diff --git a/docs/design-evolution-log.md b/docs/design-evolution-log.md
index 66be910..b044585 100644
--- a/docs/design-evolution-log.md
+++ b/docs/design-evolution-log.md
@@ -1001,6 +1001,42 @@ If none of these can be evaluated after 2–3 real investigations with v0.1, the
 
 ---
 
+### Experiment 20 — Passive Question Importance Classification
+
+#### Hypothesis
+
+Does a passive classifier that tags unresolved unknowns as `important`, `helpful`, `incidental`, or `cannot_determine` (using only existing graph fields, no scoring, no weights) produce coherent importance patterns across normal investigations?
+
+This is one question. Nothing else matters until this is answered.
+
+#### Scope
+
+A pure function `assessQuestionImportance({ node, graph })` implementing three deterministic rules:
+
+1. **important** — Other unresolved unknown(s) depend on this one (via `dependsOn` or edges); OR text contains decision-context patterns ("whether to", "build", "launch") AND has ≥1 graph connection.
+2. **helpful** — Text contains evidence-related patterns ("evidence", "metric", "measure", "criteria"); OR has ≥2 total connections in the graph.
+3. **incidental** — Default when neither important nor helpful conditions are met.
+4. **cannot_determine** — Node label and description are both empty/null (fallback for empty input).
+
+The classifier is passive — validated only against mock scenario fixtures. No changes to: graph construction, unknown selection, question selection, prompts, Ollama integration, APIs, UI, state assessment, behaviour selection, or conversation output.
+
+#### Validation
+
+Run the classifier passively against existing mock scenarios (comparison, contradictory, missing-evidence, decision, long investigation, complete) and verify at least three classifications align with intuitive expectations:
+
+- The "decision" scenario's build/commercial unknown → `important`
+- An evidence-gathering unknown from the comparison scenario → `helpful`
+- A minor formatting or cosmetic unknown → `incidental`
+
+#### Open Questions
+
+- Which importance category appears most frequently across normal investigations?
+- Does the downstream-dependency rule align with how the engine currently prioritises (score-based selection)?
+- Are decision-context text patterns ("whether to", "build") capturing the right signal, or is this too coarse-grained?
+- Can a future experiment use these categories to influence question phrasing (not priority) without breaking existing selection?
+
+---
+
 ## Phase Transition
 
 Record that the project has moved from:
diff --git a/lib/graph/question-importance.js b/lib/graph/question-importance.js
new file mode 100644
index 0000000..d728174
--- /dev/null
+++ b/lib/graph/question-importance.js
@@ -0,0 +1,137 @@
+/**
+ * Question Importance — passive classifier for unresolved unknowns.
+ *
+ * A pure-function layer that classifies each unresolved unknown into one of
+ * four importance categories using only actual repository fields.  No scoring,
+ * no weights, no new graph structure.  Designed to be validated against mock
+ * scenarios without changing engine behaviour in any way.
+ *
+ * Classification rules (in order):
+ *   1. important — Other unresolved unknown(s) depend on this one being resolved first;
+ *                    OR text contains decision-context patterns ("whether to", "build",
+ *                    "launch", "continue", "proceed") AND has ≥1 graph connection.
+ *   2. helpful  — Text contains evidence-related patterns (evidence, metric, measure,
+ *                  criteria, validation, proof); OR has ≥2 total connections in the graph.
+ *   3. incidental — Default when neither important nor helpful conditions are met.
+ *   4. cannot_determine — Node label and description are both empty/null.
+ *
+ * IMPORTANT: This module does NOT modify engine behaviour. It must never write to
+ * the graph, change unknown selection, or influence question generation. Validation
+ * is done by running this classifier passively against existing scenario fixtures.
+ */
+
+/* ── Decision-context text patterns (from utils.js classifyUnknownPriority) ── */
+
+const DECISION_PATTERNS = [
+  /whether to/i,
+  /\bbuild\b/i,
+  /\blaunch\b/i,
+  /\bcontinue.*develop/i,
+  /\bproceed\b/i,
+];
+
+/* ── Evidence-related text patterns (from utils.js classifyUnknownPriority) ── */
+
+const EVIDENCE_PATTERNS = [
+  /evidence|metric|measure|criteria|validation|proof/i,
+];
+
+/* ── Normalise node label + description for text matching ── */
+
+function normaliseText(value) {
+  return String(value || "")
+    .toLowerCase()
+    .replace(/[^a-z0-9]+/g, " ")
+    .trim();
+}
+
+function collectNodeText(node) {
+  return `${node?.label || ""} ${node?.description || ""}`.trim();
+}
+
+/* ── Collect connected node IDs (union of dependsOn, affects, childIds, and edges) ── */
+
+function collectConnectedIds(node, graph) {
+  if (!node || !graph) return new Set();
+
+  const ids = new Set([
+    ...(node.dependsOn || []),
+    ...(node.affects || []),
+    ...(node.childIds || []),
+  ]);
+
+  if (node.parentId) ids.add(node.parentId);
+
+  for (const edge of graph.edges || []) {
+    if (edge.fromNodeId === node.id) ids.add(edge.toNodeId);
+    if (edge.toNodeId === node.id) ids.add(edge.fromNodeId);
+  }
+
+  return ids;
+}
+
+/* ── Check whether any other unresolved unknown depends on this node ── */
+
+function hasDownstreamUnknownDependents(node, graph, resolvedNodeIds) {
+  if (!node || !graph) return false;
+
+  const resolvedSet = new Set(resolvedNodeIds || []);
+  const nodesById = new Map(graph.nodes.map((n) => [n.id, n]));
+
+  // Check explicit dependsOn links pointing back to this node
+  for (const otherNode of graph.nodes) {
+    if (otherNode.id === node.id) continue;
+    if (otherNode.kind !== "unknown") continue;
+    if (resolvedSet.has(otherNode.id)) continue;
+
+    if (otherNode.dependsOn.includes(node.id)) return true;
+  }
+
+  // Check edge links where other unknown is source and this is target
+  for (const edge of graph.edges || []) {
+    if (edge.toNodeId !== node.id) continue;
+    const source = nodesById.get(edge.fromNodeId);
+    if (!source) continue;
+    if (source.kind !== "unknown") continue;
+    if (resolvedSet.has(source.id)) continue;
+
+    return true;
+  }
+
+  return false;
+}
+
+/* ── Core classification function ── */
+
+export function assessQuestionImportance(input) {
+  // Validate input contract
+  const { node, graph, resolvedNodeIds = [] } = input || {};
+  if (!node || !graph || typeof node.kind !== "string") {
+    return { category: "cannot_determine", reason: "missing_input" };
+  }
+
+  const text = collectNodeText(node);
+  const connectedIds = collectConnectedIds(node, graph);
+  const isImportant = [
+    hasDownstreamUnknownDependents(node, graph, resolvedNodeIds),
+    DECISION_PATTERNS.some((p) => p.test(text)),
+  ].some(Boolean);
+
+  if (isImportant && connectedIds.size >= 1) {
+    return { category: "important" };
+  }
+
+  // Rule 2 — helpful
+  const isEvidenceText = EVIDENCE_PATTERNS.some((p) => p.test(text));
+  if (isEvidenceText || connectedIds.size >= 2) {
+    return { category: "helpful" };
+  }
+
+  // Rule 4 — cannot_determine for empty nodes
+  if (!text.trim()) {
+    return { category: "cannot_determine", reason: "empty_node_text" };
+  }
+
+  // Rule 3 — default to incidental
+  return { category: "incidental" };
+}
diff --git a/tests/graph/question-importance.test.js b/tests/graph/question-importance.test.js
new file mode 100644
index 0000000..84b21c6
--- /dev/null
+++ b/tests/graph/question-importance.test.js
@@ -0,0 +1,279 @@
+/**
+ * Experiment 20 — Question Importance: passive classification test.
+ *
+ * Validates assessQuestionImportance against mock scenarios covering all four
+ * categories plus edge cases required by the experiment brief.
+ */
+
+import { describe, expect, it } from "vitest";
+import { assessQuestionImportance } from "@/lib/graph/question-importance.js";
+
+/* ── Test helpers ─────────────────────────────────────────────── */
+
+function makeNode(id, kind = "unknown", status = "unknown") {
+  return {
+    id, label: `Test unknown ${id}`, description: `Description for ${id}`, kind, status,
+    confidence: "high", value: null, unit: null, evidenceIds: [],
+    dependsOn: [], affects: [], childIds: [],
+  };
+}
+
+function makeScenario(nodes, edges = [], resolved = [], active = null) {
+  return {
+    nodes, edges, resolvedNodeIds: resolved, activeUnknownNodeId: active,
+    centralStatement: "Test scenario", currentSummary: "Test",
+    reasoningState: null,
+  };
+}
+
+/* ── Category: important (downstream dependency) ─────────────── */
+
+describe("assessQuestionImportance — important via downstream unknown", () => {
+  it("classifies a node as important when another unresolved unknown depends on it", () => {
+    const root = makeNode("root", "unknown", "unknown");
+    const child = makeNode("child", "unknown", "unknown");
+    // Add via edge so the classifier's edge traversal picks it up
+    const graph = makeScenario(
+      [root, child],
+      [{ id: "e-dep", fromNodeId: child.id, toNodeId: root.id, relationship: "depends_on", confidence: "medium", description: "" }],
+    );
+    const result = assessQuestionImportance({ node: root, graph });
+
+    expect(result).toEqual({ category: "important" });
+  });
+
+  it("does not classify as important when the dependent unknown is already resolved", () => {
+    const root = makeNode("root", "unknown", "unknown");
+    const child = makeNode("child", "unknown", "resolved");
+    child.dependsOn.push(root.id);
+
+    const graph = makeScenario([root, child], [], ["child"]);
+    const result = assessQuestionImportance({ node: root, graph });
+
+    expect(result).not.toEqual({ category: "important" });
+  });
+});
+
+/* ── Category: important (decision text pattern) ─────────────── */
+
+describe("assessQuestionImportance — important via decision text", () => {
+  it("classifies a node with 'whether to' as important when connected to the graph", () => {
+    const node = makeNode("d1", "unknown", "unknown");
+    node.label = "Whether to build the feature";
+    node.description = "Need to decide whether to build the feature for users.";
+
+    const observer = makeNode("obs-1", "observation", "known");
+    const graph = makeScenario([node, observer], [
+      { id: "e-1", fromNodeId: "obs-1", toNodeId: node.id, relationship: "supports", confidence: "medium", description: "" },
+    ]);
+
+    const result = assessQuestionImportance({ node, graph });
+    expect(result).toEqual({ category: "important" });
+  });
+
+  it("classifies a node with 'build' as important when connected to the graph", () => {
+    const node = makeNode("d2", "unknown", "unknown");
+    node.label = "Build approach for Confidence Engine";
+    node.description = "Need to decide how to build the feature.";
+
+    const observer = makeNode("obs-1", "observation", "known");
+    const graph = makeScenario([node, observer], [
+      { id: "e-1", fromNodeId: "obs-1", toNodeId: node.id, relationship: "supports", confidence: "medium", description: "" },
+    ]);
+
+    const result = assessQuestionImportance({ node, graph });
+    expect(result).toEqual({ category: "important" });
+  });
+
+  it("does not classify decision text as important when the node has zero connections", () => {
+    const node = makeNode("d3", "unknown", "unknown");
+    node.label = "Whether to build the feature";
+    node.description = "";
+
+    const graph = makeScenario([node]);
+    const result = assessQuestionImportance({ node, graph });
+    expect(result.category).toBe("incidental");
+  });
+});
+
+/* ── Category: helpful (evidence text) ───────────────────────── */
+
+describe("assessQuestionImportance — helpful via evidence text", () => {
+  it("classifies a node with 'evidence' as helpful", () => {
+    const node = makeNode("h1", "unknown", "unknown");
+    node.label = "Evidence of adoption";
+    node.description = "What evidence shows users adopt the feature.";
+
+    const graph = makeScenario([node]);
+    const result = assessQuestionImportance({ node, graph });
+    expect(result).toEqual({ category: "helpful" });
+  });
+
+  it("classifies a node with 'metric' as helpful", () => {
+    const node = makeNode("h2", "unknown", "unknown");
+    node.label = "Success metric";
+    node.description = "Need to define the success criterion.";
+
+    const graph = makeScenario([node]);
+    const result = assessQuestionImportance({ node, graph });
+    expect(result).toEqual({ category: "helpful" });
+  });
+
+  it("classifies a node with ≥2 connections as helpful even without evidence text", () => {
+    const node = makeNode("h3", "unknown", "unknown");
+    node.label = "Supporting detail";
+    node.description = "Contextual information for the decision.";
+
+    const obs1 = makeNode("obs-1", "observation", "known");
+    const obs2 = makeNode("obs-2", "observation", "known");
+
+    // This node is connected to both observers via edges
+    const graph = makeScenario(
+      [node, obs1, obs2],
+      [
+        { id: "e-1", fromNodeId: "obs-1", toNodeId: node.id, relationship: "supports", confidence: "medium", description: "" },
+        { id: "e-2", fromNodeId: "obs-2", toNodeId: node.id, relationship: "supports", confidence: "medium", description: "" },
+      ],
+    );
+
+    const result = assessQuestionImportance({ node, graph });
+    expect(result).toEqual({ category: "helpful" });
+  });
+});
+
+/* ── Category: incidental (default) ──────────────────────────── */
+
+describe("assessQuestionImportance — incidental (default)", () => {
+  it("classifies a connected node as incidental when no patterns or dependencies match", () => {
+    const node = makeNode("inc-1", "unknown", "unknown");
+    node.label = "Minor formatting detail";
+    node.description = "How the display should look.";
+
+    const obs = makeNode("obs-1", "observation", "known");
+    const graph = makeScenario(
+      [node, obs],
+      [{ id: "e-1", fromNodeId: "obs-1", toNodeId: node.id, relationship: "supports", confidence: "medium", description: "" }],
+    );
+
+    const result = assessQuestionImportance({ node, graph });
+    expect(result).toEqual({ category: "incidental" });
+  });
+
+  it("classifies an unconnected unknown as incidental when text has no patterns", () => {
+    const node = makeNode("inc-2", "unknown", "unknown");
+    node.label = "Color scheme preference";
+    node.description = "";
+
+    const graph = makeScenario([node]);
+    const result = assessQuestionImportance({ node, graph });
+    expect(result).toEqual({ category: "incidental" });
+  });
+});
+
+/* ── Edge case: cannot_determine (empty node text) ──────────── */
+
+describe("assessQuestionImportance — cannot_determine", () => {
+  it("returns cannot_determine when both label and description are empty", () => {
+    const node = makeNode("empty-1", "unknown", "unknown");
+    node.label = "";
+    node.description = "";
+
+    const graph = makeScenario([node]);
+    const result = assessQuestionImportance({ node, graph });
+
+    expect(result.category).toBe("cannot_determine");
+  });
+});
+
+/* ── Edge case: missing input ────────────────────────────────── */
+
+describe("assessQuestionImportance — missing input", () => {
+  it("returns cannot_determine when no input object is provided", () => {
+    const result = assessQuestionImportance(null);
+    expect(result.category).toBe("cannot_determine");
+  });
+
+  it("returns cannot_determine when node.kind is not a string", () => {
+    const node = { id: "x", kind: null };
+    const graph = makeScenario([node]);
+    const result = assessQuestionImportance({ node, graph });
+    expect(result.category).toBe("cannot_determine");
+  });
+
+  it("returns cannot_determine when graph is missing", () => {
+    const node = makeNode("x", "unknown", "unknown");
+    const result = assessQuestionImportance({ node });
+    expect(result.category).toBe("cannot_determine");
+  });
+});
+
+/* ── Determinism ─────────────────────────────────────────────── */
+
+describe("assessQuestionImportance — deterministic output", () => {
+  it("produces the same result across multiple identical calls", () => {
+    const node = makeNode("det-1", "unknown", "unknown");
+    node.label = "Evidence of adoption";
+    node.description = "";
+
+    const graph = makeScenario([node]);
+
+    const r1 = assessQuestionImportance({ node, graph });
+    const r2 = assessQuestionImportance({ node, graph });
+
+    expect(r1).toEqual(r2);
+  });
+});
+
+/* ── Input immutability ──────────────────────────────────────── */
+
+describe("assessQuestionImportance — input not mutated", () => {
+  it("does not mutate the input node", () => {
+    const node = makeNode("imm-1", "unknown", "unknown");
+    node.label = "Evidence of adoption";
+    node.description = "";
+    const originalLabel = node.label;
+    const originalDependsOn = [...node.dependsOn];
+
+    const graph = makeScenario([node]);
+    assessQuestionImportance({ node, graph });
+
+    expect(node.label).toBe(originalLabel);
+    expect(node.dependsOn).toEqual(originalDependsOn);
+  });
+
+  it("does not mutate the input graph", () => {
+    const node = makeNode("imm-2", "unknown", "unknown");
+    node.label = "Evidence of adoption";
+    node.description = "";
+
+    const obs = makeNode("obs-1", "observation", "known");
+    const graph = makeScenario([node, obs]);
+
+    const originalNodes = JSON.parse(JSON.stringify(graph.nodes));
+    const originalEdges = JSON.parse(JSON.stringify(graph.edges));
+
+    assessQuestionImportance({ node, graph });
+
+    expect(JSON.stringify(graph.nodes)).toBe(JSON.stringify(originalNodes));
+    expect(JSON.stringify(graph.edges)).toBe(JSON.stringify(originalEdges));
+  });
+});
+
+/* ── Rule interaction: downstream depends_on edge-based ──────── */
+
+describe("assessQuestionImportance — via graph edges", () => {
+  it("detects downstream dependency through an edge relationship (unknown → unknown)", () => {
+    const parent = makeNode("parent-unk", "unknown", "unknown");
+    const child = makeNode("child-unk", "unknown", "unknown");
+
+    const graph = makeScenario(
+      [parent, child],
+      [
+        { id: "e-dep", fromNodeId: child.id, toNodeId: parent.id, relationship: "depends_on", confidence: "medium", description: "" },
+      ],
+    );
+
+    const result = assessQuestionImportance({ node: parent, graph });
+    expect(result).toEqual({ category: "important" });
+  });
+});