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diff --git a/tests/FundLab.Domain.Tests/StockRulesTests.fs b/tests/FundLab.Domain.Tests/StockRulesTests.fs
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+++ b/tests/FundLab.Domain.Tests/StockRulesTests.fs
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+namespace FundLab.Domain.Tests
+
+module StockRulesTests =
+
+    open System
+    open Xunit
+    open FundLab.Domain
+
+    let private terms = StockTerms.aShareDefault
+
+    [<Fact>]
+    let ``buy cost applies commission minimum and transfer fee`` () =
+        // gross 1000.00 -> commission 0.25 floors to 5.00, transfer 0.01.
+        Assert.Equal(1005.01m, StockRules.buyCost terms 100m 10m)
+        // gross 100000.00 -> commission 25.00 (above the minimum), transfer 1.00.
+        Assert.Equal(100026.00m, StockRules.buyCost terms 1000m 100m)
+
+    [<Fact>]
+    let ``sell proceeds deduct commission, stamp duty and transfer fee`` () =
+        Assert.Equal(994.49m, StockRules.sellProceeds terms 100m 10m)
+        Assert.Equal(99924.00m, StockRules.sellProceeds terms 1000m 100m)
+
+    [<Fact>]
+    let ``sell proceeds are lower than buy cost on a round trip`` () =
+        Assert.True(StockRules.sellProceeds terms 100m 10m < StockRules.buyCost terms 100m 10m)
+
+    [<Fact>]
+    let ``quantity must be a whole multiple of the lot size`` () =
+        match StockRules.validateQuantity terms 100m with
+        | Ok value -> Assert.Equal(100m, value)
+        | Error message -> failwithf "expected a valid lot, got %s" message
+
+        match StockRules.validateQuantity terms 50m with
+        | Ok _ -> failwith "expected a lot-size error"
+        | Error message -> Assert.Contains("multiple of 100", message)
+
+        match StockRules.validateQuantity terms 0m with
+        | Ok _ -> failwith "expected a positive-quantity error"
+        | Error message -> Assert.Contains("positive", message)
+
+    [<Fact>]
+    let ``settlement applies the cutoff then T+1 and skips the weekend`` () =
+        let thursday = DateOnly(2026, 9, 24)
+
+        // Before the 15:00 cutoff: T+1 lands on Friday.
+        Assert.Equal(DateOnly(2026, 9, 25), StockRules.settlementDate terms thursday (TimeOnly(10, 0)))
+
+        // At/after the cutoff: the order slips to Friday, T+1 lands on Monday.
+        Assert.Equal(DateOnly(2026, 9, 28), StockRules.settlementDate terms thursday (TimeOnly(15, 0)))
+        Assert.Equal(DateOnly(2026, 9, 28), StockRules.settlementDate terms thursday (TimeOnly(15, 30)))
+
+    [<Fact>]
+    let ``effective trade date shifts to the next trading day after the cutoff`` () =
+        let thursday = DateOnly(2026, 9, 24)
+        Assert.Equal(thursday, StockRules.effectiveTradeDate terms thursday (TimeOnly(14, 59)))
+        Assert.Equal(DateOnly(2026, 9, 25), StockRules.effectiveTradeDate terms thursday (TimeOnly(15, 0)))
+
+    [<Fact>]
+    let ``a stock bought today cannot be sold until T+1`` () =
+        let thursday = DateOnly(2026, 9, 24)
+        Assert.False(StockRules.isSellAllowed terms thursday thursday)
+        Assert.True(StockRules.isSellAllowed terms thursday (DateOnly(2026, 9, 25)))