feat(rails): add ACH_SAME_DAY payment rail

Grid exposes one ACH rail today, so a platform cannot choose between
standard and same-day settlement and the two cannot be priced apart.

Add ACH_SAME_DAY alongside ACH in PaymentRail, and to the paymentRails
array on UsdAccountInfo. The array is the one that matters at runtime:
the generated client validates paymentRails against it, so advertising
the rail on a USD account without this entry makes
GET /customers/external-accounts/{id} return 500. UsdAccountInfo is
composed by PaymentUsdAccountInfo and UsdExternalAccountInfo, so the
single entry covers all three generated models.

Describe the rails as they will actually behave on release. ACH keeps
settling same-business-day on production platforms until an announced
date, and already settles next-business-day on sandbox, so the
description says so rather than claiming next-day everywhere. An
over-cap same-day request settles on the standard schedule and is
priced accordingly; it is not rejected.

RailFeeConfig accepts the new rail, and its description now says the
two are separate rails that each need their own fee config.

The docs pages that enumerate rails list the new one: the US row of the
country support table, the paymentRail tip on the sending pages, the
rail-fee limits on the fees page, and a changelog entry.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RUF3krYG9C9hShf4iS5Sug

Author: jacklatourette

mintlify/changelog.mdx

@@ -11,6 +11,24 @@ changes and roadmap, [book a live demo](https://www.lightspark.com/contact) or
 
 <Update label="September 2026">
 
+## 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.
+- A same-day request above the NACHA per-entry limit settles on the standard schedule
+  and is priced accordingly. It is not rejected.
+- `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
 
 New `POST /cards/{id}/tokenize` exchanges the values a wallet SDK produces

mintlify/openapi.yaml

@@ -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/assessing-fees.mdx

@@ -135,7 +135,7 @@ 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. Above the NACHA per-entry same-day limit it settles on the standard schedule and is priced accordingly.
 </Tip>
 
 <Info>

mintlify/payouts-and-b2b/payment-flow/send-payment.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/country-support.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. Above the NACHA per-entry same-day limit it settles on the standard schedule and is priced accordingly.
 </Tip>
 
 <Info>

mintlify/snippets/sending/accounts.mdx

@@ -2,6 +2,7 @@ type: string
 enum:
   - ACH
   - ACH_COLOMBIA
+  - ACH_SAME_DAY
   - BANK_TRANSFER
   - BRE_B
   - CIPS
@@ -26,4 +27,15 @@ 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; a request above that
+  limit settles on the standard schedule and is priced accordingly.
+
+
+  `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.yaml

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

openapi/components/schemas/common/PaymentRail.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.