diff --git a/packages/chess/src/modifiers/custom/library.test.ts b/packages/chess/src/modifiers/custom/library.test.ts
new file mode 100644
index 0000000..141cafe
--- /dev/null
+++ b/packages/chess/src/modifiers/custom/library.test.ts
@@ -0,0 +1,205 @@
+import { beforeAll, beforeEach, describe, expect, it } from "vitest";
+import { asCustomModifierId, type CustomModifierDescriptor } from "./types.js";
+import {
+  __test__,
+  loadCustomModifierLibrary,
+  removeFromCustomModifierLibrary,
+  saveToCustomModifierLibrary,
+  setCustomModifierStarred,
+} from "./library.js";
+
+// Same Map-backed Storage shim used by the T1 library tests
+// (see ../library.test.ts) — happy-dom's bundled localStorage doesn't
+// survive every destructuring pattern reliably across vitest workers.
+beforeAll(() => {
+  const store = new Map<string, string>();
+  const shim: Storage = {
+    get length() {
+      return store.size;
+    },
+    clear() {
+      store.clear();
+    },
+    getItem(k: string) {
+      return store.get(k) ?? null;
+    },
+    setItem(k: string, v: string) {
+      store.set(k, v);
+    },
+    removeItem(k: string) {
+      store.delete(k);
+    },
+    key(i: number) {
+      return [...store.keys()][i] ?? null;
+    },
+  };
+  Object.defineProperty(globalThis, "localStorage", {
+    configurable: true,
+    value: shim,
+  });
+});
+
+beforeEach(() => {
+  localStorage.clear();
+});
+
+function descriptor(
+  overrides: Partial<CustomModifierDescriptor> = {},
+): CustomModifierDescriptor {
+  return {
+    type: "data",
+    id: asCustomModifierId(`custom:${Math.random().toString(36).slice(2, 10)}`),
+    name: "Test",
+    description: "",
+    version: 1,
+    primitives: [],
+    targetAttrs: [],
+    uiForm: "primitive-composer",
+    source: "custom",
+    ...overrides,
+  };
+}
+
+describe("custom modifier library — basic round-trip", () => {
+  it("loadCustomModifierLibrary returns [] when storage is empty", () => {
+    expect(loadCustomModifierLibrary()).toEqual([]);
+  });
+
+  it("loadCustomModifierLibrary returns [] on malformed JSON", () => {
+    localStorage.setItem(__test__.STORAGE_KEY, "{not json[");
+    expect(loadCustomModifierLibrary()).toEqual([]);
+  });
+
+  it("save then load returns the entry", () => {
+    const desc = descriptor({ name: "Roundtrip" });
+    const saved = saveToCustomModifierLibrary(desc);
+    expect(saved.ok).toBe(true);
+
+    const loaded = loadCustomModifierLibrary();
+    expect(loaded).toHaveLength(1);
+    expect(loaded[0]!.id).toBe(desc.id);
+    expect(loaded[0]!.descriptor.name).toBe("Roundtrip");
+    expect(loaded[0]!.starred).toBe(false);
+    expect(typeof loaded[0]!.updatedAt).toBe("number");
+  });
+
+  it("auto-populates createdAt on first save when absent", () => {
+    const desc = descriptor();
+    expect(desc.createdAt).toBeUndefined();
+    const saved = saveToCustomModifierLibrary(desc);
+    expect(saved.ok).toBe(true);
+    if (!saved.ok) return;
+    expect(typeof saved.entry.descriptor.createdAt).toBe("number");
+  });
+
+  it("preserves createdAt on subsequent saves", async () => {
+    const id = asCustomModifierId("custom:stable");
+    const first = saveToCustomModifierLibrary(descriptor({ id, name: "A" }));
+    expect(first.ok).toBe(true);
+    if (!first.ok) return;
+    const originalCreatedAt = first.entry.descriptor.createdAt;
+    expect(originalCreatedAt).toBeDefined();
+
+    // Wait a tick so updatedAt definitely changes.
+    await new Promise((r) => setTimeout(r, 2));
+    const second = saveToCustomModifierLibrary(
+      descriptor({ id, name: "B" }),
+    );
+    expect(second.ok).toBe(true);
+    if (!second.ok) return;
+    expect(second.entry.descriptor.createdAt).toBe(originalCreatedAt);
+    expect(second.entry.descriptor.name).toBe("B");
+  });
+});
+
+describe("custom modifier library — update, remove, star", () => {
+  it("update by id replaces the descriptor in place", () => {
+    const id = asCustomModifierId("custom:update");
+    saveToCustomModifierLibrary(descriptor({ id, name: "v1" }));
+    saveToCustomModifierLibrary(descriptor({ id, name: "v2" }));
+
+    const loaded = loadCustomModifierLibrary();
+    expect(loaded).toHaveLength(1);
+    expect(loaded[0]!.descriptor.name).toBe("v2");
+  });
+
+  it("removeFromCustomModifierLibrary deletes by id and returns true", () => {
+    const id = asCustomModifierId("custom:remove");
+    saveToCustomModifierLibrary(descriptor({ id }));
+    expect(removeFromCustomModifierLibrary(id)).toBe(true);
+    expect(loadCustomModifierLibrary()).toEqual([]);
+  });
+
+  it("removeFromCustomModifierLibrary returns false when id absent", () => {
+    expect(
+      removeFromCustomModifierLibrary(asCustomModifierId("custom:nope")),
+    ).toBe(false);
+  });
+
+  it("setCustomModifierStarred toggles the flag", () => {
+    const id = asCustomModifierId("custom:star");
+    saveToCustomModifierLibrary(descriptor({ id }));
+    setCustomModifierStarred(id, true);
+    expect(loadCustomModifierLibrary()[0]!.starred).toBe(true);
+    setCustomModifierStarred(id, false);
+    expect(loadCustomModifierLibrary()[0]!.starred).toBe(false);
+  });
+});
+
+describe("custom modifier library — capacity and validation", () => {
+  it("rejects descriptors that fail T19 validation", () => {
+    // Empty name fails the validator's name-length rule.
+    const bad = descriptor({ name: "" });
+    const result = saveToCustomModifierLibrary(bad);
+    expect(result.ok).toBe(false);
+    if (result.ok) return;
+    expect(result.errors).toBeDefined();
+    expect(result.errors!.some((e) => e.code === "descriptor.name.length")).toBe(
+      true,
+    );
+  });
+
+  it("evicts the oldest non-starred entry when at capacity", async () => {
+    // Fill with MAX_ENTRIES non-starred entries with strictly-increasing
+    // updatedAt timestamps.
+    for (let i = 0; i < __test__.MAX_ENTRIES; i += 1) {
+      saveToCustomModifierLibrary(
+        descriptor({
+          id: asCustomModifierId(`custom:fill-${i}`),
+          name: `fill-${i}`,
+        }),
+      );
+      // Tiny stagger so updatedAt timestamps differ.
+      await new Promise((r) => setTimeout(r, 1));
+    }
+    expect(loadCustomModifierLibrary()).toHaveLength(__test__.MAX_ENTRIES);
+
+    // Add one more — the oldest (fill-0) should be evicted.
+    const overflow = descriptor({
+      id: asCustomModifierId("custom:overflow"),
+      name: "overflow",
+    });
+    const result = saveToCustomModifierLibrary(overflow);
+    expect(result.ok).toBe(true);
+
+    const loaded = loadCustomModifierLibrary();
+    expect(loaded).toHaveLength(__test__.MAX_ENTRIES);
+    expect(loaded.find((e) => e.descriptor.name === "fill-0")).toBeUndefined();
+    expect(loaded.find((e) => e.descriptor.name === "overflow")).toBeDefined();
+  });
+
+  it("refuses save when the library is full of starred entries", async () => {
+    for (let i = 0; i < __test__.MAX_ENTRIES; i += 1) {
+      const id = asCustomModifierId(`custom:starred-${i}`);
+      saveToCustomModifierLibrary(descriptor({ id, name: `s-${i}` }));
+      setCustomModifierStarred(id, true);
+      await new Promise((r) => setTimeout(r, 1));
+    }
+    const result = saveToCustomModifierLibrary(
+      descriptor({ id: asCustomModifierId("custom:rejected") }),
+    );
+    expect(result.ok).toBe(false);
+    if (result.ok) return;
+    expect(result.reason).toMatch(/library full/i);
+  });
+});
diff --git a/packages/chess/src/modifiers/custom/library.ts b/packages/chess/src/modifiers/custom/library.ts
new file mode 100644
index 0000000..c8e4de1
--- /dev/null
+++ b/packages/chess/src/modifiers/custom/library.ts
@@ -0,0 +1,189 @@
+/**
+ * Custom modifier library — localStorage-backed store of user-authored
+ * custom modifier descriptors (T21).
+ *
+ * Mirrors the T1 modifier-profile library at `../library.ts`:
+ *   - Same MAX_ENTRIES (20) capacity rule with starred-aware FIFO eviction.
+ *   - Same `loadLibrary` / `saveToLibrary` / `deleteFromLibrary` /
+ *     `setStarred` / `duplicateEntry` API surface.
+ *   - Different storage key: `houserules:custom-modifiers:v1`.
+ *
+ * On `saveToLibrary` we run T19's validator (`validateCustomDescriptor`)
+ * before persisting. Invalid descriptors are rejected with the validator's
+ * error list so the caller can surface inline editor feedback. The library
+ * never persists a structurally-broken descriptor.
+ *
+ * `createdAt` on the descriptor is auto-populated on first save (when the
+ * descriptor doesn't already carry one); subsequent saves preserve the
+ * original timestamp so library order stays stable.
+ */
+import {
+  parseCustomModifierDescriptor,
+  safeParseCustomModifierDescriptor,
+} from "./schema.js";
+import type { CustomModifierDescriptor, CustomModifierId } from "./types.js";
+import {
+  validateCustomDescriptor,
+  type ValidationError,
+} from "./validate.js";
+
+const STORAGE_KEY = "houserules:custom-modifiers:v1";
+const MAX_ENTRIES = 20;
+
+export interface SavedCustomModifier {
+  readonly id: CustomModifierId;
+  readonly descriptor: CustomModifierDescriptor;
+  readonly starred: boolean;
+  readonly updatedAt: number;
+}
+
+export type SaveResult =
+  | { ok: true; entry: SavedCustomModifier }
+  | { ok: false; reason: string; errors?: ValidationError[] };
+
+/**
+ * Read every saved custom modifier from storage. Empty/missing/corrupt
+ * → `[]`. Per-entry shape failures are silently dropped so a single
+ * bad row never blocks the whole library.
+ */
+export function loadCustomModifierLibrary(): SavedCustomModifier[] {
+  try {
+    const raw = localStorage.getItem(STORAGE_KEY);
+    if (raw === null) return [];
+    const parsed = JSON.parse(raw) as unknown;
+    if (!Array.isArray(parsed)) return [];
+    return parsed.filter(isSavedCustomModifier);
+  } catch {
+    return [];
+  }
+}
+
+/** Replace the entire library array on disk. Best-effort on quota error. */
+function writeLibrary(entries: readonly SavedCustomModifier[]): void {
+  try {
+    localStorage.setItem(STORAGE_KEY, JSON.stringify(entries));
+  } catch {
+    /* quota exceeded / storage disabled — best effort */
+  }
+}
+
+/**
+ * Save a new descriptor or update an existing one (matched by descriptor
+ * id). Runs T19's structural+semantic validator first; rejects with
+ * the error list on failure. Auto-populates `descriptor.createdAt` on
+ * first save and preserves it on subsequent saves.
+ *
+ * Capacity: when adding a NEW descriptor would exceed MAX_ENTRIES, the
+ * oldest non-starred entry is evicted. If every entry is starred, the
+ * save is refused with a human-friendly reason.
+ */
+export function saveToCustomModifierLibrary(
+  descriptor: CustomModifierDescriptor,
+): SaveResult {
+  const validation = validateCustomDescriptor(descriptor);
+  if (!validation.ok) {
+    return {
+      ok: false,
+      reason: "Descriptor failed validation",
+      errors: validation.errors,
+    };
+  }
+
+  const library = loadCustomModifierLibrary();
+  const existingIdx = library.findIndex((e) => e.id === descriptor.id);
+  const now = Date.now();
+
+  // Preserve the original createdAt if this is an update; populate it
+  // for first-time saves.
+  const existingCreatedAt =
+    existingIdx >= 0 ? library[existingIdx]?.descriptor.createdAt : undefined;
+  const createdAt =
+    descriptor.createdAt ?? existingCreatedAt ?? now;
+  const stampedDescriptor: CustomModifierDescriptor = {
+    ...descriptor,
+    createdAt,
+  };
+
+  const entry: SavedCustomModifier = {
+    id: descriptor.id,
+    descriptor: stampedDescriptor,
+    starred: existingIdx >= 0 ? library[existingIdx]!.starred : false,
+    updatedAt: now,
+  };
+
+  if (existingIdx >= 0) {
+    const updated = library.slice();
+    updated[existingIdx] = entry;
+    writeLibrary(updated);
+    return { ok: true, entry };
+  }
+
+  if (library.length >= MAX_ENTRIES) {
+    const evictable = library
+      .filter((e) => !e.starred)
+      .sort((a, b) => a.updatedAt - b.updatedAt);
+    if (evictable.length === 0) {
+      return {
+        ok: false,
+        reason:
+          "Library full (20 custom modifiers). Unstar one to make room, or delete an entry.",
+      };
+    }
+    const oldest = evictable[0]!;
+    const pruned = library.filter((e) => e.id !== oldest.id);
+    pruned.push(entry);
+    writeLibrary(pruned);
+    return { ok: true, entry };
+  }
+
+  writeLibrary([...library, entry]);
+  return { ok: true, entry };
+}
+
+/** Remove a custom modifier by id. No-op if the id is unknown. Returns true if removed. */
+export function removeFromCustomModifierLibrary(
+  id: CustomModifierId,
+): boolean {
+  const library = loadCustomModifierLibrary();
+  const next = library.filter((e) => e.id !== id);
+  if (next.length === library.length) return false;
+  writeLibrary(next);
+  return true;
+}
+
+/** Toggle the starred flag on a saved custom modifier. */
+export function setCustomModifierStarred(
+  id: CustomModifierId,
+  starred: boolean,
+): void {
+  const library = loadCustomModifierLibrary();
+  const idx = library.findIndex((e) => e.id === id);
+  if (idx < 0) return;
+  const updated = library.slice();
+  updated[idx] = { ...library[idx]!, starred, updatedAt: Date.now() };
+  writeLibrary(updated);
+}
+
+// ── Internal helpers ──────────────────────────────────────────────────
+
+function isSavedCustomModifier(value: unknown): value is SavedCustomModifier {
+  if (typeof value !== "object" || value === null) return false;
+  const v = value as Record<string, unknown>;
+  if (
+    typeof v["id"] !== "string" ||
+    typeof v["starred"] !== "boolean" ||
+    typeof v["updatedAt"] !== "number"
+  ) {
+    return false;
+  }
+  // Validate the nested descriptor structurally via Zod.
+  const parseResult = safeParseCustomModifierDescriptor(v["descriptor"]);
+  if (!parseResult.success) return false;
+  // Discard the parse output — we keep the original raw shape so the
+  // entry round-trips identically (createdAt preserved, etc.).
+  void parseCustomModifierDescriptor;
+  return true;
+}
+
+// Exported for tests only.
+export const __test__ = { STORAGE_KEY, MAX_ENTRIES };