# Fee Assets

Choose whether a BUY fee is charged in quote or deducted from received base, and decode fee amounts with the correct scale.

The fee asset defines which asset is charged for an order fill. It is independent from order side, execution type, and sizing, although those choices determine which combinations are valid.

## FeeAsset values

| Value                   | Behavior                                                      |
| ----------------------- | ------------------------------------------------------------- |
| `FEE_ASSET_UNSPECIFIED` | Uses the default quote-denominated behavior                   |
| `QUOTE`                 | Charges the fee in quote                                      |
| `BASE`                  | Deducts the fee from base received; valid only for BUY orders |

SELL fees are always quote-denominated. A SELL request with `BASE` is rejected rather than rewritten.

## BUY behavior

For an exact-base BUY with `QUOTE`, the submitted base quantity remains the gross and net base quantity before any other adjustments. The account must fund trade notional plus the quote fee.

For an exact-base BUY with `BASE`, the gross base quantity is submitted for execution and the fee reduces the net base amount received.

For a quote-budget BUY with `QUOTE`, the fee counts toward the maximum quote debit. For a quote-budget BUY with `BASE`, the quote ceiling limits trade debit while the base fee reduces net base received. See [Order Sizing](https://testnet.polyester.com/docs/developer-docs/shared-concepts/order-sizing).

## Decode fee values

Use the companion `fee_asset` field whenever a response contains `fee_scaled`:

| Fee asset                          | Scale                       |
| ---------------------------------- | --------------------------- |
| `QUOTE` or `FEE_ASSET_UNSPECIFIED` | Pair `quote_quantity_scale` |
| `BASE`                             | Pair `base_quantity_scale`  |

Do not decode fee fields from the order side alone. Preserve the explicit denomination returned by the server.

## Error handling

An unsupported combination returns the stable order error `ERROR_CODE_FEE_ASSET_NOT_ALLOWED`. Do not silently convert an invalid `BASE` request to `QUOTE`.

[Preview Order](https://testnet.polyester.com/docs/developer-docs/shared-concepts/preview-order) validates whether the selected fee asset is allowed and whether the account currently has enough available balance. It does not return a fee estimate. Final fees are recorded per fill.