diff --git a/src/model/chat.test.ts b/src/model/chat.test.ts
index 0af890f..d7c4340 100644
--- a/src/model/chat.test.ts
+++ b/src/model/chat.test.ts
@@ -120,7 +120,7 @@ describe('ChatModel', () => {
     expect(apiResponseEvent).toHaveBeenCalledWith({
       timestamp: new Date().toISOString(),
       modelType: 'chat',
-      modelProvider: 'openai',
+      modelProvider: 'spy', // because we mocked the client
       params: {
         model: 'gpt-fake',
         messages: [{ role: 'user', content: 'content' }],
@@ -133,7 +133,7 @@ describe('ChatModel', () => {
 
   it('implements extend', async () => {
     type ChatContext = { userId: string; cloned?: boolean };
-    const chatModel = new ChatModel<ChatContext>({
+    const chatModel = new ChatModel<ChatContext, Model.Chat.Client, Model.Chat.Config<Model.Chat.Client>>({
       client: Client,
       context: { userId: '123' },
       params: { model: 'gpt-fake' },
@@ -220,7 +220,7 @@ describe('ChatModel', () => {
 
     // Extend the model and make another request
     const secondChatModel = chatModel.extend({
-      params: { model: 'gpt-fake-extended' },
+      params: { model: 'gpt-fake-extended'},
       context: { level: 2 },
       events: { onComplete: [newOnComplete] },
     });
diff --git a/src/model/chat.ts b/src/model/chat.ts
index 27b5b11..22f4fb5 100644
--- a/src/model/chat.ts
+++ b/src/model/chat.ts
@@ -55,7 +55,7 @@ export class ChatModel<
   CustomCtx
 > {
   modelType = 'chat' as const;
-  modelProvider = 'anthropic' as const;
+  modelProvider = 'openai';
 
   constructor(args: ChatModelArgs<CustomCtx, CustomClient, CustomConfig> = {}) {
     const {
@@ -83,6 +83,8 @@ export class ChatModel<
       ),
       ...rest,
     });
+
+    this.modelProvider = this.client.name;
   }
 
   protected async runModel<Cfg extends Model.Chat.Config<CustomClient>>(
@@ -274,17 +276,16 @@ export class ChatModel<
   }
 
   /** Clone the model and merge/override the given properties. */
-  extend(
-    args?: PartialChatModelArgs<CustomCtx, CustomClient, CustomConfig>
-  ): this {
+  extend(args?: PartialChatModelArgs<CustomCtx, CustomClient, Model.Chat.Config<CustomClient>>): this {
+    const { client, params, ...rest } = args ?? {};
     return new ChatModel({
       cacheKey: this.cacheKey,
       cache: this.cache,
-      client: this.client,
+      client: client ?? this.client,
       debug: this.debug,
       telemetry: this.telemetry,
-      ...args,
-      params: deepMerge(this.params, args?.params),
+      ...rest,
+      params: deepMerge(this.params, params),
       context:
         args?.context && Object.keys(args.context).length === 0
           ? undefined
diff --git a/src/model/model.ts b/src/model/model.ts
index 1d39bdf..ec8b684 100644
--- a/src/model/model.ts
+++ b/src/model/model.ts
@@ -83,23 +83,33 @@ export abstract class AbstractModel<
   ): Promise<MResponse>;
 
   /** Clones the model, optionally modifying its config */
-  abstract extend<
-    Args extends PartialModelArgs<MClient, MConfig, MRun, MResponse, CustomCtx>,
-  >(args?: Args): this;
+  abstract extend(
+    args?: PartialModelArgs<
+      MClient,
+      Model.Base.Config<MClient>,
+      MRun,
+      // Note: this response type maybe change over time as the user
+      // extends the model
+      // it should be inferred from some types rather than set to MResponse
+      MResponse,
+      CustomCtx
+    >
+  ): this;
 
   public abstract readonly modelType: Model.Type;
-  public abstract readonly modelProvider: Model.Provider;
+  public abstract modelProvider: Model.Provider;
 
-  protected readonly cacheKey: CacheKey<MRun & MConfig, string>;
+  // the cache key can be updated in a call to .extend so it doesn't necessarily conform to MRun & MConfig
+  protected readonly cacheKey: CacheKey<Model.Base.Run & Model.Base.Config<Model.Base.Client>, string>;
   protected readonly cache?: CacheStorage<string, MResponse>;
   public readonly client: MClient;
   public readonly context: CustomCtx;
   public readonly debug: boolean;
   public readonly params: MConfig & Partial<MRun>;
   public readonly events: Model.Events<
-    MClient,
-    MRun & MConfig,
-    MResponse,
+    Model.Base.Client,
+    Model.Base.Run & Model.Base.Config<Model.Base.Client>,
+    Model.Base.Response,
     CustomCtx,
     AResponse
   >;
@@ -107,13 +117,22 @@ export abstract class AbstractModel<
   public readonly telemetry: Telemetry.Provider;
 
   constructor(args: ModelArgs<MClient, MConfig, MRun, MResponse, CustomCtx>) {
-    this.cacheKey = args.cacheKey ?? defaultCacheKey;
+    this.cacheKey = (args.cacheKey ?? defaultCacheKey) as CacheKey<
+      Model.Base.Run & Model.Base.Config<Model.Base.Client>,
+      string
+    >;
     this.cache = args.cache;
     this.client = args.client;
     this.context = args.context ?? ({} as CustomCtx);
     this.debug = args.debug ?? false;
     this.params = args.params;
-    this.events = args.events || {};
+    this.events = (args.events || {}) as Model.Events<
+      Model.Base.Client,
+      Model.Base.Run & Model.Base.Config<Model.Base.Client>,
+      Model.Base.Response,
+      CustomCtx,
+      AResponse
+    >;
     this.tokenizer = createTokenizer(args.params.model);
     this.telemetry = args.telemetry ?? DefaultTelemetry;
   }