diff --git a/package.json b/package.json
index b5566a7..2409e54 100644
--- a/package.json
+++ b/package.json
@@ -16,12 +16,13 @@
     "@types/jest": "^29.5.1",
     "@types/node": "^20.2.4",
     "jest": "^29.5.0",
-    "openai": "^3.2.1",
+    "openai": "^3.3.0",
     "ts-jest": "^29.1.0",
     "ts-toolbelt": "^9.6.0",
     "typescript": "^5.0.4"
   },
   "dependencies": {
+    "arktype": "1.0.14-alpha",
     "zod": "^3.21.4"
   }
 }
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 13a3f5c..9869661 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -3,21 +3,23 @@ lockfileVersion: 5.4
 specifiers:
   '@types/jest': ^29.5.1
   '@types/node': ^20.2.4
+  arktype: 1.0.14-alpha
   jest: ^29.5.0
-  openai: ^3.2.1
+  openai: ^3.3.0
   ts-jest: ^29.1.0
   ts-toolbelt: ^9.6.0
   typescript: ^5.0.4
   zod: ^3.21.4
 
 dependencies:
+  arktype: 1.0.14-alpha
   zod: 3.21.4
 
 devDependencies:
   '@types/jest': 29.5.1
   '@types/node': 20.2.4
   jest: 29.5.0_@types+node@20.2.4
-  openai: 3.2.1
+  openai: 3.3.0
   ts-jest: 29.1.0_tobmchb5uviuq5lwsinkw5fvje
   ts-toolbelt: 9.6.0
   typescript: 5.0.4
@@ -769,6 +771,11 @@ packages:
       sprintf-js: 1.0.3
     dev: true
 
+  /arktype/1.0.14-alpha:
+    resolution: {integrity: sha512-theD5K4QrYCWMtQ52Masj169IgtMJ8Argld/MBS4lotEwR3b+GzfBvsqVJ1OIKhJDTdES02FLQTjcfRe00//mA==}
+    requiresBuild: true
+    dev: false
+
   /asynckit/0.4.0:
     resolution: {integrity: sha512-Oei9OH4tRh0YqU3GxhX79dM/mwVgvbZJaSNaRk+bshkj0S5cfHcgYakreBjrHwatXKbz+IoIdYLxrKim2MjW0Q==}
     dev: true
@@ -1922,8 +1929,8 @@ packages:
       mimic-fn: 2.1.0
     dev: true
 
-  /openai/3.2.1:
-    resolution: {integrity: sha512-762C9BNlJPbjjlWZi4WYK9iM2tAVAv0uUp1UmI34vb0CN5T2mjB/qM6RYBmNKMh/dN9fC+bxqPwWJZUTWW052A==}
+  /openai/3.3.0:
+    resolution: {integrity: sha512-uqxI/Au+aPRnsaQRe8CojU0eCR7I0mBiKjD3sNMzY6DaC1ZVrc85u98mtJW6voDug8fgGN+DIZmTDxTthxb7dQ==}
     dependencies:
       axios: 0.26.1
       form-data: 4.0.0
diff --git a/src/Chat.ts b/src/Chat.ts
index 8c8104a..2ebf19c 100644
--- a/src/Chat.ts
+++ b/src/Chat.ts
@@ -15,7 +15,9 @@ export class Chat<
   ) {}
 
   toArray() {
-    return (this.messages as any[]).map((m: ChatCompletionRequestMessage) => ({
+    return (this.messages as any[])
+    .filter((m) => m.content !== undefined)
+    .map((m) => ({
       role: m.role,
       content: new PromptBuilder(m.content)
         .addInputValidation<ExtractArgs<typeof m.content, typeof this.args>>()
diff --git a/src/ChatBuilder.ts b/src/ChatBuilder.ts
index bd1e2d5..ad5a12f 100644
--- a/src/ChatBuilder.ts
+++ b/src/ChatBuilder.ts
@@ -2,7 +2,7 @@ import { z } from "zod";
 import { F } from "ts-toolbelt";
 import { ChatCompletionRequestMessage } from "openai";
 import { Chat } from "./Chat";
-import { user, assistant, system } from "./ChatHelpers";
+import { User, Assistant, System, Function } from "./ChatHelpers";
 import { ExtractArgs, ExtractChatArgs, TypeToZodShape, ReplaceChatArgs } from "./types";
 
 export class ChatBuilder<
@@ -19,31 +19,45 @@ export class ChatBuilder<
     return new ChatBuilder(this.messages) as any;
   }
 
-  user<TUserText extends string>(
+  User<TUserText extends string>(
     str: TUserText
   ): ChatBuilder<
     [...TMessages, { role: "user"; content: TUserText }],
     F.Narrow<TExpectedInput> & ExtractArgs<TUserText>
   > {
-    return new ChatBuilder([...this.messages, user(str)]) as any;
+    return new ChatBuilder([...this.messages, User(str)]) as any;
   }
 
-  system<TSystemText extends string>(
+  System<TSystemText extends string>(
     str: TSystemText
   ): ChatBuilder<
     [...TMessages, { role: "system"; content: TSystemText }],
     F.Narrow<TExpectedInput> & ExtractArgs<TSystemText>
   > {
-    return new ChatBuilder([...this.messages, system(str)]) as any;
+    return new ChatBuilder([...this.messages, System(str)]) as any;
   }
 
-  assistant<TAssistantText extends string>(
+  Assistant<TAssistantText extends string>(
     str: TAssistantText
   ): ChatBuilder<
     [...TMessages, { role: "assistant"; content: TAssistantText }],
     F.Narrow<TExpectedInput> & ExtractArgs<TAssistantText>
   > {
-    return new ChatBuilder([...this.messages, assistant(str)]) as any;
+    return new ChatBuilder([...this.messages, Assistant(str)]) as any;
+  }
+  
+  // Backwards compadibility
+  user = this.User;
+  system = this.System;
+  assistant = this.Assistant;
+
+  Function<TAssistantText extends string>(
+    str: TAssistantText
+  ): ChatBuilder<
+    [...TMessages, { role: "function"; content: TAssistantText }],
+    F.Narrow<TExpectedInput> & ExtractArgs<TAssistantText>
+  > {
+    return new ChatBuilder([...this.messages, Function(str)]) as any;
   }
 
   addZodInputValidation<TShape extends TExpectedInput>(
diff --git a/src/ChatHelpers.ts b/src/ChatHelpers.ts
index fb72c0f..1085f2b 100644
--- a/src/ChatHelpers.ts
+++ b/src/ChatHelpers.ts
@@ -1,7 +1,6 @@
 // ChatMessage creation helpers
-// Ideally these would Dedent their content, but ts is checker is way too slow
 // https://tinyurl.com/message-creators-literal-types
-export function system<T extends string>(
+export function System<T extends string>(
   literals: TemplateStringsArray | T,
   ...placeholders: unknown[]
 ) {
@@ -10,7 +9,7 @@ export function system<T extends string>(
     content: dedent(literals, ...placeholders),
   };
 }
-export function user<T extends string>(
+export function User<T extends string>(
   literals: TemplateStringsArray | T,
   ...placeholders: unknown[]
 ) {
@@ -19,7 +18,7 @@ export function user<T extends string>(
     content: dedent(literals, ...placeholders),
   };
 }
-export function assistant<T extends string>(
+export function Assistant<T extends string>(
   literals: TemplateStringsArray | T,
   ...placeholders: unknown[]
 ) {
@@ -28,6 +27,20 @@ export function assistant<T extends string>(
     content: dedent(literals, ...placeholders),
   };
 }
+export function Function<T extends string>(
+  literals: TemplateStringsArray | T,
+  ...placeholders: unknown[]
+) {
+  return {
+    role: "function" as const,
+    content: dedent(literals, ...placeholders),
+  };
+}
+
+// backwards compadibility
+export const system = System;
+export const user = User;
+export const assistant = Assistant;
 
 export function dedent<T extends string>(
   templ: TemplateStringsArray | T,
diff --git a/src/PromptBuilder.ts b/src/PromptBuilder.ts
index a6c83c5..0f93e62 100644
--- a/src/PromptBuilder.ts
+++ b/src/PromptBuilder.ts
@@ -1,4 +1,5 @@
 import { z } from "zod";
+import { Type } from "arktype";
 import { F } from "ts-toolbelt";
 import { Prompt } from "./Prompt";
 import { ExtractArgs, ReplaceArgs, TypeToZodShape } from "./types";
@@ -18,23 +19,13 @@ export class PromptBuilder<
   addZodInputValidation<TShape extends TExpectedInput>(
     shape: TypeToZodShape<TShape>
   ) {
-    const zodValidator = z.object(shape as any);
-    return new (class extends PromptBuilder<TPromptTemplate, TShape> {
-      validate(args: Record<string, any>): args is TShape {
-        return zodValidator.safeParse(args).success;
-      }
-
-      get type() {
-        return this.template as ReplaceArgs<TPromptTemplate, TShape>;
-      }
+    return new ZodPromptBuilder(this.template, shape);
+  }
 
-      build<TSuppliedInputArgs extends TShape>(
-        args: F.Narrow<TSuppliedInputArgs>
-      ) {
-        zodValidator.parse(args);
-        return new Prompt(this.template, args).toString();
-      }
-    })(this.template);
+  addArkTypeInputValidation<TShape extends TExpectedInput>(
+    shape: Type<TShape>
+  ) {
+    return new ArkTypePromptBuilder(this.template, shape);
   }
 
   validate(args: Record<string, any>): args is TExpectedInput {
@@ -52,3 +43,65 @@ export class PromptBuilder<
     return new Prompt(this.template, args).toString();
   }
 }
+
+class ZodPromptBuilder<
+  TPromptTemplate extends string,
+  TExpectedInput extends ExtractArgs<TPromptTemplate, {}>
+> extends PromptBuilder<TPromptTemplate, TExpectedInput> {
+  constructor(
+    public template: TPromptTemplate,
+    public shape: TypeToZodShape<TExpectedInput>
+  ) {
+    super(template);
+  }
+  validate(args: Record<string, any>): args is TExpectedInput {
+    const zodValidator = z.object(this.shape as any);
+    return zodValidator.safeParse(args).success;
+  }
+
+  get type() {
+    return this.template as ReplaceArgs<TPromptTemplate, TExpectedInput>;
+  }
+
+  build<TSuppliedInputArgs extends TExpectedInput>(
+    args: F.Narrow<TSuppliedInputArgs>
+  ) {
+    const zodValidator = z.object(this.shape as any);
+    zodValidator.parse(args);
+    return new Prompt(this.template, args).toString();
+  }
+}
+
+class ArkTypePromptBuilder<
+  TPromptTemplate extends string,
+  TExpectedInput extends ExtractArgs<TPromptTemplate, {}>
+> extends PromptBuilder<TPromptTemplate, TExpectedInput> {
+  constructor(
+    public template: TPromptTemplate,
+    public shape: Type<TExpectedInput>
+  ) {
+    super(template);
+  }
+  validate(args: Record<string, any>): args is TExpectedInput {
+    try {
+      this.shape(args);
+      return true;
+    } catch (e) {
+      return false;
+    }
+  }
+
+  get type() {
+    return this.template as ReplaceArgs<TPromptTemplate, TExpectedInput>;
+  }
+
+  build<TSuppliedInputArgs extends TExpectedInput>(
+    args: F.Narrow<TSuppliedInputArgs>
+  ) {
+    const { problems } = this.shape(args);
+    if (problems?.summary) {
+      throw new Error(problems.summary);
+    }
+    return new Prompt(this.template, args).toString();
+  }
+}
diff --git a/src/__tests__/Chat.test.ts b/src/__tests__/Chat.test.ts
index dfb6afc..59f70a6 100644
--- a/src/__tests__/Chat.test.ts
+++ b/src/__tests__/Chat.test.ts
@@ -1,6 +1,6 @@
 import { strict as assert } from "node:assert";
 import { Chat } from "../Chat";
-import { system, user, assistant } from "../ChatHelpers";
+import { System, User, Assistant } from "../ChatHelpers";
 import { Equal, Expect } from "./types.test";
 
 describe("Chat", () => {
@@ -14,7 +14,7 @@ describe("Chat", () => {
     const chat = new Chat(
       [
         // ^?
-        user("Tell me a {{jokeType}} joke"),
+        User("Tell me a {{jokeType}} joke"),
       ],
       // @ts-expect-error
       {}
@@ -28,13 +28,13 @@ describe("Chat", () => {
     const chat = new Chat(
       [
         //     ^?
-        user(`Tell me a {{jokeType}} joke`),
+        User(`Tell me a {{jokeType}} joke`),
       ],
       {
         jokeType: "funny" as const,
       }
     ).toArray();
-    const usrMsg = user("Tell me a funny joke");
+    const usrMsg = User("Tell me a funny joke");
     //     ^?
     type test = Expect<Equal<typeof chat, [typeof usrMsg]>>;
     assert.deepEqual(chat, [usrMsg]);
@@ -44,9 +44,9 @@ describe("Chat", () => {
     const chat = new Chat(
       [
         // ^?
-        user(`Tell me a {{jokeType1}} joke`),
-        assistant(`{{var2}} joke?`),
-        system(`joke? {{var3}}`),
+        User(`Tell me a {{jokeType1}} joke`),
+        Assistant(`{{var2}} joke?`),
+        System(`joke? {{var3}}`),
       ],
       {
         jokeType1: "funny",
@@ -54,9 +54,9 @@ describe("Chat", () => {
         var3: "bar",
       } as const
     ).toArray();
-    const usrMsg = user("Tell me a funny joke");
-    const astMsg = assistant("foo joke?");
-    const sysMsg = system("joke? bar");
+    const usrMsg = User("Tell me a funny joke");
+    const astMsg = Assistant("foo joke?");
+    const sysMsg = System("joke? bar");
     type test = Expect<
       Equal<typeof chat, [typeof usrMsg, typeof astMsg, typeof sysMsg]>
     >;
@@ -67,15 +67,15 @@ describe("Chat", () => {
     const chat = new Chat(
       [
         // ^?
-        user(`Tell me a joke`),
-        assistant(`joke?`),
-        system(`joke?`),
+        User(`Tell me a joke`),
+        Assistant(`joke?`),
+        System(`joke?`),
       ],
       {}
     ).toArray();
-    const usrMsg = user("Tell me a joke");
-    const astMsg = assistant("joke?");
-    const sysMsg = system("joke?");
+    const usrMsg = User("Tell me a joke");
+    const astMsg = Assistant("joke?");
+    const sysMsg = System("joke?");
     type test = Expect<
       Equal<typeof chat, [typeof usrMsg, typeof astMsg, typeof sysMsg]>
     >;
diff --git a/src/__tests__/PromptBuilder.test.ts b/src/__tests__/PromptBuilder.test.ts
index d837194..bdcf474 100644
--- a/src/__tests__/PromptBuilder.test.ts
+++ b/src/__tests__/PromptBuilder.test.ts
@@ -1,5 +1,6 @@
 import { strict as assert } from "node:assert";
 import { z, ZodError } from "zod";
+import { type } from 'arktype';
 import { PromptBuilder } from "../PromptBuilder";
 import { Equal, Expect } from "./types.test";
 
@@ -282,7 +283,9 @@ describe("PromptBuilder with input validation using Zod", () => {
     );
     type BasicType = typeof promptBuilder.type;
     //      ^?
-    type BasicTest = Expect<Equal<BasicType, `Tell ${any} ${any} ${any} ${any} joke`>>;
+    type BasicTest = Expect<
+      Equal<BasicType, `Tell ${any} ${any} ${any} ${any} joke`>
+    >;
 
     const tsValidatedPromptBuilder = promptBuilder.addInputValidation<{
       jokeType: "funny" | "silly";
@@ -305,34 +308,37 @@ describe("PromptBuilder with input validation using Zod", () => {
   });
 
   test("Can write a function that accepts the type of a PromptBuilder then accepts any output from that builder", () => {
-    const promptBuilder = new PromptBuilder("Tell me a {{jokeType}} joke.").addInputValidation<{
-      jokeType: "funny" | "silly"
+    const promptBuilder = new PromptBuilder(
+      "Tell me a {{jokeType}} joke."
+    ).addInputValidation<{
+      jokeType: "funny" | "silly";
     }>();
     function exampleFunction(input: typeof promptBuilder.type) {}
 
     exampleFunction(promptBuilder.build({ jokeType: "funny" }));
-    exampleFunction("Tell me a funny joke.")
+    exampleFunction("Tell me a funny joke.");
     exampleFunction(promptBuilder.build({ jokeType: "silly" }));
-    exampleFunction("Tell me a silly joke.")
+    exampleFunction("Tell me a silly joke.");
     // @ts-expect-error
     exampleFunction(promptBuilder.build({ jokeType: "bad" }));
     // @ts-expect-error
-    exampleFunction("Tell me a bad joke.")
-  })
-
+    exampleFunction("Tell me a bad joke.");
+  });
 
   test("Can write a function that accepts the type of a PromptBuilder then accepts any output from that builder", () => {
-    const promptBuilder = new PromptBuilder("Tell me a {{jokeType}} joke.").addZodInputValidation({
+    const promptBuilder = new PromptBuilder(
+      "Tell me a {{jokeType}} joke."
+    ).addZodInputValidation({
       jokeType: z.union([z.literal("funny"), z.literal("silly")]),
     });
     function exampleFunction(input: typeof promptBuilder.type) {}
 
     exampleFunction(promptBuilder.build({ jokeType: "funny" }));
-    exampleFunction("Tell me a funny joke.")
+    exampleFunction("Tell me a funny joke.");
     exampleFunction(promptBuilder.build({ jokeType: "silly" }));
-    exampleFunction("Tell me a silly joke.")
+    exampleFunction("Tell me a silly joke.");
     // @ts-expect-error
-    exampleFunction("Tell me a bad joke.")
+    exampleFunction("Tell me a bad joke.");
     assert.throws(
       () => {
         // @ts-expect-error
@@ -350,5 +356,32 @@ describe("PromptBuilder with input validation using Zod", () => {
         return true;
       }
     );
-  })
+  });
+
+  test("Can add arktypes validation", () => {
+    const promptBuilder = new PromptBuilder(
+      "Tell me a {{jokeType}} joke."
+    ).addArkTypeInputValidation(
+      type({
+        jokeType: "'funny' | 'silly'",
+      })
+    );
+    function exampleFunction(input: typeof promptBuilder.type) {}
+
+    exampleFunction(promptBuilder.build({ jokeType: "funny" }));
+    exampleFunction("Tell me a funny joke.");
+    exampleFunction(promptBuilder.build({ jokeType: "silly" }));
+    exampleFunction("Tell me a silly joke.");
+    // @ts-expect-error
+    exampleFunction("Tell me a bad joke.");
+    
+    assert.throws(
+      () => {
+
+        // @ts-expect-error
+        exampleFunction(promptBuilder.build({ jokeType: "bad" }));
+      }
+    );
+
+  });
 });
diff --git a/src/index.ts b/src/index.ts
index 907f022..33bc1a3 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -3,5 +3,6 @@ import { PromptBuilder } from "./PromptBuilder";
 import { Chat } from "./Chat";
 import { system, user, assistant } from "./ChatHelpers";
 import { ChatBuilder } from "./ChatBuilder";
+import * as unstable from './unstable'
 
-export { Prompt, PromptBuilder, Chat, ChatBuilder, system, user, assistant };
+export { Prompt, PromptBuilder, Chat, ChatBuilder, system, user, assistant, unstable };
diff --git a/src/unstable/AgentBuilder.ts b/src/unstable/AgentBuilder.ts
new file mode 100644
index 0000000..3c43947
--- /dev/null
+++ b/src/unstable/AgentBuilder.ts
@@ -0,0 +1,92 @@
+import { z } from "zod";
+import { F } from "ts-toolbelt";
+import { ChatCompletionRequestMessage } from "openai";
+import { Chat } from "../Chat";
+import { user, assistant, system } from "../ChatHelpers";
+import { ExtractArgs, ExtractChatArgs, TypeToZodShape, ReplaceChatArgs } from "../types";
+
+export class AgentBuilder<
+  TMessages extends
+    | []
+    | [...ChatCompletionRequestMessage[], ChatCompletionRequestMessage],
+  TExpectedInput extends ExtractChatArgs<TMessages, any>
+> {
+  constructor(public messages: TMessages) {}
+
+  addInputValidation<
+    TSTypeValidator extends ExtractChatArgs<TMessages, TSTypeValidator>
+  >(): AgentBuilder<TMessages, TSTypeValidator> {
+    return new AgentBuilder(this.messages) as any;
+  }
+
+  addOutputValidation<
+    TSTypeValidator extends ExtractChatArgs<TMessages, TSTypeValidator>
+ >(): AgentBuilder<TMessages, TSTypeValidator> {
+
+  user<TUserText extends string>(
+    str: TUserText
+  ): AgentBuilder<
+    [...TMessages, { role: "user"; content: TUserText }],
+    F.Narrow<TExpectedInput> & ExtractArgs<TUserText>
+  > {
+    return new AgentBuilder([...this.messages, user(str)]) as any;
+  }
+
+  system<TSystemText extends string>(
+    str: TSystemText
+  ): AgentBuilder<
+    [...TMessages, { role: "system"; content: TSystemText }],
+    F.Narrow<TExpectedInput> & ExtractArgs<TSystemText>
+  > {
+    return new AgentBuilder([...this.messages, system(str)]) as any;
+  }
+
+  assistant<TAssistantText extends string>(
+    str: TAssistantText
+  ): AgentBuilder<
+    [...TMessages, { role: "assistant"; content: TAssistantText }],
+    F.Narrow<TExpectedInput> & ExtractArgs<TAssistantText>
+  > {
+    return new AgentBuilder([...this.messages, assistant(str)]) as any;
+  }
+
+  addZodInputValidation<TShape extends TExpectedInput>(
+    shape: TypeToZodShape<TShape>
+  ) {
+    const zodValidator = z.object(shape as any);
+    return new (class extends AgentBuilder<TMessages, TShape> {
+      validate(args: Record<string, any>): args is TShape {
+        return zodValidator.safeParse(args).success;
+      }
+
+      get type() {
+        return this.messages as ReplaceChatArgs<TMessages, TShape>;
+      }
+    
+      build<TSuppliedInputArgs extends TShape>(
+        args: F.Narrow<TSuppliedInputArgs>
+      ) {
+        zodValidator.parse(args);
+        return super.build(args);
+      }
+    })(this.messages);
+  }
+
+  validate(args: Record<string, any>): args is TExpectedInput {
+    // Validate can only be called on a PromptBuilder with zod input validation
+    return false;
+  }
+
+  get type() {
+    return this.messages as ReplaceChatArgs<TMessages, TExpectedInput>;
+  }
+
+  build<TSuppliedInputArgs extends TExpectedInput>(
+    args: F.Narrow<TSuppliedInputArgs>
+  ) {
+    return new Chat<TMessages, TSuppliedInputArgs>(
+      this.messages as any,
+      args
+    ).toArray();
+  }
+}
diff --git a/src/unstable/ChatModelBuilder.ts b/src/unstable/ChatModelBuilder.ts
new file mode 100644
index 0000000..e69de29
diff --git a/src/unstable/PromptModelBuilder.ts b/src/unstable/PromptModelBuilder.ts
new file mode 100644
index 0000000..e69de29
diff --git a/src/unstable/index.ts b/src/unstable/index.ts
new file mode 100644
index 0000000..4a1ec83
--- /dev/null
+++ b/src/unstable/index.ts
@@ -0,0 +1 @@
+export { AgentBuilder } from './AgentBuilder';
\ No newline at end of file