diff --git a/packages/controller/src/BaseConnection.ts b/packages/controller/src/BaseConnection.ts
index ba6ba39c..76949eb8 100644
--- a/packages/controller/src/BaseConnection.ts
+++ b/packages/controller/src/BaseConnection.ts
@@ -80,7 +80,7 @@ export default class BaseConnection extends lib.Link {
 		return modPack;
 	}
 
-	getMod(mod: { name: string, version: lib.FullVersion, sha1?: string }) {
+	getMod(mod: { name: string, version: lib.SourceVersion, sha1?: string }) {
 		let filename = lib.ModInfo.filename(mod.name, mod.version);
 		let modInfo = this._controller.modStore.files.get(filename);
 		if (!modInfo) {
diff --git a/packages/controller/src/ControlConnection.ts b/packages/controller/src/ControlConnection.ts
index 7dceb299..293f28d0 100644
--- a/packages/controller/src/ControlConnection.ts
+++ b/packages/controller/src/ControlConnection.ts
@@ -707,7 +707,7 @@ export default class ControlConnection extends BaseConnection {
 			const release = releases.releases
 				.filter(info => versionRange.testVersion(info.version))
 				.reduce<ModPortalReleaseType | undefined>((max, cur) => (
-					max && lib.integerFullVersion(max.version) > lib.integerFullVersion(cur.version) ? max : cur
+					max && lib.integerSourceVersion(max.version) > lib.integerSourceVersion(cur.version) ? max : cur
 				), undefined);
 
 			if (!release) {
diff --git a/packages/controller/src/Controller.ts b/packages/controller/src/Controller.ts
index 6dfbe643..8051a0cc 100644
--- a/packages/controller/src/Controller.ts
+++ b/packages/controller/src/Controller.ts
@@ -763,8 +763,11 @@ export default class Controller {
 		const serialized = rawJson as Static<typeof lib.ModPack.jsonSchema>[];
 		return serialized.map(json => {
 			for (const mod of json.mods) {
-				// migrate: 2.0.0.alpha.22 - json schema now enforces X.Y.Z for mod version, builtins would use X.Y only
-				mod.version = lib.normaliseFullVersion(mod.version);
+				// migrate: 2.0.0-alpha.22 - old buildins used X.Y rather than X.Y.Z
+				// All other versions are kept verbatim so they still match the mod's file name.
+				if (lib.isPartialVersion(mod.version)) {
+					mod.version = lib.normaliseFullVersion(mod.version) as lib.SourceVersion;
+				}
 			}
 			return json;
 		});
diff --git a/packages/ctl/src/commands_mod.ts b/packages/ctl/src/commands_mod.ts
index a58a9f42..83f8873a 100644
--- a/packages/ctl/src/commands_mod.ts
+++ b/packages/ctl/src/commands_mod.ts
@@ -20,7 +20,7 @@ modCommands.add(new lib.Command({
 		yargs.positional("mod-version", { describe: "Version of the mod", type: "string" });
 	}],
 	handler: async function(args: { name: string, modVersion: string }, control: Control) {
-		if (!lib.isFullVersion(args.modVersion)) {
+		if (!lib.isSourceVersion(args.modVersion)) {
 			throw new lib.CommandError("mod-version must match format digit.digit.digit");
 		}
 		let modInfo = await control.send(new lib.ModGetRequest(args.name, args.modVersion));
@@ -108,9 +108,13 @@ modCommands.add(new lib.Command({
 		},
 		control: Control
 	) {
+		const searchVersion = lib.normaliseSourceVersion(String(args.factorioVersion));
+		if (searchVersion === undefined) {
+			throw new Error(`Invalid factorio version "${args.factorioVersion}"`);
+		}
 		let response = await control.send(new lib.ModSearchRequest(
 			args.query,
-			lib.normaliseApiVersion(args.factorioVersion as any),
+			lib.normaliseMajorMinorVersion(searchVersion),
 			args.page,
 			args.pageSize,
 			args.sort,
@@ -187,7 +191,7 @@ modCommands.add(new lib.Command({
 		yargs.positional("mod-version", { describe: "Version of mod to download", type: "string" });
 	}],
 	handler: async function(args: { name: string, modVersion: string }, control: Control) {
-		if (!lib.isFullVersion(args.modVersion)) {
+		if (!lib.isSourceVersion(args.modVersion)) {
 			throw new lib.CommandError("mod-version must match format digit.digit.digit");
 		}
 		let streamId = await control.send(new lib.ModDownloadRequest(args.name, args.modVersion));
@@ -207,7 +211,7 @@ modCommands.add(new lib.Command({
 		yargs.positional("mod-version", { describe: "Version of mod to delete", type: "string" });
 	}],
 	handler: async function(args: { name: string, modVersion: string }, control: Control) {
-		if (!lib.isFullVersion(args.modVersion)) {
+		if (!lib.isSourceVersion(args.modVersion)) {
 			throw new lib.CommandError("mod-version must match format digit.digit.digit");
 		}
 		await control.send(new lib.ModDeleteRequest(args.name, args.modVersion));
diff --git a/packages/ctl/src/commands_mod_pack.ts b/packages/ctl/src/commands_mod_pack.ts
index 0374be2c..a4af8a90 100644
--- a/packages/ctl/src/commands_mod_pack.ts
+++ b/packages/ctl/src/commands_mod_pack.ts
@@ -132,7 +132,7 @@ function setModPackMods(modPack: lib.ModPack, mods: string[] | undefined) {
 		if (!version) {
 			throw new lib.CommandError("Added mod must be formatted as name:version or name:version:sha1");
 		}
-		if (!lib.isFullVersion(version)) {
+		if (!lib.isSourceVersion(version)) {
 			throw new lib.CommandError("version must match the format digit.digit.digit");
 		}
 		if (sha1 && !/^[0-9a-f]{40}$/.test(sha1)) {
diff --git a/packages/lib/src/ModStore.ts b/packages/lib/src/ModStore.ts
index 13e82716..625ae253 100644
--- a/packages/lib/src/ModStore.ts
+++ b/packages/lib/src/ModStore.ts
@@ -5,7 +5,7 @@ import { Static } from "@sinclair/typebox";
 import { logger } from "./logging";
 import { downloadFile, safeOutputFile } from "./file_ops";
 import { ModPortalReleaseSchema, ModPortalDetailsSchema, ModNameVersionPair } from "./data/messages_mod";
-import { ModInfo, ModPack, FullVersion, ApiVersion, ModVersionEquality } from "./data";
+import { ModInfo, ModPack, MajorMinorVersion, ModVersionEquality, SourceVersion } from "./data";
 
 export interface ModStoreEvents {
 	/** A stored mod was created, updated or deleted */
@@ -44,7 +44,7 @@ export default class ModStore extends events.EventEmitter<ModStoreEvents> {
 		super();
 	}
 
-	getMod(name: string, version: FullVersion, sha1?: string) {
+	getMod(name: string, version: SourceVersion, sha1?: string) {
 		const mod = this.files.get(ModInfo.filename(name, version));
 		if (!mod || sha1 && sha1 !== mod.sha1) {
 			return undefined;
@@ -86,11 +86,11 @@ export default class ModStore extends events.EventEmitter<ModStoreEvents> {
 		this.emit("change", modInfo);
 	}
 
-	async loadMod(name: string, version: FullVersion) {
+	async loadMod(name: string, version: SourceVersion) {
 		return await this.loadFile(ModInfo.filename(name, version));
 	}
 
-	async deleteMod(name: string, version: FullVersion) {
+	async deleteMod(name: string, version: SourceVersion) {
 		return await this.deleteFile(ModInfo.filename(name, version));
 	}
 
@@ -198,7 +198,7 @@ export default class ModStore extends events.EventEmitter<ModStoreEvents> {
 	private async downloadModsChunk(
 		mods: ModNameVersionPair[],
 		username: string, token: string,
-		factorioVersion: ApiVersion,
+		factorioVersion: MajorMinorVersion,
 	) {
 		const url = new URL("https://mods.factorio.com/api/mods");
 		url.searchParams.set("page_size", "max");
@@ -209,7 +209,18 @@ export default class ModStore extends events.EventEmitter<ModStoreEvents> {
 		});
 
 		if (response.status !== 200) {
-			throw Error(`Fetch: ${url} returned ${response.status} ${response.statusText}`);
+			// Surface the portal's own error (e.g. an unsupported version) rather
+			// than a generic status line.
+			let detail = `${response.status} ${response.statusText}`;
+			try {
+				const body = await response.json() as { message?: string };
+				if (body && typeof body.message === "string") {
+					detail = body.message;
+				}
+			} catch {
+				// Response body was not JSON; keep the status line.
+			}
+			throw new Error(`Mod portal request to ${url} failed: ${detail}`);
 		}
 
 		const modReleases = (await response.json() as ModsInfoResponse).results;
@@ -236,7 +247,7 @@ export default class ModStore extends events.EventEmitter<ModStoreEvents> {
 	 * @param factorioVersion - Factorio version to filter for (mods are not guaranteed to work between Middle versions)
 	 */
 	async downloadMods(
-		mods: ModNameVersionPair[], username: string, token: string, factorioVersion: ApiVersion = "1.1"
+		mods: ModNameVersionPair[], username: string, token: string, factorioVersion: MajorMinorVersion = "1.1"
 	) {
 		const chunkSize = 500;
 		const futures: Promise<void>[] = [];
diff --git a/packages/lib/src/data/ModInfo.ts b/packages/lib/src/data/ModInfo.ts
index 83fdf384..14416f54 100644
--- a/packages/lib/src/data/ModInfo.ts
+++ b/packages/lib/src/data/ModInfo.ts
@@ -8,13 +8,12 @@ import { findRoot } from "../zip_ops";
 import { ModRecord } from "./ModPack";
 
 import {
-	ApiVersion,
-	ApiVersionSchema,
-	FullVersion, FullVersionSchema, integerFullVersion,
-	integerPartialVersion,
+	MajorMinorVersion, MajorMinorVersionSchema, normaliseMajorMinorVersion, integerMajorMinorVersion,
+	SourceVersion, SourceVersionSchema, isSourceVersion, normaliseSourceVersion, integerSourceVersion,
 	ModVersionEquality,
 } from "./version";
 
+
 type ModDependencyType = "incompatible" | "optional" | "hidden" | "unordered" | "required";
 
 export type ModDependencyUnsatisfiedReason = "incompatible" | "missing_dependency" | "wrong_version";
@@ -129,16 +128,23 @@ export default class ModInfo {
 	name = "";
 
 	/**
-	 * Version of the mod.
-	 * Sourced from info.json.
+	 * Version of the mod, exactly as it appears in info.json and the mod's file
+	 * name. This is the raw, un-normalised version the game reads leniently, so
+	 * it is only safe to use for building file names and identifiers; use
+	 * {@link integerVersion} to compare it.
 	 */
-	version = "0.0.0" as FullVersion;
+	version = "0.0.0" as SourceVersion;
 
 	/**
-	 * Integer representation of the version
+	 * Integer representation of the version, normalised the lenient way the
+	 * game reads it so it can be compared and sorted.
 	 */
 	get integerVersion() {
-		return integerFullVersion(this.version);
+		return integerSourceVersion(this.version);
+	}
+
+	get normalisedVersion() {
+		return normaliseSourceVersion(this.version);
 	}
 
 	/**
@@ -172,17 +178,17 @@ export default class ModInfo {
 	description = "";
 
 	/**
-	 * Major version of Factorio this mod supports.
-	 * Sourced from info.json.
+	 * Major version of Factorio this mod supports, reduced to major.minor.
+	 * Sourced from info.json and normalised when read in fromJSON.
 	 */
-	factorioVersion = "0.12" as "0.12" | ApiVersion;
+	factorioVersion = "0.12" as MajorMinorVersion;
 
 	/**
 	 * Integer representation of the factorioVersion
 	 * @type {number}
 	 */
 	get integerFactorioVersion() {
-		return integerPartialVersion(this.factorioVersion);
+		return integerMajorMinorVersion(this.factorioVersion);
 	}
 
 	/**
@@ -221,7 +227,7 @@ export default class ModInfo {
 	 * @param version - Mod's version
 	 * @returns string containing {name}_{version}.zip
 	 */
-	static filename(name: string, version: FullVersion) {
+	static filename(name: string, version: SourceVersion) {
 		return `${name}_${version}.zip`;
 	}
 
@@ -248,16 +254,17 @@ export default class ModInfo {
 	 */
 	isDeleted = false;
 
-	// Content of info.json found in mod files
+	// Content of info.json found in mod files. The version fields use the same
+	// lenient format the game accepts; they are normalised when read in fromJSON.
 	static infoJsonSchema = Type.Object({
 		"name": Type.String(),
-		"version": FullVersionSchema,
+		"version": SourceVersionSchema,
 		"title": Type.String(),
 		"author": Type.String(),
 		"contact": Type.Optional(Type.String()),
 		"homepage": Type.Optional(Type.String()),
 		"description": Type.Optional(Type.String()),
-		"factorio_version": Type.Optional(ApiVersionSchema),
+		"factorio_version": Type.Optional(SourceVersionSchema),
 		"dependencies": Type.Optional(Type.Array(Type.String())),
 	});
 
@@ -279,13 +286,24 @@ export default class ModInfo {
 
 		// info.json fields
 		if (json.name) { modInfo.name = json.name; }
-		if (json.version) { modInfo.version = json.version; }
+		if (json.version) {
+			if (!isSourceVersion(json.version)) {
+				throw new Error(`Invalid mod version "${json.version}"`);
+			}
+			modInfo.version = json.version;
+		}
 		if (json.title) { modInfo.title = json.title; }
 		if (json.author) { modInfo.author = json.author; }
 		if (json.contact) { modInfo.contact = json.contact; }
 		if (json.homepage) { modInfo.homepage = json.homepage; }
 		if (json.description) { modInfo.description = json.description; }
-		if (json.factorio_version) { modInfo.factorioVersion = json.factorio_version; }
+		if (json.factorio_version) {
+			const factorioVersion = normaliseSourceVersion(json.factorio_version);
+			if (factorioVersion === undefined) {
+				throw new Error(`Invalid factorio_version "${json.factorio_version}"`);
+			}
+			modInfo.factorioVersion = normaliseMajorMinorVersion(factorioVersion);
+		}
 
 		// Parse the dependencies
 		try {
@@ -314,7 +332,7 @@ export default class ModInfo {
 		if (this.contact) { json.contact = this.contact; }
 		if (this.homepage) { json.homepage = this.homepage; }
 		if (this.description) { json.description = this.description; }
-		if (this.factorioVersion !== "0.12") { json.factorio_version = this.factorioVersion; }
+		if (this.factorioVersion !== "0.12") { json.factorio_version = this.factorioVersion as SourceVersion; }
 		if (this.dependencies.length !== 1 || this.dependencies[0].name !== "base") {
 			json.dependencies = this.dependencySpecifications;
 		}
diff --git a/packages/lib/src/data/ModPack.ts b/packages/lib/src/data/ModPack.ts
index d8ddd4ae..4968980d 100644
--- a/packages/lib/src/data/ModPack.ts
+++ b/packages/lib/src/data/ModPack.ts
@@ -12,7 +12,7 @@ import ModInfo, { ModDependencyUnsatisfiedReason } from "./ModInfo";
 
 import {
 	PartialVersion, PartialVersionSchema, integerPartialVersion,
-	FullVersion, FullVersionSchema, normaliseFullVersion,
+	SourceVersion, SourceVersionSchema, normaliseFullVersion,
 } from "./version";
 
 
@@ -46,8 +46,8 @@ export interface ModRecord {
 	name: string,
 	/** if mod is to be loaded. */
 	enabled: boolean,
-	/** version of the mod. */
-	version: FullVersion,
+	/** version of the mod, verbatim as it appears in the mod's file name. */
+	version: SourceVersion,
 	/** SHA1 hash of the zip file. */
 	sha1?: string,
 	/** Used inside packages\web_ui\src\components\ModPackViewPage.tsx to define an error type. */
@@ -61,7 +61,7 @@ export interface ModRecord {
 const ModRecordJsonSchema = Type.Object({
 	"name": Type.String(),
 	"enabled": Type.Boolean(),
-	"version": FullVersionSchema,
+	"version": SourceVersionSchema,
 	"sha1": Type.Optional(Type.String()),
 });
 
@@ -434,24 +434,24 @@ export default class ModPack {
 	 * @return Built in mods for the given version
 	 */
 	static getBuiltinMods(factorioVersion: PartialVersion) {
-		factorioVersion = normaliseFullVersion(factorioVersion);
+		const version = normaliseFullVersion(factorioVersion) as SourceVersion;
+		const integerVersion = integerPartialVersion(factorioVersion);
 		let defaultMods: ModRecord[] = [
 			// "core" not included because core cannot be disabled
-			{ name: "base", enabled: true, version: factorioVersion },
+			{ name: "base", enabled: true, version: version },
 		];
 
-		const integerVersion = integerPartialVersion(factorioVersion);
 		if (integerVersion >= integerPartialVersion("1.2")) {
 			defaultMods = defaultMods.concat([
-				{ name: "elevated-rails", enabled: false, version: factorioVersion },
-				{ name: "quality", enabled: false, version: factorioVersion },
-				{ name: "space-age", enabled: false, version: factorioVersion },
+				{ name: "elevated-rails", enabled: false, version: version },
+				{ name: "quality", enabled: false, version: version },
+				{ name: "space-age", enabled: false, version: version },
 			]);
 		}
 
 		if (integerVersion >= integerPartialVersion("2.1")) {
 			defaultMods = defaultMods.concat([
-				{ name: "recycler", enabled: false, version: factorioVersion },
+				{ name: "recycler", enabled: false, version: version },
 			]);
 		}
 
diff --git a/packages/lib/src/data/messages_mod.ts b/packages/lib/src/data/messages_mod.ts
index 096eace5..4d5a09e3 100644
--- a/packages/lib/src/data/messages_mod.ts
+++ b/packages/lib/src/data/messages_mod.ts
@@ -4,8 +4,8 @@ import ModPack from "./ModPack";
 import { JsonString, jsonArray } from "./composites";
 
 import {
-	FullVersion, FullVersionSchema,
-	ApiVersion, ApiVersionSchema, normaliseApiVersion,
+	MajorMinorVersion, MajorMinorVersionSchema,	normaliseMajorMinorVersion,
+	SourceVersion, SourceVersionSchema,
 	ModVersionEquality,
 } from "./version";
 
@@ -119,13 +119,13 @@ export class ModGetRequest {
 
 	constructor(
 		public name: string,
-		public version: FullVersion,
+		public version: SourceVersion,
 		public sha1?: string,
 	) { }
 
 	static jsonSchema = Type.Object({
 		"name": Type.String(),
-		"version": FullVersionSchema,
+		"version": SourceVersionSchema,
 		"sha1": Type.Optional(Type.String()),
 	});
 
@@ -154,7 +154,7 @@ export class ModSearchRequest {
 
 	constructor(
 		public query: string,
-		public factorioVersion: ApiVersion,
+		public factorioVersion: MajorMinorVersion,
 		public page: number,
 		public pageSize?: number,
 		public sort?: string,
@@ -163,7 +163,7 @@ export class ModSearchRequest {
 
 	static jsonSchema = Type.Object({
 		"query": Type.String(),
-		"factorioVersion": ApiVersionSchema,
+		"factorioVersion": MajorMinorVersionSchema,
 		"page": Type.Integer(),
 		"pageSize": Type.Optional(Type.Integer()),
 		"sort": Type.Optional(Type.String()),
@@ -205,9 +205,9 @@ export class ModSearchRequest {
 
 // Define the structure for the latest release info from the portal
 export const ModPortalReleaseSchema = Type.Object({
-	version: FullVersionSchema,
+	version: SourceVersionSchema,
 	// Match the structure from ModStore's ModRelease/ModDetails
-	info_json: Type.Object({ factorio_version: ApiVersionSchema }),
+	info_json: Type.Object({ factorio_version: SourceVersionSchema }),
 	released_at: Type.String(), // ISO 8601 date string
 	download_url: Type.String(),
 	file_name: Type.String(),
@@ -236,12 +236,12 @@ export class ModPortalGetAllRequest {
 	static permission = "core.mod.search_portal" as const;
 
 	constructor(
-		public factorioVersion: ApiVersion,
+		public factorioVersion: MajorMinorVersion,
 		public hide_deprecated?: boolean,
 	) { }
 
 	static jsonSchema = Type.Object({
-		"factorioVersion": ApiVersionSchema,
+		"factorioVersion": MajorMinorVersionSchema,
 		"hide_deprecated": Type.Optional(Type.Boolean()),
 	});
 
@@ -276,13 +276,13 @@ export class ModDownloadRequest {
 
 	constructor(
 		public name: string,
-		public version: FullVersion,
+		public version: SourceVersion,
 		public sha1?: string,
 	) { }
 
 	static jsonSchema = Type.Object({
 		"name": Type.String(),
-		"version": FullVersionSchema,
+		"version": SourceVersionSchema,
 		"sha1": Type.Optional(Type.String()),
 	});
 
@@ -302,12 +302,12 @@ export class ModDeleteRequest {
 
 	constructor(
 		public name: string,
-		public version: FullVersion,
+		public version: SourceVersion,
 	) { }
 
 	static jsonSchema = Type.Object({
 		"name": Type.String(),
-		"version": FullVersionSchema,
+		"version": SourceVersionSchema,
 	});
 
 	static fromJSON(json: Static<typeof this.jsonSchema>) {
@@ -385,12 +385,12 @@ export class ModPortalDownloadRequest {
 
 	constructor(
 		public mods: ModNameVersionPair[],
-		public factorioVersion: ApiVersion,
+		public factorioVersion: MajorMinorVersion,
 	) { }
 
 	static jsonSchema = Type.Object({
 		"mods": Type.Array(ModNameVersionPairSchema),
-		"factorioVersion": ApiVersionSchema,
+		"factorioVersion": MajorMinorVersionSchema,
 	});
 
 	static fromJSON(json: Static<typeof this.jsonSchema>) {
@@ -422,13 +422,13 @@ export class ModDependencyResolveRequest {
 
 	constructor(
 		public mods: ModDependency[],
-		public factorioVersion: ApiVersion,
+		public factorioVersion: MajorMinorVersion,
 		public checkForUpdates: boolean = false,
 	) { }
 
 	static jsonSchema = Type.Object({
 		"mods": Type.Array(ModDependency.jsonSchema),
-		"factorioVersion": ApiVersionSchema,
+		"factorioVersion": MajorMinorVersionSchema,
 		"checkForUpdates": Type.Boolean(),
 	});
 
@@ -441,7 +441,7 @@ export class ModDependencyResolveRequest {
 		return new this(
 			[...modPack.mods.values()]
 				.map(mod => new ModDependency(`${mod.name} ${equality} ${mod.version}`)),
-			normaliseApiVersion(modPack.factorioVersion),
+			normaliseMajorMinorVersion(modPack.factorioVersion),
 			checkForUpdates,
 		);
 	}
@@ -452,7 +452,7 @@ export class ModDependencyResolveRequest {
 			[...modPack.mods.values()]
 				.filter(mod => mod.enabled)
 				.map(mod => new ModDependency(`${mod.name} ${equality} ${mod.version}`)),
-			normaliseApiVersion(modPack.factorioVersion),
+			normaliseMajorMinorVersion(modPack.factorioVersion),
 			checkForUpdates,
 		);
 	}
diff --git a/packages/lib/src/data/version.ts b/packages/lib/src/data/version.ts
index 22ffdcd1..5844fb94 100644
--- a/packages/lib/src/data/version.ts
+++ b/packages/lib/src/data/version.ts
@@ -24,24 +24,55 @@ export function isVersionEquality(input: string): input is VersionEquality {
 }
 
 /**
- * Matches valid factorio versions accepts by the web API
- * https://wiki.factorio.com/Mod_portal_API#/api/mods (see version enum)
+ * Known Factorio major versions, offered as fallback options in the web
+ * interface before the live version list has been fetched. Versions are no
+ * longer validated against this list; the mod portal is the source of truth.
  */
-export const ApiVersions = ["0.13", "0.14", "0.15", "0.16", "0.17", "0.18", "1.0", "1.1", "2.0"] as const;
-export const ApiVersionSchema = Type.Union(ApiVersions.map(v => Type.Literal(v)));
-export type ApiVersion = Static<typeof ApiVersionSchema>;
+export const ApiVersions = ["0.13", "0.14", "0.15", "0.16", "0.17", "0.18", "1.0", "1.1", "2.0", "2.1"] as const;
 
-export function isApiVersion(input: string): input is ApiVersion {
-	return ApiVersions.includes(input as any);
+/**
+ * Matches a version string the same lenient way the game reads it: each number
+ * may be preceded by whitespace, the major and minor are required, the patch is
+ * optional, and any trailing content is ignored. The pattern is intentionally
+ * not anchored at the end.
+ */
+const sourceVersionRegExp = /^\s*(\d+)\.\s*(\d+)(?:\.\s*(\d+))?/;
+
+/**
+ * A raw, un-normalised version string exactly as the game reads it, such as a
+ * mod's version taken from info.json or a mod file name.
+ */
+declare const sourceVersionSymbol: unique symbol;
+export type SourceVersion = string & { [sourceVersionSymbol]: void };
+export const SourceVersionSchema = Type.Unsafe<SourceVersion>(
+	Type.String({ pattern: sourceVersionRegExp.source })
+);
+
+export function isSourceVersion(input: string): input is SourceVersion {
+	return sourceVersionRegExp.test(input);
 }
 
-export function normaliseApiVersion(version: PartialVersion) {
-	const parts = version.split(".");
-	const apiVersion = `${parts[0]}.${parts[1]}`;
-	if (!isApiVersion(apiVersion)) {
-		throw new Error(`Factorio version not supported by mod portal: ${apiVersion}`);
+export function integerSourceVersion(version: SourceVersion) {
+	return integerFullVersion(normaliseSourceVersion(version));
+}
+
+/**
+ * Normalise a version string the same lenient way the game reads it. Numbers
+ * are read as integers, so leading whitespace and zeros are dropped and any
+ * content after the patch is ignored.
+ *
+ * @returns The normalised full version, or undefined if no major and minor
+ *     could be read.
+ */
+export function normaliseSourceVersion(input: SourceVersion): FullVersion;
+export function normaliseSourceVersion(input: string): FullVersion | undefined;
+
+export function normaliseSourceVersion(input: string | SourceVersion): FullVersion | undefined {
+	const match = sourceVersionRegExp.exec(input);
+	if (match === null) {
+		return undefined;
 	}
-	return apiVersion;
+	return `${Number(match[1])}.${Number(match[2])}.${Number(match[3] ?? "0")}` as FullVersion;
 }
 
 /**
@@ -68,11 +99,38 @@ export function normaliseFullVersion(version: PartialVersion) {
 	return (parts.length === 3 ? version : `${version}.0`) as FullVersion;
 }
 
+/**
+ * Matches a major.minor version, as used by the mod portal's version filter.
+ */
+const majorMinorVersionRegExp = /^\d+\.\d+$/;
+export type MajorMinorVersion = `${number}.${number}`;
+export const MajorMinorVersionSchema = Type.Unsafe<MajorMinorVersion>(
+	Type.String({ pattern: majorMinorVersionRegExp.source })
+);
+
+export function isMajorMinorVersion(input: string): input is MajorMinorVersion {
+	return majorMinorVersionRegExp.test(input);
+}
+
+export function integerMajorMinorVersion(version: MajorMinorVersion) {
+	const [major, minor] = version.split(".").map(n => Number.parseInt(n, 10));
+	// Can't use bitwise here because this is 48-bits. sub is always 0 so is omitted.
+	return major * 0x100000000 + minor * 0x10000 as IntegerVersion;
+}
+
+/**
+ * Reduce a version to its major.minor, as used by the mod portal's version filter.
+ */
+export function normaliseMajorMinorVersion(version: PartialVersion): MajorMinorVersion {
+	const [major, minor] = version.split(".");
+	return `${major}.${minor}` as MajorMinorVersion;
+}
+
 /**
  * Matches valid factorio versions and mod dependencies specifications where 2 or 3 parts are specified.
  */
 const partialVersionRegExp = /^\d+\.\d+(?:\.\d+)?$/;
-export type PartialVersion = FullVersion | `${number}.${number}`;
+export type PartialVersion = FullVersion | MajorMinorVersion;
 export const PartialVersionSchema = Type.Unsafe<PartialVersion>(
 	Type.String({ pattern: partialVersionRegExp.source })
 );
@@ -94,8 +152,13 @@ export function integerPartialVersion(version: PartialVersion) {
  * and resolved against the live API when the instance starts.
  */
 const releaseChannelRegExp = /^[a-z][a-z0-9-]*$/;
+declare const releaseChannelSymbol: unique symbol;
+export type ReleaseChannel = string & { [releaseChannelSymbol]: void };
+export const ReleaseChannelSchema = Type.Unsafe<ReleaseChannel>(
+	Type.String({ pattern: releaseChannelRegExp.source })
+);
 
-export function isReleaseChannel(input: string): boolean {
+export function isReleaseChannel(input: string): input is ReleaseChannel {
 	return input !== "latest" && releaseChannelRegExp.test(input);
 }
 
@@ -103,15 +166,15 @@ export function isReleaseChannel(input: string): boolean {
  * Matches valid factorio target versions: a partial version, "latest", or a
  * release channel name (e.g. "stable" or "experimental").
  */
-export type TargetVersion = PartialVersion | "latest";
+export type TargetVersion = PartialVersion | ReleaseChannel | "latest";
 export const TargetVersionSchema = Type.Union([
 	PartialVersionSchema,
+	ReleaseChannelSchema,
 	Type.Literal("latest"),
-	Type.Unsafe<TargetVersion>(Type.String({ pattern: releaseChannelRegExp.source })),
 ]);
 
 export function isTargetVersion(input: string): input is TargetVersion {
-	return input === "latest" || isReleaseChannel(input) || partialVersionRegExp.test(input);
+	return input === "latest" || releaseChannelRegExp.test(input) || partialVersionRegExp.test(input);
 }
 
 /**
@@ -150,8 +213,8 @@ export class ModVersionEquality {
 		}
 	}
 
-	testVersion(version: PartialVersion) {
-		return this.testIntegerVersion(integerPartialVersion(version));
+	testVersion(version: SourceVersion) {
+		return this.testIntegerVersion(integerSourceVersion(version));
 	}
 
 	toString() {
@@ -213,8 +276,11 @@ export class ModVersionRange {
 		return this.minVersion.testIntegerVersion(other) && this.maxVersion.testIntegerVersion(other);
 	}
 
-	testVersion(version: PartialVersion) {
-		return this.testIntegerVersion(integerPartialVersion(version));
+	testVersion(version: PartialVersion | SourceVersion) {
+		if (isPartialVersion(version)) {
+			return this.testIntegerVersion(integerPartialVersion(version));
+		}
+		return this.testIntegerVersion(integerSourceVersion(version));
 	}
 
 	combineVersion(other: ModVersionEquality) {
diff --git a/packages/web_ui/src/components/InstanceList.tsx b/packages/web_ui/src/components/InstanceList.tsx
index 630d349e..449a7eb7 100644
--- a/packages/web_ui/src/components/InstanceList.tsx
+++ b/packages/web_ui/src/components/InstanceList.tsx
@@ -37,10 +37,11 @@ export default function InstanceList(props: InstanceListProps) {
 	}
 
 	function integerFactorioVersionOrDefault(instance: lib.InstanceDetails) {
-		if (instance.factorioVersion === undefined || instance.factorioVersion === "latest") {
-			return -1;
+		const v = instance.factorioVersion;
+		if (v && lib.isPartialVersion(v)) {
+			return lib.integerPartialVersion(v);
 		}
-		return lib.integerPartialVersion(instance.factorioVersion);
+		return -1;
 	}
 
 	let columns: ColumnsType<lib.InstanceDetails> = [
diff --git a/packages/web_ui/src/components/ModPackViewPage.tsx b/packages/web_ui/src/components/ModPackViewPage.tsx
index b6327a72..937450e2 100644
--- a/packages/web_ui/src/components/ModPackViewPage.tsx
+++ b/packages/web_ui/src/components/ModPackViewPage.tsx
@@ -77,12 +77,12 @@ function SearchModsTable(props: SearchModsTableProps) {
 	const [modResultPageSize, setModResultPageSize] = useState<number>(10);
 	const [modResultCount, setModResultCount] = useState<number>(2);
 	const [modResultSelectedVersion, setModResultSelectedVersion] = useState<Map<string, number>>(new Map());
-	const [factorioVersion, setFactorioVersion] = useState<lib.ApiVersion | null>(null);
+	const [factorioVersion, setFactorioVersion] = useState<lib.MajorMinorVersion | null>(null);
 
 	// Get a valid factorio version
 	useEffect(() => {
 		try {
-			setFactorioVersion(lib.normaliseApiVersion(props.modPack.factorioVersion));
+			setFactorioVersion(lib.normaliseMajorMinorVersion(props.modPack.factorioVersion));
 		} catch (err) {
 			setFactorioVersion(null);
 		}
@@ -271,7 +271,7 @@ function DownloadDependenciesButton(props: DownloadDependenciesProps) {
 	const [open, setOpen] = useState(false);
 	const [error, setError] = useState<Error | null>(null);
 	const [loading, setLoading] = useState<boolean>(false);
-	const [factorioVersion, setFactorioVersion] = useState<lib.ApiVersion | null>(null);
+	const [factorioVersion, setFactorioVersion] = useState<lib.MajorMinorVersion | null>(null);
 
 	const [mods, setMods] = useState<lib.ModInfo[]>([]); // All resolved mods and dependencies
 	const [missing, setMissing] = useState<lib.ModInfo[]>([]); // Missing mods and dependencies, excluding builtin
@@ -295,7 +295,7 @@ function DownloadDependenciesButton(props: DownloadDependenciesProps) {
 	// Get a valid factorio version
 	useEffect(() => {
 		try {
-			setFactorioVersion(lib.normaliseApiVersion(props.modPack.factorioVersion));
+			setFactorioVersion(lib.normaliseMajorMinorVersion(props.modPack.factorioVersion));
 		} catch (err) {
 			setError(new Error(`Invalid factorio version: ${props.modPack.factorioVersion}`));
 			setFactorioVersion(null);
@@ -403,7 +403,9 @@ function DownloadDependenciesButton(props: DownloadDependenciesProps) {
 		control.send(
 			new lib.ModPortalDownloadRequest(
 				missing.map(mod => ({
-					name: mod.name, version: new lib.ModVersionEquality("=", mod.version),
+					name: mod.name,
+					// mod.version is the raw version; the equality bound must be canonical.
+					version: new lib.ModVersionEquality("=", mod.normalisedVersion),
 				})),
 				factorioVersion,
 			)
@@ -577,8 +579,8 @@ function ModsTable(props: ModsTableProps) {
 		if (mod.enabled && mod.info) {
 			mod.warning = mod.info.checkDependencySatisfaction(mods.filter(m => m.enabled));
 			try {
-				const packFactorioVersion = lib.normaliseApiVersion(props.modPack.factorioVersion);
-				const modFactorioVersion = lib.normaliseApiVersion(mod.info.factorioVersion);
+				const packFactorioVersion = lib.normaliseMajorMinorVersion(props.modPack.factorioVersion);
+				const modFactorioVersion = mod.info.factorioVersion;
 				if (packFactorioVersion !== modFactorioVersion) {
 					mod.warning = "wrong_factorio_version";
 				}
diff --git a/packages/web_ui/src/components/ModsPage.tsx b/packages/web_ui/src/components/ModsPage.tsx
index e32bb3c0..8d76cc78 100644
--- a/packages/web_ui/src/components/ModsPage.tsx
+++ b/packages/web_ui/src/components/ModsPage.tsx
@@ -147,7 +147,7 @@ function SearchModsButton() {
 	const [open, setOpen] = useState(false);
 	const [form] = Form.useForm();
 	const [searchText, setSearchText] = useState("");
-	const [factorioVersion, setFactorioVersion] = useState<lib.ApiVersion>("2.0");
+	const [factorioVersion, setFactorioVersion] = useState<lib.MajorMinorVersion>("2.0");
 
 	// State for all mods fetched from backend
 	const [allMods, setAllMods] = useState<ModPortalModType[]>([]);
@@ -164,13 +164,16 @@ function SearchModsButton() {
 	const handleControllerDownload = (
 		modName: string,
 		modTitle: string | undefined,
-		modVersion: lib.FullVersion | undefined,
-		portalFactorioVersion: lib.ApiVersion,
+		modVersion: lib.SourceVersion | undefined,
+		portalFactorioVersion: lib.MajorMinorVersion,
 	) => {
 		if (modVersion) {
 			control.send(
 				new lib.ModPortalDownloadRequest(
-					[{ name: modName, version: new lib.ModVersionEquality("=", modVersion)}],
+					[{
+						name: modName,
+						version: new lib.ModVersionEquality("=", lib.normaliseSourceVersion(modVersion)),
+					}],
 					portalFactorioVersion
 				)
 			).then(() => {
@@ -271,7 +274,7 @@ function SearchModsButton() {
 	// Update search text and factorio version from form
 	const handleSearch = (changedValues: any, allValues: any) => {
 		const nameValue = allValues.name;
-		const versionValue = allValues.factorioVersion as lib.ApiVersion | undefined;
+		const versionValue = allValues.factorioVersion as lib.MajorMinorVersion | undefined;
 
 		// Only trigger state updates if values actually changed
 		if (nameValue !== searchText) {
diff --git a/test/lib/ModStore.test.js b/test/lib/ModStore.test.js
index 9c140b3a..8f2037a3 100644
--- a/test/lib/ModStore.test.js
+++ b/test/lib/ModStore.test.js
@@ -358,7 +358,7 @@ describe("lib/ModStore", function () {
 
 			await assert.rejects(
 				modStore.downloadMods(modsToDownload, username, token, factorioVersion),
-				/Fetch: https:\/\/mods\.factorio\.com\/api\/mods.* returned 503 Service Unavailable/
+				/Mod portal request to https:\/\/mods\.factorio\.com\/api\/mods.* failed: 503 Service Unavailable/
 			);
 			assert.equal(modStore.files.size, 0, "No mods should be added on API error");
 		});
diff --git a/test/lib/data/ModInfo.js b/test/lib/data/ModInfo.js
index 590068cc..752cbaea 100644
--- a/test/lib/data/ModInfo.js
+++ b/test/lib/data/ModInfo.js
@@ -341,6 +341,39 @@ describe("lib/data/ModInfo", function() {
 				dependencies: ["base", ""],
 			}), /my-mod/); // check the mod name is included in the error message
 		});
+		it("should keep the raw mod version but normalise it for comparison", function() {
+			const mod = ModInfo.fromJSON({
+				name: "my-mod",
+				version: " 1.2.3-extra",
+				factorio_version: "1.1.0",
+			});
+			// The version is kept verbatim so it still matches the mod's file name,
+			// but is read leniently the way the game does when compared.
+			assert.equal(mod.version, " 1.2.3-extra");
+			assert.equal(mod.integerVersion, ModInfo.fromJSON({ version: "1.2.3" }).integerVersion);
+			// factorio_version is not part of any file name, so it is reduced to major.minor.
+			assert.equal(mod.factorioVersion, "1.1");
+		});
+		it("should keep leading zeros in the version so the file name matches", function() {
+			// Mirrors the mods.factorio.com/mod/leading-zeros case: the version has
+			// a zero-padded component that the file name keeps verbatim.
+			const mod = ModInfo.fromJSON({ name: "leading-zeros", version: "1.01.0" });
+			assert.equal(mod.version, "1.01.0");
+			assert.equal(mod.filename, "leading-zeros_1.01.0.zip");
+			assert.equal(mod.integerVersion, ModInfo.fromJSON({ version: "1.1.0" }).integerVersion);
+		});
+		it("should throw for an unparseable mod version", function() {
+			assert.throws(
+				() => ModInfo.fromJSON({ name: "my-mod", version: "not-a-version" }),
+				/Invalid mod version/
+			);
+		});
+		it("should throw for an unparseable factorio_version", function() {
+			assert.throws(
+				() => ModInfo.fromJSON({ name: "my-mod", factorio_version: "not-a-version" }),
+				/Invalid factorio_version/
+			);
+		});
 		it("should sort integer mod versions lexicographically", function() {
 			let unsortedVersions = ["1.0.0", "1.1.0", "0.1.0", "3.0.0", "1.2.0", "0.3.1", "0.3.3", "2.1.1", "0.0.1"];
 			let sortedVersions = ["0.0.1", "0.1.0", "0.3.1", "0.3.3", "1.0.0", "1.1.0", "1.2.0", "2.1.1", "3.0.0"];
diff --git a/test/lib/data/version.test.js b/test/lib/data/version.test.js
index 39cf0e03..92bba3e9 100644
--- a/test/lib/data/version.test.js
+++ b/test/lib/data/version.test.js
@@ -17,36 +17,53 @@ describe("lib/data/version", function() {
 			}
 		});
 	});
-	describe("isApiVersion()", function() {
-		it("should correctly validate input strings", function() {
-			const valid = ["0.13", "0.14", "0.15", "0.16", "0.17", "0.18", "1.0", "1.1", "2.0"];
-			for (const test of valid) {
-				assert.equal(lib.isApiVersion(test), true, test);
-			}
-			const invalid = ["0.12", "0.19", "1.2", "2.1", "1.0.0", "1.0.0.0", "0", "", "latest"];
+	describe("normaliseSourceVersion()", function() {
+		it("should normalise lenient version strings the way the game reads them", function() {
+			// Major and minor required, patch optional and defaulting to 0.
+			assert.equal(lib.normaliseSourceVersion("1.1"), "1.1.0");
+			assert.equal(lib.normaliseSourceVersion("1.1.50"), "1.1.50");
+			assert.equal(lib.normaliseSourceVersion("0.18"), "0.18.0");
+			// Leading whitespace and zeros are dropped.
+			assert.equal(lib.normaliseSourceVersion("  1.1"), "1.1.0");
+			assert.equal(lib.normaliseSourceVersion("01.02.03"), "1.2.3");
+			assert.equal(lib.normaliseSourceVersion("1. 1. 50"), "1.1.50");
+			// Anything after the patch (or a non-numeric patch) is ignored.
+			assert.equal(lib.normaliseSourceVersion("1.1.50-beta"), "1.1.50");
+			assert.equal(lib.normaliseSourceVersion("1.1.x"), "1.1.0");
+			assert.equal(lib.normaliseSourceVersion("1.1.50.60"), "1.1.50");
+		});
+		it("should return undefined when no major.minor can be read", function() {
+			const invalid = ["1", "", "latest", "abc", ".1.1", "x.y"];
 			for (const test of invalid) {
-				assert.equal(lib.isApiVersion(test), false, test);
+				assert.equal(lib.normaliseSourceVersion(test), undefined, test);
 			}
 		});
 	});
-	describe("normaliseApiVersion()", function() {
-		it("should normalise accepted versions", function() {
-			assert.equal(lib.normaliseApiVersion("0.13"), "0.13");
-			assert.equal(lib.normaliseApiVersion("0.13.0"), "0.13");
-			assert.equal(lib.normaliseApiVersion("0.13.1"), "0.13");
-			assert.equal(lib.normaliseApiVersion("1.1"), "1.1");
-			assert.equal(lib.normaliseApiVersion("1.1.0"), "1.1");
-			assert.equal(lib.normaliseApiVersion("1.1.1"), "1.1");
-			assert.equal(lib.normaliseApiVersion("2.0"), "2.0");
-			assert.equal(lib.normaliseApiVersion("2.0.0"), "2.0");
-			assert.equal(lib.normaliseApiVersion("2.0.1"), "2.0");
-			assert.equal(lib.normaliseApiVersion("1.0.0.0"), "1.0");
+	describe("normaliseMajorMinorVersion()", function() {
+		it("should return the major.minor", function() {
+			assert.equal(lib.normaliseMajorMinorVersion("1.1"), "1.1");
+			assert.equal(lib.normaliseMajorMinorVersion("1.1.50"), "1.1");
+			assert.equal(lib.normaliseMajorMinorVersion("0.18.0"), "0.18");
 		});
-		it("should throw for invalid versions", function() {
-			const invalid = ["0.12", "0", "", "latest"];
-			for (const test of invalid) {
-				assert.throws(() => lib.normaliseApiVersion(test), undefined, test);
+	});
+	describe("isMajorMinorVersion()", function() {
+		it("should correctly validate input strings", function() {
+			const valid = ["1.1", "0.18", "2.0", "10.20"];
+			const invalid = ["1", "1.1.0", "1.1.50", "latest", "1.", ".1", ""];
+			for (const version of valid) {
+				assert(lib.isMajorMinorVersion(version), `${version} should be valid`);
 			}
+			for (const version of invalid) {
+				assert(!lib.isMajorMinorVersion(version), `${version} should be invalid`);
+			}
+		});
+	});
+	describe("integerMajorMinorVersion()", function() {
+		it("should sort versions lexicographically", function() {
+			let unsortedVersions = ["1.0", "1.1", "0.1", "3.0", "1.2", "0.3", "2.1"];
+			let sortedVersions = ["0.1", "0.3", "1.0", "1.1", "1.2", "2.1", "3.0"];
+			unsortedVersions.sort((a, b) => lib.integerMajorMinorVersion(a) - lib.integerMajorMinorVersion(b));
+			assert.deepEqual(unsortedVersions, sortedVersions);
 		});
 	});
 	describe("isFullVersion()", function() {