nautechsystems/nautilus_trader · error

Failed to parse balance hex string '{hex_string}': {e}

Error message

Failed to parse balance hex string '{hex_string}': {e}

What it means

This error is thrown when the hex string returned by an `eth_getBalance` RPC call cannot be parsed into a `U256` via `U256::from_str`. The library expects the node to return a `0x`-prefixed hex quantity; any malformed, empty, or non-hex response triggers this anyhow error wrapping the parse failure with the original error `e`.

Source

Thrown at crates/adapters/blockchain/src/rpc/http.rs:257

        &self,
        address: &Address,
        block: Option<u64>,
        timeout_secs: Option<u64>,
    ) -> anyhow::Result<U256> {
        let block_param = block_parameter(block);

        let request = serde_json::json!({
            "jsonrpc": "2.0",
            "id": 1,
            "method": "eth_getBalance",
            "params": [address, block_param]
        });
        let hex_string: String = self
            .execute_rpc_call_with_timeout(request, timeout_secs)
            .await?;

        U256::from_str(&hex_string)
            .map_err(|e| anyhow::anyhow!("Failed to parse balance hex string '{hex_string}': {e}"))
    }

    /// Retrieves logs matching the given filter criteria.
    ///
    /// This method calls the `eth_getLogs` RPC method to fetch event logs from the blockchain.
    /// It's commonly used for querying historical events like token transfers, swaps, etc.
    ///
    /// # Errors
    ///
    /// Returns an error if the RPC call fails or the response cannot be parsed.
    pub async fn get_logs(
        &self,
        address: Option<&Address>,
        topics: Option<Vec<Option<String>>>,
        from_block: u64,
        to_block: u64,
    ) -> anyhow::Result<Vec<RpcLog>> {
        let mut filter = serde_json::Map::new();

View on GitHub (pinned to 18893faf8b)

Solutions

1. Log/inspect the raw `hex_string` returned by the node to see the actual malformed value
2. Verify the RPC endpoint is a JSON-RPC compliant Ethereum node (test with curl against eth_getBalance)
3. Check for proxies/CDNs rewriting responses; point the client at the node URL directly
4. Strip unexpected prefixes/whitespace if your node adds them, or upgrade the node software

Example fix

// before: trusting node output blindly
let balance = client.get_balance(&address).await?;
// after: validate the endpoint first
let raw = client.rpc_health_check().await?; // ensure node returns 0x-prefixed hex
let balance = client.get_balance(&address).await?;

fn is_hex_quantity(s: &str) -> bool {
    s.starts_with("0x") && s.len() > 2 && s[2..].chars().all(|c| c.is_ascii_hexdigit())
}
// assert node health before balances:
// debug_assert!(is_hex_quantity(raw_balance_string));

fn looks_like_u256_hex(s: &str) -> bool {
    let body = s.strip_prefix("0x").unwrap_or(s);
    !body.is_empty() && body.chars().all(|c| c.is_ascii_hexdigit())
}

When it happens

Trigger: Calling `get_balance`/`get_balance_with_timeout` when the RPC node returns a malformed balance value (non-hex string, empty string, hex with invalid characters, or a value exceeding U256 range).

Common situations: Misconfigured or non-compliant RPC endpoint (e.g. a proxy returning HTML error pages), a custom chain node returning decimal instead of hex, or an interceptor/proxy substituting the JSON-RPC result.

Understand the failure class

Background: "Invalid ... format", "must be in format X", "does not look like a ..." — invalid argument format errors across CLI tools and libraries — this error's family across 17 libraries.

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

Related errors

• Failed to parse eth_getStorageAt response
• Finalized block {} does not contain transaction {}
• Finalized execution transaction {tx_hash} no longer has a re
• Canonical head changed during signer-nonce replacement scan
• Finalized block {} changed from {} to {} before intent valid

AI-assisted analysis of nautechsystems/nautilus_trader@18893faf8b (2026-09-08). Data as JSON: /api/errors/c53b5b13e11d45e7. Report an issue: GitHub.