How to force tool calling behavior

Prerequisites

This guide assumes familiarity with the following concepts:

• Chat models
• LangChain Tools
• How to use a model to call tools

In order to force our LLM to select a specific tool, we can use the tool_choice parameter to ensure certain behavior. First, let’s define our model and tools:

import { tool } from "@langchain/core/tools";
import { z } from "zod";

const add = tool(
  (input) => {
    return `${input.a + input.b}`;
  },
  {
    name: "add",
    description: "Adds a and b.",
    schema: z.object({
      a: z.number(),
      b: z.number(),
    }),
  }
);

const multiply = tool(
  (input) => {
    return `${input.a * input.b}`;
  },
  {
    name: "Multiply",
    description: "Multiplies a and b.",
    schema: z.object({
      a: z.number(),
      b: z.number(),
    }),
  }
);

const tools = [add, multiply];

import { ChatOpenAI } from "@langchain/openai";

const llm = new ChatOpenAI({
  model: "gpt-3.5-turbo",
});

For example, we can force our tool to call the multiply tool by using the following code:

const llmForcedToMultiply = llm.bindTools(tools, {
  tool_choice: "Multiply",
});
const multiplyResult = await llmForcedToMultiply.invoke("what is 2 + 4");
console.log(JSON.stringify(multiplyResult.tool_calls, null, 2));

[
  {
    "name": "Multiply",
    "args": {
      "a": 2,
      "b": 4
    },
    "type": "tool_call",
    "id": "call_d5isFbUkn17Wjr6yEtNz7dDF"
  }
]

Even if we pass it something that doesn’t require multiplcation - it will still call the tool!

We can also just force our tool to select at least one of our tools by passing "any" (or for OpenAI models, the equivalent, "required") to the tool_choice parameter.

const llmForcedToUseTool = llm.bindTools(tools, {
  tool_choice: "any",
});
const anyToolResult = await llmForcedToUseTool.invoke("What day is today?");
console.log(JSON.stringify(anyToolResult.tool_calls, null, 2));

[
  {
    "name": "add",
    "args": {
      "a": 2,
      "b": 3
    },
    "type": "tool_call",
    "id": "call_La72g7Aj0XHG0pfPX6Dwg2vT"
  }
]