# Read an EVM chain ID before your first integration

**Independent demonstration by Pep.Liu. Not a customer project or a security audit.**

This quickstart shows a read-only JSON-RPC request. It does not send a transaction, connect a wallet, or request a private key. Use the endpoint issued by your chosen provider and check that provider's usage limits.

## Before you start

You need curl and a JSON-RPC endpoint for your intended EVM network. Treat an endpoint containing an API key as a credential: keep it out of screenshots, support tickets, repositories, and shared shell transcripts. The example below uses `http://127.0.0.1:8765`, a local demonstration endpoint. It is not a live blockchain provider.

## Send one request

```sh
curl --fail-with-body --silent --show-error \
  --max-time 10 \
  -H 'Content-Type: application/json' \
  --data '{"jsonrpc":"2.0","id":1,"method":"eth_chainId","params":[]}' \
  http://127.0.0.1:8765
```

If your curl does not support `--fail-with-body`, upgrade curl or use `--fail` and inspect errors separately. In a live integration, replace the local address with your provider's endpoint without publishing its credentials.

The local fixture returns:

```json
{"jsonrpc":"2.0","id":1,"result":"0x1"}
```

`0x1` is the hexadecimal representation of chain ID 1, Ethereum mainnet. This sample response is simulated. On a live endpoint, compare the returned chain ID with the network you actually intend to use. A successful chain-ID response does not prove that the provider has every method or historical data your application needs.

## Recognize two kinds of failure

An HTTP error concerns the transport or provider gateway. A JSON-RPC `error` concerns the RPC request, and can arrive even with HTTP status 200. Check both. Match the response `id` with the request before using the result.

For example, the fixture returns this for an unknown method:

```json
{"jsonrpc":"2.0","id":2,"error":{"code":-32601,"message":"Method not found"}}
```

## Five checks when it does not work

1. **401 or 403:** check the provider's credential, allowed domains/IPs, and account access. Do not paste a secret into a public help thread.
2. **429:** check plan limits and provider instructions. Apply bounded backoff and a retry limit; do not retry indefinitely.
3. **Timeout:** check your connection, provider status, and timeout. An RPC endpoint is a service dependency, not a guarantee of availability.
4. **Method not found:** check spelling, the selected chain, provider method support, and whether a paid feature is required. Changing the chain ID cannot enable unsupported methods.
5. **Unexpected chain ID:** stop integration testing and select the correct endpoint. Do not send a transaction to “see whether it works.”

## Acceptance checklist

- The request has an explicit timeout and a valid JSON-RPC envelope.
- The response ID matches and the expected chain ID is documented.
- HTTP failures and JSON-RPC errors are handled separately.
- The example can be copied without exposing a credential.
- The guide labels simulated output and identifies the intended live network.

## Verification scope

The companion `rpc_fixture.py` checks the request and demonstrates success, an unsupported method, and invalid JSON locally. This verifies this document's demonstration, not a live provider, production performance, or transaction safety. No external credentials or wallets were used.

Reference for method semantics: https://ethereum.org/en/developers/docs/apis/json-rpc/#eth_chainid . Original sample text and fixture code © 2026 Pep.Liu. Sample content licensed CC BY 4.0; fixture code MIT. No third-party graphics, fonts, or copied documentation included.