razzmatazz · razzmatazz · May 22, 2026 · May 22, 2026
diff --git a/Directory.Packages.props b/Directory.Packages.props
@@ -11,14 +11,17 @@
     <NoWarn>$(NoWarn);NU1902;NU1903</NoWarn>
   </PropertyGroup>
   <ItemGroup>
+    <PackageVersion Include="FSharp.Core" Version="10.0.101" />
     <PackageVersion Include="Argu" Version="6.2.5" />
+    <PackageVersion Include="Fabulous.AST" Version="[1.2.0]" />
     <PackageVersion Include="Castle.Core" Version="5.2.1" />
     <PackageVersion Include="coverlet.collector" Version="6.0.4" />
     <PackageVersion Include="DotNet.ReproducibleBuilds" Version="1.2.25" />
     <PackageVersion Include="FSharp.Control.AsyncSeq" Version="3.2.1" />
     <PackageVersion Include="ICSharpCode.Decompiler" Version="9.1.0.7988" />
     <PackageVersion Include="Ionide.KeepAChangelog.Tasks" Version="0.1.8" />
     <PackageVersion Include="Ionide.LanguageServerProtocol" Version="0.7.0" />
+    <PackageVersion Include="StreamJsonRpc" Version="2.16.36" />
     <PackageVersion Include="Microsoft.Build" Version="$(MSBuildPackageVersion)" />
     <PackageVersion Include="Microsoft.Build.Framework" Version="$(MSBuildPackageVersion)" />
     <PackageVersion Include="Microsoft.Build.Locator" Version="$(MSBuildLocatorPackageVersion)" />

diff --git a/plans/system-text-json-migration.md b/plans/system-text-json-migration.md
diff --git a/src/CSharpLanguageServer/CSharpLanguageServer.fsproj b/src/CSharpLanguageServer/CSharpLanguageServer.fsproj
@@ -17,8 +17,7 @@
     <PackageReadmeFile>README.md</PackageReadmeFile>
     <ChangelogFile>CHANGELOG.md</ChangelogFile>
     <Nullable>enable</Nullable>
-    <MSBuildTreatWarningsAsErrors>true</MSBuildTreatWarningsAsErrors>
-    <NoWarn>$(NoWarn);NU1902;NU1903</NoWarn>
+    <NoWarn>$(NoWarn);NU1902;NU1903;FS3261;FS3262;FS3264;FS0043</NoWarn>
   </PropertyGroup>
 
   <ItemGroup>
@@ -87,12 +86,15 @@
     <None Include="..\..\README.md" Pack="true" PackagePath="\" />
   </ItemGroup>
 
+  <ItemGroup>
+    <ProjectReference Include="../Ionide.LanguageServerProtocol/src/Ionide.LanguageServerProtocol.fsproj" />
+  </ItemGroup>
+
   <ItemGroup>
     <PackageReference Include="Argu" />
     <PackageReference Include="Castle.Core" />
     <PackageReference Include="FSharp.Control.AsyncSeq" />
     <PackageReference Include="ICSharpCode.Decompiler" />
-    <PackageReference Include="Ionide.LanguageServerProtocol" />
     <PackageReference Include="Microsoft.Extensions.Caching.Memory" />
     <PackageReference Include="Microsoft.Extensions.Logging" />
     <PackageReference Include="Microsoft.Extensions.Logging.Console" />

diff --git a/src/CSharpLanguageServer/Handlers/CodeLens.fs b/src/CSharpLanguageServer/Handlers/CodeLens.fs
@@ -160,8 +160,7 @@ module CodeLens =
 
         let lensData: CodeLensData =
             p.Data
-            |> Option.map _.ToObject<CodeLensData>()
-            |> Option.bind Option.ofObj
+            |> Option.map (fun je -> deserialize<CodeLensData> je)
             |> Option.defaultValue CodeLensData.Default
 
         let! wf, solution = lensData.DocumentUri |> context.LoadWorkspaceFolder

diff --git a/src/CSharpLanguageServer/Handlers/Completion.fs b/src/CSharpLanguageServer/Handlers/Completion.fs
@@ -112,7 +112,7 @@ module Completion =
                    completionOptions.Object
                    emptyRoslynOptionSet
                    completionTrigger
-                   null
+                   (null: obj)
                    ct |]
 
             let result =

diff --git a/src/CSharpLanguageServer/Handlers/Debug.fs b/src/CSharpLanguageServer/Handlers/Debug.fs
@@ -1,7 +1,6 @@
 namespace CSharpLanguageServer.Handlers
 
-open Newtonsoft.Json.Linq
-
+open System.Text.Json
 open Ionide.LanguageServerProtocol.JsonRpc
 
 open CSharpLanguageServer.Runtime.DebugInfo
@@ -17,7 +16,7 @@ module Debug =
     let handle
         (getDebugInfo: unit -> Async<DebugInfo option>)
         (_ctx: RequestContext)
-        (_params: JObject)
+        (_params: JsonElement)
         : Async<LspResult<DebugInfo option> * LspWorkspaceUpdate> =
         async {
             let! info = getDebugInfo ()

diff --git a/src/CSharpLanguageServer/Handlers/Workspace.fs b/src/CSharpLanguageServer/Handlers/Workspace.fs
@@ -192,11 +192,17 @@ module Workspace =
         : Async<LspResult<unit> * LspWorkspaceUpdate> =
         async {
             let pushedConfig =
-                configParams.Settings
-                |> Option.ofObj
-                |> Option.bind deserialize<DidChangeConfigurationSettingsDto option>
-                |> Option.map _.csharp
-                |> Option.bind id // flatten option option, also guards against null from Newtonsoft
+                // Settings is LSPAny (JsonElement, a struct) — check it's an object before deserializing
+                let settingsOpt =
+                    if
+                        configParams.Settings.ValueKind = System.Text.Json.JsonValueKind.Null
+                        || configParams.Settings.ValueKind = System.Text.Json.JsonValueKind.Undefined
+                    then
+                        None
+                    else
+                        deserialize<DidChangeConfigurationSettingsDto option> configParams.Settings
+
+                settingsOpt |> Option.map _.csharp |> Option.bind id // flatten option option
 
             let configurationSupported =
                 context.ClientCapabilities.Workspace

diff --git a/src/CSharpLanguageServer/Lsp/Client.fs b/src/CSharpLanguageServer/Lsp/Client.fs
@@ -1,7 +1,6 @@
 module CSharpLanguageServer.Lsp.Client
 
 open System.Text.Json
-open Newtonsoft.Json.Linq
 open Microsoft.Extensions.Logging
 
 open Ionide.LanguageServerProtocol
@@ -26,28 +25,25 @@ type CSharpLspClient
 
         return
             match result with
-            | Result.Ok je -> je.GetRawText() |> JToken.Parse |> deserialize |> Result.Ok
+            | Result.Ok je -> je |> deserialize |> Result.Ok
             | Result.Error errEl ->
-                let errToken = JToken.Parse(errEl.GetRawText())
-
                 let code =
-                    errToken.SelectToken("code")
-                    |> Option.ofObj
-                    |> Option.bind (fun t ->
-                        t.ToString()
-                        |> System.Int32.TryParse
-                        |> function
-                            | true, v -> Some v
-                            | _ -> None)
-                    |> Option.defaultValue -32603 // -32603 = JSON-RPC "Internal error"
+                    match errEl.TryGetProperty("code") with
+                    | true, v ->
+                        match v.TryGetInt32() with
+                        | true, i -> i
+                        | _ -> -32603
+                    | _ -> -32603
 
                 let message =
-                    errToken.SelectToken("message")
-                    |> Option.ofObj
-                    |> Option.map _.ToObject<string>()
-                    |> Option.defaultValue "Unknown error"
+                    match errEl.TryGetProperty("message") with
+                    | true, v -> v.GetString()
+                    | _ -> "Unknown error"
 
-                let data = errToken.SelectToken("data") |> Option.ofObj
+                let data =
+                    match errEl.TryGetProperty("data") with
+                    | true, v -> Some v
+                    | _ -> None
 
                 Result.Error
                     { Code = code
@@ -56,35 +52,35 @@ type CSharpLspClient
     }
 
     override __.WindowShowMessage p =
-        sendServerNotification "window/showMessage" (serialize p |> jtokenToJe)
+        sendServerNotification "window/showMessage" (serialize p)
 
     // Note: CSharpLspClient is a pure transport adapter. It does not gate calls on
     // ClientCapabilities — that is the responsibility of callers, who have access to
     // capabilities via RequestContext. See ProgressReporter for the reference pattern.
 
     override __.WindowShowMessageRequest p : AsyncLspResult<Types.MessageActionItem option> =
-        sendServerRequest "window/showMessageRequest" (serialize p |> jtokenToJe)
+        sendServerRequest "window/showMessageRequest" (serialize p)
 
     override __.WindowLogMessage p =
-        sendServerNotification "window/logMessage" (serialize p |> jtokenToJe)
+        sendServerNotification "window/logMessage" (serialize p)
 
     override __.TelemetryEvent p =
-        sendServerNotification "telemetry/event" (serialize p |> jtokenToJe)
+        sendServerNotification "telemetry/event" (p)
 
     override __.ClientRegisterCapability p =
-        sendServerRequest "client/registerCapability" (serialize p |> jtokenToJe)
+        sendServerRequest "client/registerCapability" (serialize p)
 
     override __.ClientUnregisterCapability p =
-        sendServerRequest "client/unregisterCapability" (serialize p |> jtokenToJe)
+        sendServerRequest "client/unregisterCapability" (serialize p)
 
     override __.WorkspaceWorkspaceFolders() : AsyncLspResult<Types.WorkspaceFolder[] option> =
         sendServerRequest "workspace/workspaceFolders" nullJE
 
-    override __.WorkspaceConfiguration p : AsyncLspResult<JToken[]> =
-        sendServerRequest "workspace/configuration" (serialize p |> jtokenToJe)
+    override __.WorkspaceConfiguration p : AsyncLspResult<JsonElement[]> =
+        sendServerRequest "workspace/configuration" (serialize p)
 
     override __.WorkspaceApplyEdit p : AsyncLspResult<Types.ApplyWorkspaceEditResult> =
-        sendServerRequest "workspace/applyEdit" (serialize p |> jtokenToJe)
+        sendServerRequest "workspace/applyEdit" (serialize p)
 
     override __.WorkspaceSemanticTokensRefresh() =
         sendServerRequest "workspace/semanticTokens/refresh" nullJE
@@ -93,22 +89,22 @@ type CSharpLspClient
         sendServerRequest "workspace/diagnostic/refresh" nullJE
 
     override __.TextDocumentPublishDiagnostics p =
-        sendServerNotification "textDocument/publishDiagnostics" (serialize p |> jtokenToJe)
+        sendServerNotification "textDocument/publishDiagnostics" (serialize p)
 
     override __.WindowWorkDoneProgressCreate createParams =
-        sendServerRequest "window/workDoneProgress/create" (serialize createParams |> jtokenToJe)
+        sendServerRequest "window/workDoneProgress/create" (serialize createParams)
 
     override __.LogTrace p =
-        sendServerNotification "$/logTrace" (serialize p |> jtokenToJe)
+        sendServerNotification "$/logTrace" (serialize p)
 
     override __.Progress progressParams =
-        sendServerNotification "$/progress" (serialize progressParams |> jtokenToJe)
+        sendServerNotification "$/progress" (serialize progressParams)
 
     /// Query the client for the `csharp` workspace configuration section.
     /// Returns `None` when the call fails or the response cannot be deserialized.
     static member TryPullCSharpConfig(lspClient: ILspClient) : Async<CSharpConfiguration option> = async {
         try
-            let! (result: Result<JToken[], _>) =
+            let! (result: Result<JsonElement[], _>) =
                 lspClient.WorkspaceConfiguration(
                     { Items =
                         [| { Section = Some "csharp"
@@ -119,7 +115,7 @@ type CSharpLspClient
                 result
                 |> Option.fromResult
                 |> Option.bind Seq.tryHead
-                |> Option.bind deserialize<CSharpConfiguration option>
+                |> Option.bind (fun je -> deserialize<CSharpConfiguration option> je)
         with ex ->
             logger.LogWarning("could not retrieve `csharp` workspace configuration section: {error}", ex |> string)
 

diff --git a/src/CSharpLanguageServer/Lsp/Server.fs b/src/CSharpLanguageServer/Lsp/Server.fs
@@ -12,7 +12,6 @@ open StreamJsonRpc
 open Microsoft.Extensions.Logging
 open System.Text.Json
 open System.Text.Json.Nodes
-open Newtonsoft.Json.Linq
 
 open CSharpLanguageServer.Types
 open CSharpLanguageServer.Handlers
@@ -135,7 +134,7 @@ let configureRpcTransport
 
                 let fnParams =
                     try
-                        rawParams |> sanitize |> jeToJToken |> deserialize
+                        rawParams |> sanitize |> deserialize
                     with ex ->
                         logger.LogError(
                             ex,
@@ -154,16 +153,10 @@ let configureRpcTransport
                 stateActor.Post(LeaveRequestContext(jsonRpcCtx.RequestOrdinal, wsUpdate))
         }
 
-    let serializeNullable value =
-        if isNull (box value) then
-            Newtonsoft.Json.Linq.JValue.CreateNull() :> Newtonsoft.Json.Linq.JToken
-        else
-            serialize value
-
     let unwrapResult result =
         match result with
-        | Ok value -> value |> serializeNullable |> jtokenToJe |> Ok
-        | Error error -> error |> serialize |> jtokenToJe |> Error
+        | Ok value -> value |> serialize |> Ok
+        | Error error -> error |> serialize |> Error
 
     let callHandler requestMode fn : JsonRpcCallHandler =
         wrapHandler unwrapResult requestMode id fn

diff --git a/src/CSharpLanguageServer/Runtime/ServerStateLoop.fs b/src/CSharpLanguageServer/Runtime/ServerStateLoop.fs
@@ -7,7 +7,7 @@ open System.Threading
 open Ionide.LanguageServerProtocol.Types
 open Ionide.LanguageServerProtocol
 open Microsoft.Extensions.Logging
-open Newtonsoft.Json.Linq
+
 
 open CSharpLanguageServer.Logging
 open CSharpLanguageServer.Lsp.Workspace
@@ -400,13 +400,23 @@ let processServerEvent state postServerEvent (inbox: MailboxProcessor<ServerEven
                 ShutdownReceived = true }
 
     | ClientCapabilityChange cc ->
-        let experimentalCapsBoolValue boolPropName =
+        let experimentalCapsBoolValue (dotPath: string) =
+            // Navigate a dot-separated path (e.g. "csharp.metadataUris") through JsonElement
+            let rec navigate (parts: string list) (je: System.Text.Json.JsonElement) =
+                match parts with
+                | [] -> Some je
+                | head :: tail ->
+                    match je.TryGetProperty(head) with
+                    | true, child -> navigate tail child
+                    | _ -> None
+
             cc.Experimental
-            |> Option.map _.SelectToken(boolPropName)
-            |> Option.bind Option.ofObj
-            |> Option.map (fun t ->
-                let v = t :?> JValue
-                v.Value :?> bool)
+            |> Option.bind (fun je -> navigate (dotPath.Split('.') |> List.ofArray) je)
+            |> Option.bind (fun je ->
+                match je.ValueKind with
+                | System.Text.Json.JsonValueKind.True -> Some true
+                | System.Text.Json.JsonValueKind.False -> Some false
+                | _ -> None)
 
         let newConfig =
             { state.Config with

diff --git a/src/CSharpLanguageServer/Util.fs b/src/CSharpLanguageServer/Util.fs
@@ -5,7 +5,6 @@ open System.Threading.Tasks
 
 open System.Text.Json
 open System.Text.Json.Nodes
-open Newtonsoft.Json.Linq
 
 let noneIfEmpty (s: string) : string option =
     s
@@ -102,13 +101,6 @@ module Option =
         | Some v -> [ v ]
         | None -> []
 
-let jeToJToken (je: JsonElement) : JToken = JToken.Parse(je.GetRawText())
-
-let jtokenToJe (token: JToken) : JsonElement =
-    let json = token.ToString(Newtonsoft.Json.Formatting.None)
-    use doc = JsonDocument.Parse(json)
-    doc.RootElement.Clone()
-
 let nullJE =
     use doc = JsonDocument.Parse("null")
     doc.RootElement.Clone()