nautechsystems/nautilus_trader · critical

Chain ID mismatch at connect: expected {expected_chain_id},

Error message

Chain ID mismatch at connect: expected {expected_chain_id}, node reported {actual_chain_id}

What it means

During connect(), the client compares the configured chain.chain_id against eth_chainId reported by the RPC node and aborts before any key material is loaded or any signature is produced. This guards against signing transactions for a different network than intended, which is the classic way funds get sent to wrong-chain addresses.

Source

Thrown at crates/adapters/blockchain/src/execution/client.rs:2769

                crate::cache::database::BlockchainCacheDatabase::connect(pg_options.clone().into())
                    .await
                    .map_err(|e| {
                        anyhow::anyhow!("Failed to connect to the Postgres cache database: {e}")
                    })?;
            self.cache.database = Some(database);
            self.cache.initialize_chain().await;
            self.cache.ensure_execution_transaction_schema().await?;
        } else {
            log::warn!(
                "No Postgres cache database configured; transactions will be refused (no durable store)"
            );
        }

        // Verify the RPC chain ID against configuration before any signature
        let expected_chain_id = u64::from(self.chain.chain_id);
        let actual_chain_id = self.http_rpc_client.chain_id().await?;
        if actual_chain_id != expected_chain_id {
            anyhow::bail!(
                "Chain ID mismatch at connect: expected {expected_chain_id}, node reported {actual_chain_id}"
            );
        }

        // Load the signer key from the configured environment variable; the key is never
        // logged, serialized, or stored in configuration
        let private_key = std::env::var(&self.config.signer_private_key_env).map_err(|_| {
            anyhow::anyhow!(
                "Signer private key environment variable '{}' is not set",
                self.config.signer_private_key_env
            )
        })?;
        let signer = PrivateKeySigner::from_str(private_key.trim()).map_err(|_| {
            anyhow::anyhow!(
                "Signer private key in '{}' is not a valid hex private key",
                self.config.signer_private_key_env
            )
        })?;

View on GitHub (pinned to 2114cf6f76)

Solutions

Make the RPC URL and chain.chain_id consistent in the BlockchainExecutionConfig (query the node with eth_chainId / cast chain-id and set that exact value)
For local forks (anvil/hardhat), set the config chain id to the id the fork reports
Keep one config block per environment (mainnet/sepolia/local) and switch wholesale instead of editing single fields
Treat this error as a hard stop: do not bypass it, fix the environment instead

Example fix

// before: mismatched pair
BlockchainExecutionConfig {
    chain: Chain::mainnet(), // chain_id = 1
    chain_rpc_http: "https://sepolia.infura.io/v3/KEY".into(), // reports 11155111
}

// after: consistent testnet pair
BlockchainExecutionConfig {
    chain: Chain::sepolia(), // chain_id = 11155111
    chain_rpc_http: "https://sepolia.infura.io/v3/KEY".into(),
}

Defensive patterns

Strategy: try-catch

Validate before calling

// Pre-verify config before connect
let reported = http_client.chain_id().await?;
anyhow::ensure!(
    reported == u64::from(config.chain.chain_id),
    "RPC reports chain {reported} but config expects {}", config.chain.chain_id
);

Try / catch

match client.connect().await {
    Err(e) if e.to_string().contains("Chain ID mismatch") => {
        // Environment misconfiguration: halt deployment, never override
        fatal!("refusing to trade on mismatched network: {e}");
    }
    other => other,
}

Prevention

Smoke-test eth_chainId against config.chain_id in CI for every environment profile
Treat RPC URL and chain id as one atomic unit in config
Alert loudly on this error — it exists to protect capital

When it happens

Trigger: Calling connect() when the RPC URL serves a different network than chain.chain_id in the config: mainnet RPC paired with a testnet chain id, a fork/proxy endpoint reporting a different id, a provider failover URL pointing elsewhere, or chain presets whose id was edited incorrectly.

Common situations: Switching between mainnet and testnet by changing only one of (RPC URL, chain id); localhost Hardhat/Anvil forks that report their own chain id; mis-pasted provider keys that default to a different network; env-specific configs drifting out of sync.

Related errors

AI-assisted analysis of nautechsystems/nautilus_trader@2114cf6f76 (2026-08-21). Data as JSON: /api/errors/db35dc54b678c42b. Report an issue: GitHub.