diff --git a/packages/server/src/broadcast.ts b/packages/server/src/broadcast.ts
index 0a5665a..d8412c4 100644
--- a/packages/server/src/broadcast.ts
+++ b/packages/server/src/broadcast.ts
@@ -29,6 +29,7 @@ import { RateLimiter } from "./middleware.js";
 import {
   PROTOCOL_VERSION,
   validateMessageString,
+  type CustomModifierRegisterPayload,
   type ErrorCode,
   type Fact as WireFact,
   type GameMovePayload,
@@ -273,6 +274,9 @@ export function handleMessage(
     case "modifier-profile.consent":
       handleModifierProfileConsent(ws, msg.payload);
       break;
+    case "custom-modifier.register":
+      handleCustomModifierRegister(ws, msg.payload);
+      break;
     case "room.created":
     case "room.joined":
     case "game.state":
@@ -284,6 +288,7 @@ export function handleMessage(
     case "modifier-profile.proposal-pending":
     case "modifier-profile.rejected":
     case "modifier-profile.consent-received":
+    case "custom-modifier.registered":
     case "error":
       sendTo(
         ws,
@@ -1451,6 +1456,94 @@ function bufferDeltaForDisconnected(
   }
 }
 
+// ---------------------------------------------------------------------------
+// T3 — custom-modifier.register handler
+// ---------------------------------------------------------------------------
+
+/** Maximum custom modifier descriptors per room (T3 DoS guard). */
+const CUSTOM_MODIFIER_ROOM_CAP = 10;
+
+/**
+ * Register a user-authored custom modifier descriptor on this room.
+ * Structural validation already happened via `validateMessageString`
+ * before dispatch — here we only enforce the per-room capacity,
+ * store into the room's `customModifiers` map (creating it on first
+ * call), and broadcast to every connected client so opponents'
+ * engines mirror the same custom registry.
+ *
+ * Updates are idempotent keyed on descriptor id: re-registering with
+ * the same id REPLACES the existing entry and broadcasts the new
+ * shape; the capacity check counts distinct ids only.
+ */
+function handleCustomModifierRegister(
+  ws: ServerWebSocket<ClientData>,
+  payload: CustomModifierRegisterPayload,
+): void {
+  const { roomCode, token } = ws.data;
+  if (roomCode === undefined || token === undefined) {
+    sendTo(
+      ws,
+      errorMessage("BAD_TOKEN", "not authenticated into a room", false),
+    );
+    return;
+  }
+
+  if (payload.roomCode !== roomCode) {
+    sendTo(
+      ws,
+      errorMessage(
+        "BAD_TOKEN",
+        "custom-modifier.register roomCode does not match authenticated room",
+        false,
+      ),
+    );
+    return;
+  }
+
+  const room = roomRegistry.getRoom(roomCode);
+  if (!room) {
+    sendTo(
+      ws,
+      errorMessage("ROOM_NOT_FOUND", `room ${roomCode} not found`, false),
+    );
+    return;
+  }
+
+  // Lazily create the per-room custom registry.
+  if (room.customModifiers === undefined) {
+    room.customModifiers = new Map();
+  }
+
+  const existing = room.customModifiers.has(payload.descriptor.id);
+  if (!existing && room.customModifiers.size >= CUSTOM_MODIFIER_ROOM_CAP) {
+    sendTo(
+      ws,
+      errorMessage(
+        "CUSTOM_MODIFIER_LIMIT",
+        `custom modifier cap (${CUSTOM_MODIFIER_ROOM_CAP}) reached for room ${roomCode}`,
+        false,
+      ),
+    );
+    return;
+  }
+
+  room.customModifiers.set(payload.descriptor.id, payload.descriptor);
+
+  logger
+    .child({ roomCode, clientId: ws.data.clientId })
+    .info({ descriptorId: payload.descriptor.id }, "custom-modifier.register");
+
+  // Broadcast to every client (proposer included — confirms the
+  // server-authoritative state matches what was sent).
+  broadcastToRoom(
+    roomCode,
+    envelope("custom-modifier.registered", {
+      roomCode,
+      descriptor: payload.descriptor,
+    }),
+  );
+}
+
 /**
  * Broadcast a game.end to everyone in `code` EXCEPT the player whose
  * token is `leaverToken`. Used when a player disconnects or leaves
diff --git a/packages/server/src/protocol.ts b/packages/server/src/protocol.ts
index 7f5dfc2..d9b97f6 100644
--- a/packages/server/src/protocol.ts
+++ b/packages/server/src/protocol.ts
@@ -63,6 +63,9 @@ export const ErrorCodeSchema = z.enum([
   "MODIFIER_PROFILE_NO_KING",
   "MODIFIER_PROFILE_INVULN_KING",
   "MODIFIER_PROFILE_DEADLOCK",
+  // T3 custom modifier registration failures.
+  "CUSTOM_MODIFIER_INVALID",
+  "CUSTOM_MODIFIER_LIMIT",
 ]);
 export type ErrorCode = z.infer<typeof ErrorCodeSchema>;
 
@@ -73,6 +76,8 @@ export const MODIFIER_PROFILE_INVALID = "MODIFIER_PROFILE_INVALID" as const;
 export const MODIFIER_PROFILE_NO_KING = "MODIFIER_PROFILE_NO_KING" as const;
 export const MODIFIER_PROFILE_INVULN_KING = "MODIFIER_PROFILE_INVULN_KING" as const;
 export const MODIFIER_PROFILE_DEADLOCK = "MODIFIER_PROFILE_DEADLOCK" as const;
+export const CUSTOM_MODIFIER_INVALID = "CUSTOM_MODIFIER_INVALID" as const;
+export const CUSTOM_MODIFIER_LIMIT = "CUSTOM_MODIFIER_LIMIT" as const;
 
 export const GameEndReasonSchema = z.enum([
   "checkmate",
@@ -453,6 +458,87 @@ export type ModifierProfileConsentReceivedPayload = z.infer<
   typeof ModifierProfileConsentReceivedPayloadSchema
 >;
 
+// ---------------------------------------------------------------------------
+// T3 — Custom modifier descriptor registration
+// ---------------------------------------------------------------------------
+//
+// Users compose custom modifier descriptors from the 15 engine-shipped
+// effect primitives (T3-ADR-2). Each descriptor is a structured JSON
+// document that travels over the wire as-is and registers into the
+// room's per-engine custom registry (T3-ADR-4) — never globally, so
+// two rooms can own descriptors with the same id without collision.
+//
+// The server performs STRUCTURAL validation here (Zod shape + count
+// caps). Semantic validation (primitive-kind-in-registry, paramsSchema
+// satisfaction, recursion depth ≤ 3) happens on the client via
+// `validateCustomDescriptor` before send — rejecting malformed
+// descriptors server-side would require mirroring the primitive
+// catalog across the Zod v3/v4 boundary and isn't worth the duplication.
+// Clients that skip their own validation get the engine's degradation
+// behaviour: unknown primitives are silently skipped at apply time.
+
+const EffectPrimitiveNodeWireSchema = z.lazy(() =>
+  z.object({
+    kind: z.string().min(1),
+    // z.unknown() is used rather than z.any() for parity with the client-side
+    // schema; Zod v3 infers `unknown` fields as optional at the type level,
+    // but runtime parsing accepts any value (including undefined).
+    params: z.unknown(),
+  }),
+);
+
+/** Wire shape for a user-authored custom modifier descriptor. */
+export const CustomModifierDescriptorSchema = z.object({
+  type: z.literal("data"),
+  id: z.string().min(1),
+  name: z.string().min(1).max(40),
+  description: z.string().max(200),
+  version: z.literal(1),
+  // DoS guard: cap the primitive array length. Deep nesting is the
+  // client validator's responsibility — the server sees flat arrays
+  // because nested children are inside `params`.
+  primitives: z.array(EffectPrimitiveNodeWireSchema).max(50),
+  targetAttrs: z.array(z.string()).max(32),
+  uiForm: z.literal("primitive-composer"),
+  source: z.literal("custom"),
+  author: z.string().max(80).optional(),
+  createdAt: z.number().int().nonnegative().optional(),
+});
+export type CustomModifierDescriptorWire = z.infer<
+  typeof CustomModifierDescriptorSchema
+>;
+
+/**
+ * Client → server: register a custom modifier descriptor on this
+ * room's per-engine custom registry. Server validates structurally
+ * and rejects with `CUSTOM_MODIFIER_INVALID` on shape failure or
+ * `CUSTOM_MODIFIER_LIMIT` when the room is at capacity (10 custom
+ * descriptors). On success broadcasts `custom-modifier.registered`
+ * to every connected client in the room so opponents' engines
+ * mirror the same custom registry.
+ */
+export const CustomModifierRegisterPayloadSchema = z.object({
+  roomCode: RoomCodeSchema,
+  descriptor: CustomModifierDescriptorSchema,
+});
+export type CustomModifierRegisterPayload = z.infer<
+  typeof CustomModifierRegisterPayloadSchema
+>;
+
+/**
+ * Server → all clients in room: a new custom modifier descriptor is
+ * available for reference in subsequent ModifierProfile entries.
+ * Clients register the descriptor into their local engine's
+ * `customModifiers` registry on receipt.
+ */
+export const CustomModifierRegisteredPayloadSchema = z.object({
+  roomCode: RoomCodeSchema,
+  descriptor: CustomModifierDescriptorSchema,
+});
+export type CustomModifierRegisteredPayload = z.infer<
+  typeof CustomModifierRegisteredPayloadSchema
+>;
+
 export const RoomJoinPayloadSchema = z.object({
   code: RoomCodeSchema,
 });
@@ -631,6 +717,10 @@ export const ModifierProfileConsentMessageSchema = msg(
   "modifier-profile.consent",
   ModifierProfileConsentPayloadSchema,
 );
+export const CustomModifierRegisterMessageSchema = msg(
+  "custom-modifier.register",
+  CustomModifierRegisterPayloadSchema,
+);
 
 export const ClientMessageSchema = z.discriminatedUnion("type", [
   RoomCreateMessageSchema,
@@ -641,6 +731,7 @@ export const ClientMessageSchema = z.discriminatedUnion("type", [
   ModifierProfileUpdateMessageSchema,
   ModifierProfileProposeMessageSchema,
   ModifierProfileConsentMessageSchema,
+  CustomModifierRegisterMessageSchema,
 ]);
 export type ClientMessage = z.infer<typeof ClientMessageSchema>;
 
@@ -679,6 +770,10 @@ export const ModifierProfileConsentReceivedMessageSchema = msg(
   "modifier-profile.consent-received",
   ModifierProfileConsentReceivedPayloadSchema,
 );
+export const CustomModifierRegisteredMessageSchema = msg(
+  "custom-modifier.registered",
+  CustomModifierRegisteredPayloadSchema,
+);
 export const ErrorMessageSchema = msg("error", ErrorPayloadSchema);
 
 export const ServerMessageSchema = z.discriminatedUnion("type", [
@@ -693,6 +788,7 @@ export const ServerMessageSchema = z.discriminatedUnion("type", [
   ModifierProfileProposalPendingMessageSchema,
   ModifierProfileRejectedMessageSchema,
   ModifierProfileConsentReceivedMessageSchema,
+  CustomModifierRegisteredMessageSchema,
   ErrorMessageSchema,
 ]);
 export type ServerMessage = z.infer<typeof ServerMessageSchema>;
@@ -706,6 +802,7 @@ export const AnyMessageSchema = z.discriminatedUnion("type", [
   ModifierProfileUpdateMessageSchema,
   ModifierProfileProposeMessageSchema,
   ModifierProfileConsentMessageSchema,
+  CustomModifierRegisterMessageSchema,
   RoomCreatedMessageSchema,
   RoomJoinedMessageSchema,
   GameStateMessageSchema,
@@ -717,6 +814,7 @@ export const AnyMessageSchema = z.discriminatedUnion("type", [
   ModifierProfileProposalPendingMessageSchema,
   ModifierProfileRejectedMessageSchema,
   ModifierProfileConsentReceivedMessageSchema,
+  CustomModifierRegisteredMessageSchema,
   ErrorMessageSchema,
 ]);
 export type AnyMessage = z.infer<typeof AnyMessageSchema>;
@@ -730,6 +828,7 @@ export const KNOWN_MESSAGE_TYPES = [
   "modifier-profile.update",
   "modifier-profile.propose",
   "modifier-profile.consent",
+  "custom-modifier.register",
   "room.created",
   "room.joined",
   "game.state",
@@ -741,6 +840,7 @@ export const KNOWN_MESSAGE_TYPES = [
   "modifier-profile.proposal-pending",
   "modifier-profile.rejected",
   "modifier-profile.consent-received",
+  "custom-modifier.registered",
   "error",
 ] as const;
 export type MessageType = (typeof KNOWN_MESSAGE_TYPES)[number];
diff --git a/packages/server/src/rooms.ts b/packages/server/src/rooms.ts
index b8dbbe9..0b7655c 100644
--- a/packages/server/src/rooms.ts
+++ b/packages/server/src/rooms.ts
@@ -4,7 +4,10 @@
 // six-character upper-case alphanumeric strings ([A-Z0-9]) drawn from a CSPRNG.
 // Nothing here persists across restarts — PROTOCOL.md §Auth & Rooms mandates
 // in-memory-only storage.
-import type { Color } from "./protocol.js";
+import type {
+  Color,
+  CustomModifierDescriptorWire,
+} from "./protocol.js";
 import {
   CLASSIC_LAYOUT,
   type ModifierProfile,
@@ -44,6 +47,14 @@ export interface Room {
    * and the server replays it onto reconstructed engines.
    */
   profile?: ModifierProfile;
+  /**
+   * T3: user-authored custom modifier descriptors registered into
+   * this room via `custom-modifier.register`. Keyed by descriptor id.
+   * Scoped to the room so per-room isolation (ADR-4) holds across
+   * server restarts — a room that dies and is re-created is a new
+   * room, not a resumption. Capped at 10 entries (T3 DoS guard).
+   */
+  customModifiers?: Map<string, CustomModifierDescriptorWire>;
   /**
    * Queued profile swap awaiting the next turn boundary (T2-ADR-1).
    *
diff --git a/packages/server/src/ws.custom-modifier-register.test.ts b/packages/server/src/ws.custom-modifier-register.test.ts
new file mode 100644
index 0000000..86c2f02
--- /dev/null
+++ b/packages/server/src/ws.custom-modifier-register.test.ts
@@ -0,0 +1,278 @@
+/**
+ * T3 integration tests: `custom-modifier.register` WebSocket handler.
+ *
+ * Covers:
+ *   1. Valid register → room customModifiers populated + broadcast fires.
+ *   2. Re-register (same id) → replace in place, still broadcasts.
+ *   3. 11th distinct descriptor → CUSTOM_MODIFIER_LIMIT rejection.
+ *   4. Room mismatch → BAD_TOKEN.
+ *   5. No auth → BAD_TOKEN.
+ *   6. Unknown room → ROOM_NOT_FOUND.
+ *   7. Malformed descriptor (validator catches it at schema layer).
+ */
+import type { ServerWebSocket } from "bun";
+import { describe, expect, it } from "vitest";
+
+import {
+  handleMessage,
+  registerConnection,
+  roomRegistry,
+  type ClientData,
+} from "./broadcast.js";
+import {
+  PROTOCOL_VERSION,
+  type ClientMessage,
+  type CustomModifierDescriptorWire,
+} from "./protocol.js";
+
+// ─── Mock ServerWebSocket (same harness as other WS tests) ───────────
+
+interface MockWs extends ServerWebSocket<ClientData> {
+  readonly sent: unknown[];
+  readonly closed: boolean;
+}
+
+function makeMockWs(clientId: string): MockWs {
+  const sent: unknown[] = [];
+  const closedFlag = { value: false };
+  const ws = {
+    data: { clientId } as ClientData,
+    sent,
+    get closed(): boolean {
+      return closedFlag.value;
+    },
+    send(msg: string | Buffer): number {
+      const str = typeof msg === "string" ? msg : msg.toString("utf8");
+      sent.push(JSON.parse(str));
+      return str.length;
+    },
+    close(): void {
+      closedFlag.value = true;
+    },
+  } as unknown as MockWs;
+  return ws;
+}
+
+function nextMsgOfType(
+  ws: MockWs,
+  type: string,
+): { payload: Record<string, unknown> } {
+  const idx = ws.sent.findIndex(
+    (m) =>
+      typeof m === "object" &&
+      m !== null &&
+      (m as { type: unknown }).type === type,
+  );
+  if (idx < 0) {
+    const types = ws.sent.map((m) => (m as { type?: string }).type);
+    throw new Error(
+      `no message of type "${type}" in inbox (got ${JSON.stringify(types)})`,
+    );
+  }
+  const msg = ws.sent[idx] as { payload: Record<string, unknown> };
+  ws.sent.splice(idx, 1);
+  return msg;
+}
+
+function sendClient(
+  ws: MockWs,
+  type: ClientMessage["type"],
+  payload: unknown,
+  seq = 1,
+): void {
+  handleMessage(
+    ws,
+    JSON.stringify({
+      v: PROTOCOL_VERSION,
+      seq,
+      ts: Date.now(),
+      type,
+      payload,
+    }),
+  );
+}
+
+// ─── Descriptor fixtures ─────────────────────────────────────────────
+
+function makeDescriptor(
+  id: string,
+  overrides: Partial<CustomModifierDescriptorWire> = {},
+): CustomModifierDescriptorWire {
+  return {
+    type: "data",
+    id,
+    name: id,
+    description: "",
+    version: 1,
+    primitives: [
+      { kind: "seed-attribute", params: { attr: "HpBonus", value: 1 } },
+    ],
+    targetAttrs: ["HpBonus"],
+    uiForm: "primitive-composer",
+    source: "custom",
+    ...overrides,
+  };
+}
+
+function setupRoom(): { white: MockWs; black: MockWs; code: string } {
+  const white = makeMockWs(`T3CM-W-${crypto.randomUUID()}`);
+  const black = makeMockWs(`T3CM-B-${crypto.randomUUID()}`);
+  registerConnection(white);
+  registerConnection(black);
+
+  sendClient(white, "room.create", {});
+  const created = nextMsgOfType(white, "room.created");
+  const code = created.payload["code"] as string;
+
+  sendClient(black, "room.join", { code });
+  nextMsgOfType(black, "room.joined");
+  // Drain any game.state broadcasts from both inboxes so subsequent
+  // assertions only see custom-modifier messages.
+  while (
+    white.sent.some(
+      (m) => (m as { type?: string }).type === "game.state",
+    )
+  ) {
+    nextMsgOfType(white, "game.state");
+  }
+  while (
+    black.sent.some(
+      (m) => (m as { type?: string }).type === "game.state",
+    )
+  ) {
+    nextMsgOfType(black, "game.state");
+  }
+
+  return { white, black, code };
+}
+
+// ─── Tests ───────────────────────────────────────────────────────────
+
+describe("custom-modifier.register WS handler (T24)", () => {
+  it("accepts a valid descriptor and broadcasts to both clients", () => {
+    const { white, black, code } = setupRoom();
+    const desc = makeDescriptor("custom:t3-accept");
+
+    sendClient(white, "custom-modifier.register", {
+      roomCode: code,
+      descriptor: desc,
+    });
+
+    // Both receive the broadcast.
+    const wMsg = nextMsgOfType(white, "custom-modifier.registered");
+    expect(wMsg.payload["roomCode"]).toBe(code);
+    expect((wMsg.payload["descriptor"] as { id: string }).id).toBe(
+      "custom:t3-accept",
+    );
+
+    const bMsg = nextMsgOfType(black, "custom-modifier.registered");
+    expect((bMsg.payload["descriptor"] as { id: string }).id).toBe(
+      "custom:t3-accept",
+    );
+
+    // Room state reflects the registration.
+    const room = roomRegistry.getRoom(code);
+    expect(room?.customModifiers?.get("custom:t3-accept")?.id).toBe(
+      "custom:t3-accept",
+    );
+  });
+
+  it("re-register with same id replaces the stored descriptor", () => {
+    const { white, black, code } = setupRoom();
+    const v1 = makeDescriptor("custom:t3-replace", { name: "v1" });
+    const v2 = makeDescriptor("custom:t3-replace", { name: "v2" });
+
+    sendClient(white, "custom-modifier.register", {
+      roomCode: code,
+      descriptor: v1,
+    });
+    nextMsgOfType(white, "custom-modifier.registered");
+    nextMsgOfType(black, "custom-modifier.registered");
+
+    sendClient(white, "custom-modifier.register", {
+      roomCode: code,
+      descriptor: v2,
+    });
+    const latest = nextMsgOfType(black, "custom-modifier.registered");
+    expect((latest.payload["descriptor"] as { name: string }).name).toBe("v2");
+
+    const room = roomRegistry.getRoom(code);
+    expect(room?.customModifiers?.size).toBe(1);
+    expect(room?.customModifiers?.get("custom:t3-replace")?.name).toBe("v2");
+  });
+
+  it("rejects the 11th distinct descriptor with CUSTOM_MODIFIER_LIMIT", () => {
+    const { white, code } = setupRoom();
+
+    for (let i = 0; i < 10; i += 1) {
+      sendClient(white, "custom-modifier.register", {
+        roomCode: code,
+        descriptor: makeDescriptor(`custom:t3-bulk-${i}`),
+      });
+      nextMsgOfType(white, "custom-modifier.registered");
+    }
+
+    sendClient(white, "custom-modifier.register", {
+      roomCode: code,
+      descriptor: makeDescriptor("custom:t3-overflow"),
+    });
+
+    const err = nextMsgOfType(white, "error");
+    expect(err.payload["code"]).toBe("CUSTOM_MODIFIER_LIMIT");
+
+    const room = roomRegistry.getRoom(code);
+    expect(room?.customModifiers?.size).toBe(10);
+    expect(room?.customModifiers?.has("custom:t3-overflow")).toBe(false);
+  });
+
+  it("rejects roomCode mismatch with BAD_TOKEN", () => {
+    const { white, code } = setupRoom();
+    sendClient(white, "custom-modifier.register", {
+      roomCode: "ZZZZZZ", // doesn't match authenticated room
+      descriptor: makeDescriptor("custom:t3-wrong-room"),
+    });
+    const err = nextMsgOfType(white, "error");
+    expect(err.payload["code"]).toBe("BAD_TOKEN");
+
+    const room = roomRegistry.getRoom(code);
+    expect(room?.customModifiers?.size ?? 0).toBe(0);
+  });
+
+  it("rejects unauthenticated sockets with BAD_TOKEN", () => {
+    const stranger = makeMockWs(`T3CM-stranger-${crypto.randomUUID()}`);
+    registerConnection(stranger);
+
+    sendClient(stranger, "custom-modifier.register", {
+      roomCode: "ABCDEF",
+      descriptor: makeDescriptor("custom:t3-stranger"),
+    });
+
+    const err = nextMsgOfType(stranger, "error");
+    expect(err.payload["code"]).toBe("BAD_TOKEN");
+  });
+
+  it("rejects malformed descriptor at the schema layer (INVALID_MESSAGE)", () => {
+    const { white, code } = setupRoom();
+
+    // Missing required `primitives` field.
+    const malformed = {
+      type: "data",
+      id: "custom:t3-malformed",
+      name: "Bad",
+      description: "",
+      version: 1,
+      // primitives: missing
+      targetAttrs: [],
+      uiForm: "primitive-composer",
+      source: "custom",
+    };
+
+    sendClient(white, "custom-modifier.register", {
+      roomCode: code,
+      descriptor: malformed,
+    });
+
+    const err = nextMsgOfType(white, "error");
+    expect(err.payload["code"]).toBe("INVALID_MESSAGE");
+  });
+});