Transaction Builder - Gero Nexus Documentation

Transaction Builder add-on. Build Cardano transactions without running your own UTXO selection, fee calculation, or CBOR serialization. You send intent (recipient, amount, certificate, etc.); Nexus returns an unsigned transaction CBOR. You sign it with your wallet/keys and submit it.

Endpoints on this page require the Transaction Builder add-on. Without it they return HTTP 402 with "addon": "transactionBuilder".

Base URL: https://nexus.gerowallet.io · Auth: X-Api-Key: nxs_…

The flow

Build - POST your intent to a builder endpoint → receive unsigned CBOR.
Sign - sign the CBOR client-side (your wallet / CIP-30 / key material). Nexus never sees your keys.
Submit - POST the signed CBOR to the core API’s POST /api/transactions/submit.

Builders

Method	Path	Builds
POST	`/api/tx/build`	A transfer (payment) transaction
POST	`/api/tx/build/delegation`	Stake delegation to a pool
POST	`/api/tx/build/stake-registration`	Stake key registration / deregistration
POST	`/api/tx/build/drep-registration`	DRep registration / deregistration
POST	`/api/tx/build/vote-delegation`	Vote (governance) delegation
POST	`/api/tx/build/withdrawal`	Reward withdrawal
POST	`/api/tx/max-ada`	Maximum sendable ADA (after fees + min-UTXO) for an address

DEX aggregator

Best-execution swaps routed across Cardano DEXes, with non-custodial co-signed submission, have their own page: DEX Aggregator. Its build-tx and submit endpoints require this Transaction Builder add-on.

Example

# 1. Build an unsigned transfer
curl -X POST "https://nexus.gerowallet.io/api/tx/build" \
  -H "X-Api-Key: nxs_your_api_key_here" \
  -H "Content-Type: application/json" \
  -d '{
    "sender": "addr1q9...",
    "outputs": [{ "address": "addr1qx...", "lovelace": "5000000" }]
  }'
# → { "cbor": "84a400818258...", ... }

# 2. Sign the CBOR client-side (wallet / CIP-30)

# 3. Submit the signed tx via the core API
curl -X POST "https://nexus.gerowallet.io/api/transactions/submit" \
  -H "X-Api-Key: nxs_your_api_key_here" \
  -H "Content-Type: application/cbor" \
  --data-binary @signed-tx.cbor

Full request/response schemas for each builder are in the API Reference.