diff --git a/test/webkit/utilities.test.ts b/test/webkit/utilities.test.ts
index 48789dd..45d6a65 100644
--- a/test/webkit/utilities.test.ts
+++ b/test/webkit/utilities.test.ts
@@ -2,6 +2,7 @@
  * Copyright (C) Microsoft Corporation. All rights reserved.
  *--------------------------------------------------------*/
 
+import * as sinon from 'sinon';
 import * as mockery from 'mockery';
 import * as assert from 'assert';
 
@@ -14,9 +15,9 @@ suite('Utilities', () => {
         mockery.enable({ useCleanCache: true, warnOnReplace: false });
         mockery.registerMock('fs', { statSync: () => { } });
         mockery.registerMock('os', { platform: () => 'win32' });
-        mockery.registerMock('url', { });
-        mockery.registerMock('path', { });
-        mockery.registerAllowable(MODULE_UNDER_TEST);
+
+        mockery.registerAllowables([
+            'url', 'path', MODULE_UNDER_TEST]);
     });
 
     teardown(() => {
@@ -178,4 +179,82 @@ suite('Utilities', () => {
                 });
         });
     });
+
+    suite('webkitUrlToClientUrl()', () => {
+        const TEST_CLIENT_PATH = 'c:/site/scripts/a.js';
+        const TEST_WEBKIT_LOCAL_URL = 'file:///' + TEST_CLIENT_PATH;
+        const TEST_WEBKIT_HTTP_URL = 'http://site.com/page/scripts/a.js';
+        const TEST_CWD = 'c:/site';
+
+        function Utilities(): typeof _Utilities {
+            return require(MODULE_UNDER_TEST);
+        }
+
+        test('file:/// urls are returned canonicalized', () => {
+            assert.equal(Utilities().webkitUrlToClientUrl('', TEST_WEBKIT_LOCAL_URL), TEST_CLIENT_PATH);
+        });
+
+        test('an empty string is returned for a missing url', () => {
+            assert.equal(Utilities().webkitUrlToClientUrl('', ''), '');
+        });
+
+        test('an empty string is returned when the cwd is missing', () => {
+            assert.equal(Utilities().webkitUrlToClientUrl(null, TEST_WEBKIT_HTTP_URL), '');
+        });
+
+        test('a url without a path returns an empty string', () => {
+            assert.equal(Utilities().webkitUrlToClientUrl(TEST_CWD, 'http://site.com'), '');
+        });
+
+        test('it searches the disk for a path that exists, built from the url', () => {
+            const statSync = (path: string) => {
+                if (path !== TEST_CLIENT_PATH) throw new Error('Not found');
+            };
+            mockery.registerMock('fs', { statSync });
+            assert.equal(Utilities().webkitUrlToClientUrl(TEST_CWD, TEST_WEBKIT_HTTP_URL), TEST_CLIENT_PATH);
+        });
+
+        test(`returns an empty string when it can't resolve a url`, () => {
+            const statSync = (path: string) => {
+                throw new Error('Not found');
+            };
+            mockery.registerMock('fs', { statSync });
+            const Utilities = require(MODULE_UNDER_TEST);
+            assert.equal(Utilities.webkitUrlToClientUrl(TEST_CWD, TEST_WEBKIT_HTTP_URL), '');
+        });
+    });
+
+    suite('canonicalizeUrl()', () => {
+        function testCanUrl(inUrl: string, expectedUrl: string): void {
+            const Utilities: typeof _Utilities = require(MODULE_UNDER_TEST);
+            assert.equal(Utilities.canonicalizeUrl(inUrl), expectedUrl);
+        }
+
+        test('removes file:///', () => {
+            testCanUrl('file:///c:/file.js', 'c:/file.js');
+        });
+
+        test('enforces forward slash', () => {
+            testCanUrl('c:\\thing\\file.js', 'c:/thing/file.js');
+        });
+
+        test('removes file:///', () => {
+            testCanUrl('file:///c:/file.js', 'c:/file.js');
+        });
+
+        test('ensures local path starts with / on OSX', () => {
+            mockery.registerMock('os', { platform: () => 'darwin' });
+            testCanUrl('file:///Users/scripts/app.js', '/Users/scripts/app.js');
+        });
+
+        test('force lowercase drive letter on Win to match VS Code', () => {
+            // note default 'os' mock is win32
+            testCanUrl('file:///D:/FILE.js', 'd:/FILE.js');
+        });
+
+        test('http:// url - no change', () => {
+            const url = 'http://site.com/My/Cool/Site/script.js?stuff';
+            testCanUrl(url, url);
+        })
+    });
 });
diff --git a/webkit/utilities.ts b/webkit/utilities.ts
index b2112c6..2a6834e 100644
--- a/webkit/utilities.ts
+++ b/webkit/utilities.ts
@@ -160,7 +160,9 @@ export class Logger {
 }
 
 /**
- * http://localhost/app/scripts/code.js => d:/scripts/code.js
+ * Maps a url from webkit to an absolute local path.
+ * If not given an absolute path (with file: prefix), searches the current working directory for a matching file. 
+ * http://localhost/scripts/code.js => d:/app/scripts/code.js
  * file:///d:/scripts/code.js => d:/scripts/code.js
  */
 export function webkitUrlToClientUrl(cwd: string, url: string): string {
@@ -181,15 +183,16 @@ export function webkitUrlToClientUrl(cwd: string, url: string): string {
 
     // Search the filesystem under our cwd for the file that best matches the given url
     const pathName = nodeUrl.parse(canonicalizeUrl(url)).pathname;
-    if (!pathName) {
+    if (!pathName || pathName === '/') {
         return '';
     }
 
     const pathParts = pathName.split('/');
     while (pathParts.length > 0) {
         const clientUrl = path.join(cwd, pathParts.join('/'));
-        if (existsSync(clientUrl)) {
-            return canonicalizeUrl(clientUrl); // path.join will change / to \
+        const canClientUrl = canonicalizeUrl(clientUrl); // path.join will change / to \
+        if (existsSync(canClientUrl)) {
+            return canonicalizeUrl(canClientUrl);
         }
 
         pathParts.shift();
@@ -199,18 +202,21 @@ export function webkitUrlToClientUrl(cwd: string, url: string): string {
 }
 
 /**
- * Modify a url either from the ODP client or the webkit target to a canonical version for comparing.
- * The ODP client can handle urls in this format too.
- * file:///d:\\scripts\\code.js => d:/scripts/code.js
+ * Modify a url either from the client or the webkit target to a platform-independent format for comparing.
+ * The client can handle urls in this format too.
+ * file:///D:\\scripts\\code.js => d:/scripts/code.js
  * file:///Users/me/project/code.js => /Users/me/project/code.js
+ * c:\scripts\code.js => c:/scripts/code.js 
+ * http://site.com/scripts/code.js => (no change) 
  */
 export function canonicalizeUrl(url: string): string {
     url = url
         .replace('file:///', '')
         .replace(/\\/g, '/'); // \ to /
 
-    // Ensure osx path starts with /, it can be removed when file:/// was stripped
-    if (url[0] !== '/' && getPlatform() === Platform.OSX) {
+    // Ensure osx path starts with /, it can be removed when file:/// was stripped.
+    // Don't add if the url still has a protocol
+    if (url[0] !== '/' && url.indexOf(':') < 0 && getPlatform() === Platform.OSX) {
         url = '/' + url;
     }