diff --git a/src/browser.ts b/src/browser.ts
index 7fa23cd..45987d0 100644
--- a/src/browser.ts
+++ b/src/browser.ts
@@ -157,7 +157,7 @@ async encodeImage(image: Uint8Array | string): Promise<string> {
   ): Promise<ChatResponse | AbortableAsyncIterator<ChatResponse>> {
     if (request.messages) {
       for (const message of request.messages) {
-        if (message.images) {
+        if ("images" in message && message.images) {
           message.images = await Promise.all(
             message.images.map(this.encodeImage.bind(this)),
           )
diff --git a/src/interfaces.ts b/src/interfaces.ts
index 5b7b115..450bc00 100644
--- a/src/interfaces.ts
+++ b/src/interfaces.ts
@@ -60,13 +60,30 @@ export interface GenerateRequest {
   options?: Partial<Options>
 }
 
-export interface Message {
-  role: string
+export type Message = SystemMessage | AssistantMessage | UserMessage | ToolResponseMessage
+
+export interface SystemMessage {
+  role: "system",
+  content: string
+}
+
+export interface AssistantMessage {
+  role: "assistant"
   content: string
-  images?: Uint8Array[] | string[]
   tool_calls?: ToolCall[]
 }
 
+export interface UserMessage {
+  role: "user",
+  content: string,
+  images?: Uint8Array[] | string[]
+}
+
+export interface ToolResponseMessage {
+  role: "tool",
+  content: string
+}
+
 export interface ToolCall {
   function: {
     name: string;