--- name: dev-multiplayer-anti-cheat-validation description: Input validation and anti-cheat patterns for multiplayer servers. Use when implementing server-side validation. category: multiplayer --- # Anti-Cheat Validation Server-side validation to prevent cheating in multiplayer games. ## When to Use Use when: - Implementing input validation on the server - Adding shooting mechanics - Preventing speed hacks or teleportation ## Input Validation Pattern ```typescript function validateInput(input: PlayerInput, player: PlayerState): boolean { // Sanity checks - reject impossible inputs if (input.movementSpeed > 20) return false; // Speed hack if (input.jumpHeight > 10) return false; // Super jump hack // Movement constraints const dx = input.targetX - player.x; const dz = input.targetZ - player.z; const distance = Math.sqrt(dx * dx + dz * dz); // Can't move more than X meters per tick if (distance > 2) return false; return true; } onMessage(client: Client, data: any) { const player = this.state.players.get(client.sessionId); if (!player) return; if (data.type === 'player_input') { // VALIDATE before processing if (validateInput(data.input, player)) { player.pendingInput = data.input; } else { // Log potential cheater console.warn(`Suspicious input from ${client.sessionId}`); } } } ``` ## Shooting Validation ```typescript onMessage(client: Client, data: any) { if (data.type !== 'shoot') return; const shooter = this.state.players.get(client.sessionId); if (!shooter) return; // Validate shooter can shoot if (shooter.ink <= 0) return; if (Date.now() - shooter.lastShotTime < 100) return; // 100ms cooldown // Validate aim direction is reasonable const aim = data.aimDirection; const aimLength = Math.sqrt(aim.x ** 2 + aim.y ** 2 + aim.z ** 2); if (aimLength > 1.1 || aimLength < 0.9) return; // Must be normalized // Server creates paint projectile const projectile = { x: shooter.x, y: shooter.y + 1.5, z: shooter.z, dx: aim.x * 25, dy: aim.y * 25, dz: aim.z * 25, owner: client.sessionId, team: shooter.team, }; this.projectiles.push(projectile); shooter.ink -= 1; shooter.lastShotTime = Date.now(); } ``` ## Hit Detection with Lag Compensation ```typescript // Server validates hits by rewinding time function checkHit(shooter: PlayerState, targetId: string, aim: Vector3): boolean { const target = this.state.players.get(targetId); if (!target) return false; // Get target position at the time of shooting (lag compensation) const shotTime = Date.now(); const latency = this.getClientLatency(shooter.sessionId); const rewindTime = shotTime - latency; // Find where target was at rewindTime const historicalPosition = this.getPositionHistory(targetId, rewindTime); if (!historicalPosition) return false; // Raycast from shooter to historical position return this.raycastHits(shooter, historicalPosition, aim); } // Store position history for lag compensation private positionHistory: Map> = new Map(); update(dt: number) { const now = Date.now(); for (const [sessionId, player] of this.state.players) { if (!this.positionHistory.has(sessionId)) { this.positionHistory.set(sessionId, []); } const history = this.positionHistory.get(sessionId)!; // Store position for lag compensation (keep last 500ms) history.push({ time: now, x: player.x, y: player.y, z: player.z }); // Remove old entries while (history.length > 0 && history[0].time < now - 500) { history.shift(); } } } ``` ## Anti-Cheat Best Practices 1. **Validate all inputs** - Reject impossible values 2. **Rate limit actions** - Prevent spam exploits 3. **Track position history** - Detect teleportation 4. **Checksum game state** - Detect tampering 5. **Log suspicious activity** - For analysis/banning ## Testing Checklist - [ ] Server running successfully - [ ] Feature works through network (not just locally) - [ ] Server logs show player actions - [ ] State updates propagate to all clients - [ ] Inputs are validated server-side - [ ] Impossible inputs are rejected - [ ] No client-authoritative position updates ## Common Mistakes | ❌ Wrong | ✅ Right | |----------|----------| | Trust client position | Validate all inputs | | No rate limiting | Add cooldowns | | No logging | Log suspicious activity | | Client determines hit | Server validates hit | ## Reference - [server-authoritative.md](references/upstream/ee7092017d8fde76-SKILL.md) - Architecture principles - [prediction-shooting.md](references/upstream/8cb47f0f05b304d1-SKILL.md) - Shooting prediction --- Source attribution: feliperyba Original project: https://github.com/feliperyba/ralph-orchestra/blob/9338a5a194fac4e20e174decc6ce1b7400744866/.claude/skills/dev-multiplayer-anti-cheat-validation/SKILL.md License: MIT License information: https://opensource.org/license/mit Original upstream instructions; downloaded from Dave AI. ## Upstream license notice MIT License Copyright (c) 2026 feliperyba Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions: The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software. THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. # Dave AI resource metadata Creator / publisher: feliperyba Original project: https://github.com/feliperyba/ralph-orchestra/blob/9338a5a194fac4e20e174decc6ce1b7400744866/.claude/skills/dev-multiplayer-anti-cheat-validation/SKILL.md Attribution basis: Recovered original package and original project license at a pinned revision Fields of work: Development & Engineering Roles: Backend Developer, Game Developer, QA / Test Engineer Specialties: Backend Tools: Tool not specified Country applicability: Country not specified Classification basis: Source catalog labels Instruction SHA-256: d9a70f25e2a55f021e4d0e49958e9203ab4f714f37e81effdda8d6160c3b4ed6 Metadata reviewed: 2026-10-27 This metadata supersedes earlier Dave AI catalog attribution. Original instruction and supporting files are preserved.