diff --git a/RELEASE_NOTES.md b/RELEASE_NOTES.md
index 41a94ebb0..5ca6d71b7 100644
--- a/RELEASE_NOTES.md
+++ b/RELEASE_NOTES.md
@@ -1,5 +1,10 @@
 # Release Notes
 
+## 8.1.12 - Apr 28 2026
+
+- Fix: `JsonValue.WriteTo` now always uses `CultureInfo.InvariantCulture` when serializing `Number` (decimal) values, preventing invalid JSON output (e.g. `1,5` instead of `1.5`) when called with a `TextWriter` configured with a non-English culture.
+- Perf: `JsonValue.WriteTo` no longer allocates an intermediate `System.String(' ', n)` per indentation level; spaces are written directly to the writer.
+
 ## 8.1.11 - Apr 22 2026
 
 - Code: `HtmlParser` `EmitTag` removes dead code in the `else` branch (the expression `x.HasFormattedParent || x.IsFormattedTag` was always equivalent to `x.HasFormattedParent` since `x.IsFormattedTag` is always `false` in that branch). Uses `name` directly to avoid re-computing `CurrentTagName()` for formatted/script tag checks. Also removes redundant `.ToLowerInvariant()` calls in `IsFormattedTag` and `IsScriptTag` since tag names are already lowercased at read time.
diff --git a/src/FSharp.Data.Json.Core/JsonValue.fs b/src/FSharp.Data.Json.Core/JsonValue.fs
index 8cc035bd0..7e17c616b 100644
--- a/src/FSharp.Data.Json.Core/JsonValue.fs
+++ b/src/FSharp.Data.Json.Core/JsonValue.fs
@@ -73,11 +73,16 @@ type JsonValue =
     member x.WriteTo(w: TextWriter, saveOptions, ?indentationSpaces: int) =
         let indentationSpaces = defaultArg indentationSpaces 2
 
+        // Write `count` space characters without allocating an intermediate string.
+        let inline writeSpaces count =
+            for _ = 1 to count do
+                w.Write(' ')
+
         let newLine =
             if saveOptions = JsonSaveOptions.None then
                 fun indentation plus ->
                     w.WriteLine()
-                    System.String(' ', indentation + plus) |> w.Write
+                    writeSpaces (indentation + plus)
             else
                 fun _ _ -> ()
 
@@ -94,7 +99,7 @@ type JsonValue =
             function
             | Null -> w.Write "null"
             | Boolean b -> w.Write(if b then "true" else "false")
-            | Number number -> w.Write number
+            | Number number -> w.Write(number.ToString(CultureInfo.InvariantCulture))
             | Float v when Double.IsInfinity v || Double.IsNaN v -> w.Write "null"
             | Float number ->
                 let s = number.ToString("R", CultureInfo.InvariantCulture)
diff --git a/tests/FSharp.Data.Core.Tests/JsonValue.fs b/tests/FSharp.Data.Core.Tests/JsonValue.fs
index ef7fd760a..7161c4274 100644
--- a/tests/FSharp.Data.Core.Tests/JsonValue.fs
+++ b/tests/FSharp.Data.Core.Tests/JsonValue.fs
@@ -859,3 +859,31 @@ let ``JsonValue WriteTo with None (default) produces indented output`` () =
     let result = writer.ToString()
     result.Contains("\n") |> should equal true
     result.Contains("  ") |> should equal true
+
+[<Test>]
+let ``JsonValue WriteTo serializes decimals using InvariantCulture regardless of thread culture`` () =
+    // In cultures that use ',' as decimal separator (e.g. de-DE), TextWriter.Write(decimal)
+    // could produce invalid JSON like {"price":1,5} instead of {"price":1.5}.
+    // WriteTo must always use InvariantCulture for decimal numbers.
+    use _holder = withCulture "de-DE"
+    let json = JsonValue.Record [| "price", JsonValue.Number 1.5M |]
+    use writer = new System.IO.StringWriter()
+    json.WriteTo(writer, JsonSaveOptions.DisableFormatting)
+    let result = writer.ToString()
+    result |> should equal """{"price":1.5}"""
+
+[<Test>]
+let ``JsonValue ToString serializes decimal array using InvariantCulture`` () =
+    use _holder = withCulture "fr-FR"
+    let json = JsonValue.Array [| JsonValue.Number 1.5M; JsonValue.Number 99.99M |]
+    json.ToString(JsonSaveOptions.DisableFormatting)
+    |> should equal "[1.5,99.99]"
+
+[<Test>]
+let ``JsonValue WriteTo indentation uses correct number of spaces`` () =
+    let json = JsonValue.Record [| "x", JsonValue.Number 1M |]
+    use writer = new System.IO.StringWriter()
+    json.WriteTo(writer, JsonSaveOptions.None, 4)
+    let result = writer.ToString()
+    // With 4-space indent, the property line should start with 4 spaces
+    result |> should contain "    \"x\""