feat(rails): add ACH_SAME_DAY payment rail

mintlify/changelog.mdx

@@ -18,6 +18,24 @@ changes and roadmap, [book a live demo](https://www.lightspark.com/contact) or
 `CustomerExternalAccountCreateRequest`, in which `customerId` is required. Use
 `POST /platform/external-accounts` to create an external account owned by the
 platform itself.
+## Same-day ACH is its own payment rail
+
+`ACH_SAME_DAY` joins `ACH` in `PaymentRail`. Pin it on a quote's destination to request
+same-business-day settlement for a USD payout.
+
+- Grid never selects `ACH_SAME_DAY` for you. It is only used when you name it, so
+  existing integrations that omit `paymentRail` are unaffected.
+- The two rails are priced separately. A `RailFeeConfig` with `rail: ACH_SAME_DAY`
+  prices same-day sends; your existing `ACH` config keeps pricing standard sends.
+- `ACH_SAME_DAY` is capped at $1,000,000 per entry by the NACHA same-day limit, rising
+  to $10,000,000 on 2027-09-17. A payout above the limit is rejected rather than slowed;
+  send it over `ACH` instead.
+- `ACH` will settle on the standard next-business-day schedule. Until a date we
+  announce in advance, `ACH` continues to settle same-business-day on production
+  platforms; it already settles next-business-day on sandbox platforms. To guarantee
+  same-day settlement after that date, request `ACH_SAME_DAY`.
+
+See [Assessing fees](/payouts-and-b2b/payment-flow/assessing-fees).
 
 ## Add cards to Apple Pay, Google Pay, and Samsung Pay from your app
 

mintlify/payouts-and-b2b/payment-flow/assessing-fees.mdx

@@ -49,7 +49,8 @@ Both fee types may apply to the same transaction, e.g. a cross-currency payment
 only, and the fixed fee must be in that same currency — other currencies return
 `NOT_IMPLEMENTED`.
 
-Rail fees cover `ACH`, `RTP`, `FEDNOW` and `WIRE`, going `OUT`, with the fixed fee in USD.
+Rail fees cover `ACH`, `ACH_SAME_DAY`, `RTP`, `FEDNOW` and `WIRE`, going `OUT`, with the fixed fee in USD.
+`ACH` and `ACH_SAME_DAY` are separate rails, so price each one with its own config.
 Anything else returns `NOT_IMPLEMENTED`.
 
 Both are expanding — support for stablecoin sending currencies such as USDC is planned.

mintlify/payouts-and-b2b/payment-flow/send-payment.mdx

@@ -139,7 +139,7 @@ curl -X POST 'https://api.lightspark.com/grid/2025-10-13/quotes' \
 </Info>
 
 <Tip>
-  The `paymentRail` field is optional. If omitted, Grid selects a default rail for the destination. Specify a rail (e.g., `ACH`, `WIRE`, `RTP`, `FEDNOW`) when you need to control which payment network processes the transfer.
+  The `paymentRail` field is optional. If omitted, Grid selects a default rail for the destination. Specify a rail (e.g., `ACH`, `ACH_SAME_DAY`, `WIRE`, `RTP`, `FEDNOW`) when you need to control which payment network processes the transfer. `ACH_SAME_DAY` is never selected for you: it requests same-business-day settlement and is priced separately. It is capped at $1,000,000 per entry by the NACHA same-day limit (rising to $10,000,000 on 2027-09-17); a payout above that is rejected, so send it over `ACH` instead.
 </Tip>
 
 <Info>

mintlify/snippets/country-support.mdx

@@ -77,7 +77,7 @@ import { FeatureCard, FeatureCardGrid, FeatureCardList, FeatureCardContainer } f
 | 🇺🇬 Uganda | UG | `Mobile Money` (Airtel, MTN) `SWIFT` |
 | 🇦🇪 United Arab Emirates | AE | `Bank Transfer` `SWIFT` |
 | 🇬🇧 United Kingdom | GB | `Faster Payments` `SWIFT` |
-| 🇺🇸 United States | US | `ACH` `Wire Transfer` `RTP` `FedNow` `SWIFT` |
+| 🇺🇸 United States | US | `ACH` `Same-day ACH` `Wire Transfer` `RTP` `FedNow` `SWIFT` |
 | 🇻🇳 Vietnam | VN | `Bank Transfer` `SWIFT` |
 | 🇿🇲 Zambia | ZM | `Mobile Money` (Airtel, MTN, Zamtel) `SWIFT` |
 

mintlify/snippets/sending/accounts.mdx

@@ -68,7 +68,7 @@ curl -X POST 'https://api.lightspark.com/grid/2025-10-13/quotes' \
 </Info>
 
 <Tip>
-  The `paymentRail` field is optional. If omitted, Grid selects a default rail for the destination. Specify a rail (e.g., `ACH`, `WIRE`, `RTP`, `FEDNOW`) when you need to control which payment network processes the transfer.
+  The `paymentRail` field is optional. If omitted, Grid selects a default rail for the destination. Specify a rail (e.g., `ACH`, `ACH_SAME_DAY`, `WIRE`, `RTP`, `FEDNOW`) when you need to control which payment network processes the transfer. `ACH_SAME_DAY` is never selected for you: it requests same-business-day settlement and is priced separately. It is capped at $1,000,000 per entry by the NACHA same-day limit (rising to $10,000,000 on 2027-09-17); a payout above that is rejected, so send it over `ACH` instead.
 </Tip>
 
 <Info>

openapi/components/schemas/common/PaymentRail.yaml

@@ -2,6 +2,7 @@ type: string
 enum:
   - ACH
   - ACH_COLOMBIA
+  - ACH_SAME_DAY
   - BANK_TRANSFER
   - BRE_B
   - CIPS
@@ -26,4 +27,18 @@ enum:
 description: >-
   The payment rail used for the transfer. Payment rails represent the underlying
   payment network or system used to move funds between accounts.
+
+
+  `ACH_SAME_DAY` requests same-business-day settlement for a USD payout and is priced
+  separately. It is subject to the NACHA per-entry same-day limit, which the network
+  applies to every originator: $1,000,000 per entry, rising to $10,000,000 on
+  2027-09-17. A payout above that limit is rejected rather than slowed — check the
+  amount before requesting this rail, or send it over `ACH`, which settles on the
+  standard schedule and has no such limit.
+
+
+  `ACH` will settle on the standard next-business-day schedule. Until a date we announce
+  in advance, `ACH` continues to settle same-business-day on production platforms; it
+  already settles next-business-day on sandbox platforms. To guarantee same-day
+  settlement after that date, request `ACH_SAME_DAY`.
 example: ACH

openapi/components/schemas/common/UsdAccountInfo.yaml

@@ -10,6 +10,7 @@ allOf:
         type: string
         enum:
         - ACH
+        - ACH_SAME_DAY
         - WIRE
         - RTP
         - FEDNOW

openapi/components/schemas/config/RailFeeConfig.yaml

@@ -8,8 +8,8 @@ allOf:
       rail:
         $ref: ../common/PaymentRail.yaml
         description: >-
-          The rail used in the transaction. `ACH`, `RTP`, `FEDNOW` and `WIRE` are
-          accepted today; other rails return a `NOT_IMPLEMENTED` error.
+          The rail used in the transaction. `ACH`, `ACH_SAME_DAY`, `RTP`, `FEDNOW` and
+          `WIRE` are accepted today; other rails return a `NOT_IMPLEMENTED` error.
       direction:
         type: string
         enum:
@@ -35,4 +35,5 @@ allOf:
 description: >-
   A fee charged when a transaction uses the specified rail. There can be at most one for
   a given rail, direction and originator. For example, a config with rail: ACH, direction: OUT,
-  originator: GRID will charge a fee on all outgoing ACH transactions.
+  originator: GRID will charge a fee on all outgoing ACH transactions. `ACH` and
+  `ACH_SAME_DAY` are separate rails, so price each one with its own config.