diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 320f4e9f4ddc1..0547fdb5d4bd1 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -1624,6 +1624,7 @@ namespace ts {
             messageText: text,
             category: message.category,
             code: message.code,
+            unused: message.unused,
         };
     }
 
@@ -1653,7 +1654,8 @@ namespace ts {
 
             messageText: text,
             category: message.category,
-            code: message.code
+            code: message.code,
+            unused: message.unused,
         };
     }
 
@@ -1665,7 +1667,7 @@ namespace ts {
 
             code: chain.code,
             category: chain.category,
-            messageText: chain.next ? chain : chain.messageText
+            messageText: chain.next ? chain : chain.messageText,
         };
     }
 
diff --git a/src/compiler/diagnosticMessages.json b/src/compiler/diagnosticMessages.json
index 9f95a66711b75..912814772a2c5 100644
--- a/src/compiler/diagnosticMessages.json
+++ b/src/compiler/diagnosticMessages.json
@@ -3250,7 +3250,8 @@
     },
     "'{0}' is declared but its value is never read.": {
         "category": "Error",
-        "code": 6133
+        "code": 6133,
+        "unused": true
     },
     "Report errors on unused locals.": {
         "category": "Message",
@@ -3270,7 +3271,8 @@
     },
     "Property '{0}' is declared but its value is never read.": {
         "category": "Error",
-        "code": 6138
+        "code": 6138,
+        "unused": true
     },
     "Import emit helpers from 'tslib'.": {
         "category": "Message",
@@ -3482,7 +3484,8 @@
     },
     "All imports in import declaration are unused.": {
         "category": "Error",
-        "code": 6192
+        "code": 6192,
+        "unused": true
     },
     "Variable '{0}' implicitly has an '{1}' type.": {
         "category": "Error",
@@ -3562,7 +3565,8 @@
     },
     "Unused label.": {
         "category": "Error",
-        "code": 7028
+        "code": 7028,
+        "unused": true
     },
     "Fallthrough case in switch.": {
         "category": "Error",
diff --git a/src/compiler/types.ts b/src/compiler/types.ts
index 42d8462cecdb7..183ea4f07b682 100644
--- a/src/compiler/types.ts
+++ b/src/compiler/types.ts
@@ -4009,6 +4009,7 @@ namespace ts {
         category: DiagnosticCategory;
         code: number;
         message: string;
+        unused?: {};
     }
 
     /**
@@ -4030,6 +4031,8 @@ namespace ts {
         length: number | undefined;
         messageText: string | DiagnosticMessageChain;
         category: DiagnosticCategory;
+        /** May store more in future. For now, this will simply be `true` to indicate when a diagnostic is an unused-identifier diagnostic. */
+        unused?: {};
         code: number;
         source?: string;
     }
diff --git a/src/server/client.ts b/src/server/client.ts
index 578cbe507bc00..fc36ea316adbd 100644
--- a/src/server/client.ts
+++ b/src/server/client.ts
@@ -352,11 +352,11 @@ namespace ts.server {
             return this.getDiagnostics(file, CommandNames.SuggestionDiagnosticsSync);
         }
 
-        private getDiagnostics(file: string, command: CommandNames) {
+        private getDiagnostics(file: string, command: CommandNames): Diagnostic[] {
             const request = this.processRequest<protocol.SyntacticDiagnosticsSyncRequest | protocol.SemanticDiagnosticsSyncRequest | protocol.SuggestionDiagnosticsSyncRequest>(command, { file, includeLinePosition: true });
             const response = this.processResponse<protocol.SyntacticDiagnosticsSyncResponse | protocol.SemanticDiagnosticsSyncResponse | protocol.SuggestionDiagnosticsSyncResponse>(request);
 
-            return (<protocol.DiagnosticWithLinePosition[]>response.body).map(entry => {
+            return (<protocol.DiagnosticWithLinePosition[]>response.body).map<Diagnostic>(entry => {
                 const category = firstDefined(Object.keys(DiagnosticCategory), id =>
                     isString(id) && entry.category === id.toLowerCase() ? (<any>DiagnosticCategory)[id] : undefined);
                 return {
@@ -365,7 +365,8 @@ namespace ts.server {
                     length: entry.length,
                     messageText: entry.message,
                     category: Debug.assertDefined(category, "convertDiagnostic: category should not be undefined"),
-                    code: entry.code
+                    code: entry.code,
+                    unused: entry.unused,
                 };
             });
         }
diff --git a/src/server/protocol.ts b/src/server/protocol.ts
index 47c607966006b..667fd92fc6f60 100644
--- a/src/server/protocol.ts
+++ b/src/server/protocol.ts
@@ -455,6 +455,8 @@ namespace ts.server.protocol {
         endLocation: Location;
         category: string;
         code: number;
+        /** May store more in future. For now, this will simply be `true` to indicate when a diagnostic is an unused-identifier diagnostic. */
+        unused?: {};
     }
 
     /**
diff --git a/src/services/shims.ts b/src/services/shims.ts
index 1a43fbda14610..1e2fb7f0cc4db 100644
--- a/src/services/shims.ts
+++ b/src/services/shims.ts
@@ -588,6 +588,7 @@ namespace ts {
         length: number;
         category: string;
         code: number;
+        unused?: {};
     }
     export function realizeDiagnostics(diagnostics: ReadonlyArray<Diagnostic>, newLine: string): RealizedDiagnostic[] {
         return diagnostics.map(d => realizeDiagnostic(d, newLine));
diff --git a/tests/cases/fourslash/codeFixUnusedIdentifier_suggestion.ts b/tests/cases/fourslash/codeFixUnusedIdentifier_suggestion.ts
index 71de4fd190f50..de671de1e5db8 100644
--- a/tests/cases/fourslash/codeFixUnusedIdentifier_suggestion.ts
+++ b/tests/cases/fourslash/codeFixUnusedIdentifier_suggestion.ts
@@ -10,11 +10,13 @@ verify.getSuggestionDiagnostics([
         message: "'p' is declared but its value is never read.",
         range: r0,
         code: 6133,
+        unused: true,
     },
     {
         message: "'x' is declared but its value is never read.",
         range: r1,
         code: 6133,
+        unused: true,
     }
 ]);
 
diff --git a/tests/cases/fourslash/fourslash.ts b/tests/cases/fourslash/fourslash.ts
index fd53a5acf2e8a..58339caa4ec35 100644
--- a/tests/cases/fourslash/fourslash.ts
+++ b/tests/cases/fourslash/fourslash.ts
@@ -527,6 +527,7 @@ declare namespace FourSlashInterface {
         /** @default `test.ranges()[0]` */
         range?: Range;
         code: number;
+        unused?: true;
     }
 }
 declare function verifyOperationIsCancelled(f: any): void;