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# Custom RPC and Errors

## Overview

Client schemas add type-safe custom RPC methods without changing the Transport. Error namespaces
keep transport, execution, contract, and Action failures beside the domain that owns them.

## Recipes

These recipes assume you understand the
[provider's custom RPC contract](/docs/transports/custom) and error behavior.

### Type a Custom RPC Method

Pass a Zod RPC schema to [`Client.create`](/docs/clients/create#typing-a-custom-rpc-schema). The
schema types the method parameters and return value used by
[`client.request`](/docs/clients/create#typing-a-custom-rpc-schema).

The schema provides TypeScript inference. It does not validate requests or responses at runtime.

```ts twoslash
import { Client, http } from 'viem'
import { mainnet } from 'viem/chains'
import { z } from 'viem/zod'

const client = Client.create({
  chain: mainnet,
  transport: http('https://rpc.example'),
  schema: z.RpcSchema.from({ // [!code focus]
    app_getLabel: { // [!code focus]
      params: z.tuple([z.string()]), // [!code focus]
      returns: z.string(), // [!code focus]
    }, // [!code focus]
  }), // [!code focus]
})

const label = await client.request({ // [!code focus]
  method: 'app_getLabel', // [!code focus]
  params: ['0xA0Cf798816D4b9b9866b5330EEa46a18382f251e'], // [!code focus]
}) // [!code focus]
```

### Wrap an EIP-1193 Provider

Use the [`custom`](/docs/transports/custom) Transport to connect an injected wallet or another
EIP-1193 provider to a Client.

```ts twoslash
import { Client, custom, walletActions } from 'viem'

declare const provider: {
  request(args: { method: string; params?: unknown }): Promise<unknown>
}

const client = Client.create({
  transport: custom(provider), // [!code focus]
}).extend(walletActions())

const addresses = await client.wallet.requestAddresses() // [!code focus]
```

### Match a Nested RPC Error

Every Viem [`Errors.BaseError`](/docs/errors) preserves its cause chain. Use `walk` to find the
specific [`RpcError`](/docs/errors/rpc) when an Action adds useful context around it.

:::code-group
```ts twoslash [example.ts]
import { Errors, RpcError } from 'viem'
import { Value } from 'viem/utils'
import { client } from './viem.config'

try {
  await client.transaction.send({
    to: '0x70997970c51812dc3a010c7d01b50e0d17dc79c8',
    value: Value.fromEther('1'),
  })
} catch (error) {
  if (error instanceof Errors.BaseError) { // [!code focus]
    const nonceError = error.walk( // [!code focus]
      (cause) => cause instanceof RpcError.NonceTooLowError, // [!code focus]
    ) // [!code focus]
    if (nonceError) console.error('Refresh the nonce and retry.') // [!code focus]
  } // [!code focus]
}
```

```ts twoslash [viem.config.ts] filename="viem.config.ts"
// [!include ~/snippets/docs/viem.config.ts:setup]
```
:::

## Best Practices

### Preserve the Cause

When translating a Viem failure into an application error, attach the original error as `cause`.
Callers can still inspect its namespace and contextual fields.

### Match Concrete Errors

Avoid parsing error messages. Match the owning error class, such as
[`RpcError.ExecutionRevertedError`](/docs/errors/rpc) or
[`Transport.TimeoutError`](/docs/transports/http#errors).

## See More

<Cards>
  <Card icon="lucide:shield-check" title="Build Resilient Transports" description="Recover from transport failures without retrying terminal execution errors." to="/docs/guides/clients/resilient-transports" />

  <Card icon="lucide:network" title="Custom Chains and Transports" description="Implement a new transport and chain definition." to="/docs/guides/extending/chains-transports" />
</Cards>