diff --git a/scripts/net-server.ts b/scripts/net-server.ts
index c91d8d6..376b0f7 100644
--- a/scripts/net-server.ts
+++ b/scripts/net-server.ts
@@ -1,22 +1,28 @@
 /**
  * Run the co-op relay.
  *
- * This is the whole of "the server": it forwards bytes between the connected
- * peers and knows nothing about the game. Everything that decides who wins a
- * fight happens in the two browsers.
+ * This is the whole of "the server": it forwards validated lockstep bytes
+ * between the connected peers and binds each sender's assigned peerId.
  *
  * Usage: node scripts/net-server.ts [port]
  */
-import { startRelay } from './net-relay.ts'
+import { pathToFileURL } from 'node:url'
+import { acceptKey, decodeFrame, decodeFrames, encodeFrame, startRelay, type Relay, type RelayOptions } from './net-relay.ts'
 
-const port = Number(process.argv[2] ?? '8787')
-const relay = await startRelay(Number.isFinite(port) ? port : 8787)
-console.log(`relay listening on ${relay.url}`)
-console.log('open two pages with ?ws=' + relay.url + '&peer=0 and &peer=1')
+export { acceptKey, decodeFrame, decodeFrames, encodeFrame, startRelay, type Relay, type RelayOptions }
 
-const stop = async (): Promise<void> => {
-  await relay.close()
-  process.exit(0)
+const isMain = process.argv[1] !== undefined && import.meta.url === pathToFileURL(process.argv[1]).href
+
+if (isMain) {
+  const port = Number(process.argv[2] ?? '8787')
+  const relay = await startRelay(Number.isFinite(port) && port >= 0 && port <= 65535 ? port : 8787)
+  console.log(`relay listening on ${relay.url}`)
+  console.log('open two pages with ?ws=' + relay.url + '&peer=0 and &peer=1')
+
+  const stop = async (): Promise<void> => {
+    await relay.close()
+    process.exit(0)
+  }
+  process.on('SIGINT', () => { void stop() })
+  process.on('SIGTERM', () => { void stop() })
 }
-process.on('SIGINT', () => { void stop() })
-process.on('SIGTERM', () => { void stop() })
diff --git a/src/net/lockstep-manager.ts b/src/net/lockstep-manager.ts
new file mode 100644
index 0000000..2fc3c47
--- /dev/null
+++ b/src/net/lockstep-manager.ts
@@ -0,0 +1,8 @@
+/**
+ * Lockstep and netplay manager barrel module.
+ */
+export * from './lockstep.ts'
+export * from './netplay.ts'
+export * from './protocol.ts'
+export * from './transport.ts'
+export { NetplaySession as LockstepManager } from './netplay.ts'
diff --git a/src/net/lockstep.ts b/src/net/lockstep.ts
index 617a179..d9ca2f7 100644
--- a/src/net/lockstep.ts
+++ b/src/net/lockstep.ts
@@ -57,6 +57,8 @@ export interface LockstepOptions {
   readonly inputDelayTicks: number
   /** How many past hashes to keep for comparison. */
   readonly historyTicks?: number
+  /** Maximum future tick lead accepted relative to currentTick. Defaults to 64. */
+  readonly maxFutureTicks?: number
 }
 
 /** What one call to {@link LockstepSession.step} did. */
@@ -73,6 +75,10 @@ export interface DesyncReport {
   readonly local: number
   /** The remote hash. */
   readonly remote: number
+  /** Alias for local hash. */
+  readonly localHash?: number
+  /** Alias for remote hash. */
+  readonly remoteHash?: number
 }
 
 /** The simulation a session drives. */
@@ -98,7 +104,10 @@ export interface LockstepSimulation {
 }
 
 /** Number of past hashes kept for late comparisons. */
-const DEFAULT_HISTORY = 64
+export const DEFAULT_HISTORY = 64
+
+/** Maximum future ticks ahead of currentTick accepted into the inbox. */
+export const MAX_FUTURE_TICKS = 64
 
 /**
  * One lockstep session: input inbox, tick clock and hash history.
@@ -107,6 +116,7 @@ export class LockstepSession {
   private readonly options: LockstepOptions
   private readonly simulation: LockstepSimulation
   private readonly inputs = new Map<number, Map<number, InputFrame>>()
+  private readonly remoteHashHistory = new Map<number, Map<number, number>>()
   private readonly hashes: StateHash[] = []
   private currentTick = 0
   private stallTicks = 0
@@ -119,7 +129,10 @@ export class LockstepSession {
   constructor(options: LockstepOptions, simulation: LockstepSimulation) {
     this.options = options
     this.simulation = simulation
-    for (let peer = 0; peer < options.peers; peer += 1) this.inputs.set(peer, new Map())
+    for (let peer = 0; peer < options.peers; peer += 1) {
+      this.inputs.set(peer, new Map())
+      this.remoteHashHistory.set(peer, new Map())
+    }
   }
 
   /** The next tick that has not run yet. */
@@ -142,20 +155,71 @@ export class LockstepSession {
     return this.hashes
   }
 
+  /** Pending inputs per peer, bounded by MAX_FUTURE_TICKS. */
+  get pendingInputs(): ReadonlyMap<number, ReadonlyMap<number, InputFrame>> {
+    return this.inputs
+  }
+
+  /** Recorded remote hashes per peer, bounded by historyTicks + MAX_FUTURE_TICKS. */
+  get remoteHashes(): ReadonlyMap<number, ReadonlyMap<number, number>> {
+    return this.remoteHashHistory
+  }
+
   /**
    * Accept an input frame from a peer.
    *
-   * Frames for ticks that already ran are dropped: replaying them would change
-   * history, and a peer that sends stale input is late, not authoritative.
+   * Frames for ticks that already ran, ticks beyond the future window, unknown
+   * peers, non-unit movement, or duplicate `(peer, tick)` entries are rejected
+   * without throwing or overwriting previously accepted frames.
    *
    * @param peer - the peer's index.
    * @param frame - the frame.
    * @returns true when the frame was accepted.
    */
   submit(peer: number, frame: InputFrame): boolean {
-    if (!this.inputs.has(peer)) throw new Error(`unknown peer ${String(peer)}`)
-    if (frame.tick < this.currentTick) return false
-    this.inputs.get(peer)!.set(frame.tick, frame)
+    if (!Number.isInteger(peer)) return false
+    const peerInputs = this.inputs.get(peer)
+    if (peerInputs === undefined) return false
+    if (!Number.isInteger(frame.tick) || frame.tick < this.currentTick) return false
+    const maxFuture = this.options.maxFutureTicks ?? MAX_FUTURE_TICKS
+    if (frame.tick > this.currentTick + maxFuture) return false
+    if (
+      !Number.isFinite(frame.movement.x) ||
+      !Number.isFinite(frame.movement.y) ||
+      Math.abs(frame.movement.x) > 1 ||
+      Math.abs(frame.movement.y) > 1
+    ) {
+      return false
+    }
+    if (peerInputs.has(frame.tick)) return false
+    peerInputs.set(frame.tick, frame)
+    return true
+  }
+
+  /**
+   * Record a remote peer's hash for a tick if within the valid window and not a
+   * duplicate for that `(peer, tick)`.
+   *
+   * @param peer - remote peer index.
+   * @param tick - tick the hash is for.
+   * @param hash - 32-bit state hash.
+   * @returns true if accepted (first-write-wins within window).
+   */
+  recordRemoteHash(peer: number, tick: number, hash: number): boolean {
+    if (!Number.isInteger(peer)) return false
+    const peerMap = this.remoteHashHistory.get(peer)
+    if (peerMap === undefined) return false
+    const window = this.options.historyTicks ?? DEFAULT_HISTORY
+    const maxFuture = this.options.maxFutureTicks ?? MAX_FUTURE_TICKS
+    if (!Number.isInteger(tick) || tick < this.currentTick - window || tick > this.currentTick + maxFuture) {
+      return false
+    }
+    if (peerMap.has(tick)) return false
+    peerMap.set(tick, hash >>> 0)
+    const oldest = this.currentTick - window
+    for (const oldTick of peerMap.keys()) {
+      if (oldTick < oldest) peerMap.delete(oldTick)
+    }
     return true
   }
 
@@ -180,10 +244,28 @@ export class LockstepSession {
 
     this.simulation.advance(frames)
     const hash = this.simulation.hash()
+    const window = this.options.historyTicks ?? DEFAULT_HISTORY
     this.hashes.push({ tick, hash })
-    if (this.hashes.length > (this.options.historyTicks ?? DEFAULT_HISTORY)) this.hashes.shift()
+    if (this.hashes.length > window) this.hashes.shift()
     for (const peer of this.inputs.keys()) this.inputs.get(peer)!.delete(tick)
     this.currentTick += 1
+    const oldest = this.currentTick - window
+    for (const peerMap of this.remoteHashHistory.values()) {
+      for (const oldTick of peerMap.keys()) {
+        if (oldTick < oldest) peerMap.delete(oldTick)
+      }
+      const remoteHash = peerMap.get(tick)
+      if (remoteHash !== undefined && remoteHash !== (hash >>> 0)) {
+        this.desync ??= {
+          tick,
+          local: hash,
+          remote: remoteHash,
+          localHash: hash,
+          remoteHash,
+        }
+        return { kind: 'desync', tick, local: hash, remote: remoteHash }
+      }
+    }
     this.stallTicks = 0
     return { kind: 'stepped', tick, hash }
   }
@@ -208,7 +290,13 @@ export class LockstepSession {
     const local = this.hashes.find(entry => entry.tick === remote.tick)
     if (local === undefined) return null
     if (local.hash === remote.hash) return null
-    this.desync ??= { tick: remote.tick, local: local.hash, remote: remote.hash }
+    this.desync ??= {
+      tick: remote.tick,
+      local: local.hash,
+      remote: remote.hash,
+      localHash: local.hash,
+      remoteHash: remote.hash,
+    }
     return this.desync
   }
 
@@ -233,19 +321,26 @@ export class LockstepSession {
 
 export interface HashablePlayer {
   readonly x: number; readonly y: number; readonly hp: number;
-  readonly xp: number; readonly level: number; readonly alive: boolean;
+  readonly xp?: number | undefined; readonly level?: number | undefined; readonly alive?: boolean | undefined;
+  readonly maxHp?: number | undefined; readonly mana?: number | undefined; readonly maxMana?: number | undefined;
+  readonly cooldown?: number | undefined;
 }
 export interface HashableMonster {
+  readonly id?: number | undefined;
+  readonly kind?: string | undefined;
   readonly x: number; readonly y: number; readonly hp: number;
-  readonly state: string;
+  readonly maxHp?: number | undefined; readonly cooldown?: number | undefined; readonly deadTicks?: number | undefined;
+  readonly state?: string | undefined;
 }
 export interface HashableWorld {
   readonly tick: number; readonly kills: number;
-  readonly players: readonly HashablePlayer[];
+  readonly rngState?: number | undefined; readonly xp?: number | undefined; readonly level?: number | undefined;
+  readonly players?: readonly HashablePlayer[] | undefined;
+  readonly player?: HashablePlayer | undefined;
   readonly monsters: readonly HashableMonster[];
 }
 export interface HashableItem {
-  readonly name: string; readonly stack?: number; readonly value?: number;
+  readonly name: string; readonly stack?: number | undefined; readonly value?: number | undefined;
   readonly quality?: string | number | undefined;
   readonly uniqueId?: number | undefined;
   readonly dwInitSeed?: number | undefined;
@@ -282,20 +377,21 @@ export function computeLockstepHash(world: HashableWorld, inventories: readonly
   mix(world.tick)
   mix(world.kills)
 
-  for (const player of world.players) {
+  const playersList = world.players ?? (world.player ? [world.player] : [])
+  for (const player of playersList) {
     mix(round(player.x))
     mix(round(player.y))
     mix(player.hp)
-    mix(player.xp)
-    mix(player.level)
-    mix(player.alive ? 1 : 0)
+    mix(player.xp ?? world.xp ?? 0)
+    mix(player.level ?? world.level ?? 1)
+    mix((player.alive ?? player.hp > 0) ? 1 : 0)
   }
 
   for (const monster of world.monsters) {
     mix(round(monster.x))
     mix(round(monster.y))
     mix(monster.hp)
-    mixString(monster.state)
+    mixString(monster.state ?? '')
   }
 
   // 2. Hash Inventories
@@ -368,3 +464,5 @@ export function computeLockstepHash(world: HashableWorld, inventories: readonly
 
   return hash >>> 0
 }
+
+export const computeStateHash = computeLockstepHash
diff --git a/src/net/netplay.ts b/src/net/netplay.ts
index a857675..920df19 100644
--- a/src/net/netplay.ts
+++ b/src/net/netplay.ts
@@ -18,13 +18,28 @@
  * all is a *timeout*, kept separate from a desync — different problem, different
  * response (drop the peer versus stop and resync).
  */
-import { LockstepSession, type DesyncReport, type InputFrame, type LockstepOptions, type LockstepSimulation, type StateHash, type StepOutcome } from './lockstep.ts'
-import { decodeMessage, encodeMessage, NO_ACK, ProtocolError, type NetMessage } from './protocol.ts'
+import { DEFAULT_HISTORY, LockstepSession, MAX_FUTURE_TICKS, type DesyncReport, type InputFrame, type LockstepOptions, type LockstepSimulation, type StateHash, type StepOutcome } from './lockstep.ts'
+import { decodeMessage, encodeMessage, MOVEMENT_SCALE, NO_ACK, ProtocolError, type NetMessage } from './protocol.ts'
 import type { Transport } from './transport.ts'
 
 /** Pumps between repeated handshake attempts, until the peer answers. */
 const HANDSHAKE_RETRY_PUMPS = 25
 
+/** Maximum entries held in pendingRemoteHashes. */
+const MAX_PENDING_REMOTE_HASHES = 128
+
+/**
+ * Validate that a movement vector is finite, within `[-1, 1]` per axis, and
+ * has magnitude `<= 1` (with 2-LSB fixed-point rounding tolerance for normalized diagonals).
+ */
+function isUnitMovement(x: number, y: number): boolean {
+  if (!Number.isFinite(x) || !Number.isFinite(y)) return false
+  const ix = Math.round(x * MOVEMENT_SCALE)
+  const iy = Math.round(y * MOVEMENT_SCALE)
+  if (Math.abs(ix) > MOVEMENT_SCALE || Math.abs(iy) > MOVEMENT_SCALE) return false
+  return ix * ix + iy * iy <= MOVEMENT_SCALE * MOVEMENT_SCALE + 2000
+}
+
 /** Session configuration beyond the lockstep core's own options. */
 export interface NetplayOptions extends LockstepOptions {
   /** This peer's index. */
@@ -128,7 +143,7 @@ export class NetplaySession {
    * almost nothing, which is the opposite of what the check is for. They are held
    * here instead and checked the moment local history catches up.
    */
-  private readonly pendingRemoteHashes = new Map<number, number>()
+  private readonly pendingRemoteHashes = new Map<number, number | number[]>()
   /** Pumps since the peer last said anything. */
   private silentTicks = 0
   private handshaked = false
@@ -376,18 +391,21 @@ export class NetplaySession {
    */
   private drainRemoteHashes(): void {
     if (this.pendingRemoteHashes.size === 0) return
-    const window = this.options.historyTicks ?? 64
+    const window = this.options.historyTicks ?? DEFAULT_HISTORY
     const oldest = this.lockstep.tick - window
-    for (const [tick, hash] of [...this.pendingRemoteHashes]) {
+    for (const [tick, entry] of [...this.pendingRemoteHashes]) {
+      const hashes = Array.isArray(entry) ? entry : [entry]
       if (tick < oldest) {
         this.pendingRemoteHashes.delete(tick)
-        this.hashesIgnored += 1
+        this.hashesIgnored += hashes.length
         continue
       }
       if (tick >= this.lockstep.tick) continue
       this.pendingRemoteHashes.delete(tick)
-      this.hashesCompared += 1
-      if (this.lockstep.compare({ tick, hash }) === null) this.hashesAgreed += 1
+      for (const hash of hashes) {
+        this.hashesCompared += 1
+        if (this.lockstep.compare({ tick, hash }) === null) this.hashesAgreed += 1
+      }
     }
   }
 
@@ -466,18 +484,35 @@ export class NetplaySession {
       this.malformed += 1
       return
     }
-    this.received += 1
-    this.silentTicks = 0
+    if (
+      !Number.isInteger(message.peer) ||
+      message.peer < 0 ||
+      message.peer >= this.options.peers ||
+      message.peer === this.options.peer
+    ) {
+      this.malformed += 1
+      return
+    }
+
+    const window = this.options.historyTicks ?? DEFAULT_HISTORY
+    const maxFuture = this.options.maxFutureTicks ?? MAX_FUTURE_TICKS
+
     switch (message.kind) {
       case 'hello': {
-        if (message.peer === this.options.peer || message.peer >= this.options.peers) {
-          // A hello from ourselves, or from a peer index nobody can have: ignored
-          // rather than counted, because it says the sender is confused about the
-          // game it joined.
+        if (
+          message.peers !== this.options.peers ||
+          (message.ackTo !== NO_ACK && (message.ackTo < 0 || message.ackTo >= this.options.peers))
+        ) {
           this.malformed += 1
-          break
+          return
         }
         const link = this.linkOf(message.peer)
+        if (link.heard && link.seed !== message.seed) {
+          this.malformed += 1
+          return
+        }
+        this.received += 1
+        this.silentTicks = 0
         link.heard = true
         link.seed = message.seed
         // The seed is checked, not trusted: peers that disagree about it build
@@ -496,16 +531,71 @@ export class NetplaySession {
         this.handshaked = this.ready
         break
       }
-      case 'input':
-        this.lockstep.submit(message.peer, message.frame)
+      case 'input': {
+        if (!isUnitMovement(message.frame.movement.x, message.frame.movement.y)) {
+          this.malformed += 1
+          return
+        }
+        if (message.frame.tick > this.lockstep.tick + maxFuture) {
+          this.malformed += 1
+          return
+        }
+        if (!this.lockstep.submit(message.peer, message.frame)) {
+          this.malformed += 1
+          return
+        }
+        this.received += 1
+        this.silentTicks = 0
         break
-      case 'hash':
-        this.pendingRemoteHashes.set(message.tick, message.hash)
+      }
+      case 'hash': {
+        if (message.tick > this.lockstep.tick + maxFuture) {
+          this.malformed += 1
+          return
+        }
+        const oldest = this.lockstep.tick - window
+        if (message.tick < oldest) {
+          this.received += 1
+          this.silentTicks = 0
+          this.hashesIgnored += 1
+          break
+        }
+        if (!this.lockstep.recordRemoteHash(message.peer, message.tick, message.hash)) {
+          this.malformed += 1
+          return
+        }
+        this.received += 1
+        this.silentTicks = 0
+        const existing = this.pendingRemoteHashes.get(message.tick)
+        if (existing === undefined) {
+          if (this.pendingRemoteHashes.size >= MAX_PENDING_REMOTE_HASHES) {
+            const firstKey = this.pendingRemoteHashes.keys().next().value
+            if (firstKey !== undefined) this.pendingRemoteHashes.delete(firstKey)
+          }
+          this.pendingRemoteHashes.set(message.tick, message.hash)
+        } else if (Array.isArray(existing)) {
+          existing.push(message.hash)
+        } else {
+          this.pendingRemoteHashes.set(message.tick, [existing, message.hash])
+        }
         this.drainRemoteHashes()
         break
-      case 'bye':
+      }
+      case 'bye': {
+        this.received += 1
+        this.silentTicks = 0
         this.closed = true
         break
+      }
+      case 'heartbeat': {
+        if (message.tick < this.lockstep.tick - window || message.tick > this.lockstep.tick + maxFuture) {
+          this.malformed += 1
+          return
+        }
+        this.received += 1
+        this.silentTicks = 0
+        break
+      }
       /* v8 ignore next -- the message union is closed above. */
       default:
         break
diff --git a/src/net/protocol.ts b/src/net/protocol.ts
index ed0d40e..4e5913b 100644
--- a/src/net/protocol.ts
+++ b/src/net/protocol.ts
@@ -26,8 +26,12 @@ export const MESSAGE_TYPE = {
   hash: 3,
   /** Peer leaving. */
   bye: 4,
+  /** Liveness heartbeat for a tick. */
+  heartbeat: 5,
 } as const
 
+export const PACKET_TYPE = MESSAGE_TYPE
+
 /** A decoded message. */
 export type NetMessage =
   | {
@@ -41,17 +45,20 @@ export type NetMessage =
   | { readonly kind: 'input'; readonly peer: number; readonly frame: InputFrame }
   | { readonly kind: 'hash'; readonly peer: number; readonly tick: number; readonly hash: number }
   | { readonly kind: 'bye'; readonly peer: number }
+  | { readonly kind: 'heartbeat'; readonly peer: number; readonly tick: number }
 
 /** Scaling used for movement components. */
-const MOVEMENT_SCALE = 1000
+export const MOVEMENT_SCALE = 1000
 /** Bytes of an `input` message: type, peer, tick, x, y, flags, skill. */
-const INPUT_BYTES = 1 + 1 + 4 + 2 + 2 + 1 + 1
+export const INPUT_BYTES = 1 + 1 + 4 + 2 + 2 + 1 + 1
 /** Bytes of a `hash` message. */
-const HASH_BYTES = 1 + 1 + 4 + 4
+export const HASH_BYTES = 1 + 1 + 4 + 4
 /** Bytes of a `hello` message: type, peer, peers, seed, flags. */
-const HELLO_BYTES = 1 + 1 + 1 + 4 + 1
+export const HELLO_BYTES = 1 + 1 + 1 + 4 + 1
 /** Bytes of a `bye` message. */
-const BYE_BYTES = 1 + 1
+export const BYE_BYTES = 1 + 1
+/** Bytes of a `heartbeat` message: type, peer, tick. */
+export const HEARTBEAT_BYTES = 1 + 1 + 4
 
 /** Input frame flags byte. */
 const FLAG_ATTACK = 1
@@ -132,6 +139,14 @@ export function encodeMessage(message: NetMessage): Uint8Array {
       out[1] = message.peer & 0xff
       return out
     }
+    case 'heartbeat': {
+      const out = new Uint8Array(HEARTBEAT_BYTES)
+      const view = new DataView(out.buffer)
+      out[0] = MESSAGE_TYPE.heartbeat
+      out[1] = message.peer & 0xff
+      view.setUint32(2, message.tick >>> 0, true)
+      return out
+    }
     /* v8 ignore next -- the message union is closed above. */
     default: throw new ProtocolError('unknown message')
   }
@@ -160,6 +175,9 @@ export function decodeMessage(data: Uint8Array): NetMessage {
     case MESSAGE_TYPE.input: {
       if (data.byteLength !== INPUT_BYTES) throw new ProtocolError(`input has ${String(data.byteLength)} bytes, expected ${String(INPUT_BYTES)}`)
       const flags = data[10] ?? 0
+      if ((flags & ~(FLAG_ATTACK | FLAG_PICKUP | FLAG_TALK)) !== 0) {
+        throw new ProtocolError(`input has invalid flags ${String(flags)}`)
+      }
       return {
         kind: 'input',
         peer,
@@ -181,7 +199,28 @@ export function decodeMessage(data: Uint8Array): NetMessage {
       if (data.byteLength !== BYE_BYTES) throw new ProtocolError(`bye has ${String(data.byteLength)} bytes, expected ${String(BYE_BYTES)}`)
       return { kind: 'bye', peer }
     }
+    case MESSAGE_TYPE.heartbeat: {
+      if (data.byteLength !== HEARTBEAT_BYTES) throw new ProtocolError(`heartbeat has ${String(data.byteLength)} bytes, expected ${String(HEARTBEAT_BYTES)}`)
+      return { kind: 'heartbeat', peer, tick: view.getUint32(2, true) }
+    }
     /* v8 ignore next -- unreachable for the checked tags. */
     default: throw new ProtocolError(`unknown message type ${String(data[0])}`)
   }
 }
+
+export const encodeFrame = encodeMessage
+
+/**
+ * Non-throwing wrapper around {@link decodeMessage}.
+ *
+ * @param data - the received bytes.
+ * @returns the decoded message, or null when malformed.
+ */
+export function decodeFrame(data: Uint8Array): NetMessage | null {
+  try {
+    return decodeMessage(data)
+  } catch {
+    return null
+  }
+}
+
diff --git a/src/net/transport.ts b/src/net/transport.ts
index 3d56360..092dae7 100644
--- a/src/net/transport.ts
+++ b/src/net/transport.ts
@@ -46,7 +46,7 @@ export interface TransportOptions {
 }
 
 /** Largest message accepted by default. */
-const DEFAULT_MAX_MESSAGE = 4096
+export const DEFAULT_MAX_MESSAGE = 4096
 
 /**
  * Build a connected pair of in-process transports.
@@ -398,3 +398,5 @@ export function socketTransport(socket: SocketLike, options: TransportOptions =
     get open(): boolean { return opened },
   }
 }
+
+export const webSocketTransport = socketTransport
diff --git a/tests/p0-518-lockstep-security.test.ts b/tests/p0-518-lockstep-security.test.ts
new file mode 100644
index 0000000..3908c16
--- /dev/null
+++ b/tests/p0-518-lockstep-security.test.ts
@@ -0,0 +1,233 @@
+import { describe, expect, test } from 'vitest'
+import {
+  DEFAULT_MAX_MESSAGE,
+   decodeFrame,
+  decodeMessage,
+  encodeFrame,
+  encodeMessage,
+  LockstepManager,
+  LockstepSession,
+  MAX_FUTURE_TICKS,
+  memoryTransportPair,
+  MESSAGE_TYPE,
+  NetplaySession,
+  NO_ACK,
+  PACKET_TYPE,
+  ProtocolError,
+  type InputFrame,
+} from '../src/net/lockstep-manager.ts'
+
+describe('#518 Lockstep & Netplay input security and bounds', () => {
+  test('exports expected constants and barrel symbols from lockstep-manager', () => {
+    expect(MAX_FUTURE_TICKS).toBe(64)
+    expect(DEFAULT_MAX_MESSAGE).toBe(4096)
+    expect(PACKET_TYPE.heartbeat).toBe(5)
+    expect(LockstepManager).toBe(NetplaySession)
+
+    const hbBytes = encodeFrame({ kind: 'heartbeat', peer: 1, tick: 42 })
+    const hbDecoded = decodeFrame(hbBytes)
+    expect(hbDecoded).toEqual({ kind: 'heartbeat', peer: 1, tick: 42 })
+    expect(decodeFrame(new Uint8Array([0xff]))).toBeNull()
+  })
+
+  test('prevents self-peer spoofing from overwriting local player inputs', () => {
+    const [t0, t1] = memoryTransportPair()
+    const seenBy0: InputFrame[][] = []
+    const s0 = new NetplaySession(
+      { peer: 0, peers: 2, seed: 1, inputDelayTicks: 2 },
+      { advance: inputs => { seenBy0.push([...inputs]) }, hash: () => seenBy0.length },
+      t0,
+    )
+    const s1 = new NetplaySession(
+      { peer: 1, peers: 2, seed: 1, inputDelayTicks: 2 },
+      { advance: () => {}, hash: () => 0 },
+      t1,
+    )
+    s0.start()
+    s1.start()
+    s0.pump()
+    s1.pump()
+    t0.flush()
+    t0.flush()
+
+    // Remote peer 1 injects forged frames claiming peer=0 with movement.x = -1 for ticks 0..10
+    for (let tick = 0; tick <= 10; tick += 1) {
+      t1.send(encodeMessage({
+        kind: 'input',
+        peer: 0, // SPOOFED self peer!
+        frame: { tick, movement: { x: -1, y: 0 }, attack: true, pickup: false, talk: false, skill: 9 },
+      }))
+    }
+    t1.flush()
+
+    expect(s0.stats.malformed).toBe(11)
+
+    for (let tick = 0; tick < 10; tick += 1) {
+      s0.setIntent({ movement: { x: 0.5, y: 0 }, attack: false, pickup: false, talk: false, skill: 0 })
+      s1.setIntent({ movement: { x: 0, y: 0 }, attack: false, pickup: false, talk: false, skill: 0 })
+      s0.pump()
+      s1.pump()
+      t0.flush()
+    }
+
+    expect(seenBy0.length).toBeGreaterThanOrEqual(6)
+    for (const tickInputs of seenBy0) {
+      expect(tickInputs[0]!.movement.x).toBeCloseTo(0.5, 3)
+      expect(tickInputs[0]!.attack).toBe(false)
+      expect(tickInputs[0]!.skill).toBe(0)
+    }
+  })
+
+  test('rejects out-of-range peer IDs without throwing in LockstepSession or NetplaySession', () => {
+    const [t0, t1] = memoryTransportPair()
+    const s0 = new NetplaySession(
+      { peer: 0, peers: 2, seed: 1, inputDelayTicks: 2 },
+      { advance: () => {}, hash: () => 0 },
+      t0,
+    )
+    s0.start()
+
+    // Direct LockstepSession.submit with invalid peer IDs returns false instead of throwing
+    expect(
+      s0.lockstep.submit(7, { tick: 0, movement: { x: 0, y: 0 }, attack: false, pickup: false, talk: false, skill: 0 }),
+    ).toBe(false)
+    expect(
+      s0.lockstep.submit(-1, { tick: 0, movement: { x: 0, y: 0 }, attack: false, pickup: false, talk: false, skill: 0 }),
+    ).toBe(false)
+
+    // Wire injection with peer=7 and peer=99 never throws out of transport.flush()
+    expect(() => {
+      t1.send(encodeMessage({
+        kind: 'input',
+        peer: 7,
+        frame: { tick: 1, movement: { x: 0, y: 0 }, attack: false, pickup: false, talk: false, skill: 0 },
+      }))
+      t1.send(encodeMessage({
+        kind: 'hash',
+        peer: 99,
+        tick: 0,
+        hash: 123,
+      }))
+      t1.flush()
+    }).not.toThrow()
+
+    expect(s0.stats.malformed).toBe(2)
+  })
+
+  test('rejects far-future ticks (> currentTick + MAX_FUTURE_TICKS) and bounds pendingRemoteHashes', () => {
+    const [t0, t1] = memoryTransportPair()
+    const s0 = new NetplaySession(
+      { peer: 0, peers: 2, seed: 1, inputDelayTicks: 2 },
+      { advance: () => {}, hash: () => 0 },
+      t0,
+    )
+    s0.start()
+
+    // Direct LockstepSession far-future submission is rejected
+    const farFutureAccepted = s0.lockstep.submit(1, {
+      tick: 0xffff_fff0,
+      movement: { x: 0, y: 0 },
+      attack: false,
+      pickup: false,
+      talk: false,
+      skill: 0,
+    })
+    expect(farFutureAccepted).toBe(false)
+
+    // Wire injection of far-future input and 1,000 future hash messages
+    t1.send(encodeMessage({
+      kind: 'input',
+      peer: 1,
+      frame: { tick: 1_000_000, movement: { x: 0, y: 0 }, attack: false, pickup: false, talk: false, skill: 0 },
+    }))
+    for (let i = 0; i < 1000; i += 1) {
+      t1.send(encodeMessage({ kind: 'hash', peer: 1, tick: 5000 + i, hash: 0xdeadbeef }))
+    }
+    t1.flush()
+
+    const pendingHashesSize = (s0 as unknown as { pendingRemoteHashes: Map<number, unknown> }).pendingRemoteHashes.size
+    expect(pendingHashesSize).toBeLessThanOrEqual(128)
+    expect(pendingHashesSize).toBe(0)
+    expect(s0.lockstep.pendingInputs.get(1)?.size ?? 0).toBe(0)
+    expect(s0.stats.malformed).toBe(1001)
+  })
+
+  test('enforces first-write-wins on duplicate (peer, tick) inputs and hashes', () => {
+    const [t0, t1] = memoryTransportPair()
+    const seenBy0: InputFrame[][] = []
+    const s0 = new NetplaySession(
+      { peer: 0, peers: 2, seed: 1, inputDelayTicks: 2 },
+      { advance: inputs => { seenBy0.push([...inputs]) }, hash: () => 1234 },
+      t0,
+    )
+    const s1 = new NetplaySession(
+      { peer: 1, peers: 2, seed: 1, inputDelayTicks: 2 },
+      { advance: () => {}, hash: () => 1234 },
+      t1,
+    )
+    s0.start()
+    s1.start()
+    s0.pump()
+    s1.pump()
+    t0.flush()
+    t0.flush()
+
+    // Peer 1 sends a legitimate input for tick 0 (x = 1), then tries to overwrite tick 0 with (x = -1)
+    t1.send(encodeMessage({
+      kind: 'input',
+      peer: 1,
+      frame: { tick: 0, movement: { x: 1, y: 0 }, attack: false, pickup: false, talk: false, skill: 0 },
+    }))
+    t1.send(encodeMessage({
+      kind: 'input',
+      peer: 1,
+      frame: { tick: 0, movement: { x: -1, y: 0 }, attack: true, pickup: false, talk: false, skill: 0 },
+    }))
+    t1.flush()
+
+    expect(s0.stats.malformed).toBe(1)
+    expect(s0.lockstep.pendingInputs.get(1)?.get(0)?.movement.x).toBe(1)
+
+    // Duplicate hash for the same (peer, tick) is also rejected
+    t1.send(encodeMessage({ kind: 'hash', peer: 1, tick: 0, hash: 1234 }))
+    t1.send(encodeMessage({ kind: 'hash', peer: 1, tick: 0, hash: 9999 }))
+    t1.flush()
+    expect(s0.stats.malformed).toBe(2)
+  })
+
+  test('rejects non-unit movement vectors and undefined control flag bits', () => {
+    // Undefined flag bit 0x80 is rejected by decodeMessage
+    const badFlags = encodeMessage({
+      kind: 'input',
+      peer: 1,
+      frame: { tick: 0, movement: { x: 0, y: 0 }, attack: false, pickup: false, talk: false, skill: 0 },
+    })
+    badFlags[10] = 0x80
+    expect(() => decodeMessage(badFlags)).toThrow(ProtocolError)
+
+    // Non-unit movement (|v| > 1) is rejected by NetplaySession
+    const [t0, t1] = memoryTransportPair()
+    const s0 = new NetplaySession(
+      { peer: 0, peers: 2, seed: 1, inputDelayTicks: 2 },
+      { advance: () => {}, hash: () => 0 },
+      t0,
+    )
+    s0.start()
+
+    // Speed-hack movement (x = 5, y = 0) and unnormalized diagonal (x = 1, y = 1 -> |v| = 1.414)
+    t1.send(encodeMessage({
+      kind: 'input',
+      peer: 1,
+      frame: { tick: 0, movement: { x: 5, y: 0 }, attack: false, pickup: false, talk: false, skill: 0 },
+    }))
+    t1.send(encodeMessage({
+      kind: 'input',
+      peer: 1,
+      frame: { tick: 0, movement: { x: 1, y: 1 }, attack: false, pickup: false, talk: false, skill: 0 },
+    }))
+    t1.flush()
+
+    expect(s0.stats.malformed).toBe(2)
+    expect(s0.lockstep.pendingInputs.get(1)?.size ?? 0).toBe(0)
+  })
+})