diff --git a/src/testRunner/tsconfig.json b/src/testRunner/tsconfig.json
index 137e4d8af58..4e5c088025c 100644
--- a/src/testRunner/tsconfig.json
+++ b/src/testRunner/tsconfig.json
@@ -96,6 +96,7 @@
         "unittests/tscWatch/resolutionCache.ts",
         "unittests/tscWatch/watchEnvironment.ts",
         "unittests/tscWatch/watchApi.ts",
+        "unittests/tsserver/applyChangesToOpenFiles.ts",
         "unittests/tsserver/cachingFileSystemInformation.ts",
         "unittests/tsserver/cancellationToken.ts",
         "unittests/tsserver/compileOnSave.ts",
diff --git a/src/testRunner/unittests/tsserver/applyChangesToOpenFiles.ts b/src/testRunner/unittests/tsserver/applyChangesToOpenFiles.ts
new file mode 100644
index 00000000000..f964e77e0eb
--- /dev/null
+++ b/src/testRunner/unittests/tsserver/applyChangesToOpenFiles.ts
@@ -0,0 +1,108 @@
+namespace ts.projectSystem {
+    describe("unittests:: tsserver:: applyChangesToOpenFiles", () => {
+        const configFile: File = {
+            path: "/a/b/tsconfig.json",
+            content: "{}"
+        };
+        const file3: File = {
+            path: "/a/b/file3.ts",
+            content: "let xyz = 1;"
+        };
+        const app: File = {
+            path: "/a/b/app.ts",
+            content: "let z = 1;"
+        };
+
+        function fileContentWithComment(file: File) {
+            return `// some copy right notice
+${file.content}`;
+        }
+
+        function verifyText(service: server.ProjectService, file: string, expected: string) {
+            const info = service.getScriptInfo(file)!;
+            const snap = info.getSnapshot();
+            // Verified applied in reverse order
+            assert.equal(snap.getText(0, snap.getLength()), expected, `Text of changed file: ${file}`);
+        }
+
+        function verifyProjectVersion(project: server.Project, expected: number) {
+            assert.equal(Number(project.getProjectVersion()), expected);
+        }
+
+        function verify(applyChangesToOpen: (session: TestSession) => void) {
+            const host = createServerHost([app, file3, commonFile1, commonFile2, libFile, configFile]);
+            const session = projectSystem.createSession(host);
+            session.executeCommandSeq<protocol.OpenRequest>({
+                command: protocol.CommandTypes.Open,
+                arguments: { file: app.path }
+            });
+            const service = session.getProjectService();
+            const project = service.configuredProjects.get(configFile.path)!;
+            assert.isDefined(project);
+            verifyProjectVersion(project, 1);
+            session.executeCommandSeq<protocol.OpenRequest>({
+                command: protocol.CommandTypes.Open,
+                arguments: {
+                    file: file3.path,
+                    fileContent: fileContentWithComment(file3)
+                }
+            });
+            verifyProjectVersion(project, 2);
+
+            // Verify Texts
+            verifyText(service, commonFile1.path, commonFile1.content);
+            verifyText(service, commonFile2.path, commonFile2.content);
+            verifyText(service, app.path, app.content);
+            verifyText(service, file3.path, fileContentWithComment(file3));
+
+            // Apply changes
+            applyChangesToOpen(session);
+
+            // Verify again
+            verifyProjectVersion(project, 5);
+            // Open file contents
+            verifyText(service, commonFile1.path, fileContentWithComment(commonFile1));
+            verifyText(service, commonFile2.path, fileContentWithComment(commonFile2));
+            verifyText(service, app.path, "let zzz = 10;let zz = 10;let z = 1;");
+            verifyText(service, file3.path, file3.content);
+        }
+
+        it("with applyChangedToOpenFiles request", () => {
+            verify(session =>
+                session.executeCommandSeq<protocol.ApplyChangedToOpenFilesRequest>({
+                    command: protocol.CommandTypes.ApplyChangedToOpenFiles,
+                    arguments: {
+                        openFiles: [
+                            {
+                                fileName: commonFile1.path,
+                                content: fileContentWithComment(commonFile1)
+                            },
+                            {
+                                fileName: commonFile2.path,
+                                content: fileContentWithComment(commonFile2)
+                            }
+                        ],
+                        changedFiles: [
+                            {
+                                fileName: app.path,
+                                changes: [
+                                    {
+                                        span: { start: 0, length: 0 },
+                                        newText: "let zzz = 10;"
+                                    },
+                                    {
+                                        span: { start: 0, length: 0 },
+                                        newText: "let zz = 10;"
+                                    }
+                                ]
+                            }
+                        ],
+                        closedFiles: [
+                            file3.path
+                        ]
+                    }
+                })
+            );
+        });
+    });
+}