diff --git a/package.json b/package.json
index 2701624..04f547e 100644
--- a/package.json
+++ b/package.json
@@ -28,6 +28,7 @@
     "openai-fetch": "^1.2.0",
     "p-map": "^6.0.0",
     "parse-json": "^7.0.0",
+    "type-fest": "^3.9.0",
     "zod": "^3.21.4",
     "zod-validation-error": "^1.3.0"
   },
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index ad15e5b..11cc221 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -16,6 +16,9 @@ dependencies:
   parse-json:
     specifier: ^7.0.0
     version: 7.0.0
+  type-fest:
+    specifier: ^3.9.0
+    version: 3.9.0
   zod:
     specifier: ^3.21.4
     version: 3.21.4
diff --git a/src/llm.ts b/src/llm.ts
index f43ceb8..cd5c2e3 100644
--- a/src/llm.ts
+++ b/src/llm.ts
@@ -1,7 +1,10 @@
+import type { SetRequired } from 'type-fest'
 import { ZodRawShape, ZodTypeAny, z } from 'zod'
 
 import * as types from './types'
 
+const defaultOpenAIModel = 'gpt-3.5-turbo'
+
 export class Agentic {
   _client: types.openai.OpenAIClient
   _verbosity: number
@@ -10,21 +13,19 @@ export class Agentic {
     'provider' | 'model' | 'modelParams' | 'timeoutMs' | 'retryConfig'
   >
 
-  constructor(
-    client: types.openai.OpenAIClient,
-    opts: {
-      verbosity?: number
-      defaults?: Pick<
-        types.BaseLLMOptions,
-        'provider' | 'model' | 'modelParams' | 'timeoutMs' | 'retryConfig'
-      >
-    } = {}
-  ) {
-    this._client = client
+  constructor(opts: {
+    openai: types.openai.OpenAIClient
+    verbosity?: number
+    defaults?: Pick<
+      types.BaseLLMOptions,
+      'provider' | 'model' | 'modelParams' | 'timeoutMs' | 'retryConfig'
+    >
+  }) {
+    this._client = opts.openai
     this._verbosity = opts.verbosity ?? 0
     this._defaults = {
       provider: 'openai',
-      model: 'gpt-3.5-turbo',
+      model: defaultOpenAIModel,
       modelParams: {},
       timeoutMs: 30000,
       retryConfig: {
@@ -54,7 +55,7 @@ export class Agentic {
       options = promptOrChatCompletionParams
 
       if (!options.messages) {
-        throw new Error()
+        throw new Error('messages must be provided')
       }
     }
 
@@ -68,7 +69,7 @@ export class Agentic {
 
 export abstract class BaseLLMCallBuilder<
   TInput extends ZodRawShape | ZodTypeAny = ZodTypeAny,
-  TOutput extends ZodRawShape | ZodTypeAny = ZodTypeAny,
+  TOutput extends ZodRawShape | ZodTypeAny = z.ZodType<string>,
   TModelParams extends Record<string, any> = Record<string, any>
 > {
   _options: types.BaseLLMOptions<TInput, TOutput, TModelParams>
@@ -118,7 +119,7 @@ export abstract class BaseLLMCallBuilder<
 
 export abstract class ChatModelBuilder<
   TInput extends ZodRawShape | ZodTypeAny = ZodTypeAny,
-  TOutput extends ZodRawShape | ZodTypeAny = ZodTypeAny,
+  TOutput extends ZodRawShape | ZodTypeAny = z.ZodType<string>,
   TModelParams extends Record<string, any> = Record<string, any>
 > extends BaseLLMCallBuilder<TInput, TOutput, TModelParams> {
   _messages: types.ChatMessage[]
@@ -132,11 +133,11 @@ export abstract class ChatModelBuilder<
 
 export class OpenAIChatModelBuilder<
   TInput extends ZodRawShape | ZodTypeAny = ZodTypeAny,
-  TOutput extends ZodRawShape | ZodTypeAny = ZodTypeAny
+  TOutput extends ZodRawShape | ZodTypeAny = z.ZodType<string>
 > extends ChatModelBuilder<
   TInput,
   TOutput,
-  Omit<types.openai.ChatCompletionParams, 'messages'>
+  SetRequired<Omit<types.openai.ChatCompletionParams, 'messages'>, 'model'>
 > {
   _client: types.openai.OpenAIClient
 
@@ -150,6 +151,7 @@ export class OpenAIChatModelBuilder<
   ) {
     super({
       provider: 'openai',
+      model: defaultOpenAIModel,
       ...options
     })
 
@@ -159,8 +161,26 @@ export class OpenAIChatModelBuilder<
   override async call(
     input?: types.ParsedData<TInput>
   ): Promise<types.ParsedData<TOutput>> {
-    // this._options.output?.describe
-    // TODO
-    return true as types.ParsedData<TOutput>
+    // TODO: construct messages
+
+    const completion = await this._client.createChatCompletion({
+      model: defaultOpenAIModel, // TODO: this shouldn't be necessary
+      ...this._options.modelParams,
+      messages: this._messages
+    })
+
+    if (this._options.output) {
+      const schema =
+        this._options.output instanceof z.ZodType
+          ? this._options.output
+          : z.object(this._options.output)
+
+      // TODO: convert string => object if necessary
+      // TODO: handle errors, retry logic, and self-healing
+
+      return schema.parse(completion.message.content)
+    } else {
+      return completion.message.content as any
+    }
   }
 }
diff --git a/src/temp.ts b/src/temp.ts
index c735131..3f455e4 100644
--- a/src/temp.ts
+++ b/src/temp.ts
@@ -5,9 +5,10 @@ import { z } from 'zod'
 import { Agentic } from './llm'
 
 dotenv.config()
+
 async function main() {
   const openai = new OpenAIClient({ apiKey: process.env.OPENAI_API_KEY! })
-  const $ = new Agentic(openai)
+  const $ = new Agentic({ openai })
 
   const ex0 = await $.gpt4(`give me a single boolean value`)
     .output(z.boolean())
@@ -15,6 +16,13 @@ async function main() {
     .call()
 
   console.log(ex0)
+
+  const ex1 = await $.gpt4(`give me fake data conforming to this schema`)
+    .output(z.object({ foo: z.string(), bar: z.number() }))
+    // .retry({ attempts: 3 })
+    .call()
+
+  console.log(ex1)
 }
 
 main()
diff --git a/src/types.ts b/src/types.ts
index d0c1720..d9ef18c 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -3,7 +3,6 @@ import {
   SafeParseReturnType,
   ZodObject,
   ZodRawShape,
-  ZodSchema,
   ZodTypeAny,
   output,
   z
@@ -27,7 +26,7 @@ export type SafeParsedData<T extends ZodRawShape | ZodTypeAny> =
 
 export interface BaseLLMOptions<
   TInput extends ZodRawShape | ZodTypeAny = ZodTypeAny,
-  TOutput extends ZodRawShape | ZodTypeAny = ZodTypeAny,
+  TOutput extends ZodRawShape | ZodTypeAny = z.ZodType<string>,
   TModelParams extends Record<string, any> = Record<string, any>
 > {
   provider?: string
@@ -43,7 +42,7 @@ export interface BaseLLMOptions<
 
 export interface LLMOptions<
   TInput extends ZodRawShape | ZodTypeAny = ZodTypeAny,
-  TOutput extends ZodRawShape | ZodTypeAny = ZodTypeAny,
+  TOutput extends ZodRawShape | ZodTypeAny = z.ZodType<string>,
   TModelParams extends Record<string, any> = Record<string, any>
 > extends BaseLLMOptions<TInput, TOutput, TModelParams> {
   promptTemplate?: string
@@ -51,7 +50,7 @@ export interface LLMOptions<
   promptSuffix?: string
 }
 
-export type ChatMessageRole = 'user' | 'system' | 'assistant' | 'tool'
+export type ChatMessageRole = 'user' | 'system' | 'assistant'
 
 export interface ChatMessage {
   role: ChatMessageRole
@@ -60,7 +59,7 @@ export interface ChatMessage {
 
 export interface ChatModelOptions<
   TInput extends ZodRawShape | ZodTypeAny = ZodTypeAny,
-  TOutput extends ZodRawShape | ZodTypeAny = ZodTypeAny,
+  TOutput extends ZodRawShape | ZodTypeAny = z.ZodType<string>,
   TModelParams extends Record<string, any> = Record<string, any>
 > extends BaseLLMOptions<TInput, TOutput, TModelParams> {
   messages: ChatMessage[]