diff --git a/index.d.ts b/index.d.ts
index 51c984e..f41811e 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -158,6 +158,14 @@ export interface Message {
   content: string
   copilot_references: MessageCopilotReference[]
   copilot_confirmations?: MessageCopilotConfirmation[]
+  tool_calls?: {
+    "function": {
+      "arguments": string,
+      "name": string
+    },
+    "id": string,
+    "type": "function"
+  }[]
   name?: string
 }
 
@@ -251,9 +259,21 @@ export type ModelName =
   | "gpt-4"
   | "gpt-3.5-turbo"
 
+export interface PromptFunction {
+  type: "function"
+  function: {
+    name: string;
+    description?: string;
+    /** @see https://platform.openai.com/docs/guides/structured-outputs/supported-schemas */
+    parameters?: Record<string, unknown>;
+    strict?: boolean | null;
+  }
+}
+
 export type PromptOptions = {
   model: ModelName
   token: string
+  tools?: PromptFunction[]
   request?: {
     fetch?: Function
   }
diff --git a/index.test-d.ts b/index.test-d.ts
index f8a16bd..267d21c 100644
--- a/index.test-d.ts
+++ b/index.test-d.ts
@@ -295,4 +295,24 @@ export async function promptTest() {
 
   // @ts-expect-error - token argument is required
   prompt("What is the capital of France?", { model: "" })
+}
+
+export async function promptWithToolsTest() {
+  await prompt("What is the capital of France?", {
+    model: "gpt-4",
+    token: "secret",
+    tools: [
+      {
+        type: "function",
+        function: {
+          name: "",
+          description: "",
+          parameters: {
+
+          },
+          strict: true,
+        }
+      }
+    ]
+  })
 }
\ No newline at end of file
diff --git a/lib/prompt.js b/lib/prompt.js
index 5e9bf8d..77bbab9 100644
--- a/lib/prompt.js
+++ b/lib/prompt.js
@@ -3,6 +3,11 @@
 /** @type {import('..').PromptInterface} */
 export async function prompt(userPrompt, promptOptions) {
   const promptFetch = promptOptions.request?.fetch || fetch;
+
+  const systemMessage = promptOptions.tools
+    ? "You are a helpful assistant. Use the supplied tools to assist the user."
+    : "You are a helpful assistant.";
+
   const response = await promptFetch(
     "https://api.githubcopilot.com/chat/completions",
     {
@@ -17,7 +22,7 @@ export async function prompt(userPrompt, promptOptions) {
         messages: [
           {
             role: "system",
-            content: "You are a helpful assistant.",
+            content: systemMessage,
           },
           {
             role: "user",
@@ -25,6 +30,8 @@ export async function prompt(userPrompt, promptOptions) {
           },
         ],
         model: promptOptions.model,
+        toolChoice: promptOptions.tools ? "auto" : undefined,
+        tools: promptOptions.tools,
       }),
     }
   );
diff --git a/test/prompt.test.js b/test/prompt.test.js
index 81ba603..fb8d723 100644
--- a/test/prompt.test.js
+++ b/test/prompt.test.js
@@ -33,7 +33,7 @@ test("minimal usage", async (t) => {
             content: "What is the capital of France?",
           },
         ],
-        model: "gpt-4o-mini",
+        model: "gpt-4",
       }),
     })
     .reply(
@@ -57,7 +57,82 @@ test("minimal usage", async (t) => {
 
   const result = await prompt("What is the capital of France?", {
     token: "secret",
-    model: "gpt-4o-mini",
+    model: "gpt-4",
+    request: { fetch: fetchMock },
+  });
+
+  t.assert.deepEqual(result, {
+    requestId: "<request-id>",
+    message: {
+      content: "<response text>",
+    },
+  });
+});
+
+test("function calling", async (t) => {
+  const mockAgent = new MockAgent();
+  function fetchMock(url, opts) {
+    opts ||= {};
+    opts.dispatcher = mockAgent;
+    return fetch(url, opts);
+  }
+
+  mockAgent.disableNetConnect();
+  const mockPool = mockAgent.get("https://api.githubcopilot.com");
+  mockPool
+    .intercept({
+      method: "post",
+      path: `/chat/completions`,
+      body: JSON.stringify({
+        messages: [
+          {
+            role: "system",
+            content:
+              "You are a helpful assistant. Use the supplied tools to assist the user.",
+          },
+          { role: "user", content: "Call the function" },
+        ],
+        model: "gpt-4",
+        toolChoice: "auto",
+        tools: [
+          {
+            type: "function",
+            function: { name: "the_function", description: "The function" },
+          },
+        ],
+      }),
+    })
+    .reply(
+      200,
+      {
+        choices: [
+          {
+            message: {
+              content: "<response text>",
+            },
+          },
+        ],
+      },
+      {
+        headers: {
+          "content-type": "application/json",
+          "x-request-id": "<request-id>",
+        },
+      }
+    );
+
+  const result = await prompt("Call the function", {
+    token: "secret",
+    model: "gpt-4",
+    tools: [
+      {
+        type: "function",
+        function: {
+          name: "the_function",
+          description: "The function",
+        },
+      },
+    ],
     request: { fetch: fetchMock },
   });