Unit testing

Unit tests exercise small, deterministic pieces of your agent in isolation. By replacing the real LLM with an in-memory fake (AKA fixture), you can script exact responses (text, tool calls, and errors) so tests are fast, free, and repeatable without API keys.

Mock chat model with `fakeModel`

fakeModel is a builder-style fake chat model that lets you script exact responses (text, tool calls, errors) and assert what the model received. It extends BaseChatModel, so it works anywhere a real model is expected.

import { fakeModel } from "langchain";

Quick start

Create a model, queue responses with .respond(), and invoke. Each invoke() consumes the next queued response in order:

import { fakeModel } from "langchain";
import { AIMessage, HumanMessage } from "@langchain/core/messages";

const model = fakeModel()
  .respond(new AIMessage("I can help with that."))
  .respond(new AIMessage("Here's what I found."))
  .respond(new AIMessage("You're welcome!"));

const r1 = await model.invoke([new HumanMessage("Can you help?")]);
// r1.content === "I can help with that."

const r2 = await model.invoke([new HumanMessage("What did you find?")]);
// r2.content === "Here's what I found."

const r3 = await model.invoke([new HumanMessage("Thanks!")]);
// r3.content === "You're welcome!"

If the model is invoked more times than there are queued responses, it throws a descriptive error:

const model = fakeModel()
  .respond(new AIMessage("only one"));

await model.invoke([new HumanMessage("first")]);  // works
await model.invoke([new HumanMessage("second")]); // throws: "no response queued for invocation 1"

Tool calling responses

.respond() supports tool calls by passing an AIMessage with tool_calls:

import { fakeModel } from "langchain";
import { AIMessage, HumanMessage } from "@langchain/core/messages";

const model = fakeModel()
  .respond(new AIMessage({
    content: "",
    tool_calls: [
      { name: "get_weather", args: { city: "San Francisco" }, id: "call_1", type: "tool_call" },
    ],
  }))
  .respond(new AIMessage("It's 72°F and sunny in San Francisco."));

const r1 = await model.invoke([new HumanMessage("What's the weather in SF?")]);
console.log(r1.tool_calls[0].name); // "get_weather"

const r2 = await model.invoke([new HumanMessage("Thanks")]);
console.log(r2.content); // "It's 72°F and sunny in San Francisco."

.respondWithTools() is a shorthand for the same thing. Instead of constructing the full AIMessage, provide just the tool name and arguments:

// These two queue entries produce identical responses:

model.respond(new AIMessage({
  content: "",
  tool_calls: [
    { name: "get_weather", args: { city: "SF" }, id: "call_1", type: "tool_call" },
  ],
}));

// Equivalent shorthand:
model.respondWithTools([  
  { name: "get_weather", args: { city: "SF" }, id: "call_1" },
]);

The id field is optional. If omitted, a unique ID is auto-generated.

.respond() and .respondWithTools() can be mixed freely in any order. This is particularly useful for testing agentic loops where the model alternates between tool calls and text responses.

Simulate errors

Errors at specific turns

Passing an Error to .respond() makes the model throw on that specific invocation. Errors can appear at any position in the sequence:

import { fakeModel } from "langchain";
import { AIMessage, HumanMessage } from "@langchain/core/messages";

const model = fakeModel()
  .respond(new Error("rate limit exceeded"))  // Turn 1: throws
  .respond(new AIMessage("Recovered!"));      // Turn 2: succeeds

try {
  await model.invoke([new HumanMessage("first")]);
} catch (e) {
  console.log(e.message); // "rate limit exceeded"
}

const result = await model.invoke([new HumanMessage("retry")]);
console.log(result.content); // "Recovered!"

Errors on every call

.alwaysThrow() makes every invocation throw, regardless of the queue. This is useful for testing error handling and retry logic:

import { fakeModel } from "langchain";
import { HumanMessage } from "@langchain/core/messages";

const model = fakeModel().alwaysThrow(new Error("service unavailable"));

await model.invoke([new HumanMessage("a")]); // throws "service unavailable"
await model.invoke([new HumanMessage("b")]); // throws "service unavailable"

Dynamic responses with factory functions

.respond() also accepts a function that computes the response based on the input messages. The function receives the full message array and returns either a BaseMessage or an Error:

import { fakeModel } from "langchain";
import { AIMessage, HumanMessage } from "@langchain/core/messages";

const model = fakeModel()
  .respond((messages) => {
    const last = messages[messages.length - 1].text;
    return new AIMessage(`You said: ${last}`);
  });

const result = await model.invoke([new HumanMessage("hello")]);
console.log(result.content); // "You said: hello"

Factory functions can also return errors:

import { fakeModel } from "langchain";
import { AIMessage, HumanMessage } from "@langchain/core/messages";

const model = fakeModel()
  .respond((messages) => {
    const content = messages[messages.length - 1].text;
    if (content.includes("forbidden")) {
      return new Error("Content policy violation");
    }
    return new AIMessage("OK");
  });

await model.invoke([new HumanMessage("forbidden topic")]); // throws "Content policy violation"

Each function is a single queue entry, consumed once. To reuse the same dynamic logic for multiple turns, queue multiple respond function calls.

Structured output

For code that uses .withStructuredOutput(), configure the fake return value with .structuredResponse():

import { fakeModel } from "langchain";
import { HumanMessage } from "@langchain/core/messages";
import { z } from "zod";

const model = fakeModel()
  .structuredResponse({ temperature: 72, unit: "fahrenheit" });

const structured = model.withStructuredOutput(
  z.object({
    temperature: z.number(),
    unit: z.string(),
  })
);

const result = await structured.invoke([new HumanMessage("Weather?")]);
console.log(result);
// { temperature: 72, unit: "fahrenheit" }

The schema passed to .withStructuredOutput() is ignored. The model always returns the value configured with .structuredResponse(). This keeps tests focused on application logic rather than parsing.

Assert what the model received

fakeModel records every invocation, including the messages and options passed to the model. This works like a spy or mock in traditional testing frameworks:

import { fakeModel } from "langchain";
import { AIMessage, HumanMessage } from "@langchain/core/messages";

const model = fakeModel()
  .respond(new AIMessage("first"))
  .respond(new AIMessage("second"));

await model.invoke([new HumanMessage("question 1")]);
await model.invoke([new HumanMessage("question 2")]);

console.log(model.callCount); // 2

console.log(model.calls[0].messages[0].content); // "question 1"
console.log(model.calls[1].messages[0].content); // "question 2"

Calls are recorded even when the model throws:

import { fakeModel } from "langchain";
import { HumanMessage } from "@langchain/core/messages";

const model = fakeModel().respond(new Error("boom"));

try {
  await model.invoke([new HumanMessage("will fail")]);
} catch {
  // error handled
}

console.log(model.callCount); // 1
console.log(model.calls[0].messages[0].content); // "will fail"

Use with `bindTools`

Agent frameworks like LangChain agents and LangGraph call model.bindTools(tools) internally. fakeModel handles this automatically. The bound model shares the same response queue and call recording as the original, so no special setup is needed:

import { fakeModel } from "langchain";
import { AIMessage, HumanMessage } from "@langchain/core/messages";
import { tool } from "@langchain/core/tools";
import { z } from "zod";

const searchTool = tool(async ({ query }) => `Results for: ${query}`, {
  name: "search",
  description: "Search the web",
  schema: z.object({ query: z.string() }),
});

const model = fakeModel()
  .respondWithTools([{ name: "search", args: { query: "weather" }, id: "1" }])
  .respond(new AIMessage("The weather is sunny."));

const bound = model.bindTools([searchTool]);

const r1 = await bound.invoke([new HumanMessage("weather?")]);
console.log(r1.tool_calls[0].name); // "search"

const r2 = await bound.invoke([new HumanMessage("thanks")]);
console.log(r2.content); // "The weather is sunny."

// Call recording is shared. Inspect via the original model.
console.log(model.callCount); // 2

Full example: test a tool-calling agent with vitest

import { describe, test, expect } from "vitest";
import { fakeModel } from "langchain";
import { AIMessage, HumanMessage, ToolMessage } from "@langchain/core/messages";
import { tool } from "@langchain/core/tools";
import { z } from "zod";

const getWeather = tool(
  async ({ city }) => `72°F and sunny in ${city}`,
  {
    name: "get_weather",
    description: "Get weather for a city",
    schema: z.object({ city: z.string() }),
  }
);

async function runAgent(
  model: ReturnType<typeof fakeModel>,
  input: string
) {
  const messages: any[] = [new HumanMessage(input)];
  const bound = model.bindTools([getWeather]);

  while (true) {
    const response = await bound.invoke(messages);
    messages.push(response);

    if (!response.tool_calls?.length) {
      return { messages, finalResponse: response };
    }

    for (const tc of response.tool_calls) {
      const result = await getWeather.invoke(tc.args);
      messages.push(new ToolMessage({
        content: result as string,
        tool_call_id: tc.id!,
      }));
    }
  }
}

describe("weather agent", () => {
  test("calls get_weather and returns a final answer", async () => {
    const model = fakeModel()
      .respondWithTools([
        { name: "get_weather", args: { city: "SF" }, id: "call_1" },
      ])
      .respond(new AIMessage("It's 72°F and sunny in SF!"));

    const { finalResponse } = await runAgent(model, "Weather in SF?");

    expect(finalResponse.content).toBe("It's 72°F and sunny in SF!");
    expect(model.callCount).toBe(2);

    const secondCall = model.calls[1].messages;
    const toolMsg = secondCall.find((m: any) => m._getType() === "tool");
    expect(toolMsg?.content).toContain("72°F and sunny in SF");
  });

  test("handles model errors gracefully", async () => {
    const model = fakeModel()
      .respond(new Error("rate limit"));

    await expect(
      runAgent(model, "Weather?")
    ).rejects.toThrow("rate limit");

    expect(model.callCount).toBe(1);
  });
});

Next steps

Learn how to test your agent with real model provider APIs in Integration testing.

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Get started

Core components

Frontend

Middleware

Advanced usage

Agent development

Deploy with LangSmith

Mock chat model with `fakeModel`

Quick start

Tool calling responses

Simulate errors

Errors at specific turns

Errors on every call

Dynamic responses with factory functions

Structured output

Assert what the model received

Use with `bindTools`

Next steps

Get started

Core components

Frontend

Middleware

Advanced usage

Agent development

Deploy with LangSmith

​Mock chat model with fakeModel

​Quick start

​Tool calling responses

​Simulate errors

​Errors at specific turns

​Errors on every call

​Dynamic responses with factory functions

​Structured output

​Assert what the model received

​Use with bindTools

​Next steps

Mock chat model with `fakeModel`

Quick start

Tool calling responses

Simulate errors

Errors at specific turns

Errors on every call

Dynamic responses with factory functions

Structured output

Assert what the model received

Use with `bindTools`

Next steps