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 result = await llmForcedToMultiply.invoke("what is 2 + 4");
console.log(JSON.stringify(result.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 in the “any” (or “required” which is OpenAI specific) keyword to the tool_choice parameter.

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

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

note

We've recently standardized the tool_choice field to be consistent across all models which support it. The following table shows the different packages and minium versions required to use the new standardized tool_choice field:

Package Name	Min Package Version	Min Core Version
@langchain/aws	0.0.3	0.2.17
@langchain/openai	0.2.4	0.2.17
@langchain/groq	0.0.14	0.2.17