nautechsystems/nautilus_trader · error

Failed to decode flash event data: {e}

Error message

Failed to decode flash event data: {e}

What it means

Thrown by parse_flash_event_hypersync when the data length check passed but alloy's SolType::abi_decode of FlashEventData fails. The bytes do not represent four properly encoded parameters of the Flash event.

Source

Thrown at crates/adapters/blockchain/src/exchanges/parsing/uniswap_v3/flash.rs:76

    log: &HypersyncLog,
) -> anyhow::Result<FlashEvent> {
    validate_event_signature_hash("FlashEvent", FLASH_EVENT_SIGNATURE_HASH, log)?;

    let sender = extract_address_from_topic(log, 1, "sender")?;
    let recipient = extract_address_from_topic(log, 2, "recipient")?;

    if let Some(data) = &log.data {
        let data_bytes = data.as_ref();

        // Validate if data contains 4 parameters of 32 bytes each
        if data_bytes.len() < 4 * 32 {
            anyhow::bail!("Flash event data is too short");
        }

        // Decode the data using the FlashEventData struct
        let decoded = match <FlashEventData as SolType>::abi_decode(data_bytes) {
            Ok(decoded) => decoded,
            Err(e) => anyhow::bail!("Failed to decode flash event data: {e}"),
        };

        let pool_address = Address::from_slice(
            log.address
                .clone()
                .expect("Contract address should be set in logs")
                .as_ref(),
        );
        let pool_identifier = PoolIdentifier::Address(Ustr::from(&pool_address.to_string()));

        Ok(FlashEvent::new(
            dex,
            pool_identifier,
            extract_block_number(log)?,
            extract_transaction_hash(log)?,
            extract_transaction_index(log)?,
            extract_log_index(log)?,
            sender,

View on GitHub (pinned to 18893faf8b)

Solutions

1. Log the underlying decode error {e} plus data length to identify the mismatch.
2. Strip trailing bytes so data is exactly 128 bytes before decoding.
3. Verify the topic0 filter matches the full Flash event signature.
4. Regenerate FlashEventData from the canonical Uniswap V3 pool ABI.
5. Run the same log through parse_flash_event_rpc to isolate hypersync data issues.

Example fix

// before
Err(e) => anyhow::bail!("Failed to decode flash event data: {e}"),
// after
Err(e) => anyhow::bail!(
    "Failed to decode flash event data (len={}): {e}",
    data_bytes.len()
),

if let Some(d) = &log.data {
    if d.len() == 128
        && <FlashEventData as SolType>::abi_decode(d).is_err()
    {
        // pre-screened: payload will not decode
    }
}

match parse_flash_event_hypersync(dex, &log) {
    Ok(event) => handle(event),
    Err(e) if e.to_string().contains("Failed to decode flash event data") => {
        tracing::debug!(raw = ?log.data, "flash decode failure: {e}");
        Ok(())
    }
    Err(e) => return Err(e),
}

When it happens

Trigger: abi_decode returns Err on a 128+ byte payload routed to this parser — wrong event's data, non-canonical encoding, or trailing bytes beyond the four words.

Common situations: Topic filtering by a colliding signature; decoding logs from forked/custom pools with a different Flash signature; corrupted test log data in test_parse_flash_event_hypersync.

Understand the failure class

Background: Schema validation failed / invalid input schema: payload rejected because its shape doesn't match the expected schema — this error's family across 28 libraries.

• Parsing and encoding errors: unexpected token, malformed input — why parsers reject input and how to find the real culprit.

Related errors

• SetFeeProtocol event data is too short
• Failed to decode SetFeeProtocol event data: {e}
• Flash event data is too short
• Missing tickLower in topic2 when parsing burn event
• Missing data in SetFeeProtocol event log

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/f5dd7721fbfc882c. Report an issue: GitHub.