diff --git a/assistant/src/memory/v3/__tests__/index-composition.test.ts b/assistant/src/memory/v3/__tests__/index-composition.test.ts
new file mode 100644
index 00000000000..aa08205e35f
--- /dev/null
+++ b/assistant/src/memory/v3/__tests__/index-composition.test.ts
@@ -0,0 +1,233 @@
+/**
+ * Tests for `assistant/src/memory/v3/index-composition.ts`.
+ *
+ * `composeNodeIndex` is a pure function over an already-built `TreeIndex` and
+ * `PageIndex`, so these tests hand-build both fixtures (no filesystem / no I/O)
+ * and assert on the rendered string.
+ *
+ * Coverage matrix:
+ *   - mixed node:/page: children render one summary line each, in authored
+ *     order, with the node's routing hints appended as a trailer.
+ *   - a `page:` ref whose slug is absent from the index is silently omitted.
+ *   - a `node:` ref whose id is absent from the tree is silently omitted.
+ *   - empty / missing children → just the routing hints, or the empty string
+ *     when there are none either.
+ *   - a `node:` child with no summary falls back to the first non-empty body
+ *     line; with neither, only its header is emitted.
+ */
+
+import { describe, expect, test } from "bun:test";
+
+import type { PageIndex, PageIndexEntry } from "../../v2/page-index.js";
+import { composeNodeIndex } from "../index-composition.js";
+import type { ChildRef, TreeIndex } from "../tree-index.js";
+import type { TreeNode } from "../types.js";
+
+// ---------------------------------------------------------------------------
+// Fixture builders
+// ---------------------------------------------------------------------------
+
+function treeNode(
+  id: string,
+  opts: { summary?: string; routing_hints?: string; body?: string } = {},
+): TreeNode {
+  return {
+    id,
+    frontmatter: {
+      children: [],
+      summary: opts.summary,
+      routing_hints: opts.routing_hints,
+    },
+    body: opts.body ?? "",
+  };
+}
+
+/**
+ * Build a `TreeIndex` from a list of nodes and an explicit child-ref list for
+ * the node under test. Only the fields `composeNodeIndex` reads (`nodes`,
+ * `childrenByNode`) are populated; the reverse-adjacency maps are left empty.
+ */
+function treeIndex(
+  nodes: TreeNode[],
+  childrenByNode: Record<string, ChildRef[]>,
+): TreeIndex {
+  return {
+    nodes: new Map(nodes.map((n) => [n.id, n])),
+    childrenByNode: new Map(Object.entries(childrenByNode)),
+    parentsByNode: new Map(),
+    pageParents: new Map(),
+    root: "_root",
+  };
+}
+
+function pageEntry(slug: string, summary: string): PageIndexEntry {
+  return { id: 1, slug, summary, edges: [], modifiedAt: 0 };
+}
+
+function pageIndex(entries: PageIndexEntry[]): PageIndex {
+  return {
+    entries,
+    bySlug: new Map(entries.map((e) => [e.slug, e])),
+    byId: new Map(entries.map((e) => [e.id, e])),
+    rendered: "",
+  };
+}
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+describe("composeNodeIndex", () => {
+  test("composes mixed node:/page: children in authored order with routing hints", () => {
+    const tree = treeIndex(
+      [
+        treeNode("people", {
+          summary: "People you know",
+          routing_hints: "for work contacts see node:colleagues",
+        }),
+        treeNode("colleagues", { summary: "Work relationships" }),
+      ],
+      {
+        people: [
+          { kind: "node", ref: "colleagues" },
+          { kind: "page", ref: "alice" },
+        ],
+      },
+    );
+    const pages = pageIndex([
+      pageEntry("alice", "Alice — neighbor and friend"),
+    ]);
+
+    const block = composeNodeIndex("people", tree, pages);
+
+    expect(block).toBe(
+      [
+        "[node:colleagues] Work relationships",
+        "[page:alice] Alice — neighbor and friend",
+        "Routing hints: for work contacts see node:colleagues",
+      ].join("\n"),
+    );
+  });
+
+  test("emits children in authored order regardless of map insertion", () => {
+    const tree = treeIndex(
+      [treeNode("a", { summary: "Node A" }), treeNode("root", {})],
+      {
+        root: [
+          { kind: "page", ref: "zeta" },
+          { kind: "node", ref: "a" },
+          { kind: "page", ref: "beta" },
+        ],
+      },
+    );
+    const pages = pageIndex([
+      pageEntry("beta", "Beta page"),
+      pageEntry("zeta", "Zeta page"),
+    ]);
+
+    const block = composeNodeIndex("root", tree, pages);
+
+    expect(block).toBe(
+      [
+        "[page:zeta] Zeta page",
+        "[node:a] Node A",
+        "[page:beta] Beta page",
+      ].join("\n"),
+    );
+  });
+
+  test("silently omits a page ref absent from the index", () => {
+    const tree = treeIndex([treeNode("root", {})], {
+      root: [
+        { kind: "page", ref: "present" },
+        { kind: "page", ref: "missing" },
+      ],
+    });
+    const pages = pageIndex([pageEntry("present", "I exist")]);
+
+    const block = composeNodeIndex("root", tree, pages);
+
+    expect(block).toBe("[page:present] I exist");
+  });
+
+  test("silently omits a node ref absent from the tree", () => {
+    const tree = treeIndex([treeNode("present", { summary: "Here" })], {
+      root: [
+        { kind: "node", ref: "present" },
+        { kind: "node", ref: "ghost" },
+      ],
+    });
+    const pages = pageIndex([]);
+
+    const block = composeNodeIndex("root", tree, pages);
+
+    expect(block).toBe("[node:present] Here");
+  });
+
+  test("empty children → just the routing hints", () => {
+    const tree = treeIndex(
+      [treeNode("leaf", { routing_hints: "this is a leaf branch" })],
+      { leaf: [] },
+    );
+
+    const block = composeNodeIndex("leaf", tree, pageIndex([]));
+
+    expect(block).toBe("Routing hints: this is a leaf branch");
+  });
+
+  test("no children and no routing hints → empty string", () => {
+    const tree = treeIndex([treeNode("bare", {})], { bare: [] });
+
+    expect(composeNodeIndex("bare", tree, pageIndex([]))).toBe("");
+  });
+
+  test("node with no childrenByNode entry composes from routing hints alone", () => {
+    const tree = treeIndex(
+      [treeNode("orphan", { routing_hints: "hint only" })],
+      {},
+    );
+
+    expect(composeNodeIndex("orphan", tree, pageIndex([]))).toBe(
+      "Routing hints: hint only",
+    );
+  });
+
+  test("node child with no summary falls back to first non-empty body line", () => {
+    const tree = treeIndex(
+      [
+        treeNode("root", {}),
+        treeNode("bodyonly", {
+          body: "\n  \nFirst real line\nSecond line",
+        }),
+      ],
+      { root: [{ kind: "node", ref: "bodyonly" }] },
+    );
+
+    const block = composeNodeIndex("root", tree, pageIndex([]));
+
+    expect(block).toBe("[node:bodyonly] First real line");
+  });
+
+  test("node child with empty summary string falls back to body line", () => {
+    const tree = treeIndex(
+      [
+        treeNode("root", {}),
+        treeNode("blank", { summary: "   ", body: "fallback line" }),
+      ],
+      { root: [{ kind: "node", ref: "blank" }] },
+    );
+
+    expect(composeNodeIndex("root", tree, pageIndex([]))).toBe(
+      "[node:blank] fallback line",
+    );
+  });
+
+  test("node child with neither summary nor body emits only its header", () => {
+    const tree = treeIndex(
+      [treeNode("root", {}), treeNode("empty", { body: "   \n\t" })],
+      { root: [{ kind: "node", ref: "empty" }] },
+    );
+
+    expect(composeNodeIndex("root", tree, pageIndex([]))).toBe("[node:empty]");
+  });
+});
diff --git a/assistant/src/memory/v3/index-composition.ts b/assistant/src/memory/v3/index-composition.ts
new file mode 100644
index 00000000000..e1c16e3da8c
--- /dev/null
+++ b/assistant/src/memory/v3/index-composition.ts
@@ -0,0 +1,113 @@
+/**
+ * Memory v3 — Compositional index rendering.
+ *
+ * A v3 tree node has no stored "index" of its own. Instead, a parent node's
+ * index is *composed at read time* by concatenating one description line per
+ * child (a `node:` sub-node's summary or a `page:` leaf's summary) plus a thin
+ * `Routing hints:` trailer drawn from the node's own frontmatter. Nothing here
+ * is persisted — the block is generated fresh every time a descent prompt needs
+ * it, so it always reflects the current state of the children.
+ *
+ * {@link composeNodeIndex} is a **pure function** over an already-built
+ * {@link TreeIndex} (from `tree-index.ts`) and {@link PageIndex} (from
+ * `../v2/page-index.ts`). It does no I/O: the tree walk / driver PR is
+ * responsible for building those indices and feeding them in.
+ *
+ * Resolution rules, per child ref of `nodeId` (in authored order):
+ *   - `kind:"node"` → look up the child in `tree.nodes`; emit
+ *     `"[node:<id>] <summary>"` where summary is the child's
+ *     `frontmatter.summary` if non-empty, else the first non-empty line of its
+ *     body. A node with neither still emits its header (`"[node:<id>]"`).
+ *   - `kind:"page"` → look up `pages.bySlug.get(ref)`; emit
+ *     `"[page:<slug>] <entry.summary>"`.
+ *   - Either lookup missing → emit nothing for that ref. Reporting dangling
+ *     refs is validation's job, not this renderer's.
+ *
+ * The node's own `routing_hints` (when present) are appended last under a
+ * `Routing hints:` trailer. A node with no resolvable children and no routing
+ * hints composes to the empty string.
+ */
+
+import type { PageIndex } from "../v2/page-index.js";
+import type { TreeIndex } from "./tree-index.js";
+import type { TreeNode } from "./types.js";
+
+/** Trailer label introducing a node's own routing hints. */
+const ROUTING_HINTS_LABEL = "Routing hints:";
+
+/**
+ * Resolve a node's display summary: its frontmatter `summary` if non-empty,
+ * otherwise the first non-empty line of its body, otherwise the empty string.
+ * Whitespace is trimmed so a leading blank line in the body never wins.
+ */
+function nodeSummary(node: TreeNode): string {
+  const summary = node.frontmatter.summary?.trim();
+  if (summary) return summary;
+  for (const line of node.body.split("\n")) {
+    const trimmed = line.trim();
+    if (trimmed) return trimmed;
+  }
+  return "";
+}
+
+/**
+ * Render one child ref into its index line, or `null` when the ref's target is
+ * absent from the supplied indices (validation owns reporting those).
+ *
+ * A resolvable `node:` child always yields a line — its header (`[node:<id>]`)
+ * with a trailing summary when one exists. A `page:` child yields
+ * `[page:<slug>] <summary>`; the v2 page index already truncates `summary`.
+ */
+function renderChild(
+  kind: "page" | "node",
+  ref: string,
+  tree: TreeIndex,
+  pages: PageIndex,
+): string | null {
+  if (kind === "node") {
+    const child = tree.nodes.get(ref);
+    if (!child) return null;
+    const summary = nodeSummary(child);
+    return summary ? `[node:${ref}] ${summary}` : `[node:${ref}]`;
+  }
+  const entry = pages.bySlug.get(ref);
+  if (!entry) return null;
+  return `[page:${ref}] ${entry.summary}`;
+}
+
+/**
+ * Compose the prompt-ready index block for `nodeId` from its children's
+ * descriptions plus the node's own routing hints.
+ *
+ * Pure and deterministic: children are emitted in authored order (the order
+ * `tree.childrenByNode` preserves from the node's `children` frontmatter), refs
+ * whose targets are absent are silently skipped, and the node's
+ * `routing_hints` (if present) are appended under a {@link ROUTING_HINTS_LABEL}
+ * trailer. A node with no entry in `childrenByNode`, no resolvable children,
+ * and no routing hints composes to the empty string.
+ *
+ * The result is a plain string with no trailing newline, suitable to drop
+ * directly into an LLM descent prompt.
+ */
+export function composeNodeIndex(
+  nodeId: string,
+  tree: TreeIndex,
+  pages: PageIndex,
+): string {
+  const blocks: string[] = [];
+
+  const childRefs = tree.childrenByNode.get(nodeId) ?? [];
+  for (const { kind, ref } of childRefs) {
+    const line = renderChild(kind, ref, tree, pages);
+    if (line !== null) blocks.push(line);
+  }
+
+  const routingHints = tree.nodes
+    .get(nodeId)
+    ?.frontmatter.routing_hints?.trim();
+  if (routingHints) {
+    blocks.push(`${ROUTING_HINTS_LABEL} ${routingHints}`);
+  }
+
+  return blocks.join("\n");
+}