---
title: API Reference
sidebar:
  icon: book-marked
description: A short list of every public export, with links to the pages that explain them.
---

## `tacho`

| export                    | from    |                                                                                                                  |
| ------------------------- | ------- | ---------------------------------------------------------------------------------------------------------------- |
| `tacho()`                 | `tacho` | Procedure builder. Call it as `rpc({ ... })` to make a [router](/tacho/router).                                  |
| `router(def)`             | `tacho` | Identity helper. Same as `rpc({ ... })`.                                                                         |
| `RpcError`                | `tacho` | Throw from a procedure or [middleware](/tacho/middleware). See [errors](/tacho/error-handling).                  |
| `JSON_RPC_ERROR`          | `tacho` | `-32700` parse, `-32600` invalid request, `-32601` method not found, `-32602` invalid params, `-32603` internal. |
| `APP_ERROR_RANGE`         | `tacho` | `{ min: -32099, max: -32000 }` for your own codes.                                                               |
| `toOpenRpc(router, info)` | `tacho` | Walk a router into an [OpenRPC](/tacho/openrpc) document.                                                        |

## Transports

| export                  | from                    |                                                                                            |
| ----------------------- | ----------------------- | ------------------------------------------------------------------------------------------ |
| `handle(router, opts?)` | `tacho/transport/fetch` | `(Request) => Promise<Response>`. [HTTP](/tacho/transport/http).                           |
| `createClient(opts)`    | `tacho/client/http`     | Typed proxy over POST.                                                                     |
| `handle(router, opts?)` | `tacho/transport/ws`    | [crossws](https://github.com/h3js/crossws) hooks. [WebSocket](/tacho/transport/websocket). |
| `createClient(opts)`    | `tacho/client/ws`       | One socket. `.ready` and `.close()`.                                                       |

A `File` or `Blob` in params or the result is [Files](/tacho/files). An `async function*` is [SSE](/tacho/sse-streaming).

## Low-level

For a custom transport.

```ts twoslash
import {
  createProxyClient,
  resolveProcedure,
  rpcResult,
  runBatch,
  runOne,
  tacho,
} from "tacho";

const rpc = tacho();
const router = rpc({ ping: rpc.run(() => "pong" as const) });
const ctx = { req: new Request("http://localhost") };
// ---cut---
resolveProcedure(router, "ping");
await runOne(router, { jsonrpc: "2.0", method: "ping", id: 1 }, ctx);
await runBatch(
  router,
  [{ jsonrpc: "2.0", method: "ping", id: 1 }],
  ctx,
);
rpcResult({ result: "pong" });
rpcResult({ error: { message: "nope", code: -32603 } });

const client = createProxyClient<typeof router>(
  async (method, params) => {
    const res = await runOne(
      router,
      { jsonrpc: "2.0", method, params, id: 1 },
      ctx,
    );
    return rpcResult(res ?? {});
  },
);
await client.ping();
```

| export                             |                                                          |
| ---------------------------------- | -------------------------------------------------------- |
| `resolveProcedure(router, method)` | Procedure, or `undefined`.                               |
| `runOne(router, request, ctx)`     | One JSON-RPC call. No `id` → `undefined` (notification). |
| `runBatch(router, requests, ctx)`  | Many calls. All notifications → `undefined`.             |
| `rpcResult(body)`                  | Result, or throws `RpcError`.                            |
| `createProxyClient(send)`          | Typed proxy over your own `send`.                        |