diff --git a/src/lib/demil.ts b/src/lib/demil.ts
new file mode 100644
index 0000000..978b735
--- /dev/null
+++ b/src/lib/demil.ts
@@ -0,0 +1,152 @@
+import semver from 'semver';
+export interface StartMissionOptions {
+	steamID?: string;
+	seed?: number;
+	force?: boolean;
+}
+export interface StartMissionByNameOptions {
+	seed?: number;
+	force?: boolean;
+}
+
+export interface DeMiLResultBase {
+	Version?: string;
+	IsVersionInRange?: boolean;
+}
+
+export interface StartMissionSuccessResult extends DeMiLResultBase {
+	MissionID: string;
+	Seed: string;
+}
+export interface StartMissionMissingModulesResult extends DeMiLResultBase {
+	MissionID: string;
+	MissingModules: string[];
+}
+export interface StartMissionTooManyModulesResult extends DeMiLResultBase {
+	MissionID: string;
+	MaximumSupportedModulesCount: number;
+	MissionModulesCount: number;
+}
+export interface StartMissionTooManyModulesFrontfaceResult extends DeMiLResultBase {
+	MissionID: string;
+	MaximumSupportedFrontfaceModulesCount: number;
+	MissionModulesCount: number;
+}
+export interface StartMissionTooManyBombsResult extends DeMiLResultBase {
+	MissionID: string;
+	MaximumSupportedBombsCount: number;
+	MissionBombsCount: number;
+}
+export type StartMissionResult =
+	| StartMissionSuccessResult
+	| StartMissionMissingModulesResult
+	| StartMissionTooManyModulesResult
+	| StartMissionTooManyModulesFrontfaceResult
+	| StartMissionTooManyBombsResult;
+
+export interface DeMiLErrorResponse extends DeMiLResultBase {
+	ERROR: string;
+	Stacktrace?: string;
+}
+
+export class DeMiLError extends Error {
+	public internalStack?: string;
+	public version?: string;
+	public isVersionInRange: boolean;
+
+	public constructor(obj: DeMiLErrorResponse) {
+		super(obj.ERROR);
+		this.internalStack = obj.Stacktrace;
+		this.version = obj.Version;
+		this.isVersionInRange = obj.IsVersionInRange ?? false;
+	}
+
+	public static isDeMiLErrorResponse(obj: any): obj is DeMiLErrorResponse {
+		return obj.hasOwnProperty('ERROR') && typeof obj['ERROR'] === 'string';
+	}
+
+	static modNotFoundRegex = /Mod with steamID (\d+) not found/;
+	public static ParseIfErrorResponse(obj: any): DeMiLError | undefined {
+		if (this.isDeMiLErrorResponse(obj)) {
+			const modNotFoundMatch = obj.ERROR.match(DeMiLError.modNotFoundRegex);
+
+			if (modNotFoundMatch !== null) {
+				return new DeMiLModNotFoundError({ ...obj, steamID: modNotFoundMatch[1] });
+			}
+			return new DeMiLError(obj);
+		}
+		return;
+	}
+}
+
+export class DeMiLModNotFoundError extends DeMiLError {
+	public steamID?: string;
+
+	public constructor(obj: DeMiLErrorResponse & { steamID: string }) {
+		super(obj);
+		this.steamID = obj.steamID;
+	}
+}
+
+const versionRange = '>=2.1.1';
+
+export class DeMiLClient {
+	constructor(private port: number) {}
+
+	/**
+	 * Start the mission with specified mission name.
+	 * @param missionName Mission name for the mission. If mission with the exact name exist, or there are only one mission that partially match the name, the mission will be activated.
+	 * @param steamID SteamID of the mission pack that the mission is in.
+	 * @param options Options to start the mission.
+	 * @param options.seed Specify mission seed.
+	 * @param options.force Start the mission without checking the mission can be run in the current setting. Default: false
+	 * @returns Mission ID and seed
+	 * @see {@link https://github.com/tepel-chen/DeMiLService/wiki/API-Reference#httplocalhostportstartmissionmissionidmissionid}
+	 */
+	async startMissionByName(
+		missionName: string,
+		steamID: string,
+		options: StartMissionByNameOptions = {}
+	): Promise<StartMissionResult> {
+		const parsedOptions = Object.fromEntries(
+			Object.entries({
+				...options,
+				seed: options.seed?.toString(),
+				force: options.force ? 'true' : 'false',
+				missionName,
+				steamID
+			}).filter((kvp): kvp is [string, string] => typeof kvp[1] === 'string')
+		);
+
+		return this.get<StartMissionResult>('startMission', parsedOptions);
+	}
+
+	private async get<T extends DeMiLResultBase>(path: string, params: Record<string, string> = {}): Promise<T> {
+		const urlParams = new URLSearchParams(params);
+		const url = new URL('http://localhost');
+		url.port = this.port.toString();
+		url.pathname = path;
+		url.search = urlParams.toString();
+
+		const data: DeMiLResultBase = await (await fetch(url)).json();
+
+		if (typeof data.Version === 'string' && semver.satisfies(data.Version, versionRange)) {
+			data.IsVersionInRange = true;
+		} else {
+			data.IsVersionInRange = false;
+		}
+
+		const demilError = DeMiLError.ParseIfErrorResponse(data);
+		if (demilError) throw demilError;
+		return data as T;
+	}
+
+	/**
+	 * Check if StartMissionResult was successful
+	 * @param result
+	 * @returns true if tartMissionResult was successful
+	 */
+	static isStartMissionSuccess(result: StartMissionResult): result is StartMissionSuccessResult {
+		return result.hasOwnProperty('Seed');
+	}
+}
diff --git a/src/lib/repo.ts b/src/lib/repo.ts
index 1e10232..1233c22 100644
--- a/src/lib/repo.ts
+++ b/src/lib/repo.ts
@@ -5,6 +5,7 @@ export type RepoModule = {
 	RuleSeedSupport: string | null;
 	Type: string;
 	Quirks: string | null;
+	SteamID: string | null;
 	X: number;
 	Y: number;
 };
diff --git a/src/lib/util.ts b/src/lib/util.ts
index 5bf836d..7fd8ed7 100644
--- a/src/lib/util.ts
+++ b/src/lib/util.ts
@@ -279,6 +279,7 @@ export function getModule(moduleID: string, modules: Record<string, RepoModule>
 		RuleSeedSupport: null,
 		Type: moduleID.match(/needy/gi) ? 'Needy' : 'Regular',
 		Quirks: null,
+		SteamID: null,
 		X: 0,
 		Y: 0
 	};
diff --git a/src/routes/mission/[...mission]/+page.svelte b/src/routes/mission/[...mission]/+page.svelte
index 0d10a5b..44631ed 100644
--- a/src/routes/mission/[...mission]/+page.svelte
+++ b/src/routes/mission/[...mission]/+page.svelte
@@ -17,6 +17,9 @@
 	import { writable } from 'svelte/store';
 	import { browser } from '$app/environment';
 	import Select from '$lib/controls/Select.svelte';
+	import * as DeMiL from '$lib/demil';
+	import toast from 'svelte-french-toast';
+	import DeMiLErrorDialog from './_DeMiLErrorDialog.svelte';
 
 	type Variant = Pick<Mission, 'name' | 'completions' | 'tpSolve'>;
 	export let data;
@@ -32,6 +35,12 @@
 	const dateOptions: Intl.DateTimeFormatOptions = { year: 'numeric', month: 'short', day: 'numeric' };
 	// const dateOptions: Intl.DateTimeFormatOptions = { year: 'numeric', month: 'short', day: 'numeric', hour: 'numeric', minute: 'numeric' };
 
+	let demilHelpState: 'Error' | 'InvalidVersion' | 'NotInstalled' | 'MissingModules' | 'MissionNotFound' | undefined;
+	let missingModules: RepoModule[];
+	let demilErrorMessage: string;
+	let demilVersion: string | undefined;
+	const steamID = mission.missionPack.steamId;
+
 	function poolClass(mods: string[] = [], module: RepoModule | null = null): string {
 		let classes = '';
 		if (module || mods.length == 1) {
@@ -72,6 +81,40 @@
 		};
 	}
 
+	let demilClient = new DeMiL.DeMiLClient(8095);
+	async function startMission() {
+		demilHelpState = undefined;
+		try {
+			const missionResult = await demilClient.startMissionByName(mission.name, steamID);
+			demilVersion = !missionResult.IsVersionInRange ? missionResult.Version ?? '<2.1.0' : undefined;
+			if (missionResult.hasOwnProperty('MissingModules')) {
+				missingModules = (missionResult as DeMiL.StartMissionMissingModulesResult).MissingModules.map(mod =>
+					getModule(mod, modules)
+				);
+				demilHelpState = 'MissingModules';
+			} else if (missionResult.hasOwnProperty('MissionModulesCount')) {
+				let moduleCountError = missionResult as DeMiL.StartMissionTooManyModulesResult;
+				demilErrorMessage = `Failed to start mission. A bomb that can support more modules is required. Current bombs only support up to ${moduleCountError.MaximumSupportedModulesCount} modules, and the mission has ${moduleCountError.MissionModulesCount} modules.`;
+				demilHelpState = 'Error';
+			} else if (missionResult.hasOwnProperty('MissionBombsCount')) {
+				let bombCountError = missionResult as DeMiL.StartMissionTooManyBombsResult;
+				demilErrorMessage = `A room that can support more bombs is required. Current rooms only support up to ${bombCountError.MaximumSupportedBombsCount} bombs, and the mission has ${bombCountError.MissionBombsCount} bombs.`;
+				demilHelpState = 'Error';
+			} else {
+				toast.success(`Started mission ${missionResult.MissionID}`);
+			}
+		} catch (e) {
+			console.error(e);
+			if (e instanceof TypeError) {
+				demilHelpState = 'NotInstalled';
+			} else if (e instanceof DeMiL.DeMiLError && e.message.match(/Mod with steamID \d+ not found/)) {
+				demilHelpState = 'MissionNotFound';
+			} else if (e instanceof Error) {
+				demilHelpState = 'Error';
+				demilErrorMessage = 'DeMiL threw an error: ' + e.message;
+			}
+		}
+	}
 	sortBombs(mission, modules);
 
 	type BombFrac = {
@@ -130,6 +173,7 @@
 		{#if mission.logfile !== null}
 			<a class="logfile" href={mission.logfile}>Logfile</a>
 		{/if}
+		<button class="start-mission" on:click={startMission}>Start Mission</button>
 	</div>
 	{#if hasPermission($page.data.user, Permission.VerifyMission)}
 		<a href={$page.url.href + '/edit'} class="top-right">Edit</a>
@@ -154,6 +198,7 @@
 		{/if}
 	</div>
 {/if}
+<DeMiLErrorDialog {demilHelpState} {steamID} {missingModules} {demilErrorMessage} {demilVersion} />
 <div class="main-content">
 	<div class="bombs">
 		<div class="block legend-bar flex">
@@ -281,7 +326,8 @@
 		color: var(--text-color);
 	}
 	a.logfile,
-	.date {
+	.date,
+	.start-mission {
 		margin-left: 20px;
 	}
 	a.variant {
diff --git a/src/routes/mission/[...mission]/_DeMiLErrorDialog.svelte b/src/routes/mission/[...mission]/_DeMiLErrorDialog.svelte
new file mode 100644
index 0000000..f23b447
--- /dev/null
+++ b/src/routes/mission/[...mission]/_DeMiLErrorDialog.svelte
@@ -0,0 +1,90 @@
+<script lang="ts">
+	import Dialog from '$lib/controls/Dialog.svelte';
+	import type { RepoModule } from '$lib/repo';
+
+	export let demilHelpState:
+		| 'Error'
+		| 'InvalidVersion'
+		| 'NotInstalled'
+		| 'MissingModules'
+		| 'MissionNotFound'
+		| undefined;
+	export let steamID: string;
+	export let missingModules: RepoModule[];
+	export let demilErrorMessage: string;
+	export let demilVersion: string | undefined;
+
+	let dialog: HTMLDialogElement;
+
+	$: {
+		if (dialog !== undefined) {
+			if (demilHelpState !== undefined) {
+				dialog.showModal();
+			} else {
+				dialog.close();
+			}
+		}
+	}
+</script>
+
+<Dialog bind:dialog>
+	{#if typeof demilHelpState !== 'undefined'}
+		<div class="block">
+			{#if demilHelpState === 'NotInstalled'}
+				<div>
+					You need to install <a
+						href="https://steamcommunity.com/sharedfiles/filedetails/?id=2930718104"
+						target="_blank"
+						rel="noopener noreferrer">DeMiL</a> to start a mission from this webpage and run the game.
+				</div>
+			{/if}
+			{#if demilHelpState === 'MissionNotFound'}
+				<div>
+					Mission pack is not installed. Download mission pack from <a
+						href="https://steamcommunity.com/sharedfiles/filedetails/?id={steamID}"
+						target="_blank"
+						rel="noopener noreferrer">Steam page</a
+					>.
+				</div>
+			{/if}
+			{#if demilHelpState === 'MissingModules'}
+				<div>Failed to start mission. Missing modules (click to open steam page):</div>
+				<div class="missing-modules">
+					{#each missingModules as mod}
+						<div>
+							{#if mod.SteamID !== null}
+								<a
+									href="https://steamcommunity.com/sharedfiles/filedetails/?id={mod.SteamID}"
+									target="_blank"
+									rel="noopener noreferrer">{mod.Name}</a>
+							{:else}
+								{mod.Name}
+							{/if}
+						</div>
+					{/each}
+				</div>
+			{/if}
+			{#if demilHelpState === 'Error'}
+				<div>{demilErrorMessage}</div>
+			{/if}
+			{#if typeof demilVersion === 'string'}
+				<div>
+					Your <a href="https://steamcommunity.com/sharedfiles/filedetails/?id=2930718104">DeMiL</a> looks outdated.
+					(Version: {demilVersion}) Please try
+					<a href="https://help.steampowered.com/en/faqs/view/0C48-FCBD-DA71-93EB">verifying integrity of game files</a
+					>. If it still doesn't work, please contact t-chen#5876 in KTaNE discord server.
+				</div>
+			{/if}
+		</div>
+	{/if}
+</Dialog>
+
+<style>
+	.missing-modules {
+		display: flex;
+		flex-wrap: wrap;
+	}
+	.missing-modules > div {
+		margin-right: 32px;
+	}
+</style>