diff --git a/src/usimp.ts b/src/usimp.ts
index 6c3eaec..576bcd6 100644
--- a/src/usimp.ts
+++ b/src/usimp.ts
@@ -1,46 +1,135 @@
 "use strict";
 
-interface DomainJson {
+const DNS_RE = /^([a-z0-9_-]+\.)+[a-z]{2,}$/i;
+const UUID_RE = /^[0-9a-f]{8}(-[0-9a-f]{4}){3}-[0-9a-f]{12}$/i;
+
+type DomainJson = {
     name: string,
     id: string,
 }
 
-interface DomainServerJson {
+function isDomainJson(json: any): json is DomainJson {
+    return (
+        json !== null && typeof json === 'object' && json.constructor === Object &&
+        typeof json.name === 'string' && typeof json.id === 'string' &&
+        DNS_RE.test(json.name) && UUID_RE.test(json.id)
+    );
+}
+
+type DomainServerJson = {
     host: string,
     id: string,
     priority: number,
     weight: number,
     protocols: {
-        https: number | undefined,
-        wss: number | undefined,
-        http: number | undefined,
-        ws: number | undefined,
-        udp: number | undefined
+        https?: number | null,
+        wss?: number | null,
+        http?: number | null,
+        ws?: number | null,
+        udp?: number | null
     }
 }
 
-interface OutputEnvelopeJson {
+function isDomainServerJson(json: any): json is DomainServerJson {
+    if (
+        json === null || typeof json !== 'object' || json.constructor !== Object ||
+        typeof json.host !== 'string' || typeof json.id !== 'string' || typeof json.priority !== 'number' ||
+        typeof json.weight !== 'number' ||
+        json.protocols === null || typeof json.protocols !== 'object' || json.protocols.constructor !== Object ||
+        !DNS_RE.test(json.host) || !UUID_RE.test(json.id) || json.priority < 0 || json.weight < 0
+    ) return false;
+
+
+    return (
+        (json.protocols.https === undefined || json.protocols.https === null || (
+            typeof json.protocols.https === 'number' && json.protocols.https > 0 && json.protocols.https <= 65535
+        )) && (json.protocols.wss === undefined || json.protocols.wss === null || (
+            typeof json.protocols.wss === 'number' && json.protocols.wss > 0 && json.protocols.wss <= 65535
+        )) && (json.protocols.http === undefined || json.protocols.http === null || (
+            typeof json.protocols.http === 'number' && json.protocols.http > 0 && json.protocols.http <= 65535
+        )) && (json.protocols.ws === undefined || json.protocols.ws === null || (
+            typeof json.protocols.ws === 'number' && json.protocols.ws > 0 && json.protocols.ws <= 65535
+        )) && (json.protocols.udp === undefined || json.protocols.udp === null || (
+            typeof json.protocols.udp === 'number' && json.protocols.udp > 0 && json.protocols.udp <= 65535
+        ))
+    );
+}
+
+type WellKnownJson = {
+    domain: DomainJson,
+    domain_servers: DomainServerJson[],
+}
+
+function isWellKnownJson(json: any): json is WellKnownJson {
+    if (
+        json === null || typeof json !== 'object' || json.constructor !== Object ||
+        !json.hasOwnProperty('domain') || !json.hasOwnProperty('domain_servers') ||
+        json.domain_servers === null || typeof json.domain_servers !== 'object' ||
+        json.domain_servers.constructor !== Array
+    ) return false;
+
+    return isDomainJson(json.domain) && json.domain_servers.every((item: unknown) => isDomainServerJson(item));
+}
+
+type OutputEnvelopeJson<T> = {
     status: string,
     request_nr: number,
-    data: any,
-    action: string | null | undefined,
-    error: {
+    data: T,
+    action?: string | null,
+    error?: {
         code: string,
         message: string | null,
         description: string | null,
     } | null,
 }
 
-interface EventJson {
-    data: {
-        message: string,
+function isOutputEnvelopeJson<T>(json: any): json is OutputEnvelopeJson<T> {
+    if (
+        json === null || typeof json !== 'object' || json.constructor !== Object ||
+        typeof json.status !== 'string' || typeof json.request_nr !== 'number' || json.request_nr < 0 ||
+        (typeof json.action !== 'string' && json.action !== null && json.action === undefined) ||
+        json.data === null || typeof json.data !== 'object' || json.data.constructor !== Object
+    ) return false;
+
+    if (json.error !== null && json.error !== undefined) {
+        if (
+            typeof json.error !== 'object' || json.error.constructor !== Object ||
+            typeof json.error.code !== 'string' ||
+            (typeof json.error.message !== 'string' && json.error.message !== null) ||
+            (typeof json.error.description !== 'string' && json.error.description !== null)
+        ) return false;
     }
-    id: string,
+
+    return true;
 }
 
-interface WellKnownJson {
-    domain: DomainJson,
-    domain_servers: DomainServerJson[]
+type EventPushEnvelopeJson = {
+    events: EventJson[],
+}
+
+function isEventPushEnvelopeJson(json: any): json is EventPushEnvelopeJson {
+    if (
+        json === null || typeof json !== 'object' || json.constructor !== Object ||
+        json.events === null || typeof json.events !== 'object' || json.events.constructor !== Array
+    ) return false;
+
+    return json.events.every((item: unknown) => isEventJson(item));
+}
+
+type EventJson = {
+    id: string,
+    data: {
+        message: string,
+    },
+}
+
+function isEventJson(json: any): json is EventJson {
+    return !(
+        json === null || typeof json !== 'object' || json.constructor !== Object ||
+        typeof json.id !== 'string' || !UUID_RE.test(json.id) ||
+        json.data === null || typeof json.data !== 'object' || json.data.constructor !== Object ||
+        typeof json.data.message !== 'string'
+    );
 }
 
 export class Domain {
@@ -53,10 +142,9 @@ export class Domain {
         return `[${this.name}]`;
     }
 
-    constructor(jsonData: DomainJson, domainServers: DomainServerJson[]) {
-        // FIXME check values
-        this.name = jsonData.name;
-        this.id = jsonData.id;
+    constructor(json: DomainJson, domainServers: DomainServerJson[]) {
+        this.name = json.name;
+        this.id = json.id;
         this.servers = [];
         for (const domainServer of domainServers) {
             this.servers.push(new DomainServer(domainServer));
@@ -78,8 +166,12 @@ export class Domain {
             throw new Error("Invalid response");
         }
 
-        const data: WellKnownJson = await response.json();
-        return new Domain(data.domain, data.domain_servers);
+        const data = await response.json();
+        if (isWellKnownJson(data)) {
+            return new Domain(data.domain, data.domain_servers);
+        } else {
+            throw Error("Invalid contents in usimp.json");
+        }
     }
 
     chooseDomainServer(): DomainServer {
@@ -112,23 +204,29 @@ export class DomainServer {
     priority: number;
     weight: number;
     protocols: {
-        https: number | undefined,
-        wss: number | undefined,
-        http: number | undefined,
-        ws: number | undefined,
-        udp: number | undefined
+        https: number | null,
+        wss: number | null,
+        http: number | null,
+        ws: number | null,
+        udp: number | null
     };
 
     toString() {
         return `[${this.host}/${this.priority}/${this.weight}]`;
     }
 
-    constructor(jsonData: DomainServerJson) {
-        this.host = jsonData.host;
-        this.id = jsonData.id;
-        this.priority = jsonData.priority;
-        this.weight = jsonData.weight;
-        this.protocols = jsonData.protocols;
+    constructor(json: DomainServerJson) {
+        this.host = json.host;
+        this.id = json.id;
+        this.priority = json.priority;
+        this.weight = json.weight;
+        this.protocols = {
+            https: json.protocols.https || null,
+            wss: json.protocols.wss || null,
+            http: json.protocols.http || null,
+            ws: json.protocols.ws || null,
+            udp: json.protocols.udp || null,
+        };
     }
 }
 
@@ -145,9 +243,51 @@ export class Room {
 }
 
 export class Event {
+    id: string;
+    data: {
+        message: string;
+    };
 
+    constructor(json: EventJson) {
+        this.data = {
+            message: json.data.message,
+        };
+        this.id = json.id;
+    }
 }
 
+type EventHandler = (event: Event) => void;
+
+export class OutputEnvelope<T> {
+    status: string;
+    requestNr: number;
+    data: T;
+    action: string | null;
+    error: {
+        code: string,
+        message: string | null,
+        description: string | null,
+    } | null;
+
+    constructor(json: OutputEnvelopeJson<T>) {
+        this.status = json.status;
+        this.requestNr = json.request_nr;
+        this.data = json.data;
+        this.action = json.action || null;
+        if (json.error) {
+            this.error = {
+                code: json.error.code,
+                message: json.error.message,
+                description: json.error.description,
+            };
+        } else {
+            this.error = null;
+        }
+    }
+}
+
+type OutputEnvelopeHandler<T> = (envelope: OutputEnvelope<T>) => void;
+
 export class Session {
     domain: Domain;
     server: DomainServer | null;
@@ -158,12 +298,12 @@ export class Session {
     numRequests: number;
     requestNumDiscriminator: number;
     subscriptions: {
-        callback: (a: EventJson) => void,
+        callback: EventHandler,
         requestNr: number | undefined,
         abortController: AbortController | undefined,
     }[];
     subscriptionEndpoints: {
-        callback: (a: EventJson) => void,
+        callback: EventHandler,
     }[];
 
     constructor(domain: Domain) {
@@ -303,11 +443,11 @@ export class Session {
 
     async send(
         endpoint: string,
-        data: any,
+        data: Object,
         timeout: number = 2000,
         forceHttp: boolean = false,
         abortController: AbortController | undefined = undefined,
-        cb: ((a: OutputEnvelopeJson) => void) | undefined = undefined,
+        cb: OutputEnvelopeHandler<unknown> | undefined = undefined,
     ) {
         this.numRequests++;
         while (true) {
@@ -324,8 +464,12 @@ export class Session {
                     } else {
                         this.websocket.addEventListener("message", (msg) => {
                             const data = JSON.parse(msg.data);
-                            if (data['request_nr'] === req_nr) {
-                                cb(data);
+                            if (data !== null && data.constructor === Object && data['request_nr'] === req_nr) {
+                                if (isOutputEnvelopeJson<unknown>(data)) {
+                                    cb(new OutputEnvelope<unknown>(data));
+                                } else {
+                                    throw new Error("Invalid server response");
+                                }
                             }
                         });
                     }
@@ -415,7 +559,7 @@ export class Session {
         return response;
     }
 
-    async newEvent(roomId: string, data: any, id: string | undefined = undefined) {
+    async newEvent(roomId: string, data: Object, id: string | undefined = undefined) {
         return this.send("new_event", {
             room_id: roomId,
             events: [{
@@ -425,17 +569,21 @@ export class Session {
         });
     }
 
-    async subscribe(cb: (a: EventJson) => void) {
+    async subscribe(cb: EventHandler) {
         this.subscriptionEndpoints.push({callback: cb});
         await this._subscribe(cb);
     }
 
-    private async _subscribe(cb: (a: EventJson) => void) {
+    private async _subscribe(cb: EventHandler) {
         if (this.websocket !== null) {
             const subscription = await this.send('subscribe', {}, 60_000, false, undefined, (res) => {
                 if (res.action === 'push') {
-                    for (const event of res.data.events) {
-                        cb(event);
+                    if (isEventPushEnvelopeJson(res.data)) {
+                        for (const event of res.data.events) {
+                            cb(new Event(event));
+                        }
+                    } else {
+                        throw new Error("Invalid server response");
                     }
                 }
             });