> ## Documentation Index
> Fetch the complete documentation index at: https://nexus.gerowallet.io/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Transaction Builder

> Server-side UTXO selection and unsigned CBOR for Cardano transactions

**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.

<Note>
  Endpoints on this page require the **Transaction Builder** add-on. Without it they return [HTTP 402](/docs/addons/overview#how-access-is-enforced) with `"addon": "transactionBuilder"`.
</Note>

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

## The flow

1. **Build** - POST your intent to a builder endpoint → receive unsigned CBOR.
2. **Sign** - sign the CBOR client-side (your wallet / CIP-30 / key material). Nexus never sees your keys.
3. **Submit** - POST the signed CBOR to the core API's [`POST /api/transactions/submit`](/docs/api-reference).

## 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](/docs/addons/aggregator)**. Its `build-tx` and `submit` endpoints require this Transaction Builder add-on.

## Example

```bash theme={null}
# 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](/docs/api-reference).