleInference does not create unknowns
+  it("test 7: existing possibleInference separation remains intact — rule #27 still separates interpretation from userSupportedMeaning", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    expect(prompt).toContain(
+      "Put any stronger interpretation in answerMeaning.possibleInference, not in userSupportedMeaning",
+    );
+    // The 57J.46 rule must NOT reference possibleInference as a trigger:
+    const guidance = getAdditionalGuidance(prompt);
+    expect(guidance).not.toContain("possibleInference");
+  });
+
+  // Test 8 — actual resolution path preserved
+  it("test 8: prompt still allows resolution when the user's answer genuinely resolves an existing unknown", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    // The "resolve existing" guidance must remain intact:
+    expect(prompt).toContain(
+      "include that existing node ID in resolvedUnknownNodeIds",
+    );
+    expect(prompt).toContain("update that node rather than creating only a parallel observation");
+    // Rule #5 (resolve answered unknown first) must still exist:
+    expect(prompt).toContain("Resolve the answered unknown first when the answer supports it");
+  });
+
+  // Test 9 — duplicate validator/contract preserved
+  it("test 9: existing duplicate-avoidance wording remains unchanged", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    expect(prompt).toContain("Do not add duplicate unknowns");
+    expect(prompt).toContain("genuinely new concepts");
+    const guidance = getAdditionalGuidance(prompt);
+    expect(guidance).toContain("prefer updatedNodes and resolvedUnknownNodeIds over creating duplicate nodes");
+  });
+
+  // Test 10 — scope remains uncertainty-only (not universal to all categories)
+  it("test 10: the new action-order rule does not apply universally to facts, constraints, decisions, or other meaning categories", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    const guidance = getAdditionalGuidance(prompt);
+    // The rule must be scoped to "unresolved uncertainty" specifically:
+    expect(guidance).toContain("explicitly unresolved uncertainty");
+    // It must not say "any answer" or "all categories":
+    expect(guidance).not.toContain("any answer");
+    expect(guidance).not.toContain("every change");
+    expect(guidance).not.toContain("all categories");
+    // Rule #7 (new unknown conditions) and other category rules must be untouched:
+    expect(prompt).toContain(
+      "Add new unknown nodes only when the answer introduces a new decision, claim, object, measure, dependency, or unresolved term directly relevant to the case",
+    );
+  });
+
+  // Test 11 — fidelity / possibleInference separation preserved
+  it("test 11: fidelity rule separating userSupportedMeaning from possibleInference is untouched", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    expect(prompt).toContain(
+      "answerMeaning.userSupportedMeaning must state only what the user's answer directly supports",
+    );
+  });
+
+  // Test 12 — traceability preserved
+  it("test 12: traceability requirement for new unknowns remains intact", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    expect(prompt).toContain(
+      "Every new unknown must be directly traceable to the user's answer and its description must state why that uncertainty matters",
+    );
+  });
+
+  // Test 13 — no noop validator changed
+  it("test 13: the 'rule #6 does not apply → empty arrays' permission is preserved unchanged", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    const guidance = getAdditionalGuidance(prompt);
+    expect(guidance).toContain(
+      "If rule #6 does not apply (the answer contains no user-supported meaning that requires graph progress) and there is no other justification for change, return empty arrays for every category",
+    );
+  });
+
+  // Test 14 — no semantic classifier or keyword logic added
+  it("test 14: no new deterministic semantic matcher or keyword matching was added", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    expect(prompt).not.toContain("threshold");
+    expect(prompt).not.toContain("synonym");
+    expect(prompt).not.toContain("keyword match");
+  });
+
+  // Test 15 — provider-agnostic preserved
+  it("test 15: no provider-specific wording added", () => {
+    const prompt = buildGraphUpdatePrompt(makeContext());
+    expect(prompt).not.toContain("qwen");
+    expect(prompt).not.toContain("claude");
+    expect(prompt).not.toContain("gpt");
+    expect(prompt).not.toContain("ollama");
+  });
+});