Leader Info API

The Leader Info API provides the next 10 block leaders on Monad, including whether each leader is running the FastLane system. This allows searchers to anticipate upcoming FastLane-enabled blocks and plan their strategies accordingly.

Endpoint

GET https://monad-leader.fastlane.xyz/v1/leaderinfo

Rate limit: 3 requests per second per IP address.

Response

The response is a JSON array of the next 10 block leaders:

[
  {
    "block_number": "0x3e2fe71",
    "round": "0x3e44b91",
    "epoch": "0x519",
    "leader_secp_pubkey": "0246a4697342f9701b39edf6faf7fb8ea31c45aed660a6520e2fc46843b76aed5b",
    "leader_coinbase": "0xDbfE85b112cB67cD4BCCE563eBEb7914450d1C82",
    "leader_id": 42,
    "fastlane_enabled": true
  },
  {
    "block_number": "0x3e2fe72",
    "round": "0x3e44b92",
    "epoch": "0x519",
    "leader_secp_pubkey": "035fbf41201677e2b597c24d0f5db8171dc9092b5d05d7d17fd9bb20bf34698cf4",
    "leader_coinbase": "0x520fd81DAd59A6fD6b7Fd87ED9B0Bd667719DFe9",
    "leader_id": 7,
    "fastlane_enabled": false
  }
]

Fields

Field	Type	Description
block_number	string (hex)	Predicted block number. This is speculative — it assumes no consensus rounds are skipped. If a round times out, subsequent block numbers shift accordingly.
round	string (hex)	Consensus round number. The leader for each round is deterministic and always correct. However, a round may be skipped if consensus times out, in which case no block is produced for that round.
epoch	string (hex)	Consensus epoch. The validator set is stable within an epoch.
leader_secp_pubkey	string (hex, no 0x prefix)	Compressed secp256k1 public key (33 bytes) of the validator elected as leader for this round.
leader_coinbase	string | null	The validator's coinbase address (where they receive block rewards). May be null.
leader_id	number | null	The validator's ID as defined in the Monad staking contract. May be null if the validator is not found in the metadata.
fastlane_enabled	boolean	Whether this validator is running the FastLane system. When true, the block produced by this leader will include FastLane auction ordering.

How it works

Monad uses weighted round-robin leader election seeded by the consensus round number. This makes the leader for any future round deterministic and predictable given the current validator set and their stakes.

There are two key numbers in the response:

• round: The consensus round. The leader-to-round mapping is always correct. However, if consensus times out on a round (the leader fails to propose or the network can't reach agreement), that round is skipped and no block is produced. The next round then proceeds with its own leader.

• block_number: The predicted execution-layer block number. Unlike rounds, block numbers are always sequential with no gaps. Because the API cannot know in advance which rounds will be skipped, block numbers are speculative, they assume every round succeeds. When a round is skipped, all subsequent block numbers shift down by one.

Usage tips

• Poll the endpoint regularly.
• Use fastlane_enabled to identify upcoming blocks where FastLane auction bids will be processed.
• The leader_secp_pubkey can be used to identify validators across responses, even if their coinbase address is not known.
• All hex quantities (block_number, round, epoch) are 0x-prefixed. The leader_secp_pubkey is not 0x-prefixed.