diff --git a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalHover.cs b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalHover.cs
index ea9a5ec9094f6..de87ad88193bc 100644
--- a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalHover.cs
+++ b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalHover.cs
@@ -10,6 +10,7 @@ namespace Roslyn.LanguageServer.Protocol
     /// <summary>
     /// Extension to Hover which adds additional data for colorization.
     /// </summary>
+    [DataContract]
     internal class VSInternalHover : Hover
     {
         /// <summary>
diff --git a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionContext.cs b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionContext.cs
index 284ffa5d505ae..a3ab7943d4821 100644
--- a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionContext.cs
+++ b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionContext.cs
@@ -12,6 +12,7 @@ namespace Roslyn.LanguageServer.Protocol
     /// Context for inline completion request.
     /// See https://github.com/microsoft/vscode/blob/075ba020e8493f40dba89891b1a08453f2c067e9/src/vscode-dts/vscode.proposed.inlineCompletions.d.ts#L27.
     /// </summary>
+    [DataContract]
     internal class VSInternalInlineCompletionContext
     {
         /// <summary>
diff --git a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionItem.cs b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionItem.cs
index 73fb5c5285852..740c6799c26d9 100644
--- a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionItem.cs
+++ b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionItem.cs
@@ -13,6 +13,7 @@ namespace Roslyn.LanguageServer.Protocol
     ///
     /// See https://github.com/microsoft/vscode/blob/075ba020e8493f40dba89891b1a08453f2c067e9/src/vscode-dts/vscode.proposed.inlineCompletions.d.ts#L78.
     /// </summary>
+    [DataContract]
     internal class VSInternalInlineCompletionItem
     {
         /// <summary>
@@ -45,4 +46,4 @@ internal class VSInternalInlineCompletionItem
         [JsonProperty(DefaultValueHandling = DefaultValueHandling.Ignore)]
         public InsertTextFormat? TextFormat { get; set; } = InsertTextFormat.Plaintext;
     }
-}
\ No newline at end of file
+}
diff --git a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionList.cs b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionList.cs
index 62f4b20ba0d07..57c7957df661b 100644
--- a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionList.cs
+++ b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionList.cs
@@ -12,6 +12,7 @@ namespace Roslyn.LanguageServer.Protocol
     ///
     /// See https://github.com/microsoft/vscode/blob/075ba020e8493f40dba89891b1a08453f2c067e9/src/vscode-dts/vscode.proposed.inlineCompletions.d.ts#L72.
     /// </summary>
+    [DataContract]
     internal class VSInternalInlineCompletionList
     {
         /// <summary>
@@ -21,4 +22,4 @@ internal class VSInternalInlineCompletionList
         [JsonProperty(Required = Required.Always)]
         public VSInternalInlineCompletionItem[] Items { get; set; }
     }
-}
\ No newline at end of file
+}
diff --git a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionRequest.cs b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionRequest.cs
index fe3840400b762..a57c4f294d4db 100644
--- a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionRequest.cs
+++ b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalInlineCompletionRequest.cs
@@ -12,6 +12,7 @@ namespace Roslyn.LanguageServer.Protocol
     ///
     /// See https://github.com/microsoft/vscode/blob/075ba020e8493f40dba89891b1a08453f2c067e9/src/vscode-dts/vscode.proposed.inlineCompletions.d.ts#L24.
     /// </summary>
+    [DataContract]
     internal class VSInternalInlineCompletionRequest : ITextDocumentParams
     {
         /// <summary>
diff --git a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalReferenceParams.cs b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalReferenceParams.cs
index a302b2baeb3ce..ef0a031bbb6dd 100644
--- a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalReferenceParams.cs
+++ b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalReferenceParams.cs
@@ -10,6 +10,7 @@ namespace Roslyn.LanguageServer.Protocol
     /// <summary>
     /// Class which represents extensions of <see cref="ReferenceParams"/> passed as parameter of find reference requests.
     /// </summary>
+    [DataContract]
     internal class VSInternalReferenceParams : ReferenceParams
     {
         /// <summary>
diff --git a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalSelectedCompletionInfo.cs b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalSelectedCompletionInfo.cs
index 566acebde174c..963cc264e0acf 100644
--- a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalSelectedCompletionInfo.cs
+++ b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalSelectedCompletionInfo.cs
@@ -12,6 +12,7 @@ namespace Roslyn.LanguageServer.Protocol
     ///
     /// See https://github.com/microsoft/vscode/blob/075ba020e8493f40dba89891b1a08453f2c067e9/src/vscode-dts/vscode.proposed.inlineCompletions.d.ts#L48.
     /// </summary>
+    [DataContract]
     internal class VSInternalSelectedCompletionInfo
     {
         /// <summary>
@@ -42,4 +43,4 @@ internal class VSInternalSelectedCompletionInfo
         [JsonProperty(DefaultValueHandling = DefaultValueHandling.Ignore)]
         public bool IsSnippetText { get; set; }
     }
-}
\ No newline at end of file
+}
diff --git a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalTextDocumentClientCapabilities.cs b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalTextDocumentClientCapabilities.cs
index 1ae592269a85e..abc8b2b3749ed 100644
--- a/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalTextDocumentClientCapabilities.cs
+++ b/src/Features/LanguageServer/Protocol/Protocol/Internal/VSInternalTextDocumentClientCapabilities.cs
@@ -10,6 +10,7 @@ namespace Roslyn.LanguageServer.Protocol
     /// <summary>
     /// Text document capabilities specific to Visual Studio.
     /// </summary>
+    [DataContract]
     internal class VSInternalTextDocumentClientCapabilities : TextDocumentClientCapabilities
     {
         /// <summary>