LangChain Structured Output in NestJS: Typed with Zod

Description and use

In NestJS with LangChain and Zod, the agent returns a typed OrderDraft object instead of raw text, simplifying order processing.

Problem

Obtaining structured, typed output from a LangChain agent in a NestJS backend instead of raw text.

Outcome

The agent now returns a typed OrderDraft object, simplifying backend logic and improving reliability.

Workflow

• Define a Zod schema OrderDraft with sku (string) and quantity (positive integer).
• Import OrderDraft into the OrdersController.
• Call agent.run() with the customer message and pass {output: OrderDraft} to enforce structured output.
• Return the output object from the controller method.
• Add OrdersController to AppModule controllers.
• Test the endpoint with curl to verify JSON response.
• Optionally run tests with LangChain fakeModel() to validate without API key.

Evidence

To get an object instead of text from a LangChain agent in NestJS, pass a Zod schema as output to run().

Source

Same message, same model. Without a schema, the agent answers in text, and your code has to dig the SKU out of a sentence. With one, it answers an object, checked against the schema and typed by it.

Source