diff --git a/src/llms/chat.ts b/src/llms/chat.ts
index 7e6cb6e..0a75ac9 100644
--- a/src/llms/chat.ts
+++ b/src/llms/chat.ts
@@ -9,7 +9,11 @@ import * as types from '@/types'
 import { parseOutput } from '@/llms/parse-output'
 import { BaseTask } from '@/task'
 import { getCompiledTemplate } from '@/template'
-import { extractFunctionIdentifierFromString, stringifyForModel } from '@/utils'
+import {
+  extractFunctionIdentifierFromString,
+  isFunction,
+  stringifyForModel
+} from '@/utils'
 
 import { BaseLLM } from './llm'
 import {
@@ -23,7 +27,7 @@ export abstract class BaseChatCompletion<
   TModelParams extends Record<string, any> = Record<string, any>,
   TChatCompletionResponse extends Record<string, any> = Record<string, any>
 > extends BaseLLM<TInput, TOutput, TModelParams> {
-  protected _messages: types.ChatMessage[]
+  protected _messages: types.ChatMessageInput<TInput>[]
   protected _tools?: BaseTask<any, any>[]
 
   constructor(
@@ -116,14 +120,17 @@ export abstract class BaseChatCompletion<
       input = this.inputSchema.parse(input)
     }
 
-    const messages = this._messages
+    const messages: types.ChatMessage[] = this._messages
       .map((message) => {
         return {
           ...message,
           content: message.content
-            ? getCompiledTemplate(dedent(message.content))(input).trim()
+            ? // support functions which return a string
+              isFunction(message.content)
+              ? message.content(input!)
+              : getCompiledTemplate(dedent(message.content))(input).trim()
             : ''
-        }
+        } as types.ChatMessage
       })
       .filter((message) => message.content || message.function_call)
 
@@ -170,6 +177,7 @@ export abstract class BaseChatCompletion<
       'additionalProperties',
       '$schema'
     ])
+
     let label: string
     if (outputSchema instanceof z.ZodArray) {
       label = 'JSON array (minified)'
diff --git a/src/llms/openai.ts b/src/llms/openai.ts
index 09d6202..72f4a3d 100644
--- a/src/llms/openai.ts
+++ b/src/llms/openai.ts
@@ -14,7 +14,7 @@ const openaiModelsSupportingFunctions = new Set([
 ])
 
 export class OpenAIChatCompletion<
-  TInput extends types.TaskInput = any,
+  TInput extends types.TaskInput = void,
   TOutput extends types.TaskOutput = string
 > extends BaseChatCompletion<
   TInput,
diff --git a/src/task.ts b/src/task.ts
index 6252af2..02af003 100644
--- a/src/task.ts
+++ b/src/task.ts
@@ -156,7 +156,7 @@ export abstract class BaseTask<
   /**
    * Calls this task with the given `input` and returns the result only.
    */
-  public async call(input?: TInput): Promise<TOutput> {
+  public async call(input: TInput): Promise<TOutput> {
     const res = await this.callWithMetadata(input)
     return res.result
   }
@@ -165,7 +165,7 @@ export abstract class BaseTask<
    * Calls this task with the given `input` and returns the result along with metadata.
    */
   public async callWithMetadata(
-    input?: TInput,
+    input: TInput,
     parentCtx?: types.TaskCallContext<any>
   ): Promise<types.TaskResponse<TOutput>> {
     this.validate()
diff --git a/src/types.ts b/src/types.ts
index 4b45700..9870c07 100644
--- a/src/types.ts
+++ b/src/types.ts
@@ -73,12 +73,18 @@ export interface LLMOptions<
 export type ChatMessage = openai.ChatMessage
 export type ChatMessageRole = openai.ChatMessageRole
 
+export type ChatMessageInput<TInput extends TaskInput = void> =
+  | ChatMessage
+  | {
+      content: (input: TInput | any) => string
+    }
+
 export interface ChatModelOptions<
   TInput extends TaskInput = void,
   TOutput extends TaskOutput = string,
   TModelParams extends Record<string, any> = Record<string, any>
 > extends BaseLLMOptions<TInput, TOutput, TModelParams> {
-  messages: ChatMessage[]
+  messages: ChatMessageInput<TInput>[]
   tools?: BaseTask<any, any>[]
 }
 
@@ -136,7 +142,7 @@ export interface TaskCallContext<
   TInput extends TaskInput = void,
   TMetadata extends TaskResponseMetadata = TaskResponseMetadata
 > {
-  input?: TInput
+  input: TInput
   retryMessage?: string
 
   attemptNumber: number
diff --git a/src/utils.ts b/src/utils.ts
index 3e8a2c2..18c74c7 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -210,3 +210,8 @@ export function throttleKy(
     }
   })
 }
+
+// eslint-disable-next-line @typescript-eslint/ban-types
+export function isFunction(value: any): value is Function {
+  return typeof value === 'function'
+}
diff --git a/test/llms/openai.test.ts b/test/llms/openai.test.ts
index 1b8da04..d2a015d 100644
--- a/test/llms/openai.test.ts
+++ b/test/llms/openai.test.ts
@@ -224,3 +224,29 @@ test('OpenAIChatCompletion ⇒ missing template variable', async (t) => {
     message: 'Template error: "numFacts" not defined in input - 1:10'
   })
 })
+
+test('OpenAIChatCompletion ⇒ function inputs', async (t) => {
+  t.timeout(2 * 60 * 1000)
+  const agentic = createTestAgenticRuntime()
+
+  const builder = new OpenAIChatCompletion({
+    agentic,
+    modelParams: {
+      temperature: 0,
+      max_tokens: 30
+    },
+    messages: [
+      {
+        role: 'user',
+        content: (input) => `tell me about ${input.topic}`
+      }
+    ]
+  })
+    .input(z.object({ topic: z.string() }))
+    .output(z.string())
+
+  const result = await builder.call({ topic: 'cats' })
+  t.truthy(typeof result === 'string')
+
+  expectTypeOf(result).toMatchTypeOf<string>()
+})