# Zund Docs

> Zund is an Ethereum block builder for everyone: no whitelist, no API key, same rules for every bundle. JSON-RPC endpoint: https://zundbuilder.com (POST). Live since 2026-09-23: bids every slot to Flashbots, Ultra Sound, Titan Relay, Aestus and Agnostic. Blocks carry extraData "Zund (zundbuilder.com)". Public numbers: https://zundbuilder.com/stats.json. Wallet private RPC (Zund Protect): https://zundbuilder.com/protect. Send-once router to all builders (Zund Router): https://zundbuilder.com/router. Contact: contact@zundbuilder.com.

- Introduction: https://zundbuilder.com/#intro
- Quickstart: https://zundbuilder.com/#quickstart
- Principles: https://zundbuilder.com/#principles
- How blocks are built: https://zundbuilder.com/#how
- API overview: https://zundbuilder.com/#api
- eth_sendBundle (txs, blockNumber, revertingTxHashes, droppingTxHashes, minTimestamp, maxTimestamp, replacementUuid, replacementNonce, refundRecipient; returns bundleHash + instant simulation): https://zundbuilder.com/#eth_sendBundle
- eth_sendEndOfBlockBundle (tried on final block state; targetPools): https://zundbuilder.com/#eth_sendEndOfBlockBundle
- eth_sendRawTransaction: https://zundbuilder.com/#eth_sendRawTransaction
- eth_cancelBundle (replacementUuid, requires X-Flashbots-Signature): https://zundbuilder.com/#eth_cancelBundle
- zund_getBundleStats (status, simulation, landing block/position/builder): https://zundbuilder.com/#zund_getBundleStats
- zund_rules: https://zundbuilder.com/#zund_rules
- Errors & limits (30 req/s/IP, 1 MB): https://zundbuilder.com/#errors
- Bundle tracing: https://zundbuilder.com/#tracing
- Refunds (refundPercent in the same block + 90% of surplus): https://zundbuilder.com/#refunds
- Builder identity (coinbase 0x130e5956994e4e9262161c1abb86af739d2f42e2, extraData "Zund (zundbuilder.com)", BLS key 0x8d4eeeaf7a0f9cf05eb3b145c05d0e775b04481749450952f2b319bf4d69b9f16a538db7b30fb4b976e65873a57ced04): https://zundbuilder.com/#identity
- Ready for what's next (ePBS, FOCIL, BAL, encrypted mempools): https://zundbuilder.com/#future
- Status & roadmap: https://zundbuilder.com/#roadmap

## Zund Protect
Private wallet RPC: https://zundbuilder.com/protect (chainId 1). Transactions never go to the public mempool; shared privately with Zund, Titan, Quasar and Flashbots Protect.

- Live numbers: https://zundbuilder.com/#stats

## Zund Router
Send a bundle once to https://zundbuilder.com/router (POST JSON-RPC: eth_sendBundle, eth_cancelBundle, zund_routerStatus, zund_routerInfo). In-memory safety check (sandwich bundles refused and forwarded to nobody), then the identical body and X-Flashbots-Signature go in parallel over warm keep-alive connections to Zund, Titan (EU), Quasar, BuilderNet, Eureka, beaverbuild, bombora (EU) and Flashbots (Flashbots and BuilderNet only when signed). Answer is immediate (~2 ms, status dispatched); add ?wait=1 to wait for every builder. Router overhead vs a warm direct connection ~1 ms. Free, no API key, 30 req/s/IP. Docs: https://zundbuilder.com/#router