Classifier

When the error isn't a decodable EVM revert, the decoder still gives you a useful message via classifyError.

Rule table

Each kind maps to a fixed, friendly English message. See src/core/errorClassifier.ts for the exact regex / code rules and messages.

Decoder pipeline

decoder.decode(error) runs these steps in order, returning the first one that produces a result:

1. Cause traversal — walks error.cause, error.cause.cause, ... and any errors[] siblings. Cycle-safe via a WeakSet. Hard-capped at MAX_CAUSE_DEPTH (16) to bound work on pathological inputs.
2. Revert extraction — reads data / rawData / returnData / output fields, or hex embedded in message / shortMessage / details / reason.
3. Standard decoders — if the selector is 0x08c379a0 (Error(string)) or 0x4e487b71 (Panic(uint256)), decode without needing an ABI. Panic codes are translated into human messages (overflow, division-by-zero, out-of-bounds, etc.).
4. Custom ABI decoding — if the selector is registered, run viem's decodeErrorResult against the matching ABI and return named args.
5. Classifier fallback — match the error tree against the rule table above (regex over shortMessage / message / reason / details and EIP-1193 codes).
6. Unknown — return a best-effort message extracted from the error (or the configured fallbackMessage).

Customising the fallback

const decoder = forViem({
  fallbackMessage: 'Something went wrong. Please try again.',
});

The fallback only applies when every earlier step has nothing useful to say — usually that means a thrown value with no message, no shortMessage, no data, and no recognisable selector or text.