diff --git a/src/harness/fourslash.ts b/src/harness/fourslash.ts
index 95a320ac052..44927a7a49a 100644
--- a/src/harness/fourslash.ts
+++ b/src/harness/fourslash.ts
@@ -14,6 +14,7 @@
 //
 
 /// <reference path='..\services\services.ts' />
+/// <reference path='..\services\shims.ts' />
 /// <reference path='harnessLanguageService.ts' />
 /// <reference path='harness.ts' />
 /// <reference path='fourslashRunner.ts' />
@@ -126,12 +127,13 @@ module FourSlash {
         outDir: 'outDir',
         sourceMap: 'sourceMap',
         sourceRoot: 'sourceRoot',
+        allowNonTsExtensions: 'allowNonTsExtensions',
         resolveReference: 'ResolveReference',  // This flag is used to specify entry file for resolve file references. The flag is only allow once per test file
     };
 
     // List of allowed metadata names
     var fileMetadataNames = [testOptMetadataNames.fileName, testOptMetadataNames.emitThisFile, testOptMetadataNames.resolveReference];
-    var globalMetadataNames = [testOptMetadataNames.baselineFile, testOptMetadataNames.declaration,
+    var globalMetadataNames = [testOptMetadataNames.allowNonTsExtensions, testOptMetadataNames.baselineFile, testOptMetadataNames.declaration,
         testOptMetadataNames.mapRoot, testOptMetadataNames.module, testOptMetadataNames.out,
         testOptMetadataNames.outDir, testOptMetadataNames.sourceMap, testOptMetadataNames.sourceRoot]
 
@@ -141,6 +143,9 @@ module FourSlash {
         for (var prop in globalOptions) {
             if (globalOptions.hasOwnProperty(prop)) {
                 switch (prop) {
+                    case testOptMetadataNames.allowNonTsExtensions:
+                        settings.allowNonTsExtensions = true;
+                        break;
                     case testOptMetadataNames.declaration:
                         settings.declaration = true;
                         break;
@@ -788,6 +793,16 @@ module FourSlash {
             return "\nActual " + name + ":\n\t" + actualValue + "\nExpected value:\n\t" + expectedValue;
         }
 
+        public getSemanticDiagnostics(expected: string) {
+            var diagnostics = this.languageService.getSemanticDiagnostics(this.activeFile.fileName);
+            var realized = ts.realizeDiagnostics(diagnostics, "\r\n");
+            var actual = JSON.stringify(realized, null, "  ");
+            if (actual !== expected) {
+                ts.sys.writeFile("c:\\temp\\out.txt", actual);
+            }
+            assert.equal(actual, expected);
+        }
+
         public verifyQuickInfoString(negative: boolean, expectedText?: string, expectedDocumentation?: string) {
             [expectedText, expectedDocumentation].forEach(str => {
                 if (str) {
diff --git a/src/services/services.ts b/src/services/services.ts
index 88a56a850cb..8715a171c8b 100644
--- a/src/services/services.ts
+++ b/src/services/services.ts
@@ -2420,12 +2420,10 @@ module ts {
                     case SyntaxKind.ExportAssignment:
                         diagnostics.push(createDiagnosticForNode(node, Diagnostics.export_can_only_be_used_in_TypeScript));
                         return;
-                    case SyntaxKind.TypeParameter:
-                        diagnostics.push(createDiagnosticForNode(node, Diagnostics.type_parameter_declarations_can_only_be_used_in_TypeScript));
-                        return;
                     case SyntaxKind.ClassDeclaration:
                         let classDeclaration = <ClassDeclaration>node;
-                        if (checkModifiers(classDeclaration.modifiers)) {
+                        if (checkModifiers(classDeclaration.modifiers) ||
+                            checkTypeParameters(classDeclaration.typeParameters)) {
                             return;
                         }
                         break;
@@ -2456,6 +2454,7 @@ module ts {
                     case SyntaxKind.FunctionDeclaration:
                         let functionDeclaration = <FunctionLikeDeclaration>node;
                         if (checkModifiers(functionDeclaration.modifiers) ||
+                            checkTypeParameters(functionDeclaration.typeParameters) ||
                             checkTypeAnnotation(functionDeclaration.type)) {
                             return;
                         }
@@ -2476,7 +2475,8 @@ module ts {
                     case SyntaxKind.NewExpression:
                         let expression = <CallExpression>node;
                         if (expression.typeArguments && expression.typeArguments.length > 0) {
-                            diagnostics.push(createFileDiagnostic(sourceFile, expression.typeArguments.pos, expression.typeArguments.end,
+                            let start = expression.typeArguments.pos;
+                            diagnostics.push(createFileDiagnostic(sourceFile, start, expression.typeArguments.end - start,
                                 Diagnostics.type_arguments_can_only_be_used_in_TypeScript));
                             return;
                         }
@@ -2484,7 +2484,8 @@ module ts {
                     case SyntaxKind.Parameter:
                         let parameter = <ParameterDeclaration>node;
                         if (parameter.modifiers) {
-                            diagnostics.push(createFileDiagnostic(sourceFile, parameter.modifiers.pos, parameter.modifiers.end,
+                            let start = parameter.modifiers.pos;
+                            diagnostics.push(createFileDiagnostic(sourceFile, start, parameter.modifiers.end - start,
                                 Diagnostics.parameter_modifiers_can_only_be_used_in_TypeScript));
                             return;
                         }
@@ -2511,6 +2512,15 @@ module ts {
                 forEachChild(node, walk);
             }
 
+            function checkTypeParameters(typeParameters: NodeArray<TypeParameterDeclaration>): boolean {
+                if (typeParameters) {
+                    let start = typeParameters.pos;
+                    diagnostics.push(createFileDiagnostic(sourceFile, start, typeParameters.end - start, Diagnostics.type_parameter_declarations_can_only_be_used_in_TypeScript));
+                    return true;
+                }
+                return false;
+            }
+
             function checkTypeAnnotation(type: TypeNode): boolean {
                 if (type) {
                     diagnostics.push(createDiagnosticForNode(type, Diagnostics.types_can_only_be_used_in_TypeScript));
diff --git a/src/services/shims.ts b/src/services/shims.ts
index fc47ff95280..f2082ee39cb 100644
--- a/src/services/shims.ts
+++ b/src/services/shims.ts
@@ -325,6 +325,22 @@ module ts {
         }
     }
 
+    /* @internal */
+    export function realizeDiagnostics(diagnostics: Diagnostic[], newLine: string): { message: string; start: number; length: number; category: string; } []{
+        return diagnostics.map(d => realizeDiagnostic(d, newLine));
+    }
+
+    function realizeDiagnostic(diagnostic: Diagnostic, newLine: string): { message: string; start: number; length: number; category: string; } {
+        return {
+            message: flattenDiagnosticMessageText(diagnostic.messageText, newLine),
+            start: diagnostic.start,
+            length: diagnostic.length,
+            /// TODO: no need for the tolowerCase call
+            category: DiagnosticCategory[diagnostic.category].toLowerCase(),
+            code: diagnostic.code
+        };
+    }
+
     class LanguageServiceShimObject extends ShimBase implements LanguageServiceShim {
         private logger: Logger;
 
@@ -385,18 +401,7 @@ module ts {
 
         private realizeDiagnostics(diagnostics: Diagnostic[]): { message: string; start: number; length: number; category: string; }[]{
             var newLine = this.getNewLine();
-            return diagnostics.map(d => this.realizeDiagnostic(d, newLine));
-        }
-
-        private realizeDiagnostic(diagnostic: Diagnostic, newLine: string): { message: string; start: number; length: number; category: string; } {
-            return {
-                message: flattenDiagnosticMessageText(diagnostic.messageText, newLine),
-                start: diagnostic.start,
-                length: diagnostic.length,
-                /// TODO: no need for the tolowerCase call
-                category: DiagnosticCategory[diagnostic.category].toLowerCase(),
-                code: diagnostic.code
-            };
+            return ts.realizeDiagnostics(diagnostics, newLine);
         }
 
         public getSyntacticClassifications(fileName: string, start: number, length: number): string {
diff --git a/tests/cases/fourslash/fourslash.ts b/tests/cases/fourslash/fourslash.ts
index 087c43786f0..8ade18fc200 100644
--- a/tests/cases/fourslash/fourslash.ts
+++ b/tests/cases/fourslash/fourslash.ts
@@ -439,6 +439,10 @@ module FourSlashInterface {
             displayParts: ts.SymbolDisplayPart[], documentation: ts.SymbolDisplayPart[]) {
             FourSlash.currentTestState.verifyQuickInfoDisplayParts(kind, kindModifiers, textSpan, displayParts, documentation);
         }
+
+        public getSemanticDiagnostics(expected: string) {
+            FourSlash.currentTestState.getSemanticDiagnostics(expected);
+        }
     }
 
     export class edit {
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics1.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics1.ts
new file mode 100644
index 00000000000..e73b487347a
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics1.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// import a = b;
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'import ... =' can only be used in TypeScript.",
+    "start": 0,
+    "length": 13,
+    "category": "error",
+    "code": 8002
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics2.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics2.ts
new file mode 100644
index 00000000000..e239577e5e4
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics2.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// export = b;
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'export=' can only be used in TypeScript.",
+    "start": 0,
+    "length": 11,
+    "category": "error",
+    "code": 8003
+  }
+]`);
\ No newline at end of file
diff --git a/tests/cases/fourslash/getJavaScriptSemanticDiagnostics3.ts b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics3.ts
new file mode 100644
index 00000000000..d74e97b36cc
--- /dev/null
+++ b/tests/cases/fourslash/getJavaScriptSemanticDiagnostics3.ts
@@ -0,0 +1,15 @@
+/// <reference path="fourslash.ts" />
+
+// @allowNonTsExtensions: true
+// @Filename: a.tsjs
+//// class C<T>;
+
+verify.getSemanticDiagnostics(`[
+  {
+    "message": "'type parameter declarations' can only be used in TypeScript.",
+    "start": 8,
+    "length": 1,
+    "category": "error",
+    "code": 8004
+  }
+]`);
\ No newline at end of file