From f3ceac57f983aa822dac1467e92f4adb8ef26857 Mon Sep 17 00:00:00 2001
From: Michel Bieleveld <michel.bieleveld@ensyr.com>
Date: Tue, 10 Mar 2026 22:36:47 -0300
Subject: [PATCH 1/3] Added check for existing JsonConverter

---
 src/FSharp.SystemTextJson/All.fs              |  3 ++
 src/FSharp.SystemTextJson/TypeCache.fs        | 12 ++++++
 .../FSharp.SystemTextJson.Tests/Test.Union.fs | 39 +++++++++++++++++++
 3 files changed, 54 insertions(+)

diff --git a/src/FSharp.SystemTextJson/All.fs b/src/FSharp.SystemTextJson/All.fs
index dda4656..2394d9d 100644
--- a/src/FSharp.SystemTextJson/All.fs
+++ b/src/FSharp.SystemTextJson/All.fs
@@ -2,6 +2,7 @@ namespace System.Text.Json.Serialization
 
 open System
 open System.Collections.Generic
+open System.Reflection
 open System.Runtime.CompilerServices
 open System.Runtime.InteropServices
 open System.Text.Json
@@ -20,6 +21,8 @@ type JsonFSharpConverter(fsOptions: JsonFSharpOptions, [<Optional>] overrides: I
     member _.Overrides = fsOptions.Overrides
 
     override _.CanConvert(typeToConvert) =
+        not (TypeCache.hasCustomJsonConverter typeToConvert)
+        &&
         match TypeCache.getKind typeToConvert with
         | TypeCache.TypeKind.List -> fsOptions.Types.HasFlag JsonFSharpTypes.Lists
         | TypeCache.TypeKind.Set -> fsOptions.Types.HasFlag JsonFSharpTypes.Sets
diff --git a/src/FSharp.SystemTextJson/TypeCache.fs b/src/FSharp.SystemTextJson/TypeCache.fs
index cd904e6..bd8f8d9 100644
--- a/src/FSharp.SystemTextJson/TypeCache.fs
+++ b/src/FSharp.SystemTextJson/TypeCache.fs
@@ -1,5 +1,7 @@
 namespace System.Text.Json.Serialization
 
+open System.Reflection
+
 module TypeCache =
     open FSharp.Reflection
 
@@ -38,6 +40,16 @@ module TypeCache =
                     elif FSharpType.IsRecord(ty, true) then TypeKind.Record
                     else TypeKind.Other
             )
+    
+    let hasCustomJsonConverter =
+        let cache = Dict<System.Type, bool>()
+        fun (ty: System.Type) ->
+            cache.GetOrAdd(
+                ty,
+                fun ty ->
+                    CustomAttributeData.GetCustomAttributes(ty)
+                    |> Seq.exists (fun cad -> cad.AttributeType = typeof<JsonConverterAttribute>)
+            )
 
     let isUnion ty =
         getKind ty = TypeKind.Union
diff --git a/tests/FSharp.SystemTextJson.Tests/Test.Union.fs b/tests/FSharp.SystemTextJson.Tests/Test.Union.fs
index aacd204..71242ad 100644
--- a/tests/FSharp.SystemTextJson.Tests/Test.Union.fs
+++ b/tests/FSharp.SystemTextJson.Tests/Test.Union.fs
@@ -2482,6 +2482,45 @@ module Struct =
             """{"Case":"Unwrapped","Fields":["foo"]}""",
             JsonSerializer.Serialize(Unwrapped "foo", noNewtypeOptions)
         )
+        
+        
+    
+
+    [<JsonConverter(typeof<TruthConverter>)>]
+    type Truth =
+        | True
+        | False
+        | FileNotFound
+    and TruthConverter() =
+        inherit JsonConverter<Truth>()
+
+        override _.Write(writer, value, _options) =
+            writer.WriteStringValue(
+                match value with
+                | FileNotFound -> "lost"
+                | True -> "true"
+                | False -> "false"
+            )
+
+        override _.Read(reader, _typeToConvert, _options) =
+            match reader.GetString() with
+            | "lost" -> FileNotFound
+            | "true" -> True
+            | "false" -> False
+            | other -> failwithf "Unknown Truth value: %s" other
+        
+    [<Fact>]
+    let ``custom converter wins over FSharp.SystemTextJson`` () =
+        let options =
+            JsonFSharpOptions.Default().ToJsonSerializerOptions()
+
+        Assert.Equal("\"lost\"", JsonSerializer.Serialize( FileNotFound, options))
+        Assert.Equal("\"true\"", JsonSerializer.Serialize( True, options))
+        Assert.Equal("\"false\"", JsonSerializer.Serialize( False, options))
+
+        Assert.Equal(FileNotFound, JsonSerializer.Deserialize<Truth>("\"lost\"",options))
+        Assert.Equal(True, JsonSerializer.Deserialize<Truth>("\"true\"",options))
+        Assert.Equal(False, JsonSerializer.Deserialize<Truth>("\"false\"",options))
 
     module UnwrapRecord =
 

From 362c23efa505bcaf20544646368bde56ca0f65bc Mon Sep 17 00:00:00 2001
From: Michel Bieleveld <michel.bieleveld@ensyr.com>
Date: Tue, 10 Mar 2026 22:41:51 -0300
Subject: [PATCH 2/3] Added check for existing JsonConverter, cleaned up open

---
 src/FSharp.SystemTextJson/All.fs | 1 -
 1 file changed, 1 deletion(-)

diff --git a/src/FSharp.SystemTextJson/All.fs b/src/FSharp.SystemTextJson/All.fs
index 2394d9d..357d5bb 100644
--- a/src/FSharp.SystemTextJson/All.fs
+++ b/src/FSharp.SystemTextJson/All.fs
@@ -2,7 +2,6 @@ namespace System.Text.Json.Serialization
 
 open System
 open System.Collections.Generic
-open System.Reflection
 open System.Runtime.CompilerServices
 open System.Runtime.InteropServices
 open System.Text.Json

From 35fe7147179202ebe13cae7e32e0b31f98feb2f1 Mon Sep 17 00:00:00 2001
From: Michel Bieleveld <michel.bieleveld@ensyr.com>
Date: Tue, 10 Mar 2026 22:43:21 -0300
Subject: [PATCH 3/3] Added check for existing JsonConverter, cleaned up open

---
 src/FSharp.SystemTextJson/TypeCache.fs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/FSharp.SystemTextJson/TypeCache.fs b/src/FSharp.SystemTextJson/TypeCache.fs
index bd8f8d9..97de001 100644
--- a/src/FSharp.SystemTextJson/TypeCache.fs
+++ b/src/FSharp.SystemTextJson/TypeCache.fs
@@ -1,9 +1,9 @@
 namespace System.Text.Json.Serialization
 
-open System.Reflection
 
 module TypeCache =
     open FSharp.Reflection
+    open System.Reflection
 
     // Have to use concurrentdictionary here because dictionaries thrown on non-locked access:
     (* Error Message: