diff --git a/MODS.md b/MODS.md
index 9aa3fc9..0302319 100644
--- a/MODS.md
+++ b/MODS.md
@@ -826,25 +826,36 @@ server's atlas, which gets a hash-named URL the same way.
 ```
 client                                     server
   │  hello { name, protocol }               │
-  ├────────────────────────────────────────►│
-  │  welcome { seed, atlas, mods[] }        │   mods[i] = { id, name, version, hash,
-  │◄────────────────────────────────────────┤              data, client?, worldgen? }  (urls)
+  ├────────────────────────────────────────►│   a different protocol gets rejected { reason }
+  │  welcome { protocol, seed, atlas,       │   mods[i] = { id, name, version, hash,
+  │            mods[] }                     │              data, client?, worldgen?, sha256 }
+  │◄────────────────────────────────────────┤   atlas = { png, json, sha256 }  (paths on the server)
   │                                         │
   │  [cross-origin: confirm screen]         │
-  │  fetch data, atlas, scripts             │
-  │  check hashes, register data,           │
-  │  run client setup(), start workers      │
+  │  download the atlas and every mod file, │
+  │  check each one's sha256, register data │
+  │  in the listed order, run client        │
+  │  setup(), start chunk workers           │
   │                                         │
   │  ready                                  │
   ├────────────────────────────────────────►│
-  │  join { players, changes, inventory }   │   player_join fires on the server here
+  │  join { id, name, players, changes,     │   the player is created here, player_join fires,
+  │         spawn, selected_slot }          │   and their inventory follows as container messages
   │◄────────────────────────────────────────┤
 ```
 
-- The client registers the base game, then each mod's data **in the order the server lists them**.
+- The client registers each mod's data **in the order the server lists them**.
+- Every file is checked against its SHA-256 before it's used, and scripts are imported from the checked bytes (as
+  `blob:` URLs), so what runs is exactly what was checked. The client code is in `client/handshake.ts` and
+  `client/mods.ts`.
 - If any download, hash check or `setup` fails, the client disconnects and shows which mod failed. It never joins with
   some mods missing.
-- The `protocol` in `hello` is the game's protocol version. A mismatch is rejected before any mod code is downloaded.
+- The `protocol` in `hello` is the game's protocol version (`PROTOCOL_VERSION` in `common/protocol.ts`). A mismatch is
+  rejected before anything is downloaded.
+- Until `ready`, the connection isn't a player: other players don't see it, and anything it sends besides `ready` is
+  ignored. A client that doesn't send `ready` within 60 seconds is rejected.
+- Mod files and the atlas are served with `Access-Control-Allow-Origin: *` and cached for a year, since their paths
+  change whenever their content does.
 - Leaving a server reloads the page, so one server's mod code never stays loaded while playing on another.
 
 ## Security
@@ -1107,8 +1118,8 @@ loading mods (the base game and the template), their scripts and worldgen, saves
 
 ## Implementation plan
 
-Steps 1–4 are done and 5, 6 and 9 mostly, enough that a mod made from the template loads and runs. Each step keeps the
-game working:
+Steps 1–5 are done and 6 and 9 mostly, enough that a mod made from the template loads and runs. Each step keeps the game
+working:
 
 1. **Game server core.** Move `client/generation.ts` to `common/` (it already only needs constants and the rng package)
    and have the server generate terrain. Move world state, chunks, tile data and player inventories into a game server
@@ -1123,9 +1134,7 @@ game working:
    build the combined atlas (textures are currently all named `bworld:<file>`), bundle scripts per side, and write
    hashed output to `build/mods/` and `server_mods/`. _Done._
 5. **Delivery.** Split `welcome` into `welcome` / `ready` / `join`. Clients download, verify and run mods before
-   joining. Add the confirm screen for cross-origin servers and CORS headers on the server. _Partly done:_ `welcome`
-   lists the mods and clients download and run them before creating the world, but the server doesn't wait for `ready`
-   (its messages just queue up meanwhile), hashes aren't checked, and there's no confirm screen or CORS yet.
+   joining. Add the confirm screen for cross-origin servers and CORS headers on the server. _Done._
 6. **Server scripts.** Components, events, commands, system, storage and `ctx.recipes`. Move `FURNACE_RECIPES` and
    `FUEL_VALUES` into the recipe registry. _Mostly done_ (`server/game/mod_runtime.ts`). Not yet: `on_click` (clients
    don't report left clicks on blocks), item `on_use` (there's no item use action), `world.get_state` / `set_state`
diff --git a/build.ts b/build.ts
index ff9a6fa..aff341c 100644
--- a/build.ts
+++ b/build.ts
@@ -1,7 +1,7 @@
 import { copy } from "@std/fs";
 import { createCanvas, loadImage } from "@gfx/canvas-wasm";
 import { ENGINE_TEXTURE_DIR, load_and_check, load_order, LoadedMod } from "./tools/check_mods.ts";
-import type { ModData, ModListing } from "./common/mod_loader.ts";
+import type { AtlasListing, ModData, ModListing } from "./common/mod_loader.ts";
 
 // all overridable so tests can build somewhere else
 const BUILD_FOLDER = Deno.env.get("BUILD_DIR") ?? "build";
@@ -11,6 +11,7 @@ const SERVER_MODS_FOLDER = Deno.env.get("SERVER_MODS_DIR") ?? "server_mods";
 
 // what the server reads at startup, see server/main.ts
 export interface ServerModIndex {
+	atlas: AtlasListing;
 	mods: { listing: ModListing; server?: string }[];
 }
 
@@ -74,8 +75,9 @@ function calculate_atlas_size(count: number) {
 	};
 }
 
-// one atlas with the engine's textures (engine:<file>) and every mod's (<mod>:<file>)
-async function build_atlas(mods: LoadedMod[]) {
+// one atlas with the engine's textures (engine:<file>) and every mod's (<mod>:<file>).
+// named by its hash, so clients can cache it forever and download it from other origins
+async function build_atlas(mods: LoadedMod[]): Promise<AtlasListing> {
 	const textures = new Map<string, string>();
 	for (const entry of Deno.readDirSync(ENGINE_TEXTURE_DIR)) {
 		if (entry.isFile && entry.name.endsWith(".png")) {
@@ -115,8 +117,13 @@ async function build_atlas(mods: LoadedMod[]) {
 		atlas_info[id] = { x: column, y: row };
 	}
 
-	Deno.writeFileSync(`${BUILD_FOLDER}/assets/sprites/textures.png`, canvas.toBuffer());
-	Deno.writeTextFileSync(`${BUILD_FOLDER}/assets/sprites/textures.json`, JSON.stringify(atlas_info));
+	const png = new Uint8Array(canvas.toBuffer());
+	const json = new TextEncoder().encode(JSON.stringify(atlas_info));
+	const hashes = { png: await sha256(png), json: await sha256(json) };
+	const name = `assets/sprites/textures.${(hashes.png + hashes.json).slice(0, 12)}`;
+	Deno.writeFileSync(`${BUILD_FOLDER}/${name}.png`, png);
+	Deno.writeFileSync(`${BUILD_FOLDER}/${name}.json`, json);
+	return { png: `${name}.png`, json: `${name}.json`, sha256: hashes };
 }
 
 async function build_sprites(mods: LoadedMod[]) {
@@ -125,16 +132,16 @@ async function build_sprites(mods: LoadedMod[]) {
 			await copy(`assets/sprites/${entry.name}`, `${BUILD_FOLDER}/assets/sprites/${entry.name}`);
 		}
 	}
-	await build_atlas(mods);
+	return await build_atlas(mods);
 }
 
-async function build_assets(mods: LoadedMod[]) {
+async function build_assets(mods: LoadedMod[]): Promise<AtlasListing> {
 	Deno.mkdirSync(`${BUILD_FOLDER}/assets`, { recursive: true });
 	await copy("assets/ASSETS.md", `${BUILD_FOLDER}/assets/ASSETS.md`);
 
 	await build_fonts();
 	Deno.mkdirSync(`${BUILD_FOLDER}/assets/sprites`, { recursive: true });
-	await build_sprites(mods);
+	return await build_sprites(mods);
 }
 
 // every mod in mods/, checked and sorted so dependencies load first. a broken mod fails the whole build,
@@ -156,15 +163,21 @@ async function bundle_script(path: string, platform: "browser" | "deno"): Promis
 	return result.outputFiles[0].text();
 }
 
+async function sha256(bytes: Uint8Array<ArrayBuffer> | string) {
+	const data = typeof bytes === "string" ? new TextEncoder().encode(bytes) : bytes;
+	const digest = await crypto.subtle.digest("SHA-256", data);
+	return [...new Uint8Array(digest)].map((b) => b.toString(16).padStart(2, "0")).join("");
+}
+
 async function short_hash(parts: string[]) {
 	const digest = await crypto.subtle.digest("SHA-256", new TextEncoder().encode(parts.join("\0")));
 	return [...new Uint8Array(digest)].slice(0, 6).map((b) => b.toString(16).padStart(2, "0")).join("");
 }
 
 // build/mods/<id>/<hash>/ gets what players download, server_mods/ the server scripts
-async function build_mods(mods: LoadedMod[]) {
+async function build_mods(mods: LoadedMod[], atlas: AtlasListing) {
 	clear_folder(SERVER_MODS_FOLDER);
-	const index: ServerModIndex = { mods: [] };
+	const index: ServerModIndex = { atlas, mods: [] };
 
 	for (const mod of mods) {
 		const manifest = mod.manifest as { name: string; version: string; scripts?: Record<string, string> };
@@ -193,14 +206,17 @@ async function build_mods(mods: LoadedMod[]) {
 			version: manifest.version,
 			hash,
 			data: `${public_dir}/data.json`,
+			sha256: { data: await sha256(data_json) },
 		};
 		Deno.writeTextFileSync(`${BUILD_FOLDER}/${listing.data}`, data_json);
 		if (client) {
 			listing.client = `${public_dir}/client.js`;
+			listing.sha256.client = await sha256(client);
 			Deno.writeTextFileSync(`${BUILD_FOLDER}/${listing.client}`, client);
 		}
 		if (worldgen) {
 			listing.worldgen = `${public_dir}/worldgen.js`;
+			listing.sha256.worldgen = await sha256(worldgen);
 			Deno.writeTextFileSync(`${BUILD_FOLDER}/${listing.worldgen}`, worldgen);
 		}
 
@@ -234,9 +250,9 @@ async function build() {
 		const now = performance.now();
 		clear_folder(BUILD_FOLDER);
 		const mods = load_mods();
-		await build_assets(mods);
+		const atlas = await build_assets(mods);
 		await build_client();
-		await build_mods(mods);
+		await build_mods(mods, atlas);
 		console.log(`Built in ${(performance.now() - now).toFixed(2)}ms`);
 	} catch (e) {
 		// no index means the server refuses to start, instead of running without some mods
diff --git a/client/confirm_mods.ts b/client/confirm_mods.ts
new file mode 100644
index 0000000..7ad1c76
--- /dev/null
+++ b/client/confirm_mods.ts
@@ -0,0 +1,50 @@
+import type { ServerAddress, Welcome } from "./handshake.ts";
+
+// asks before running a server's mods when the page came from somewhere else, see "Security" in MODS.md.
+// resolves with whether the player wants to join
+export function confirm_mods(address: ServerAddress, welcome: Welcome): Promise<boolean> {
+	return new Promise((resolve) => {
+		const overlay = document.createElement("div");
+		overlay.style.cssText =
+			"position:fixed;inset:0;display:flex;align-items:center;justify-content:center;padding:16px;" +
+			"background:rgba(0,0,0,0.85);color:white;font:16px system-ui,sans-serif;";
+
+		const panel = document.createElement("div");
+		panel.style.cssText = "max-width:480px;display:flex;flex-direction:column;gap:12px;";
+
+		const title = document.createElement("div");
+		title.style.cssText = "font-size:20px;";
+		title.textContent = `Join ${address.base.host}?`;
+
+		const warning = document.createElement("div");
+		warning.style.cssText = "opacity:0.8;";
+		warning.textContent = "This server runs these mods in your browser. They can do anything this page can, " +
+			"including reading what it saved. Only join servers you trust.";
+
+		const list = document.createElement("ul");
+		list.style.cssText = "margin:0;padding-left:20px;";
+		for (const mod of welcome.mods) {
+			const item = document.createElement("li");
+			item.textContent = `${mod.name} ${mod.version} (${mod.id})`;
+			list.append(item);
+		}
+
+		const buttons = document.createElement("div");
+		buttons.style.cssText = "display:flex;gap:8px;justify-content:flex-end;";
+		const button = (label: string, answer: boolean) => {
+			const element = document.createElement("button");
+			element.textContent = label;
+			element.style.cssText = "font:inherit;padding:6px 16px;cursor:pointer;";
+			element.addEventListener("click", () => {
+				overlay.remove();
+				resolve(answer);
+			});
+			return element;
+		};
+		buttons.append(button("Cancel", false), button("Join", true));
+
+		panel.append(title, warning, list, buttons);
+		overlay.append(panel);
+		document.body.append(overlay);
+	});
+}
diff --git a/client/handshake.ts b/client/handshake.ts
new file mode 100644
index 0000000..9c2eea2
--- /dev/null
+++ b/client/handshake.ts
@@ -0,0 +1,191 @@
+// connecting to a server and getting what it needs before joining, see "Delivery to clients" in MODS.md:
+// hello -> welcome, download and check everything, ready -> join
+import type { AtlasListing } from "$/common/mod_loader.ts";
+import { ClientMessage, PROTOCOL_VERSION, ServerMessage } from "$/common/protocol.ts";
+
+const CONNECT_TIMEOUT_MS = 5000;
+const TRUST_KEY_PREFIX = "bworld:trusted:";
+
+export type Welcome = Extract<ServerMessage, { type: "welcome" }>;
+export type Join = Extract<ServerMessage, { type: "join" }>;
+
+export interface ServerAddress {
+	ws_url: string;
+	// where the server's files are, mod paths are relative to this
+	base: URL;
+	// the page came from somewhere else than the server, so its mods need the player's ok
+	cross_origin: boolean;
+}
+
+// something went wrong in a way the player should see
+export class HandshakeError extends Error {}
+
+export function server_address(page = new URL(location.href)): ServerAddress {
+	// ?server=host:port, otherwise the server that served the page
+	const host = page.searchParams.get("server") ?? page.host;
+	const secure = page.protocol === "https:";
+	const base = new URL(`${secure ? "https" : "http"}://${host}/`);
+	return {
+		ws_url: `${secure ? "wss" : "ws"}://${host}/ws`,
+		base,
+		cross_origin: base.origin !== page.origin,
+	};
+}
+
+// a socket whose messages all land in one queue, so none get lost between the handshake and the game
+export class ServerSocket {
+	socket: WebSocket;
+	messages: ServerMessage[] = [];
+	closed = false;
+	#waiters: (() => void)[] = [];
+
+	constructor(socket: WebSocket) {
+		this.socket = socket;
+		socket.addEventListener("message", (event) => {
+			this.messages.push(JSON.parse(event.data));
+			this.#wake();
+		});
+		socket.addEventListener("close", () => {
+			this.closed = true;
+			this.#wake();
+		});
+	}
+
+	send(message: ClientMessage) {
+		if (this.socket.readyState === WebSocket.OPEN) {
+			this.socket.send(JSON.stringify(message));
+		}
+	}
+
+	close() {
+		this.socket.close();
+	}
+
+	// takes the first message of one of these types out of the queue, waiting for it if needed
+	async next<T extends ServerMessage["type"]>(...types: T[]): Promise<Extract<ServerMessage, { type: T }>> {
+		while (true) {
+			const index = this.messages.findIndex((m) => (types as string[]).includes(m.type));
+			if (index !== -1) {
+				return this.messages.splice(index, 1)[0] as Extract<ServerMessage, { type: T }>;
+			}
+			if (this.closed) {
+				throw new HandshakeError("The server closed the connection");
+			}
+			await new Promise<void>((resolve) => this.#waiters.push(resolve));
+		}
+	}
+
+	#wake() {
+		for (const waiter of this.#waiters.splice(0)) waiter();
+	}
+}
+
+export async function connect(
+	address: ServerAddress,
+	name: string,
+): Promise<{ socket: ServerSocket; welcome: Welcome }> {
+	const socket = await new Promise<WebSocket>((resolve, reject) => {
+		let ws: WebSocket;
+		try {
+			ws = new WebSocket(address.ws_url);
+		} catch {
+			reject(new HandshakeError(`Couldn't connect to the server at ${address.ws_url}`));
+			return;
+		}
+		const timeout = setTimeout(() => {
+			ws.close();
+			reject(new HandshakeError(`The server at ${address.ws_url} didn't answer`));
+		}, CONNECT_TIMEOUT_MS);
+		ws.addEventListener("open", () => {
+			clearTimeout(timeout);
+			resolve(ws);
+		});
+		ws.addEventListener("error", () => {
+			clearTimeout(timeout);
+			reject(new HandshakeError(`Couldn't connect to the server at ${address.ws_url}`));
+		});
+	});
+
+	const server = new ServerSocket(socket);
+	server.send({ type: "hello", name, protocol: PROTOCOL_VERSION });
+	const answer = await server.next("welcome", "rejected");
+	if (answer.type === "rejected") {
+		server.close();
+		throw new HandshakeError(answer.reason);
+	}
+	return { socket: server, welcome: answer };
+}
+
+// tell the server everything's loaded and wait to be let in
+export async function join(socket: ServerSocket): Promise<Join> {
+	socket.send({ type: "ready" });
+	const answer = await socket.next("join", "rejected");
+	if (answer.type === "rejected") {
+		socket.close();
+		throw new HandshakeError(answer.reason);
+	}
+	return answer;
+}
+
+// downloads a file and checks it's the one the server listed. use the bytes returned, never fetch it again
+export async function fetch_verified(url: URL, sha256: string, what: string): Promise<Uint8Array<ArrayBuffer>> {
+	let response: Response;
+	try {
+		response = await fetch(url);
+	} catch {
+		throw new HandshakeError(`Couldn't download ${what} from ${url}`);
+	}
+	if (!response.ok) {
+		throw new HandshakeError(`Couldn't download ${what} (${response.status})`);
+	}
+	const bytes = new Uint8Array(await response.arrayBuffer());
+	const digest = [...new Uint8Array(await crypto.subtle.digest("SHA-256", bytes))]
+		.map((b) => b.toString(16).padStart(2, "0"))
+		.join("");
+	if (digest !== sha256) {
+		throw new HandshakeError(`${what} doesn't match what the server listed`);
+	}
+	return bytes;
+}
+
+// a url for code that was already downloaded and checked, so importing it can't fetch something else
+export function code_url(bytes: Uint8Array<ArrayBuffer>): string {
+	return URL.createObjectURL(new Blob([bytes], { type: "text/javascript" }));
+}
+
+export async function load_atlas(address: ServerAddress, atlas: AtlasListing) {
+	const [png, json] = await Promise.all([
+		fetch_verified(new URL(atlas.png, address.base), atlas.sha256.png, "the texture atlas"),
+		fetch_verified(new URL(atlas.json, address.base), atlas.sha256.json, "the texture atlas"),
+	]);
+	return {
+		image: await createImageBitmap(new Blob([png], { type: "image/png" })),
+		regions: JSON.parse(new TextDecoder().decode(json)) as Record<string, { x: number; y: number }>,
+	};
+}
+
+// players ok a cross-origin server's mods once, per server and exact mod versions
+
+function trust_key(address: ServerAddress) {
+	return TRUST_KEY_PREFIX + address.base.origin;
+}
+
+function mod_versions(welcome: Welcome) {
+	return welcome.mods.map((mod) => `${mod.id}@${mod.hash}`).sort().join(",");
+}
+
+export function is_trusted(address: ServerAddress, welcome: Welcome): boolean {
+	try {
+		return localStorage.getItem(trust_key(address)) === mod_versions(welcome);
+	} catch {
+		return false;
+	}
+}
+
+export function remember_trust(address: ServerAddress, welcome: Welcome) {
+	try {
+		localStorage.setItem(trust_key(address), mod_versions(welcome));
+	} catch {
+		// private windows and blocked storage just ask again next time
+	}
+}
diff --git a/client/main.ts b/client/main.ts
index b514cc9..4422fef 100644
--- a/client/main.ts
+++ b/client/main.ts
@@ -1,8 +1,11 @@
 import { AssetManager } from "./assets.ts";
 import { ClientWorld } from "./client_world.ts";
 import { InputManager } from "./input_manager.ts";
-import { Connection, get_player_name, get_server_url } from "./network.ts";
-import { begin_drawing, clear_background, end_drawing, init_font, init_window } from "./renderer/mod.ts";
+import { Connection, get_player_name } from "./network.ts";
+import { connect, HandshakeError, is_trusted, join, load_atlas, remember_trust, server_address } from "./handshake.ts";
+import { confirm_mods } from "./confirm_mods.ts";
+import { ModLoadError } from "$/common/mod_loader.ts";
+import { begin_drawing, clear_background, end_drawing, init_font, init_window, load_texture } from "./renderer/mod.ts";
 import { is_stopped, show_fatal_error } from "./fatal.ts";
 import { load_client_mods, set_mods_world } from "./mods.ts";
 
@@ -76,9 +79,6 @@ InputManager.initialize(canvas);
 
 AssetManager.instance.load("bworld:assets_text", "/assets/ASSETS.md");
 
-AssetManager.instance.load("bworld:textures", "/assets/sprites/textures.png");
-AssetManager.instance.load("bworld:textures_info", "/assets/sprites/textures.json");
-
 AssetManager.instance.load("bworld:player", "/assets/sprites/player.png");
 AssetManager.instance.load("bworld:roguelike", "/assets/sprites/roguelike.png");
 AssetManager.instance.load("bworld:ui", "/assets/sprites/ui.png");
@@ -90,24 +90,37 @@ await AssetManager.instance.load_all();
 
 init_font();
 
-// the game only runs against a server, it owns the world and everything in it
-const connection = await Connection.open(get_server_url(), get_player_name());
-let mods_loaded = false;
-if (connection) {
-	console.log(`Connected to ${get_server_url()}`);
+// the game only runs against a server, it owns the world and everything in it. see "Delivery to clients" in MODS.md
+async function join_server(): Promise<Connection> {
+	const address = server_address();
+	const { socket, welcome } = await connect(address, get_player_name());
+	console.log(`Connected to ${address.ws_url}`);
+
 	try {
-		// blocks, items and textures all come from mods, so this happens before anything else
-		await load_client_mods(connection.mods);
-		console.log(`Mods: ${connection.mods.map((mod) => `${mod.id} ${mod.version}`).join(", ") || "none"}`);
-		mods_loaded = true;
+		if (address.cross_origin && !is_trusted(address, welcome)) {
+			if (!await confirm_mods(address, welcome)) {
+				throw new HandshakeError(`You didn't join ${address.base.host}`);
+			}
+			remember_trust(address, welcome);
+		}
+
+		// textures, blocks and items all come from the server's mods, so this happens before anything else
+		const atlas = await load_atlas(address, welcome.atlas);
+		AssetManager.instance.assets["bworld:textures"] = load_texture(atlas.image);
+		AssetManager.instance.assets["bworld:textures_info"] = atlas.regions;
+		await load_client_mods(welcome.mods, address.base);
+		console.log(`Mods: ${welcome.mods.map((mod) => `${mod.id} ${mod.version}`).join(", ") || "none"}`);
+
+		const joined = await join(socket);
+		return new Connection(socket, welcome, joined);
 	} catch (e) {
-		console.error(e);
-		show_fatal_error(`Couldn't load the server's mods: ${e instanceof Error ? e.message : e}`);
-		connection.socket.close();
+		socket.close();
+		throw e;
 	}
 }
 
-if (connection && mods_loaded) {
+try {
+	const connection = await join_server();
 	const client_world = new ClientWorld(connection);
 	set_mods_world(client_world);
 	client_world.add_chat("Connected to the server");
@@ -116,6 +129,12 @@ if (connection && mods_loaded) {
 	loop.start();
 
 	console.log("Game started");
-} else if (!connection) {
-	show_fatal_error(`Couldn't connect to the server at ${get_server_url()}`);
+} catch (e) {
+	console.error(e);
+	const message = e instanceof HandshakeError
+		? e.message
+		: e instanceof ModLoadError
+		? `Couldn't load the server's mods: ${e.message}`
+		: `Something went wrong joining the server: ${e instanceof Error ? e.message : e}`;
+	show_fatal_error(message);
 }
diff --git a/client/mods.ts b/client/mods.ts
index 8a1125f..459e7f1 100644
--- a/client/mods.ts
+++ b/client/mods.ts
@@ -8,6 +8,7 @@ import type { OreJson } from "$/common/mod_data.ts";
 import { AIR_ID } from "$/common/protocol.ts";
 import { Position } from "$/common/components/position.ts";
 import type { ClientWorld } from "./client_world.ts";
+import { code_url, fetch_verified } from "./handshake.ts";
 
 // what the chunk workers need to generate the same world as the server
 export const worldgen_mods: { scripts: { mod: string; url: string }[]; ores: OreJson[] } = { scripts: [], ores: [] };
@@ -19,26 +20,36 @@ export function set_mods_world(client_world: ClientWorld) {
 	world = client_world;
 }
 
-export async function load_client_mods(listings: ModListing[]) {
-	const site = new URL("/", location.href);
-
-	const datas = await Promise.all(listings.map(async (listing) => {
-		const response = await fetch(new URL(listing.data, site));
-		if (!response.ok) {
-			throw new ModLoadError(listing.id, `couldn't download its data (${response.status})`);
-		}
-		return { id: listing.id, data: await response.json() as ModData };
+// downloads every mod's files and checks them against the hashes the server listed, then registers the data and
+// runs the client scripts. everything a mod runs is imported from the checked bytes, never fetched twice
+export async function load_client_mods(listings: ModListing[], base: URL) {
+	const downloads = await Promise.all(listings.map(async (listing) => {
+		const get = async (path: string | undefined, sha256: string | undefined, what: string) => {
+			if (!path) return undefined;
+			try {
+				return await fetch_verified(new URL(path, base), sha256 ?? "", what);
+			} catch (e) {
+				throw new ModLoadError(listing.id, (e as Error).message);
+			}
+		};
+		const [data, client, worldgen] = await Promise.all([
+			get(listing.data, listing.sha256.data, "its data"),
+			get(listing.client, listing.sha256.client, "its client script"),
+			get(listing.worldgen, listing.sha256.worldgen, "its worldgen script"),
+		]);
+		return { listing, data: JSON.parse(new TextDecoder().decode(data)) as ModData, client, worldgen };
 	}));
-	const recipes = register_mod_data(datas);
+
+	const recipes = register_mod_data(downloads.map(({ listing, data }) => ({ id: listing.id, data })));
 
 	worldgen_mods.ores = recipes.ores;
-	worldgen_mods.scripts = listings.flatMap((listing) =>
-		listing.worldgen ? [{ mod: listing.id, url: new URL(listing.worldgen, site).href }] : []
+	worldgen_mods.scripts = downloads.flatMap(({ listing, worldgen }) =>
+		worldgen ? [{ mod: listing.id, url: code_url(worldgen) }] : []
 	);
 
-	for (const listing of listings) {
-		if (!listing.client) continue;
-		const module = await import(new URL(listing.client, site).href);
+	for (const { listing, client } of downloads) {
+		if (!client) continue;
+		const module = await import(code_url(client));
 		if (typeof module.setup !== "function") {
 			throw new ModLoadError(listing.id, "the client script doesn't export a setup function");
 		}
diff --git a/client/network.ts b/client/network.ts
index f15e394..4692f8a 100644
--- a/client/network.ts
+++ b/client/network.ts
@@ -1,7 +1,6 @@
 import { BlockChange, ClientMessage, PlayerInfo, ServerMessage } from "$/common/protocol.ts";
 import type { ModListing } from "$/common/mod_loader.ts";
-
-const CONNECT_TIMEOUT_MS = 3000;
+import type { Join, ServerSocket, Welcome } from "./handshake.ts";
 
 export interface RemotePlayer extends PlayerInfo {
 	// where we draw them, eased towards x/y/z so movement isnt choppy
@@ -12,7 +11,7 @@ export interface RemotePlayer extends PlayerInfo {
 }
 
 export class Connection {
-	socket: WebSocket;
+	#server: ServerSocket;
 	id: string;
 	name: string;
 	seed: string;
@@ -22,35 +21,31 @@ export class Connection {
 	selected_slot: number;
 	players = new Map<string, RemotePlayer>();
 
-	// handled by the network system inside the game loop, not whenever the socket feels like it
-	incoming: ServerMessage[] = [];
-	closed = false;
-
-	constructor(socket: WebSocket, welcome: Extract<ServerMessage, { type: "welcome" }>) {
-		this.socket = socket;
-		this.id = welcome.id;
-		this.name = welcome.name;
+	constructor(server: ServerSocket, welcome: Welcome, join: Join) {
+		this.#server = server;
+		this.id = join.id;
+		this.name = join.name;
 		this.seed = welcome.seed;
 		this.mods = welcome.mods;
-		this.initial_changes = welcome.changes;
-		this.spawn = welcome.spawn;
-		this.selected_slot = welcome.selected_slot;
-		for (const player of welcome.players) {
+		this.initial_changes = join.changes;
+		this.spawn = join.spawn;
+		this.selected_slot = join.selected_slot;
+		for (const player of join.players) {
 			this.add_player(player);
 		}
+	}
 
-		socket.addEventListener("message", (event) => {
-			this.incoming.push(JSON.parse(event.data));
-		});
-		socket.addEventListener("close", () => {
-			this.closed = true;
-		});
+	// handled by the network system inside the game loop, not whenever the socket feels like it
+	get incoming(): ServerMessage[] {
+		return this.#server.messages;
+	}
+
+	get closed() {
+		return this.#server.closed;
 	}
 
 	send(message: ClientMessage) {
-		if (this.socket.readyState === WebSocket.OPEN) {
-			this.socket.send(JSON.stringify(message));
-		}
+		this.#server.send(message);
 	}
 
 	add_player(player: PlayerInfo) {
@@ -62,40 +57,6 @@ export class Connection {
 			color: color_from_name(player.name),
 		});
 	}
-
-	static open(url: string, name: string): Promise<Connection | undefined> {
-		return new Promise((resolve) => {
-			let socket: WebSocket;
-			try {
-				socket = new WebSocket(url);
-			} catch {
-				resolve(undefined);
-				return;
-			}
-
-			const timeout = setTimeout(() => {
-				socket.close();
-				resolve(undefined);
-			}, CONNECT_TIMEOUT_MS);
-
-			socket.addEventListener("open", () => {
-				socket.send(JSON.stringify({ type: "hello", name } satisfies ClientMessage));
-			});
-
-			socket.addEventListener("message", (event) => {
-				const message: ServerMessage = JSON.parse(event.data);
-				if (message.type === "welcome") {
-					clearTimeout(timeout);
-					resolve(new Connection(socket, message));
-				}
-			}, { once: true });
-
-			socket.addEventListener("error", () => {
-				clearTimeout(timeout);
-				resolve(undefined);
-			});
-		});
-	}
 }
 
 function color_from_name(name: string): [number, number, number] {
@@ -122,13 +83,6 @@ function color_from_name(name: string): [number, number, number] {
 	return [r + m, g + m, b + m];
 }
 
-export function get_server_url(): string {
-	const params = new URLSearchParams(location.search);
-	const server = params.get("server");
-	const protocol = location.protocol === "https:" ? "wss:" : "ws:";
-	return `${protocol}//${server ?? location.host}/ws`;
-}
-
 export function get_player_name(): string {
 	return new URLSearchParams(location.search).get("name") ?? "";
 }
diff --git a/client/renderer/textures.ts b/client/renderer/textures.ts
index 60b89f3..7d0b829 100644
--- a/client/renderer/textures.ts
+++ b/client/renderer/textures.ts
@@ -9,7 +9,7 @@ import {
 } from "./core.ts";
 import { Texture } from "./types.ts";
 
-export function load_texture(image: HTMLImageElement): Texture {
+export function load_texture(image: HTMLImageElement | ImageBitmap): Texture {
 	const texture = create_texture(image.width, image.height);
 	device.queue.copyExternalImageToTexture({ source: image }, { texture }, [image.width, image.height]);
 
diff --git a/common/mod_loader.ts b/common/mod_loader.ts
index e59b43c..847d1e7 100644
--- a/common/mod_loader.ts
+++ b/common/mod_loader.ts
@@ -20,15 +20,25 @@ export interface ModData {
 	ores: OreJson[];
 }
 
-// a mod as the server lists it to clients, in load order. paths are relative to the site root
+// a mod as the server lists it to clients, in load order. paths are relative to the server's root
 export interface ModListing {
 	id: string;
 	name: string;
 	version: string;
+	// short hash of everything public, the folder it's served from
 	hash: string;
 	data: string;
 	client?: string;
 	worldgen?: string;
+	// sha-256 in hex of each file, clients check these before using them
+	sha256: { data: string; client?: string; worldgen?: string };
+}
+
+// the texture atlas with every mod's textures, built by the server
+export interface AtlasListing {
+	png: string;
+	json: string;
+	sha256: { png: string; json: string };
 }
 
 export class RecipeBook {
diff --git a/common/protocol.ts b/common/protocol.ts
index cbea30f..41c5ec1 100644
--- a/common/protocol.ts
+++ b/common/protocol.ts
@@ -1,10 +1,14 @@
 // messages sent between the client and the server, as json over a websocket
 import type { Faces } from "./constants.ts";
 import type { ItemData } from "./inventory.ts";
-import type { ModListing } from "./mod_loader.ts";
+import type { AtlasListing, ModListing } from "./mod_loader.ts";
 
 export const AIR_ID = "bworld:air";
 
+// bump when a client and server of different versions can't play together.
+// the server rejects a different version before the client downloads anything
+export const PROTOCOL_VERSION = 1;
+
 export interface PlayerInfo {
 	id: string;
 	name: string;
@@ -44,7 +48,9 @@ export interface ScreenLayout {
 
 // clients send what the player is trying to do, the server decides what happens
 export type ClientMessage =
-	| { type: "hello"; name: string }
+	| { type: "hello"; name: string; protocol: number }
+	// sent once the client has downloaded, checked and loaded everything in welcome
+	| { type: "ready" }
 	| { type: "move"; x: number; y: number; z: number; yaw: number; pitch: number }
 	| { type: "break_block"; x: number; y: number; z: number }
 	// right click on a block: interact with it, or place the held block against `face`
@@ -56,14 +62,23 @@ export type ClientMessage =
 	| { type: "chat"; text: string };
 
 export type ServerMessage =
+	// what to download before joining, see "Delivery to clients" in MODS.md
 	| {
 		type: "welcome";
+		protocol: number;
+		seed: string;
+		atlas: AtlasListing;
+		// in load order, the client loads them before joining the world
+		mods: ModListing[];
+	}
+	// the connection is closed right after
+	| { type: "rejected"; reason: string }
+	// answers ready, the player is in the world from here
+	| {
+		type: "join";
 		id: string;
 		// the server may change the name asked for, like alice to alice2
 		name: string;
-		seed: string;
-		// in load order, the client loads them before joining the world
-		mods: ModListing[];
 		players: PlayerInfo[];
 		changes: BlockChange[];
 		spawn: { x: number; y: number; z: number; yaw: number; pitch: number };
diff --git a/server/game/game_server.ts b/server/game/game_server.ts
index f34005f..0c5d9df 100644
--- a/server/game/game_server.ts
+++ b/server/game/game_server.ts
@@ -18,9 +18,10 @@ import {
 	CRAFTING_RESULT_SLOT,
 	MAX_CHAT_LENGTH,
 	MAX_NAME_LENGTH,
+	PROTOCOL_VERSION,
 	ServerMessage,
 } from "$/common/protocol.ts";
-import type { ModListing, RecipeBook } from "$/common/mod_loader.ts";
+import type { AtlasListing, ModListing, RecipeBook } from "$/common/mod_loader.ts";
 import type { WorldgenSetup } from "$/common/generation.ts";
 import { BLOCK_BEHAVIORS } from "./blocks.ts";
 import { ModRuntime, run_guarded } from "./mod_runtime.ts";
@@ -34,6 +35,8 @@ const EYE_HEIGHT = 1.69;
 // tiles further than this many chunks from every player don't tick
 const SIMULATION_DISTANCE = 6;
 const MAX_GIVE = 64 * 36;
+// how long a client gets between welcome and ready to download and load everything
+const READY_TIMEOUT_TICKS = 60 * TICKS_PER_SECOND;
 
 // everything the game server needs from whatever runs it
 export interface GameHost {
@@ -63,6 +66,7 @@ export interface SavedWorld {
 // the loaded mods, see load_mods.ts
 export interface GameMods {
 	listings: ModListing[];
+	atlas: AtlasListing;
 	recipes: RecipeBook;
 	worldgen?: WorldgenSetup;
 	runtime: ModRuntime;
@@ -72,6 +76,8 @@ export class GameServer {
 	world: ServerWorld;
 	#host: GameHost;
 	#players = new Map<number, ServerPlayer>();
+	// connections that got welcome and are downloading mods, by conn
+	#pending = new Map<number, { name: unknown; since: number }>();
 	#saved_players: Record<string, SavedPlayer> = {};
 	#tick = 0;
 	#mods: GameMods;
@@ -108,6 +114,7 @@ export class GameServer {
 	on_connect(_conn: number) {}
 
 	on_disconnect(conn: number) {
+		this.#pending.delete(conn);
 		const player = this.#players.get(conn);
 		if (!player) {
 			return;
@@ -135,9 +142,7 @@ export class GameServer {
 
 		const player = this.#players.get(conn);
 		if (!player) {
-			if (message.type === "hello") {
-				this.#join(conn, message.name);
-			}
+			this.#handshake(conn, message);
 			return;
 		}
 
@@ -182,6 +187,13 @@ export class GameServer {
 
 		this.mods.tick();
 
+		for (const [conn, pending] of this.#pending) {
+			if (this.#tick - pending.since > READY_TIMEOUT_TICKS) {
+				this.#pending.delete(conn);
+				this.#reject(conn, "Took too long to load the server's mods");
+			}
+		}
+
 		for (const player of this.#players.values()) {
 			this.#sync(player);
 		}
@@ -308,6 +320,52 @@ export class GameServer {
 
 	// messages
 
+	// hello -> welcome, then ready -> join. see "Delivery to clients" in MODS.md
+	#handshake(conn: number, message: ClientMessage) {
+		if (message.type === "hello") {
+			if (this.#pending.has(conn)) {
+				return;
+			}
+			if (message.protocol !== PROTOCOL_VERSION) {
+				this.#reject(
+					conn,
+					`This server needs a game version with protocol ${PROTOCOL_VERSION}, yours has ${
+						message.protocol ?? "none"
+					}. Reload the page to update.`,
+				);
+				return;
+			}
+			this.#pending.set(conn, { name: message.name, since: this.#tick });
+			this.#host.send(
+				conn,
+				JSON.stringify(
+					{
+						type: "welcome",
+						protocol: PROTOCOL_VERSION,
+						seed: this.world.seed,
+						atlas: this.#mods.atlas,
+						mods: this.#mods.listings,
+					} satisfies ServerMessage,
+				),
+			);
+			return;
+		}
+
+		if (message.type === "ready") {
+			const pending = this.#pending.get(conn);
+			if (!pending) {
+				return;
+			}
+			this.#pending.delete(conn);
+			this.#join(conn, pending.name);
+		}
+	}
+
+	#reject(conn: number, reason: string) {
+		this.#host.send(conn, JSON.stringify({ type: "rejected", reason } satisfies ServerMessage));
+		this.#host.close(conn);
+	}
+
 	#join(conn: number, raw_name: unknown) {
 		const player = new ServerPlayer(conn, this.#unique_name(raw_name));
 		const saved = this.#saved_players[player.name];
@@ -316,11 +374,9 @@ export class GameServer {
 		}
 
 		this.#send(player, {
-			type: "welcome",
+			type: "join",
 			id: player.id,
 			name: player.name,
-			seed: this.world.seed,
-			mods: this.#mods.listings,
 			players: [...this.#players.values()].map((p) => p.info()),
 			changes: this.world.all_changes(),
 			spawn: { x: player.x, y: player.y, z: player.z, yaw: player.yaw, pitch: player.pitch },
diff --git a/server/game/host_protocol.ts b/server/game/host_protocol.ts
index a226496..659ec12 100644
--- a/server/game/host_protocol.ts
+++ b/server/game/host_protocol.ts
@@ -1,4 +1,4 @@
-import type { ModData, ModListing } from "$/common/mod_loader.ts";
+import type { AtlasListing, ModData, ModListing } from "$/common/mod_loader.ts";
 
 // messages between server/main.ts (the host) and the game server worker
 
@@ -7,6 +7,7 @@ export type HostToGame =
 		type: "init";
 		save: string | undefined;
 		default_seed: string;
+		atlas: AtlasListing;
 		// in load order, with the scripts' code since the worker can't read files
 		mods: { listing: ModListing; data: ModData; server_code?: string; worldgen_code?: string }[];
 	}
diff --git a/server/game/load_mods.ts b/server/game/load_mods.ts
index 3db2cef..9177a4d 100644
--- a/server/game/load_mods.ts
+++ b/server/game/load_mods.ts
@@ -1,7 +1,7 @@
 // starts a game server with its mods: registers their data, then imports worldgen and server scripts.
 // the worker loads built mods, tests can load mods straight from their source folders
 import { register_mod_data } from "$/common/mod_loader.ts";
-import type { ModData, ModListing } from "$/common/mod_loader.ts";
+import type { AtlasListing, ModData, ModListing } from "$/common/mod_loader.ts";
 import { load_worldgen } from "$/common/worldgen_loader.ts";
 import { add_base_item_hooks } from "./blocks.ts";
 import { GameHost, GameServer } from "./game_server.ts";
@@ -19,6 +19,7 @@ export async function start_game(
 	host: GameHost,
 	save: string | undefined,
 	default_seed: string,
+	atlas: AtlasListing,
 	mods: ServerModSource[],
 ): Promise<GameServer> {
 	const recipes = register_mod_data(mods.map((mod) => ({ id: mod.listing.id, data: mod.data })));
@@ -32,6 +33,7 @@ export async function start_game(
 	const runtime = new ModRuntime();
 	const game = new GameServer(host, save, default_seed, {
 		listings: mods.map((mod) => mod.listing),
+		atlas,
 		recipes,
 		worldgen,
 		runtime,
diff --git a/server/game/worker.ts b/server/game/worker.ts
index 13eebe5..e8a27c5 100644
--- a/server/game/worker.ts
+++ b/server/game/worker.ts
@@ -54,6 +54,7 @@ async function init(message: Extract<HostToGame, { type: "init" }>) {
 			},
 			message.save,
 			message.default_seed,
+			message.atlas,
 			message.mods.map((mod) => ({
 				listing: mod.listing,
 				data: mod.data,
diff --git a/server/main.ts b/server/main.ts
index f4b6e47..00756e9 100644
--- a/server/main.ts
+++ b/server/main.ts
@@ -70,7 +70,7 @@ game.onmessage = (event: MessageEvent<GameToHost>) => {
 };
 
 // what deno task build made. no index means the build failed or never ran, and the game never runs without its mods
-function read_mods(): Extract<HostToGame, { type: "init" }>["mods"] {
+function read_mods(): Pick<Extract<HostToGame, { type: "init" }>, "mods" | "atlas"> {
 	let index: ServerModIndex;
 	try {
 		index = JSON.parse(Deno.readTextFileSync(`${SERVER_MODS_DIR}/index.json`));
@@ -78,12 +78,15 @@ function read_mods(): Extract<HostToGame, { type: "init" }>["mods"] {
 		console.error(`No ${SERVER_MODS_DIR}/index.json, run deno task build first (it also reports mod errors)`);
 		Deno.exit(1);
 	}
-	return index.mods.map(({ listing, server }) => ({
-		listing,
-		data: JSON.parse(Deno.readTextFileSync(`${STATIC_ROOT}/${listing.data}`)),
-		server_code: server ? Deno.readTextFileSync(server) : undefined,
-		worldgen_code: listing.worldgen ? Deno.readTextFileSync(`${STATIC_ROOT}/${listing.worldgen}`) : undefined,
-	}));
+	return {
+		atlas: index.atlas,
+		mods: index.mods.map(({ listing, server }) => ({
+			listing,
+			data: JSON.parse(Deno.readTextFileSync(`${STATIC_ROOT}/${listing.data}`)),
+			server_code: server ? Deno.readTextFileSync(server) : undefined,
+			worldgen_code: listing.worldgen ? Deno.readTextFileSync(`${STATIC_ROOT}/${listing.worldgen}`) : undefined,
+		})),
+	};
 }
 
 game.onerror = (event) => {
@@ -91,10 +94,25 @@ game.onerror = (event) => {
 	Deno.exit(1);
 };
 
-const mods = read_mods();
+const { mods, atlas } = read_mods();
 console.log(`Mods: ${mods.map((mod) => `${mod.listing.id} ${mod.listing.version}`).join(", ") || "none"}`);
 const save = read_world();
-post({ type: "init", save, default_seed: Deno.env.get("SEED") ?? crypto.randomUUID(), mods });
+post({ type: "init", save, default_seed: Deno.env.get("SEED") ?? crypto.randomUUID(), atlas, mods });
+
+// mod files and the atlas are named by their hash, so they never change and pages on other origins can load them
+async function serve_static(req: Request, url: URL) {
+	const response = await serveDir(req, { fsRoot: STATIC_ROOT, quiet: true });
+	const shared = url.pathname.startsWith("/mods/") || url.pathname.startsWith("/assets/");
+	if (shared && response.ok) {
+		const headers = new Headers(response.headers);
+		headers.set("Access-Control-Allow-Origin", "*");
+		if (url.pathname.startsWith("/mods/") || /^\/assets\/sprites\/textures\.[0-9a-f]+\./.test(url.pathname)) {
+			headers.set("Cache-Control", "public, max-age=31536000, immutable");
+		}
+		return new Response(response.body, { status: response.status, statusText: response.statusText, headers });
+	}
+	return response;
+}
 
 function handle_socket(socket: WebSocket) {
 	const conn = next_conn++;
@@ -147,5 +165,5 @@ Deno.serve({ port: PORT, onListen: ({ port }) => console.log(`bworld server on h
 		return Response.redirect(new URL("/client/", url), 302);
 	}
 
-	return serveDir(req, { fsRoot: STATIC_ROOT, quiet: true });
+	return serve_static(req, url);
 });
diff --git a/tests/handshake_test.ts b/tests/handshake_test.ts
new file mode 100644
index 0000000..7299d3e
--- /dev/null
+++ b/tests/handshake_test.ts
@@ -0,0 +1,72 @@
+import { assert, assertEquals } from "@std/assert";
+import { PROTOCOL_VERSION } from "$/common/protocol.ts";
+import { TICKS_PER_SECOND } from "$/common/constants.ts";
+import { test_game } from "./helpers.ts";
+
+Deno.test("a different protocol version is rejected before anything else", async () => {
+	const { game, take, send, closed } = await test_game("mods");
+	game.on_connect(1);
+	send(1, { type: "hello", name: "old", protocol: PROTOCOL_VERSION - 1 });
+	const [message] = take(1);
+	assertEquals(message.type, "rejected");
+	assert(message.reason.includes(`protocol ${PROTOCOL_VERSION}`), message.reason);
+	assert(closed.has(1));
+
+	game.on_connect(2);
+	send(2, { type: "hello", name: "ancient" });
+	assertEquals(take(2)[0].type, "rejected");
+});
+
+Deno.test("welcome lists what to download, and nothing happens until ready", async () => {
+	const { game, take, send, join } = await test_game("mods");
+	join(1, "alice");
+	take(1);
+
+	let joined = 0;
+	game.mods.after.player_join.for_mod("test").subscribe(() => joined++);
+
+	game.on_connect(2);
+	send(2, { type: "hello", name: "bob", protocol: PROTOCOL_VERSION });
+	const [welcome] = take(2);
+	assertEquals(welcome.type, "welcome");
+	assertEquals(welcome.mods.map((m: { id: string }) => m.id), ["bworld"]);
+	assert(welcome.atlas.png && welcome.atlas.sha256);
+	assert(!("players" in welcome) && !("changes" in welcome), "welcome shouldn't have world state");
+
+	// not in the world yet: others don't hear about bob, and bob can't act
+	send(2, { type: "chat", text: "am I here?" });
+	send(2, { type: "break_block", x: 0, y: 60, z: 0 });
+	assertEquals(take(1), []);
+	assertEquals(take(2), []);
+	assertEquals(joined, 0);
+
+	send(2, { type: "ready" });
+	const messages = take(2);
+	assertEquals(messages[0].type, "join");
+	assertEquals(messages[0].name, "bob");
+	assertEquals(messages[0].players.map((p: { name: string }) => p.name), ["alice"]);
+	assert(messages.some((m) => m.type === "container" && m.container === "inventory"));
+	assertEquals(joined, 1);
+	assert(take(1).some((m) => m.type === "player_join" && m.player.name === "bob"));
+
+	// a second ready or hello does nothing
+	send(2, { type: "ready" });
+	send(2, { type: "hello", name: "bob", protocol: PROTOCOL_VERSION });
+	assertEquals(take(2).filter((m) => m.type === "join" || m.type === "welcome"), []);
+	assertEquals(joined, 1);
+});
+
+Deno.test("clients that never get ready are dropped", async () => {
+	const { game, take, send, closed } = await test_game("mods");
+	game.on_connect(1);
+	send(1, { type: "hello", name: "slow", protocol: PROTOCOL_VERSION });
+	take(1);
+	for (let i = 0; i < 59 * TICKS_PER_SECOND; i++) game.tick();
+	assert(!closed.has(1), "dropped too early");
+	for (let i = 0; i < 2 * TICKS_PER_SECOND; i++) game.tick();
+	assert(closed.has(1));
+	assertEquals(take(1)[0].type, "rejected");
+	// too late now
+	send(1, { type: "ready" });
+	assertEquals(take(1), []);
+});
diff --git a/tests/helpers.ts b/tests/helpers.ts
index 0bda5cd..194a20f 100644
--- a/tests/helpers.ts
+++ b/tests/helpers.ts
@@ -4,6 +4,7 @@ import { load_and_check, load_order } from "$/tools/check_mods.ts";
 import type { ServerModSource } from "$/server/game/load_mods.ts";
 import { GameServer } from "$/server/game/game_server.ts";
 import { start_game } from "$/server/game/load_mods.ts";
+import { PROTOCOL_VERSION } from "$/common/protocol.ts";
 
 // mods straight from their source folders, skipping the build: scripts are imported as typescript
 export function mod_sources(mods_dir: string): ServerModSource[] {
@@ -20,6 +21,7 @@ export function mod_sources(mods_dir: string): ServerModSource[] {
 				version: String(mod.manifest?.version),
 				hash: "source",
 				data: `mods/${mod.id}/source/data.json`,
+				sha256: { data: "" },
 			},
 			data: {
 				blocks: mod.blocks.map((b) => b.json),
@@ -37,14 +39,18 @@ export function mod_sources(mods_dir: string): ServerModSource[] {
 export async function test_game(mods_dir: string, save?: string, seed = "test-seed") {
 	EverythingRegistry.clear();
 	const outbox = new Map<number, unknown[]>();
+	const closed = new Set<number>();
 	const host = {
 		send: (conn: number, data: string) => {
 			if (!outbox.has(conn)) outbox.set(conn, []);
 			outbox.get(conn)!.push(JSON.parse(data));
 		},
-		close() {},
+		close(conn: number) {
+			closed.add(conn);
+		},
 	};
-	const game: GameServer = await start_game(host, save, seed, mod_sources(mods_dir));
+	const atlas = { png: "atlas.png", json: "atlas.json", sha256: { png: "", json: "" } };
+	const game: GameServer = await start_game(host, save, seed, atlas, mod_sources(mods_dir));
 	// deno-lint-ignore no-explicit-any
 	const take = (conn: number): any[] => {
 		const messages = outbox.get(conn) ?? [];
@@ -52,7 +58,14 @@ export async function test_game(mods_dir: string, save?: string, seed = "test-se
 		return messages;
 	};
 	const send = (conn: number, message: unknown) => game.on_message(conn, JSON.stringify(message));
-	return { game, take, send };
+	// the whole handshake: hello, welcome, ready, join. returns join
+	const join = (conn: number, name: string) => {
+		game.on_connect(conn);
+		send(conn, { type: "hello", name, protocol: PROTOCOL_VERSION });
+		send(conn, { type: "ready" });
+		return take(conn).find((m) => m.type === "join");
+	};
+	return { game, take, send, join, closed };
 }
 
 export function copy_dir(from: string, to: string) {
diff --git a/tests/mod_loading_test.ts b/tests/mod_loading_test.ts
index 78f2aba..2d58191 100644
--- a/tests/mod_loading_test.ts
+++ b/tests/mod_loading_test.ts
@@ -5,6 +5,7 @@ import { block_from_json, block_to_json, item_from_json, item_to_json } from "$/
 import { generate_raw_chunk } from "$/common/generation.ts";
 import { load_worldgen } from "$/common/worldgen_loader.ts";
 import { create_mod } from "$/tools/new_mod.ts";
+import { PROTOCOL_VERSION } from "$/common/protocol.ts";
 import { copy_dir, test_game } from "./helpers.ts";
 
 // mods/bworld plus a mod made from the template, in a temp folder
@@ -59,9 +60,10 @@ Deno.test("a mod made from the template loads and runs", async () => {
 	);
 
 	game.on_connect(1);
-	send(1, { type: "hello", name: "alice" });
+	send(1, { type: "hello", name: "alice", protocol: PROTOCOL_VERSION });
 	const welcome = take(1)[0];
 	assertEquals(welcome.mods.map((m: Msg) => m.id), ["bworld", "copper_tools"]);
+	send(1, { type: "ready" });
 
 	// the command, by name and by namespaced name
 	send(1, { type: "chat", text: "/hello" });
@@ -180,9 +182,8 @@ export function setup(ctx: ServerContext) {
 }
 `,
 	);
-	let { game, send, take } = await test_game(dir);
-	game.on_connect(1);
-	send(1, { type: "hello", name: "bob" });
+	let { game, send, take, join } = await test_game(dir);
+	join(1, "bob");
 	for (let i = 0; i < 5; i++) game.tick();
 	game.set_block(0, 100, 0, "copper_tools:example_block");
 	send(1, { type: "move", x: 0.5, y: 100, z: 1.5, yaw: 0, pitch: 0 });
@@ -190,9 +191,8 @@ export function setup(ctx: ServerContext) {
 	assert(take(1).some((m) => m.text === "clicks 1, placed at 5"));
 
 	const saved = game.save();
-	({ game, send, take } = await test_game(dir, saved));
-	game.on_connect(1);
-	send(1, { type: "hello", name: "bob" });
+	({ game, send, take, join } = await test_game(dir, saved));
+	join(1, "bob");
 	send(1, { type: "use_block", x: 0, y: 100, z: 0, face: "top" });
 	assert(take(1).some((m) => m.text === "clicks 2, placed at 5"));
 	Deno.removeSync(dir, { recursive: true });