When you define an API route with typed parameters, payloads, and responses, KosmoJS generates a corresponding fetch client - automatically, as part of the same build step.

The result is a fully-typed client that mirrors your route definition exactly. Parameters, payload shape, response type - all derived from the same source. Change your API, and the client updates with it. No manual sync required.

What Gets Generated

Each route's client module exports:

🔹 HTTP method functions - GET, POST, PUT, etc., accepting parameters and payloads typed to match your route definition and returning typed response promises. Details ›

🔹 path and href utilities - construct relative or absolute URLs with proper parameter substitution and optional query string support. Details ›

🔹 validationSchemas - the same schemas used for server-side validation, exposed for client-side form validation with check, errors, errorMessage, errorSummary, and validate methods. Details ›

Using the Generated Client

Import the fetch map and pick the client for your route by path:

pages/example/index.tsx

import fetchClients from "_/fetch";

const response = await fetchClients["users/[id]"].GET([123]);

The generator places its output in the lib directory alongside other generated artifacts (validation routines, OpenAPI spec). Everything is updated automatically in the background as you modify routes during development.

Isomorphic Fetch

The same fetch client works in both CSR and SSR - identical code, no server functions or RPC layer to define. A component or loader that calls fetchClients["users/[id]"].GET([123]) runs unchanged whether it executes in the browser or on the server during SSR.

What differs is the transport underneath. In the browser the client issues a normal same-origin network request. During SSR the API server is bundled into the SSR bundle, and the client dispatches to the matched API route in-process - a direct function call through the full chain (middleware, validation, handler), with no network layer and no round-trip latency.

The client wraps fetch and swaps transport based on environment; it does not patch the global. Native fetch behaves exactly as it always does - only the generated clients switch to in-process dispatch on the server.