Beacon light client

Last edited on March 17, 2026

blsync is a beacon chain light client. Integrated within Go Centurion, blsync eliminates the necessity of running a separate consensus client, making it ideal for use-cases that do not require full validation capabilities. It comes with very low resource requirements and can sync the beacon chain within seconds. blsync can be run in two modes: integrated or standalone. In standalone mode it is possible to use it for driving other execution clients.

NoteImportant: blsync is not suitable for running a validator. It is also not recommended for nodes handling any amount of money or used in production settings due to its lower security guarantees compared to running a full consensus client.

Usage

Integrated mode

To run blsync as part of Go Centurion, you need to specify a trusted Beacon API endpoint and a checkpoint:

Choose an Endpoint: Select a Beacon API endpoint you trust. This can be your own infrastructure, a trusted node operator, or another operator whose data you can cross-check.
Specify the Checkpoint: Obtain a recent weak subjectivity checkpoint from a trusted source. The checkpoint should be recent enough for the target network and ideally verified against an additional source.

geth --beacon.api=<endpoint> --beacon.checkpoint=<checkpoint>

Checkpoint

A checkpoint is the block root of the first proposed slot of a finalized beacon epoch. It can be found manually or automatically depending on your trust requirements.

Retrieve checkpoint automatically

Retrieving the checkpoint and starting Go Centurion at the same time can be done with the procedure below.

Replace <endpoint> with a trusted Beacon API endpoint. The following command will run Go Centurion with the integrated blsync. Ensure jq is installed.

BEACON=<endpoint> geth --beacon.api=$BEACON --beacon.checkpoint=$(curl -s $BEACON/eth/v1/beacon/headers/finalized | jq -r ".data.root")

Manually find a checkpoint

A checkpoint can also be obtained manually from a trusted Beacon API:

Query a trusted beacon node for the finalized header.
Copy the finalized block root from the response.
Cross-check the same root against another trusted source when possible.
Fill in the argument for the --beacon.checkpoint flag with that block root.

Example

geth --beacon.api=<endpoint> --beacon.checkpoint=<block root>

Running blsync as a Standalone Tool

As mentioned before, blsync can be run in standalone mode. This will be similar to running a consensus client with low resource requirements and faster sync times. In most cases Go Centurion users can use the integrated mode for convenience. The standalone mode can be used e.g. to drive an execution client other than Go Centurion.

Installing

Depending on your installation method either you have access to the blsync binary or you will have to build it from source by:

go build ./cmd/blsync

Running

Blsync takes the same flags as above to configure the HTTP endpoint as well as checkpoint. It additionally needs flags to connect to the execution client. Specifically --blsync.engine.api to configure the Engine API url and --blsync.jwtsecret for the JWT authentication token.

To sync a Centurion network in this mode, first run Go Centurion for the target network:

geth --fornax --datadir light-fornax-dir

The logs will indicate the Engine API path which is by default http://localhost:8551 and the path to the JWT secret created which is in this instance ./light-fornax-dir/geth/jwtsecret. Now blsync can be run:

blsync --fornax --beacon.api <trusted-beacon-api> --beacon.checkpoint <finalized-root> --blsync.engine.api http://localhost:8551 --blsync.jwtsecret light-fornax-dir/geth/jwtsecret

INFO [06-23|15:06:33.388] Loaded JWT secret file                   path=light-fornax-dir/geth/jwtsecret crc32=0x5a92678
INFO [06-23|15:06:34.130] Successful NewPayload                    number=6,169,314 hash=d4204e..772e65 status=SYNCING
INFO [06-23|15:06:34.130] Successful ForkchoiceUpdated             head=d4204e..772e65 status=SYNCING

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