The Ethereum Sepolia Beacon Chain API gives you consensus layer access on the Sepolia testnet. It is a REST API, not a JSON-RPC interface. You use it to query validators, slots, finality checkpoints and withdrawal data on Sepolia before testing the same workflows on mainnet.
This guide covers what the Sepolia Beacon API is for, how to authenticate, which endpoints matter for testnet workflows, and practical examples in curl and JavaScript. For endpoint details and chain specs, see the Ethereum Sepolia Beacon Chain endpoint page.
What the Sepolia Beacon API Is For
Sepolia runs a full Ethereum node stack: an execution layer and a consensus layer. The execution layer handles smart contracts and transactions, accessed via JSON-RPC. The consensus layer manages validator coordination, proof-of-stake, and finality. That is what the Beacon API exposes.
You need the Sepolia Beacon API for:
- Withdrawal flow testing. EIP-4895 introduced native ETH withdrawals from validators. Testing withdrawal credential types, BLS-to-execution changes and partial withdrawal mechanics requires Beacon API access on a testnet that supports the full withdrawal flow.
- Validator lifecycle testing. If your protocol activates, monitors or exits validators, Sepolia’s Beacon API is where you validate that logic before mainnet.
- Bridge finality testing. Bridges using Ethereum finality checkpoints as a safety signal need to query the Beacon API. Testing that logic on Sepolia before mainnet is the right workflow.
- Staking protocol staging. Liquid staking protocols that track validator queue position and activation status can test against Sepolia’s smaller validator set before dealing with the full mainnet queue depth.
Sepolia’s validator set is permissioned and significantly smaller than mainnet. This is useful for withdrawal testing because validator lifecycle moves faster than on mainnet. Activation queues are shorter, which means you can test the full deposit-to-active-to-exited cycle in a reasonable timeframe.
For dApp and smart contract testing without a consensus layer requirement, use the Ethereum Sepolia execution endpoint. The Beacon API adds the consensus layer on top.
Authentication
The Sepolia Beacon API authenticates the same way as the execution layer endpoint: the API key is part of the URL path.
Correct: https://endpoints.boltrpc.io/rpc/YOUR_API_KEY/ethereum-sepolia-beacon with the REST path appended
Wrong: sending the key as an Authorization: Bearer header or a ?auth= query parameter (returns 401 on Beacon endpoints)
Both use the same API key from your BoltRPC account. Only the chain slug differs between execution and consensus endpoints.
Key Beacon API Endpoints
The base URL for the Sepolia Beacon API is:
https://endpoints.boltrpc.io/rpc/YOUR_API_KEY/ethereum-sepolia-beaconAll requests are HTTP GET unless noted.
Node sync status
GET /eth/v1/node/syncingReturns the current head slot and sync status. Use this to verify your connection before running queries.
Finality checkpoints
GET /eth/v1/beacon/states/{state_id}/finality_checkpointsReturns the latest justified and finalized checkpoints. Use finalized as the state identifier for settled data. Use head only when you explicitly need the latest unconfirmed slot.
Validator status
GET /eth/v1/beacon/states/head/validators/{validator_index}Returns status, balance and withdrawal credentials for a specific validator. Status values include pending_queued, active_ongoing, active_exiting, exited_unslashed.
Block headers
GET /eth/v1/beacon/headers/headReturns the latest beacon block header with slot number, proposer index and parent root.
Full beacon block
GET /eth/v2/beacon/blocks/{block_id}Returns a full beacon block including the execution payload. Use v2 (not v1) for post-Merge blocks.
Withdrawal credentials check
GET /eth/v1/beacon/states/head/validators?status=active_ongoingReturns active validators. Filter on withdrawal credentials type to identify validators not yet migrated to 0x01 credentials for withdrawal eligibility.
Connecting Step by Step
curl
Check sync status:
curl "https://endpoints.boltrpc.io/rpc/YOUR_API_KEY/ethereum-sepolia-beacon/eth/v1/node/syncing"Get finality checkpoints:
curl "https://endpoints.boltrpc.io/rpc/YOUR_API_KEY/ethereum-sepolia-beacon/eth/v1/beacon/states/finalized/finality_checkpoints"Query a specific validator:
curl "https://endpoints.boltrpc.io/rpc/YOUR_API_KEY/ethereum-sepolia-beacon/eth/v1/beacon/states/head/validators/1234"Get the latest block header:
curl "https://endpoints.boltrpc.io/rpc/YOUR_API_KEY/ethereum-sepolia-beacon/eth/v1/beacon/headers/head"JavaScript (fetch)
const KEY = "YOUR_API_KEY";
const BASE = `https://endpoints.boltrpc.io/rpc/${KEY}/ethereum-sepolia-beacon`;
async function beaconGet(path) {
const res = await fetch(`${BASE}${path}`);
if (!res.ok) throw new Error(`Beacon API error: ${res.status}`);
return res.json();
}
// Check sync status
const sync = await beaconGet("/eth/v1/node/syncing");
console.log("Head slot:", sync.data.head_slot);
console.log("Syncing:", sync.data.is_syncing);
// Get finality checkpoint
const checkpoints = await beaconGet(
"/eth/v1/beacon/states/finalized/finality_checkpoints"
);
console.log("Finalized epoch:", checkpoints.data.finalized.epoch);
// Monitor validator status
const validator = await beaconGet(
"/eth/v1/beacon/states/head/validators/1234"
);
console.log("Status:", validator.data.status);
console.log("Balance:", validator.data.balance, "Gwei");Python (requests)
import requests
KEY = "YOUR_API_KEY"
BASE = f"https://endpoints.boltrpc.io/rpc/{KEY}/ethereum-sepolia-beacon"
def beacon_get(path):
resp = requests.get(f"{BASE}{path}", timeout=10)
resp.raise_for_status()
return resp.json()
# Get current slot
header = beacon_get("/eth/v1/beacon/headers/head")
slot = header["data"]["header"]["message"]["slot"]
print(f"Current Sepolia slot: {slot}")
# Get finality checkpoint
checkpoints = beacon_get(
"/eth/v1/beacon/states/finalized/finality_checkpoints"
)
finalized_epoch = checkpoints["data"]["finalized"]["epoch"]
print(f"Finalized epoch: {finalized_epoch}")Testing Withdrawals on Sepolia
Sepolia is the standard testnet for testing EIP-4895 withdrawal mechanics before mainnet. The Beacon API is how you track withdrawal credential status and verify the withdrawal process.
Check withdrawal credentials for a validator:
const validator = await beaconGet(
"/eth/v1/beacon/states/head/validators/1234"
);
const credentials = validator.data.validator.withdrawal_credentials;
const credType = credentials.startsWith("0x01") ? "0x01 (ETH address)" : "0x00 (BLS)";
console.log("Withdrawal credential type:", credType);
// 0x01 credentials are required for withdrawals to triggerTrack validator exit progression:
// Poll this endpoint to monitor exit queue progress
async function trackValidatorStatus(validatorIndex) {
const data = await beaconGet(
`/eth/v1/beacon/states/head/validators/${validatorIndex}`
);
const status = data.data.status;
const exitEpoch = data.data.validator.exit_epoch;
const withdrawableEpoch = data.data.validator.withdrawable_epoch;
console.log(`Status: ${status}`);
console.log(`Exit epoch: ${exitEpoch}`);
console.log(`Withdrawable epoch: ${withdrawableEpoch}`);
}Common Issues
401 on every request
The key is missing from the URL path. Beacon endpoints authenticate with the API key embedded in the URL, the same pattern as the execution layer. Sending the key as an Authorization: Bearer header or a ?auth= query parameter returns a 401.
Using head when you need settled data
In withdrawal testing, querying head state can return data from a slot that has not finalized. For anything that requires confirmed state (withdrawal credential checks before processing a withdrawal), use finalized as the state identifier.
v1 block endpoint missing execution payload
/eth/v1/beacon/blocks/{id} does not include the execution payload on post-Merge blocks. Switch to /eth/v2/beacon/blocks/{id} to get the full block including execution data.
Validator not found during queue testing
On Sepolia, validators may still be in pending_queued status when you query them. The smaller validator set means activation queues move faster than mainnet, but there is still a processing delay after deposit. Check the status field before assuming a validator is active.
FAQ
What is the Sepolia Beacon Chain API URL?
The BoltRPC endpoint is https://endpoints.boltrpc.io/rpc/YOUR_API_KEY/ethereum-sepolia-beacon. The API key is part of the URL path, so no extra header or query parameter is needed.
Does the Sepolia Beacon API support WebSocket? No. The Ethereum Beacon Chain API is a REST API only. All requests use standard HTTP GET calls. There is no WebSocket interface on the consensus layer.
How is the Sepolia Beacon API different from the mainnet Beacon API? The API interface is identical. The difference is the data: Sepolia has a smaller permissioned validator set, shorter activation queues and less finalization latency than mainnet. This makes it faster to test the full validator lifecycle on Sepolia.
What state identifier should I use in production testing?
Use finalized for data you need to be settled. Use head only when you specifically need the latest slot regardless of finalization status.
Ready to test on the Sepolia Beacon Chain? Start your free 2-week trial at dashboard.boltrpc.io. Same API key as your Sepolia execution endpoint. No credit card required.