diff --git a/src/SwaggerProvider.Runtime/RuntimeHelpers.fs b/src/SwaggerProvider.Runtime/RuntimeHelpers.fs
index 7cb53cb3..922fa6f4 100644
--- a/src/SwaggerProvider.Runtime/RuntimeHelpers.fs
+++ b/src/SwaggerProvider.Runtime/RuntimeHelpers.fs
@@ -378,13 +378,32 @@ module RuntimeHelpers =
         let props = getProperties(obj.GetType())
         let sb = System.Text.StringBuilder()
 
-        let appendFormattedArray(v: obj) =
+        let rec appendFormattedArray(v: obj) =
             let vTy = v.GetType()
             sb.Append('[') |> ignore
             let mutable firstEl = true
             let elTy = vTy.GetElementType()
             let isDateOnly = not(isNull elTy) && elTy.FullName = dateOnlyTypeName
             let isTimeOnly = not(isNull elTy) && elTy.FullName = timeOnlyTypeName
+            // Detect array<Option<T>> so elements are correctly unwrapped and formatted.
+            // Without this, Option<DateOnly>/Option<TimeOnly> elements fall through to
+            // obj.ToString(), producing locale-specific "Some(07/04/2025)" instead of ISO 8601.
+            let isOptionEl =
+                not(isNull elTy)
+                && elTy.IsGenericType
+                && elTy.GetGenericTypeDefinition() = typedefof<option<_>>
+
+            let optionTagReader =
+                if isOptionEl then
+                    Some(optionTagReaderCache.GetOrAdd(elTy, optionTagReaderFactory))
+                else
+                    None
+
+            let optionValueProp =
+                if isOptionEl then
+                    Some(optionValueCache.GetOrAdd(elTy, optionValueFactory))
+                else
+                    None
 
             for x in (v :?> Array) |> Seq.cast<obj> do
                 if not firstEl then
@@ -398,6 +417,21 @@ module RuntimeHelpers =
                     sb.Append(formatDateOrTimeValue "yyyy-MM-dd" elTy x) |> ignore
                 elif isTimeOnly then
                     sb.Append(formatDateOrTimeValue "HH:mm:ss.FFFFFFF" elTy x) |> ignore
+                elif isOptionEl then
+                    if optionTagReader.Value x = 1 then // 1 = Some
+                        let inner = optionValueProp.Value.GetValue(x)
+
+                        if isNull inner then
+                            sb.Append("null") |> ignore
+                        else
+                            let innerTy = inner.GetType()
+
+                            if innerTy.IsArray then
+                                appendFormattedArray inner
+                            else
+                                appendFormattedValue sb inner innerTy
+                    else
+                        sb.Append("null") |> ignore
                 else
                     sb.Append(x.ToString()) |> ignore
 
diff --git a/tests/SwaggerProvider.Tests/RuntimeHelpersTests.fs b/tests/SwaggerProvider.Tests/RuntimeHelpersTests.fs
index 63d304f7..326da46d 100644
--- a/tests/SwaggerProvider.Tests/RuntimeHelpersTests.fs
+++ b/tests/SwaggerProvider.Tests/RuntimeHelpersTests.fs
@@ -1184,6 +1184,15 @@ type FmtOptionStringArray(tags: string[] option) =
 type FmtOptionDateOnlyArray(dates: DateOnly[] option) =
     member _.Dates = dates
 
+type FmtOptionDateOnlyElements(dates: DateOnly option[]) =
+    member _.Dates = dates
+
+type FmtOptionTimeOnlyElements(times: TimeOnly option[]) =
+    member _.Times = times
+
+type FmtOptionStringElements(names: string option[]) =
+    member _.Names = names
+
 
 module FormatObjectTests =
 
@@ -1310,6 +1319,34 @@ module FormatObjectTests =
 
         formatObject obj |> shouldEqual "{Dates=[2025-01-01; 2025-12-31]}"
 
+    [<Fact>]
+    let ``formatObject formats array of Option DateOnly Some elements as ISO 8601``() =
+        let obj =
+            FmtOptionDateOnlyElements([| Some(DateOnly(2025, 1, 1)); Some(DateOnly(2025, 12, 31)) |])
+
+        formatObject obj |> shouldEqual "{Dates=[2025-01-01; 2025-12-31]}"
+
+    [<Fact>]
+    let ``formatObject formats array of Option DateOnly with None elements as null``() =
+        let obj = FmtOptionDateOnlyElements([| Some(DateOnly(2025, 7, 4)); None |])
+        formatObject obj |> shouldEqual "{Dates=[2025-07-04; null]}"
+
+    [<Fact>]
+    let ``formatObject formats array of Option TimeOnly Some elements using HH:mm:ss.FFFFFFF``() =
+        let obj =
+            FmtOptionTimeOnlyElements([| Some(TimeOnly(14, 30, 0)); Some(TimeOnly(9, 5, 3, 123)) |])
+
+        formatObject obj |> shouldEqual "{Times=[14:30:00; 09:05:03.123]}"
+
+    [<Fact>]
+    let ``formatObject formats array of Option TimeOnly with None elements as null``() =
+        let obj = FmtOptionTimeOnlyElements([| None; Some(TimeOnly(8, 0, 0)) |])
+        formatObject obj |> shouldEqual "{Times=[null; 08:00:00]}"
+
+    [<Fact>]
+    let ``formatObject formats array of Option string elements``() =
+        let obj = FmtOptionStringElements([| Some "hello"; None; Some "world" |])
+        formatObject obj |> shouldEqual "{Names=[\"hello\"; null; \"world\"]}"
 
 
     [<Fact>]