typescript-eslint · JoshuaKGoldberg · Oct 3, 2025 · Sep 26, 2025 · Sep 26, 2025 · Sep 30, 2025
diff --git a/packages/typescript-estree/src/useProgramFromProjectService.ts b/packages/typescript-estree/src/useProgramFromProjectService.ts
@@ -96,8 +96,24 @@ function openClientFileFromProjectService(
     }
 
     if (!isDefaultProjectAllowed) {
+      const baseMessage = `${wasNotFound}. Consider either including it in the tsconfig.json or including it in allowDefaultProject.`;
+      const allowDefaultProject =
+        parseSettings.projectService?.allowDefaultProject;
+
+      if (!allowDefaultProject) {
+        throw new Error(baseMessage);
+      }
+
+      const relativeFilePath = path.relative(
+        parseSettings.tsconfigRootDir,
+        filePathAbsolute,
+      );
+
       throw new Error(
-        `${wasNotFound}. Consider either including it in the tsconfig.json or including it in allowDefaultProject.`,
+        [
+          baseMessage,
+          `allowDefaultProject is set to ${JSON.stringify(allowDefaultProject)}, which does not match '${relativeFilePath}'.`,
+        ].join('\n'),
       );
     }
   }

diff --git a/packages/typescript-estree/tests/lib/useProgramFromProjectService.test.ts b/packages/typescript-estree/tests/lib/useProgramFromProjectService.test.ts
@@ -167,7 +167,7 @@ describe(useProgramFromProjectService, () => {
     expect(service.reloadProjects).not.toHaveBeenCalled();
   });
 
-  it('throws an error after reloading projects when hasFullTypeInformation is enabled, the file is neither in the project service nor allowDefaultProject, and the last reload was recent', () => {
+  it('throws a non-file-specific error after reloading projects when hasFullTypeInformation is enabled, the file is neither in the project service nor an empty allowDefaultProject, and the last reload was recent', () => {
     const { service } = createMockProjectService();
 
     service.openClientFile.mockReturnValueOnce({}).mockReturnValueOnce({});
@@ -189,6 +189,40 @@ describe(useProgramFromProjectService, () => {
     expect(service.reloadProjects).toHaveBeenCalledOnce();
   });
 
+  it('throws a file-specific error after reloading projects when hasFullTypeInformation is enabled, the file is neither in the project service nor a populated allowDefaultProject, and the last reload was recent', () => {
+    const { service } = createMockProjectService();
+    const allowDefaultProject = ['a.js', 'b.js'];
+
+    service.openClientFile.mockReturnValueOnce({}).mockReturnValueOnce({});
+
+    expect(() =>
+      useProgramFromProjectService(
+        createProjectServiceSettings({
+          allowDefaultProject,
+          lastReloadTimestamp: 0,
+          service,
+        }),
+        {
+          ...mockParseSettings,
+          projectService: {
+            allowDefaultProject,
+            lastReloadTimestamp: 0,
+            maximumDefaultProjectFileMatchCount: 8,
+            service,
+          },
+        },
+        true,
+        new Set(),
+      ),
+    ).toThrow(
+      [
+        `${mockParseSettings.filePath} was not found by the project service. Consider either including it in the tsconfig.json or including it in allowDefaultProject.`,
+        `allowDefaultProject is set to ["a.js","b.js"], which does not match '${path.normalize('path/PascalCaseDirectory/camelCaseFile.ts')}'.`,
+      ].join('\n'),
+    );
+    expect(service.reloadProjects).toHaveBeenCalledOnce();
+  });
+
   it('returns a created program after reloading projects when hasFullTypeInformation is enabled, the file is only in the project service after reload, and the last reload was recent', () => {
     const { service } = createMockProjectService();
     const program = { getSourceFile: vi.fn() };