Cardano Transaction Builder API: Intent In, Unsigned CBOR Out
Cardano Transaction Builder API: Intent In, Unsigned CBOR Out
Building a valid Cardano transaction means solving UTXO selection, calculating fees, handling change outputs, respecting min-UTXO rules, and serializing it all to CBOR. Libraries help, but you still own the logic, the edge cases, and the maintenance as the protocol evolves through eras. For teams whose product is not "a transaction library," that is undifferentiated heavy lifting.
The Nexus Transaction Builder inverts it: you send intent, Nexus returns an unsigned transaction. Your keys never leave your side.
The flow, in three steps
Build. POST your intent, recipient and amount, a pool to delegate to, a governance action, to a builder endpoint. Nexus handles UTXO selection, fee calculation, and change outputs, and returns unsigned transaction CBOR.
Sign. Sign the CBOR client-side with your wallet, CIP-30 connector, or key material. Nexus never sees private keys; there is nothing on its servers to steal.
Submit. POST the signed CBOR to the submit endpoint. Done.
What you can build
Six operation families cover the transactions real Cardano products actually send:
Transfers, ADA and native-token payments, including multi-output sends.
Stake delegation, delegate to a pool.
Stake key registration, register or deregister a stake key.
DRep registration, register or deregister as a delegated representative (Conway era).
Vote delegation, delegate governance voting power.
Reward withdrawal, withdraw staking rewards.
A utility endpoint also computes the maximum sendable ADA for an address after fees and min-UTXO requirements, the exact number wallet UIs need for a "Max" button.
And swaps. The builder includes a DEX aggregator flow: list swappable tokens, quote a route across Cardano DEXes, build the unsigned swap transaction, and submit the signed result. That is a swap feature for your product in four REST calls.
How it works
# 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 client-side (wallet / CIP-30)
# 3. Submit the signed transaction
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
Because reads are consistent, a submitted transaction is queryable immediately, chained flows (build a second transaction spending the first one's output) behave predictably.
What teams build with it
Wallets and dApps ship send, stake, vote, and swap features without maintaining a transaction-construction stack.
Exchanges and fintechs add Cardano withdrawals and delegation with server-side building and client-side signing, keeping keys inside their existing custody boundary.
Governance tools register DReps and delegate votes programmatically as Conway-era participation grows.
Backends in any language get CBOR-correct transactions from plain JSON over REST, with no Cardano-specific library required.
Pricing
Transaction Builder is a $19/month add-on on Builder ($29), Growth ($79), and Scale ($199), included in Enterprise ($499), drawing from your tier's unified request pool. The DEX aggregator swap flow is included with it.
Build one today
The 7-day Builder trial includes the full API surface. Get your key at nexus.gerowallet.io, build an unsigned transaction against preprod, and see the whole flow work before you commit a dollar.