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diff --git a/src/FundLab.Domain/Sip.fs b/src/FundLab.Domain/Sip.fs
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+namespace FundLab.Domain
+
+open System
+
+type SipFrequency =
+    | Weekly
+    | BiWeekly
+    | Monthly
+
+module SipPolicy =
+    let cashMaximum = 999999999999999999.99m
+
+    let validateAmount (amount: decimal) : Result<unit, string> =
+        if amount <= 0m then
+            Error "sip amount must be positive"
+        elif Decimal.Round(amount, 2) <> amount then
+            Error "sip amount exceeds cash precision"
+        elif amount > cashMaximum then
+            Error "sip amount exceeds database precision"
+        else
+            Ok ()
+
+    let parseFrequency (text: string) : SipFrequency option =
+        match text with
+        | null -> None
+        | "weekly" -> Some Weekly
+        | "biweekly" -> Some BiWeekly
+        | "monthly" -> Some Monthly
+        | _ -> None
+
+    let frequencyText (frequency: SipFrequency) : string =
+        match frequency with
+        | Weekly -> "weekly"
+        | BiWeekly -> "biweekly"
+        | Monthly -> "monthly"
+
+    let private isWeekend (date: DateOnly) =
+        date.DayOfWeek = DayOfWeek.Saturday || date.DayOfWeek = DayOfWeek.Sunday
+
+    let private rollToWeekday (date: DateOnly) : DateOnly =
+        let mutable candidate = date
+
+        while isWeekend candidate do
+            candidate <- candidate.AddDays 1
+
+        candidate
+
+    let private intervalDays (frequency: SipFrequency) : int =
+        match frequency with
+        | Weekly -> 7
+        | BiWeekly -> 14
+        | Monthly -> 0
+
+    let private addMonthsPreservingDay (anchor: DateOnly) (months: int) : DateOnly =
+        let targetMonthIndex = anchor.Year * 12 + (anchor.Month - 1) + months
+        let year = targetMonthIndex / 12
+        let month = targetMonthIndex % 12 + 1
+        let daysInTargetMonth = DateTime.DaysInMonth(year, month)
+        DateOnly(year, month, min anchor.Day daysInTargetMonth)
+
+    /// Next due trade date for a plan: the first run date of the plan's cadence that is
+    /// on or after `fromDate`, rolled forward off weekends (no-lookahead weekday rule).
+    let nextTradeDate (frequency: SipFrequency) (anchorDate: DateOnly) (fromDate: DateOnly) : DateOnly =
+        match frequency with
+        | Weekly
+        | BiWeekly ->
+            let interval = intervalDays frequency
+            let anchorTicks = anchorDate.DayNumber
+            let fromTicks = fromDate.DayNumber
+            let steps = max 0 ((fromTicks - anchorTicks + interval - 1) / interval)
+            rollToWeekday (anchorDate.AddDays(steps * interval))
+        | Monthly ->
+            let monthsBetween =
+                (fromDate.Year - anchorDate.Year) * 12 + (fromDate.Month - anchorDate.Month)
+
+            let candidateMonths =
+                if fromDate.Day <= anchorDate.Day || monthsBetween < 0 then
+                    monthsBetween
+                else
+                    monthsBetween + 1
+
+            let months = max 0 candidateMonths
+            rollToWeekday (addMonthsPreservingDay anchorDate months)
+
+    /// Execution is out of scope for this slice. These pure helpers define the
+    /// due/settlement contract that the (future) executor will implement: when a
+    /// plan is due, the net amount debited equals the plan amount exactly, and no
+    /// units, NAV or realized income are touched until that executor lands (3d-6+).
+    let isDue (nextTradeDate: DateOnly) (today: DateOnly) : bool = today >= nextTradeDate
+
+    let executionNetAmount (plannedAmount: decimal) : Result<decimal, string> =
+        match validateAmount plannedAmount with
+        | Error message -> Error message
+        | Ok() -> Ok plannedAmount