Stop writing TypeScript interfaces by hand - generate them from JSON, CSV, and OpenAPI in one command

Nathmaxx — Fri, 08 May 2026 07:55:01 +0000

You're integrating a third-party API. You open the docs, find a sample response, and start typing:

interface Product {
    id: number;
    name: string;
    price: number;
    // wait, is currency a string or an object?
    // what's the shape of variants[]?
    // does stock ever go null?
}

Twenty minutes later you have a fragile interface you're not fully confident in — and the moment the API ships a new field, it's already stale.

There's a better way.

snaptype: point it at the source, get types back

snaptype is a CLI that generates TypeScript interfaces and Zod schemas by reading your actual data — live API endpoints, OpenAPI specs, JSON files, CSV files.

No config. No code to write. One command.

`from-url` — types straight from a live endpoint

This is the one I use most. Give it any JSON endpoint:

npx snaptype from-url https://api.example.com/products --zod -o src/types/product.ts

snaptype fetches the response and generates:

export interface Product {
    id: number;
    name: string;
    price: number;
    currency: "USD" | "EUR" | "GBP";
    stock: number | null;
    createdAt: string;
    variants: Variant[];
}

export interface Variant {
    sku: string;
    color: string;
    size: "S" | "M" | "L" | "XL";
}

export const ProductSchema = z.object({
    id: z.number(),
    name: z.string(),
    price: z.number(),
    currency: z.enum(["USD", "EUR", "GBP"]),
    stock: z.number().nullable(),
    createdAt: z.iso.datetime(),
    variants: z.array(VariantSchema),
});

A few things worth noticing:

Nested objects are expanded — variants[] became its own Variant interface automatically
Enums are inferred — currency and size had a small set of repeated values, so snaptype made them union types instead of string
Nullables are detected — stock was null in some responses, so it became number | null
Semantic types — createdAt became z.iso.datetime(), not a generic z.string()

This is inference against real data, not a naive field-by-field mapping.

Real-world example: GitHub API

Point it at any real endpoint — here the GitHub user API:

npx snaptype from-url https://api.github.com/users/torvalds --zod -o src/types/github-user.ts

Done. You now have typed access to every field in that response — including nested objects — with Zod schemas you can use directly for runtime validation.

When GitHub ships a new field, re-run the command. No manual diffing.

`from-openapi` — one file per schema, from any spec

If the API you're integrating has an OpenAPI or Swagger spec (most do), this is even better. Point it at the spec file:

npx snaptype from-openapi ./openapi.yaml -o src/types/

Or directly at the hosted spec URL:

npx snaptype from-openapi https://petstore3.swagger.io/api/v3/openapi.json -o src/types/

snaptype reads every schema definition in the spec and generates one typed file per schema. YAML and JSON both work.

For a spec that defines Pet, Order, and User, you get three files:

// src/types/Pet.ts
export interface Pet {
  id?:        number;
  name:       string;
  photoUrls:  string[];
  status?:    "available" | "pending" | "sold";
  tags?:      Tag[];
  category?:  Category;
}

// src/types/Order.ts
export interface Order {
  id?:        number;
  petId?:     number;
  quantity?:  number;
  shipDate?:  string;
  status?:    "placed" | "approved" | "delivered";
  complete?:  boolean;
}

Before, setting up a typed client from an OpenAPI spec meant:

Finding (or writing) a code generator
Configuring it
Running it
Cleaning up the generated mess
Updating it every time the spec changes

Now it's one command.

Works with private and authenticated APIs

One-off request — pass headers directly with -H:

npx snaptype from-url https://api.example.com/me \
  -H "Authorization: Bearer $TOKEN" \
  --zod -o src/types/me.ts

Multiple endpoints on the same API — store the headers in .snaptyperc so you don't repeat yourself:

{
    "baseUrl": "https://api.example.com",
    "headers": {
        "Authorization": "Bearer my-token",
        "X-Tenant": "acme"
    }
}

Then every from-url call picks them up automatically:

npx snaptype from-url /users -o src/types/user.ts
npx snaptype from-url /products -o src/types/product.ts
npx snaptype from-url /orders -o src/types/order.ts

Headers set in .snaptyperc are defaults — anything passed with -H on the CLI takes priority if the same key appears in both.

stdin fallback — if the endpoint requires a flow you can't replicate in a single request (custom auth, cookies, proxy), pipe the response in from curl:

curl -s -H "Authorization: Bearer $TOKEN" https://api.example.com/me \
  | npx snaptype from-stdin --zod -o src/types/me.ts

JSON files and CSV too

If you're working from a local file rather than a live endpoint:

# Local JSON file
npx snaptype from-json ./user.json --zod -o src/types/user.ts

# CSV export from a database or spreadsheet
npx snaptype from-csv ./orders.csv --zod -o src/types/order.ts

The inference is the same — enums, nullables, semantic types, nested objects.

Zero config to start

No setup needed. If you want to pin defaults, add a .snaptyperc:

{
    "zod": true,
    "outDir": "src/types"
}

Then every command just picks up those defaults automatically.

Try it right now

Run it directly with npx — no install needed:

npx snaptype from-url https://api.github.com/users/torvalds --zod -o src/types/github-user.ts

Or add it as a dev dependency if you want a fixed version and faster runs:

npm install -D snaptype
snaptype from-url https://api.github.com/users/torvalds --zod -o src/types/github-user.ts

Full docs at snaptype.dev · Source on GitHub.

If it saves you time, a ⭐ on the repo goes a long way — it helps other developers find the tool.

Built this after one too many mornings spent re-typing interfaces from API docs. If from-url or from-openapi saves you time on a real project, drop a comment — I'm curious what APIs people are working with.

Feedback welcome

snaptype is still early-stage and I'm actively shaping what it becomes. If you hit a use case it doesn't handle, a command that behaves unexpectedly, or a feature you'd find genuinely useful — open an issue on GitHub or just leave a comment below. Every piece of feedback helps.

DEV Community: Nathmaxx