diff --git a/lib/jrpc.js b/lib/jrpc.js
index 3d284c8..3668e5f 100755
--- a/lib/jrpc.js
+++ b/lib/jrpc.js
@@ -9876,7 +9876,7 @@ function parse(tokens, file) {
 // src/ir.ts
 var import_debug = __toESM(require_src());
 var log = (0, import_debug.default)("app");
-var builtin = ["number", "string", "boolean"];
+var BUILTIN = ["number", "string", "boolean"];
 var IRError = class extends Error {
   constructor(statement, message) {
     super("Error building IR: " + message);
@@ -9885,6 +9885,7 @@ var IRError = class extends Error {
 };
 function get_ir(parsed) {
   log("Generatie IR from parse output");
+  let builtin = [...BUILTIN];
   let defined = [];
   let types = [];
   let enums = [];
@@ -9936,18 +9937,18 @@ function get_ir(parsed) {
         throw new IRError(statement, `Type ${statement.name} already defined!`);
       }
       let last = -1;
-      let values = statement.values.map((valueS) => {
+      let values = statement.values.map((values2) => {
         let value = last + 1;
-        if (valueS.value) {
-          if (valueS.value <= last) {
+        if (values2.value) {
+          if (values2.value <= last) {
             throw new IRError(statement, "Enum value must be larger than the previous one!");
           } else {
-            value = valueS.value;
+            value = values2.value;
           }
         }
         last = value;
         return {
-          name: valueS.name,
+          name: values2.name,
           value
         };
       });
@@ -10045,6 +10046,9 @@ function get_ir(parsed) {
       ]);
     } else if (statement.type == "define") {
       options[statement.key] = statement.value;
+      if (statement.key == "use_messagepack" && statement.value == "true") {
+        builtin.push("bytes");
+      }
     } else {
       throw new IRError(statement, "Invalid statement!");
     }
@@ -10118,7 +10122,8 @@ var conversion = {
   boolean: "boolean",
   number: "number",
   string: "string",
-  void: "void"
+  void: "void",
+  bytes: "Uint8Array"
 };
 function toJSType(type) {
   return conversion[type] || type;
@@ -10435,7 +10440,8 @@ var conversion2 = {
   boolean: "bool",
   number: "double",
   string: "string",
-  void: "void"
+  void: "void",
+  bytes: ""
 };
 function toCSharpType(type) {
   return conversion2[type] || type;
@@ -10446,6 +10452,9 @@ var CSharpTarget = class extends CompileTarget {
     return this.options.csharp_namespace || "JRPC";
   }
   start() {
+    if (this.options.use_messagepack == true) {
+      throw new Error("C# has no support for MessagePack yet!");
+    }
     this.writeFile(this.namespace + ".csproj", this.getTemplate("CSharp/CSharp.csproj"));
     const fixNS = (input) => input.replace("__NAMESPACE__", this.namespace);
     const copyClass = (name) => this.writeFile(name + ".cs", fixNS(this.getTemplate(`CSharp/${name}.cs`)));
diff --git a/package.json b/package.json
index 02ffde6..d4be947 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
    "name": "@hibas123/jrpcgen",
-   "version": "1.0.19",
+   "version": "1.0.20",
    "main": "lib/index.js",
    "license": "MIT",
    "packageManager": "yarn@3.1.1",
diff --git a/src/compile.ts b/src/compile.ts
index 2014e53..76193a9 100644
--- a/src/compile.ts
+++ b/src/compile.ts
@@ -11,7 +11,7 @@ import {
 
 export abstract class CompileTarget<T = any> {
    abstract name: string;
-   constructor(private outputFolder: string, protected options: T) {
+   constructor(private outputFolder: string, protected options: T & { use_messagepack: boolean }) {
       if (!FS.existsSync(outputFolder)) {
          FS.mkdirSync(outputFolder, {
             recursive: true,
diff --git a/src/ir.ts b/src/ir.ts
index dc2d810..bbb1399 100644
--- a/src/ir.ts
+++ b/src/ir.ts
@@ -2,7 +2,7 @@ import type { Parsed, StatementNode } from "./parser";
 import dbg from "debug";
 const log = dbg("app");
 
-const builtin = ["number", "string", "boolean"];
+const BUILTIN = ["number", "string", "boolean"];
 
 export class IRError extends Error {
    constructor(public statement: StatementNode, message: string) {
@@ -68,6 +68,7 @@ export type IR = {
 
 export default function get_ir(parsed: Parsed): IR {
    log("Generatie IR from parse output");
+   let builtin = [...BUILTIN];
    let defined: string[] = [];
    let types: string[] = [];
    let enums: string[] = [];
@@ -148,23 +149,23 @@ export default function get_ir(parsed: Parsed): IR {
          }
 
          let last = -1;
-         let values = statement.values.map<EnumValueDefinition>((valueS) => {
+         let values = statement.values.map<EnumValueDefinition>((values) => {
             let value = last + 1;
-            if (valueS.value) {
-               if (valueS.value <= last) {
+            if (values.value) {
+               if (values.value <= last) {
                   throw new IRError(
                      statement,
                      "Enum value must be larger than the previous one!"
                   );
                } else {
-                  value = valueS.value;
+                  value = values.value;
                }
             }
 
             last = value;
 
             return {
-               name: valueS.name,
+               name: values.name,
                value,
             };
          });
@@ -282,6 +283,9 @@ export default function get_ir(parsed: Parsed): IR {
          ]);
       } else if(statement.type == "define") {
          options[statement.key] = statement.value;
+         if(statement.key == "use_messagepack" && statement.value == "true") {
+            builtin.push("bytes");
+         }
       } else {
          throw new IRError(statement, "Invalid statement!");
       }
diff --git a/src/targets/csharp.ts b/src/targets/csharp.ts
index 1a44a30..b21d3ab 100644
--- a/src/targets/csharp.ts
+++ b/src/targets/csharp.ts
@@ -15,6 +15,7 @@ const conversion = {
    number: "double",
    string: "string",
    void: "void",
+   bytes: ""
 };
 
 function toCSharpType(type: string): string {
@@ -29,6 +30,9 @@ export class CSharpTarget extends CompileTarget<{ csharp_namespace: string }> {
    }
 
    start(): void {
+      if(this.options.use_messagepack == true) {
+         throw new Error("C# has no support for MessagePack yet!");
+      }
       this.writeFile(
          this.namespace + ".csproj",
          this.getTemplate("CSharp/CSharp.csproj")
diff --git a/src/targets/typescript.ts b/src/targets/typescript.ts
index c3dbc3d..eea40c5 100644
--- a/src/targets/typescript.ts
+++ b/src/targets/typescript.ts
@@ -16,6 +16,7 @@ const conversion = {
    number: "number",
    string: "string",
    void: "void",
+   bytes: "Uint8Array"
 };
 
 function toJSType(type: string): string {