diff --git a/packages/json-cas-fs/src/store.test.ts b/packages/json-cas-fs/src/store.test.ts
index 00e6da5..5aeefca 100644
--- a/packages/json-cas-fs/src/store.test.ts
+++ b/packages/json-cas-fs/src/store.test.ts
@@ -1,5 +1,5 @@
 import { afterEach, beforeEach, describe, expect, test } from "bun:test";
-import { mkdtempSync, rmSync } from "node:fs";
+import { existsSync, mkdtempSync, readdirSync, rmSync } from "node:fs";
 import { tmpdir } from "node:os";
 import { join } from "node:path";
 import type { CasNode } from "@uncaged/json-cas";
@@ -177,6 +177,87 @@ describe("createFsStore – has and list", () => {
   });
 });
 
+// ──────────────────────────────────────────────────────────────────────────────
+// listByType and index migration
+// ──────────────────────────────────────────────────────────────────────────────
+describe("createFsStore – listByType", () => {
+  let dir: string;
+  beforeEach(() => {
+    dir = makeTmpDir();
+  });
+  afterEach(() => {
+    rmSync(dir, { recursive: true, force: true });
+  });
+
+  test("returns empty array for unknown type", () => {
+    const store = createFsStore(dir);
+    expect(store.listByType("0000000000000")).toEqual([]);
+  });
+
+  test("returns all hashes for the given type", async () => {
+    const store = createFsStore(dir);
+    const typeHash = await computeSelfHash({ name: "t" });
+    const otherType = await computeSelfHash({ name: "other" });
+
+    const h1 = await store.put(typeHash, { a: 1 });
+    const h2 = await store.put(typeHash, { a: 2 });
+    await store.put(otherType, { b: 1 });
+
+    const byType = store.listByType(typeHash);
+    expect(byType).toHaveLength(2);
+    expect(byType).toContain(h1);
+    expect(byType).toContain(h2);
+  });
+
+  test("listByType survives round-trip across store instances", async () => {
+    const typeHash = await computeSelfHash({ name: "persist-by-type" });
+
+    const store1 = createFsStore(dir);
+    const h1 = await store1.put(typeHash, { x: 1 });
+    const h2 = await store1.put(typeHash, { x: 2 });
+
+    const store2 = createFsStore(dir);
+    const byType = store2.listByType(typeHash);
+    expect(byType).toHaveLength(2);
+    expect(byType).toContain(h1);
+    expect(byType).toContain(h2);
+  });
+
+  test("idempotent put does not duplicate in listByType", async () => {
+    const typeHash = await computeSelfHash({ name: "idempotent-index" });
+
+    const store1 = createFsStore(dir);
+    const hash = await store1.put(typeHash, { n: 7 });
+    await store1.put(typeHash, { n: 7 });
+
+    const store2 = createFsStore(dir);
+    expect(store2.listByType(typeHash)).toEqual([hash]);
+  });
+
+  test("rebuilds _index from .bin files when index is missing", async () => {
+    const typeHash = await computeSelfHash({ name: "migrate" });
+
+    const store1 = createFsStore(dir);
+    const h1 = await store1.put(typeHash, { a: 1 });
+    const h2 = await store1.put(typeHash, { a: 2 });
+
+    rmSync(join(dir, "_index"), { recursive: true, force: true });
+
+    const store2 = createFsStore(dir);
+    expect(store2.listByType(typeHash)).toEqual([h1, h2]);
+    expect(existsSync(join(dir, "_index", typeHash))).toBe(true);
+    expect(readdirSync(join(dir, "_index"))).toContain(typeHash);
+  });
+
+  test("bootstrap node is listed under its self type after reload", async () => {
+    const store1 = createFsStore(dir);
+    const hash = await bootstrap(store1);
+
+    const store2 = createFsStore(dir);
+    expect(store2.listByType(hash)).toEqual([hash]);
+  });
+});
+
 // ──────────────────────────────────────────────────────────────────────────────
 // verify on disk-loaded nodes
 // ──────────────────────────────────────────────────────────────────────────────
diff --git a/packages/json-cas-fs/src/store.ts b/packages/json-cas-fs/src/store.ts
index 2f20ea1..af6bfc9 100644
--- a/packages/json-cas-fs/src/store.ts
+++ b/packages/json-cas-fs/src/store.ts
@@ -1,4 +1,6 @@
 import {
+  appendFileSync,
+  existsSync,
   mkdirSync,
   readdirSync,
   readFileSync,
@@ -11,6 +13,8 @@ import type { CasNode, Hash, Store } from "@uncaged/json-cas";
 import { cborEncode, computeHash, computeSelfHash } from "@uncaged/json-cas";
 import { decode } from "cborg";
 
+const INDEX_DIR = "_index";
+
 function loadDir(dir: string, data: Map<Hash, CasNode>): void {
   let entries: string[];
   try {
@@ -31,9 +35,81 @@ function loadDir(dir: string, data: Map<Hash, CasNode>): void {
   }
 }
 
+function parseIndexFile(content: string): Hash[] {
+  if (content.length === 0) return [];
+  return content.split("\n").filter((line) => line.length > 0) as Hash[];
+}
+
+function loadTypeIndex(indexDir: string): Map<Hash, Hash[]> {
+  const typeIndex = new Map<Hash, Hash[]>();
+  let entries: string[];
+  try {
+    entries = readdirSync(indexDir);
+  } catch {
+    return typeIndex;
+  }
+  for (const typeHash of entries) {
+    try {
+      const content = readFileSync(join(indexDir, typeHash), "utf8");
+      typeIndex.set(typeHash as Hash, parseIndexFile(content));
+    } catch {
+      // skip unreadable index files
+    }
+  }
+  return typeIndex;
+}
+
+function buildTypeIndexFromNodes(data: Map<Hash, CasNode>): Map<Hash, Hash[]> {
+  const typeIndex = new Map<Hash, Hash[]>();
+  for (const [hash, node] of data) {
+    const list = typeIndex.get(node.type) ?? [];
+    list.push(hash);
+    typeIndex.set(node.type, list);
+  }
+  return typeIndex;
+}
+
+function writeTypeIndex(indexDir: string, typeIndex: Map<Hash, Hash[]>): void {
+  mkdirSync(indexDir, { recursive: true });
+  for (const [typeHash, hashes] of typeIndex) {
+    const body = hashes.length > 0 ? `${hashes.join("\n")}\n` : "";
+    writeFileSync(join(indexDir, typeHash), body, "utf8");
+  }
+}
+
+function loadOrMigrateTypeIndex(
+  dir: string,
+  data: Map<Hash, CasNode>,
+): Map<Hash, Hash[]> {
+  const indexDir = join(dir, INDEX_DIR);
+  if (!existsSync(indexDir)) {
+    const typeIndex = buildTypeIndexFromNodes(data);
+    if (typeIndex.size > 0) {
+      writeTypeIndex(indexDir, typeIndex);
+    }
+    return typeIndex;
+  }
+  return loadTypeIndex(indexDir);
+}
+
+function appendToTypeIndex(
+  indexDir: string,
+  typeIndex: Map<Hash, Hash[]>,
+  type: Hash,
+  hash: Hash,
+): void {
+  mkdirSync(indexDir, { recursive: true });
+  appendFileSync(join(indexDir, type), `${hash}\n`, "utf8");
+  const list = typeIndex.get(type) ?? [];
+  list.push(hash);
+  typeIndex.set(type, list);
+}
+
 export function createFsStore(dir: string): Store {
   const data = new Map<Hash, CasNode>();
   loadDir(dir, data);
+  const indexDir = join(dir, INDEX_DIR);
+  const typeIndex = loadOrMigrateTypeIndex(dir, data);
 
   return {
     async put(typeHash: Hash | null, payload: unknown): Promise<Hash> {
@@ -55,6 +131,8 @@ export function createFsStore(dir: string): Store {
           cborEncode({ type, payload, timestamp: node.timestamp }),
         );
         renameSync(tmp, dest);
+
+        appendToTypeIndex(indexDir, typeIndex, type, hash);
       }
 
       return hash;
@@ -71,5 +149,9 @@ export function createFsStore(dir: string): Store {
     list(): Hash[] {
       return [...data.keys()];
     },
+
+    listByType(typeHash: Hash): Hash[] {
+      return typeIndex.get(typeHash) ?? [];
+    },
   };
 }
diff --git a/packages/json-cas/src/index.test.ts b/packages/json-cas/src/index.test.ts
index bb6b2dc..fbcd248 100644
--- a/packages/json-cas/src/index.test.ts
+++ b/packages/json-cas/src/index.test.ts
@@ -150,6 +150,48 @@ describe("createMemoryStore – has and list", () => {
   });
 });
 
+// ──────────────────────────────────────────────────────────────────────────────
+// Step 4b: store.listByType()
+// ──────────────────────────────────────────────────────────────────────────────
+describe("createMemoryStore – listByType", () => {
+  test("returns empty array for unknown type", () => {
+    const store = createMemoryStore();
+    expect(store.listByType("0000000000000")).toEqual([]);
+  });
+
+  test("returns all hashes for the given type", async () => {
+    const store = createMemoryStore();
+    const typeHash = await computeSelfHash({ name: "t" });
+    const otherType = await computeSelfHash({ name: "other" });
+
+    const h1 = await store.put(typeHash, { a: 1 });
+    const h2 = await store.put(typeHash, { a: 2 });
+    await store.put(otherType, { b: 1 });
+
+    const byType = store.listByType(typeHash);
+    expect(byType).toHaveLength(2);
+    expect(byType).toContain(h1);
+    expect(byType).toContain(h2);
+  });
+
+  test("idempotent put does not duplicate in listByType", async () => {
+    const store = createMemoryStore();
+    const typeHash = await computeSelfHash({ name: "t" });
+
+    const h1 = await store.put(typeHash, { n: 1 });
+    await store.put(typeHash, { n: 1 });
+
+    expect(store.listByType(typeHash)).toEqual([h1]);
+  });
+
+  test("bootstrap node is listed under its self type", async () => {
+    const store = createMemoryStore();
+    const hash = await bootstrap(store);
+
+    expect(store.listByType(hash)).toEqual([hash]);
+  });
+});
+
 // ──────────────────────────────────────────────────────────────────────────────
 // Step 5: verify()
 // ──────────────────────────────────────────────────────────────────────────────
diff --git a/packages/json-cas/src/store.ts b/packages/json-cas/src/store.ts
index ac20f4a..417eb35 100644
--- a/packages/json-cas/src/store.ts
+++ b/packages/json-cas/src/store.ts
@@ -3,6 +3,16 @@ import type { CasNode, Hash, Store } from "./types.js";
 
 export function createMemoryStore(): Store {
   const data = new Map<Hash, CasNode>();
+  const byType = new Map<Hash, Set<Hash>>();
+
+  function indexHash(type: Hash, hash: Hash): void {
+    let set = byType.get(type);
+    if (!set) {
+      set = new Set();
+      byType.set(type, set);
+    }
+    set.add(hash);
+  }
 
   return {
     async put(typeHash: Hash | null, payload: unknown): Promise<Hash> {
@@ -14,6 +24,7 @@ export function createMemoryStore(): Store {
       if (!data.has(hash)) {
         const type = typeHash === null ? hash : typeHash;
         data.set(hash, { type, payload, timestamp: Date.now() });
+        indexHash(type, hash);
       }
 
       return hash;
@@ -30,5 +41,10 @@ export function createMemoryStore(): Store {
     list(): Hash[] {
       return [...data.keys()];
     },
+
+    listByType(typeHash: Hash): Hash[] {
+      const set = byType.get(typeHash);
+      return set ? [...set] : [];
+    },
   };
 }
diff --git a/packages/json-cas/src/types.ts b/packages/json-cas/src/types.ts
index 28543b5..44afc65 100644
--- a/packages/json-cas/src/types.ts
+++ b/packages/json-cas/src/types.ts
@@ -24,4 +24,5 @@ export type Store = {
   get(hash: Hash): CasNode | null;
   has(hash: Hash): boolean;
   list(): Hash[];
+  listByType(typeHash: Hash): Hash[];
 };