diff --git a/src/server/protocol.ts b/src/server/protocol.ts
index 8e3c6c2c07f..41d4741559f 100644
--- a/src/server/protocol.ts
+++ b/src/server/protocol.ts
@@ -1828,6 +1828,11 @@ namespace ts.server.protocol {
          * The error code of the diagnostic message.
          */
         code?: number;
+
+        /**
+         * The category of the diagnostic message, e.g. "error" vs. "warning"
+         */
+        category: string;
     }
 
     export interface DiagnosticEventBody {
diff --git a/src/server/session.ts b/src/server/session.ts
index 194c6f667bb..72080d28fe7 100644
--- a/src/server/session.ts
+++ b/src/server/session.ts
@@ -67,7 +67,8 @@ namespace ts.server {
             start: scriptInfo.positionToLineOffset(diag.start),
             end: scriptInfo.positionToLineOffset(diag.start + diag.length),
             text: ts.flattenDiagnosticMessageText(diag.messageText, "\n"),
-            code: diag.code
+            code: diag.code,
+            category: DiagnosticCategory[diag.category].toLowerCase()
         };
     }
 
@@ -75,7 +76,8 @@ namespace ts.server {
         return {
             start: undefined,
             end: undefined,
-            text: ts.flattenDiagnosticMessageText(diag.messageText, "\n")
+            text: ts.flattenDiagnosticMessageText(diag.messageText, "\n"),
+            category: DiagnosticCategory[diag.category].toLowerCase()
         };
     }