> ## Documentation Index > Fetch the complete documentation index at: https://docs.portalhq.io/llms.txt > Use this file to discover all available pages before exploring further. # Build a transaction > Creates an unsigned transaction for transferring assets to another address on a specific chain. You can then use this unsigned transaction to sign and submit the transaction. The response shape varies depending on the chain type (EIP-155, Solana, Bitcoin, Stellar, or Tron). ## OpenAPI ````yaml /openapi/custodian-api.yaml post /custodians/me/clients/{clientId}/chains/{chain}/assets/send/build-transaction openapi: 3.1.0 info: title: Portal Custodian API version: '3.0' description: > The Portal Custodian API provides endpoints for managing clients, building transactions, retrieving wallet data, managing delegations, alert webhooks, and gas sponsorship. All endpoints require authentication via a Portal API Key (also known as a Custodian API Key) passed as a Bearer token. ## Base URL `https://api.portalhq.io/api/v3` ## Authentication Include your Portal API Key as a Bearer token in the `Authorization` header of every request. ## Chain ID Format Chain parameters use either friendly names (e.g. `ethereum`, `solana`) or CAIP-2 format (e.g. `eip155:1`, `solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp`). When using CAIP-2 format in URLs, ensure the colon is URI-encoded (`%3A`). servers: - url: https://api.portalhq.io/api/v3 description: Production security: - bearerAuth: [] tags: - name: Clients description: Create and manage Portal clients - name: Transactions description: Build and evaluate blockchain transactions - name: Wallet Metadata description: Retrieve wallet balances and NFTs - name: Wallet Shares description: Manage backup shares and wallet ejection - name: Delegations description: Manage token delegations and delegated transfers - name: Session Keys description: Build and send EIP-7702 session key user operations - name: Alert Webhooks description: Manage alert webhooks, external addresses, and replay failed events - name: Gas Sponsorship description: View and manage gas sponsorship across chains paths: /custodians/me/clients/{clientId}/chains/{chain}/assets/send/build-transaction: post: tags: - Transactions summary: Build a transaction description: > Creates an unsigned transaction for transferring assets to another address on a specific chain. You can then use this unsigned transaction to sign and submit the transaction. The response shape varies depending on the chain type (EIP-155, Solana, Bitcoin, Stellar, or Tron). operationId: custodianBuildTransaction parameters: - $ref: '#/components/parameters/clientId' - $ref: '#/components/parameters/chain' requestBody: required: true content: application/json: schema: $ref: '#/components/schemas/BuildTransactionRequest' example: to: '0xdFd8302f44727A6348F702fF7B594f127dE3A902' token: USDC amount: '0.01' responses: '200': description: > Unsigned transaction built successfully. The response structure depends on the chain type. content: application/json: schema: oneOf: - $ref: '#/components/schemas/Eip155TransactionResult' - $ref: '#/components/schemas/SolanaTransactionResult' - $ref: '#/components/schemas/BitcoinTransactionResult' examples: eip155: summary: EIP-155 (Ethereum, Polygon, Base, etc.) value: transaction: from: '0x21d8d1d2d9907c051670d8c9a80bd2192d273f8d' to: '0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48' data: >- 0xa9059cbb000000000000000000000000dfd8302f44727a6348f702ff7b594f127de3a9020000000000000000000000000000000000000000000000000000000000002710 metadata: rawAmount: '10000' formattedAmount: '0.01' tokenDecimals: 6 tokenAddress: '0xa0b86991c6218b36c1d19d4a2e9eb0ce3606eb48' solana: summary: Solana value: transaction: >- AQAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAABAAUIdDKlh5tOZP... metadata: amount: '0.01' mintAddress: EPjFWdd5AufqSSqeM2qN1xzybapC8G4wEGGkZwyTDt1v lastValidBlockHeight: '259030944' bitcoin: summary: Bitcoin value: transaction: signatureHashes: - signatureHash1 - signatureHash2 rawTxHex: rawTxHex publicKey: jsonStringifiedPublicKey metadata: chainId: bip122:000000000933ea01ad0ee984209779ba-p2wpkh amount: '0.00001' fromAddress: tb1qpl54na90wlmdxj5z4wtz7cpp6p32dc9tajwa5g toAddress: tb1q7ynrnrywae5ypxk6fqgzqhqy202ehk7hntqc9j rawAmount: '1000' feeInSatoshis: '420' changeInSatoshis: '4584' tokenDecimals: 8 tokenSymbol: BTC '400': description: | Bad request - invalid parameters. Common causes include: - Invalid destination address format - Amount must be greater than zero - Unsupported chain - Token not supported on the specified chain content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' '401': description: Unauthorized - invalid or missing authentication token content: application/json: schema: $ref: '#/components/schemas/ErrorResponse' components: parameters: clientId: name: clientId in: path required: true description: The unique identifier of the client. schema: type: string chain: name: chain in: path required: true description: > The blockchain chain identifier. Use either a friendly name or CAIP-2 format. When using CAIP-2 format in URLs, ensure the colon (`:`) is URI-encoded as `%3A`. **Supported chains:** - `ethereum` (`eip155:1`) - `sepolia` (`eip155:11155111`) - `base` (`eip155:8453`) - `base-sepolia` (`eip155:84532`) - `polygon` (`eip155:137`) - `polygon-amoy` (`eip155:80002`) - `celo` (`eip155:42220`) - `celo-alfajores` (`eip155:44787`) - `monad` (`eip155:143`) - `monad-testnet` (`eip155:10143`) - `solana` (`solana:5eykt4UsFv8P8NJdTREpY1vzqKqZKvdp`) - `solana-devnet` (`solana:EtWTRABZaYq6iMfeYKouRu166VU2xqa1`) - `tron` (`tron:mainnet`) - `tron-nile` (`tron:nile`) - `tron-shasta` (`tron:shasta`) - `stellar` (`stellar:pubnet`) - `stellar-testnet` (`stellar:testnet`) - `bitcoin-segwit` (`bip122:000000000019d6689c085ae165831e93-p2wpkh`) - `bitcoin-segwit-testnet` (`bip122:000000000933ea01ad0ee984209779ba-p2wpkh`) schema: type: string examples: ethereum: value: ethereum summary: Ethereum mainnet (friendly name) eip155: value: eip155:1 summary: Ethereum mainnet (CAIP-2) solana: value: solana summary: Solana mainnet (friendly name) schemas: BuildTransactionRequest: type: object required: - to - token - amount properties: to: type: string description: The recipient's wallet address. token: type: string description: | The token symbol (e.g. `USDC`) or contract/mint address. amount: type: string description: | The amount to transfer in the token's primary denomination. Must be greater than zero. Eip155TransactionResult: type: object properties: transaction: type: object properties: from: type: string to: type: string data: type: string metadata: type: object properties: rawAmount: type: string formattedAmount: type: string tokenDecimals: type: integer tokenAddress: type: string SolanaTransactionResult: type: object properties: transaction: type: string description: Base64-encoded Solana transaction metadata: type: object properties: amount: type: string mintAddress: type: string lastValidBlockHeight: type: string BitcoinTransactionResult: type: object properties: transaction: type: object properties: signatureHashes: type: array items: type: string rawTxHex: type: string publicKey: type: string metadata: type: object properties: chainId: type: string amount: type: string fromAddress: type: string toAddress: type: string rawAmount: type: string feeInSatoshis: type: string changeInSatoshis: type: string tokenDecimals: type: integer tokenSymbol: type: string ErrorResponse: type: object properties: error: type: string description: Error message describing what went wrong securitySchemes: bearerAuth: type: http scheme: bearer description: | Portal API Key (Custodian API Key). Pass as a Bearer token in the Authorization header. ````