0x Swap Allowance Holder Firm Quote

Choose this endpoint over the indicative price sibling when you need an executable, binding quote with actual calldata to submit a transaction — not just an estimate for display purposes. Use when you are about to execute a swap and need precise output amounts, gas data, and allowance holder details. Prefer this when integrating with the Allowance Holder contract flow for managing ERC-20 token approvals.

• Insufficient liquidity for the requested token pair returns an error or empty route
• Invalid or unsupported token addresses return a 400 validation error
• Quote expires between fetch and execution, requiring a refresh
• Requested sell amount below minimum threshold returns an error
• Unsupported chain ID returns a 400 error
• Missing required query parameters return a 422 unprocessable entity error
• Payment not provided or insufficient USDC returns a 402 Payment Required