Structured output with zod

Describe the deck outline once with zod and use LangChain's withStructuredOutput to get a checked JavaScript object from Gemini instead of JSON text.

Lesson 26 of 4411 min+30 XPHands-on

In lesson 7.2 you wrote the outline schema by hand in Gemini's format, called the API, and parsed the JSON text. LangChain collapses that into two steps: describe the shape, then ask for it.

zod schema

z.object({ title: z.string(), … })

JSON schema

sent with the request

Gemini

constrained to that shape

Parsed object

outline.slides[0].heading

You describe the shape once in zod. LangChain converts it to Gemini's JSON schema, sends it, and hands you back a parsed object instead of a string.

The schema in zod

the same outline, in zod

z.object({
  title: z.string().describe("The deck title, under 8 words"),
  slides: z.array(
    z.object({
      component: z.enum(SLIDE_COMPONENTS),
      heading: z.string(),
      points: z.array(z.string()),
    }),
  ),
})

z.object, z.array, z.string and z.enum describe the same shape as before, in plain JavaScript.
.describe("…") adds a description to the schema Gemini receives. The model reads it, so "under 8 words" really does shorten titles.

Asking for it

model.withStructuredOutput(OutlineSchema) returns a new runnable. Invoke it with the request, and what comes back is already a JavaScript object.

Under the hood — Is the object checked against the schema?

Gemini is constrained to the JSON schema while it writes, and LangChain parses the result. If the reply somehow doesn't match (a truncated response, for example), you get an error instead of a half-built object. That's what you want: fail loudly at the boundary, not three steps later.

Key takeaways

Describe the outline once with zod; LangChain turns it into Gemini's JSON schema.
withStructuredOutput returns a parsed object, not a string you have to JSON.parse.
.describe() text is part of the prompt: the model reads it.

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