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# Handling reverts

Every call the SDK builds carries the contract's ABI, so viem decodes a custom error's name and
arguments. [`getRailnetError`](/utilities/errors) reads that revert out of whatever viem threw and
attaches a hint when the SDK has one.

## Simulate before you send

`simulateContract` runs the call and returns the revert before the user is asked to sign. Every
example in these docs simulates first for that reason, and the React path does the same with
`useSimulateContract` ([Sending from React](/guides/sendingFromReact)).

```ts
import { simulateContract } from 'viem/actions'
import { buildDepositConduitCall, getRailnetError } from '@railnetorg/railnet-sdk'

try {
  await simulateContract(client, { ...buildDepositConduitCall(parameters), account })
} catch (error) {
  const reverted = getRailnetError(error)

  if (reverted?.name === 'InsufficientAllowance') {
    return approveThenRetry()
  }

  throw new Error(reverted?.hint ?? 'the deposit failed', { cause: error })
}
```

`getRailnetError` returns `null` for anything that was not a contract revert. A rejected
signature, a transport error, and a failed gas estimate all land here.

## Reverts viem leaves undecoded

viem decodes a revert against the ABI of the call, and a contract's ABI lists only the errors its
own code reverts with. Two kinds arrive as raw bytes. A rejection encoded from `ErrorLib` and
reverted through assembly is on no contract's ABI at all; `InvalidOutput` and `InvalidEstimation`,
the two most common deposit failures, are of that kind. An error thrown further down the call is
on another contract's ABI. `AssetNotAuthorized` comes from the AssetRegistry during
`conduitFactory.spawn`, `EnforcedPause` from the beacon behind a vehicle proxy. `getRailnetError`
falls back to `protocolErrorsAbi`, which lists every error the protocol declares, so both decode.

## The ones that catch people

The errors that most often reach an integration, with the mechanism behind each, are listed on
[Errors](/utilities/errors#the-ones-that-catch-people).