diff --git a/src/server/session.ts b/src/server/session.ts
index aae9ab8980f..7dccda3c82c 100644
--- a/src/server/session.ts
+++ b/src/server/session.ts
@@ -258,10 +258,6 @@ namespace ts.server {
         readonly symLinkedProjects: MultiMap<Project>;
     };
 
-    function isProjectsArray(projects: Projects): projects is ReadonlyArray<Project> {
-        return !!(<ReadonlyArray<Project>>projects).length;
-    }
-
     /**
      * This helper function processes a list of projects and return the concatenated, sortd and deduplicated output of processing each project.
      */
@@ -273,8 +269,8 @@ namespace ts.server {
         comparer?: (a: U, b: U) => number,
         areEqual?: (a: U, b: U) => boolean,
     ): U[] {
-        const outputs = flatMap(isProjectsArray(projects) ? projects : projects.projects, project => action(project, defaultValue));
-        if (!isProjectsArray(projects) && projects.symLinkedProjects) {
+        const outputs = flatMap(isArray(projects) ? projects : projects.projects, project => action(project, defaultValue));
+        if (!isArray(projects) && projects.symLinkedProjects) {
             projects.symLinkedProjects.forEach((projects, path) => {
                 const value = getValue(path as Path);
                 outputs.push(...flatMap(projects, project => action(project, value)));
@@ -370,7 +366,7 @@ namespace ts.server {
     }
 
     function forEachProjectInProjects(projects: Projects, path: string | undefined, cb: (project: Project, path: string | undefined) => void): void {
-        for (const project of isProjectsArray(projects) ? projects : projects.projects) {
+        for (const project of isArray(projects) ? projects : projects.projects) {
             cb(project, path);
         }
         if (!isArray(projects) && projects.symLinkedProjects) {
@@ -1106,7 +1102,7 @@ namespace ts.server {
             return project.getLanguageService().getRenameInfo(file, position);
         }
 
-        private getProjects(args: protocol.FileRequestArgs): Projects {
+        private getProjects(args: protocol.FileRequestArgs, getScriptInfoEnsuringProjectsUptoDate?: boolean, ignoreNoProjectError?: boolean): Projects {
             let projects: ReadonlyArray<Project> | undefined;
             let symLinkedProjects: MultiMap<Project> | undefined;
             if (args.projectFileName) {
@@ -1116,13 +1112,18 @@ namespace ts.server {
                 }
             }
             else {
-                const scriptInfo = this.projectService.getScriptInfo(args.file)!;
+                const scriptInfo = getScriptInfoEnsuringProjectsUptoDate ?
+                    this.projectService.getScriptInfoEnsuringProjectsUptoDate(args.file) :
+                    this.projectService.getScriptInfo(args.file);
+                if (!scriptInfo) {
+                    return ignoreNoProjectError ? emptyArray : Errors.ThrowNoProject();
+                }
                 projects = scriptInfo.containingProjects;
                 symLinkedProjects = this.projectService.getSymlinkedProjects(scriptInfo);
             }
             // filter handles case when 'projects' is undefined
             projects = filter(projects, p => p.languageServiceEnabled && !p.isOrphan());
-            if ((!projects || !projects.length) && !symLinkedProjects) {
+            if (!ignoreNoProjectError && (!projects || !projects.length) && !symLinkedProjects) {
                 return Errors.ThrowNoProject();
             }
             return symLinkedProjects ? { projects: projects!, symLinkedProjects } : projects!; // TODO: GH#18217
@@ -1465,18 +1466,16 @@ namespace ts.server {
         }
 
         private getCompileOnSaveAffectedFileList(args: protocol.FileRequestArgs): ReadonlyArray<protocol.CompileOnSaveAffectedFileListSingleProject> {
-            const info = this.projectService.getScriptInfoEnsuringProjectsUptoDate(args.file);
+            const projects = this.getProjects(args, /*getScriptInfoEnsuringProjectsUptoDate*/ true, /*ignoreNoProjectError*/ true);
+            const info = this.projectService.getScriptInfo(args.file);
             if (!info) {
                 return emptyArray;
             }
 
-            // if specified a project, we only return affected file list in this project
-            const projects = args.projectFileName ? [this.projectService.findProject(args.projectFileName)!] : info.containingProjects;
-            const symLinkedProjects = !args.projectFileName && this.projectService.getSymlinkedProjects(info);
             return combineProjectOutput(
                 info,
                 path => this.projectService.getScriptInfoForPath(path)!,
-                symLinkedProjects ? { projects, symLinkedProjects } : projects,
+                projects,
                 (project, info) => {
                     let result: protocol.CompileOnSaveAffectedFileListSingleProject | undefined;
                     if (project.compileOnSaveEnabled && project.languageServiceEnabled && !project.isOrphan() && !project.getCompilationSettings().noEmit) {
diff --git a/src/testRunner/unittests/tsserverProjectSystem.ts b/src/testRunner/unittests/tsserverProjectSystem.ts
index fe6cfd25712..6d39ecb76d1 100644
--- a/src/testRunner/unittests/tsserverProjectSystem.ts
+++ b/src/testRunner/unittests/tsserverProjectSystem.ts
@@ -3495,6 +3495,102 @@ namespace ts.projectSystem {
                 openFilesForSession([{ file, projectRootPath }], session);
             }
         });
+
+        describe("CompileOnSaveAffectedFileListRequest with and without projectFileName in request", () => {
+            const projectRoot = "/user/username/projects/myproject";
+            const core: File = {
+                path: `${projectRoot}/core/core.ts`,
+                content: "let z = 10;"
+            };
+            const app1: File = {
+                path: `${projectRoot}/app1/app.ts`,
+                content: "let x = 10;"
+            };
+            const app2: File = {
+                path: `${projectRoot}/app2/app.ts`,
+                content: "let y = 10;"
+            };
+            const app1Config: File = {
+                path: `${projectRoot}/app1/tsconfig.json`,
+                content: JSON.stringify({
+                    files: ["app.ts", "../core/core.ts"],
+                    compilerOptions: { outFile : "build/output.js" },
+                    compileOnSave: true
+                })
+            };
+            const app2Config: File = {
+                path: `${projectRoot}/app2/tsconfig.json`,
+                content: JSON.stringify({
+                    files: ["app.ts", "../core/core.ts"],
+                    compilerOptions: { outFile: "build/output.js" },
+                    compileOnSave: true
+                })
+            };
+            const files = [libFile, core, app1, app2, app1Config, app2Config];
+
+            function insertString(session: TestSession, file: File) {
+                session.executeCommandSeq<protocol.ChangeRequest>({
+                    command: protocol.CommandTypes.Change,
+                    arguments: {
+                        file: file.path,
+                        line: 1,
+                        offset: 1,
+                        endLine: 1,
+                        endOffset: 1,
+                        insertString: "let k = 1"
+                    }
+                });
+            }
+
+            function getSession() {
+                const host = createServerHost(files);
+                const session = createSession(host);
+                openFilesForSession([app1, app2, core], session);
+                const service = session.getProjectService();
+                checkNumberOfProjects(session.getProjectService(), { configuredProjects: 2 });
+                const project1 = service.configuredProjects.get(app1Config.path)!;
+                const project2 = service.configuredProjects.get(app2Config.path)!;
+                checkProjectActualFiles(project1, [libFile.path, app1.path, core.path, app1Config.path]);
+                checkProjectActualFiles(project2, [libFile.path, app2.path, core.path, app2Config.path]);
+                insertString(session, app1);
+                insertString(session, app2);
+                assert.equal(project1.dirty, true);
+                assert.equal(project2.dirty, true);
+                return session;
+            }
+
+            it("when projectFile is specified", () => {
+                const session = getSession();
+                const response = session.executeCommandSeq<protocol.CompileOnSaveAffectedFileListRequest>({
+                    command: protocol.CommandTypes.CompileOnSaveAffectedFileList,
+                    arguments: {
+                        file: core.path,
+                        projectFileName: app1Config.path
+                    }
+                }).response;
+                assert.deepEqual(response, [
+                    { projectFileName: app1Config.path, fileNames: [core.path, app1.path], projectUsesOutFile: true }
+                ]);
+                assert.equal(session.getProjectService().configuredProjects.get(app1Config.path)!.dirty, false);
+                assert.equal(session.getProjectService().configuredProjects.get(app2Config.path)!.dirty, true);
+            });
+
+            it("when projectFile is not specified", () => {
+                const session = getSession();
+                const response = session.executeCommandSeq<protocol.CompileOnSaveAffectedFileListRequest>({
+                    command: protocol.CommandTypes.CompileOnSaveAffectedFileList,
+                    arguments: {
+                        file: core.path
+                    }
+                }).response;
+                assert.deepEqual(response, [
+                    { projectFileName: app1Config.path, fileNames: [core.path, app1.path], projectUsesOutFile: true },
+                    { projectFileName: app2Config.path, fileNames: [core.path, app2.path], projectUsesOutFile: true }
+                ]);
+                assert.equal(session.getProjectService().configuredProjects.get(app1Config.path)!.dirty, false);
+                assert.equal(session.getProjectService().configuredProjects.get(app2Config.path)!.dirty, false);
+            });
+        });
     });
 
     describe("tsserverProjectSystem Proper errors", () => {