Function●Since v1.1

fakeModel

Creates a new FakeBuiltModel for testing.

Returns a chainable builder — queue responses, then pass the model anywhere a chat model is expected. Responses are consumed in FIFO order, one per invoke() call.

API summary

Method	Description
`fakeModel()`	Creates a new fake chat model. Returns a chainable builder.
`.respond(message)`	Queue an `AIMessage` (or any `BaseMessage`) to return on the next invocation.
`.respond(error)`	Queue an `Error` to throw on the next invocation.
`.respond(factory)`	Queue a function `(messages) => BaseMessage \\| Error` for dynamic responses.
`.respondWithTools(toolCalls)`	Shorthand for `.respond()` with tool calls. Each entry needs `name` and `args`; `id` is optional.
`.alwaysThrow(error)`	Make every invocation throw this error, regardless of the queue.
`.structuredResponse(value)`	Set the value returned by `.withStructuredOutput()`.
`.bindTools(tools)`	Bind tools to the model. Returns a `RunnableBinding` that shares the response queue and call recording.
`.withStructuredOutput(schema)`	Returns a runnable that produces the `.structuredResponse()` value.
`.calls`	Array of `{ messages, options }` for every invocation (read-only).
`.callCount`	Number of times the model has been invoked.

fakeModel(): FakeBuiltModel

Example

const model = fakeModel()
  .respondWithTools([{ name: "search", args: { query: "weather" } }])
  .respond(new AIMessage("Sunny and warm."));

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

const r2 = await model.invoke([new HumanMessage("Thanks")]);
// r2.content === "Sunny and warm."

View source on GitHub

fakeModel

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Example

LangChain Assistant

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fakeModel

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Example

fakeModel

API summary

Used in Docs

Example

Menu

fakeModel

API summary

Used in Docs

Example