diff --git a/packages/playwright-core/src/utils/fileUtils.ts b/packages/playwright-core/src/utils/fileUtils.ts
index 751878a869..0b2f28143b 100644
--- a/packages/playwright-core/src/utils/fileUtils.ts
+++ b/packages/playwright-core/src/utils/fileUtils.ts
@@ -52,3 +52,7 @@ export async function copyFileAndMakeWritable(from: string, to: string) {
 export function sanitizeForFilePath(s: string) {
   return s.replace(/[\x00-\x2C\x2E-\x2F\x3A-\x40\x5B-\x60\x7B-\x7F]+/g, '-');
 }
+
+export function toPosixPath(aPath: string): string {
+  return aPath.split(path.sep).join(path.posix.sep);
+}
diff --git a/packages/playwright/src/common/suiteUtils.ts b/packages/playwright/src/common/suiteUtils.ts
index 7994c10e04..6c3015772f 100644
--- a/packages/playwright/src/common/suiteUtils.ts
+++ b/packages/playwright/src/common/suiteUtils.ts
@@ -15,7 +15,7 @@
 */
 
 import path from 'path';
-import { calculateSha1 } from 'playwright-core/lib/utils';
+import { calculateSha1, toPosixPath } from 'playwright-core/lib/utils';
 import type { Suite, TestCase } from './test';
 import type { FullProjectInternal } from './config';
 import type { Matcher, TestFileFilter } from '../util';
@@ -39,9 +39,10 @@ export function filterTestsRemoveEmptySuites(suite: Suite, filter: (test: TestCa
   suite._entries = suite._entries.filter(e => entries.has(e)); // Preserve the order.
   return !!suite._entries.length;
 }
+
 export function bindFileSuiteToProject(project: FullProjectInternal, suite: Suite): Suite {
-  const relativeFile = path.relative(project.project.testDir, suite.location!.file).split(path.sep).join('/');
-  const fileId = calculateSha1(relativeFile).slice(0, 20);
+  const relativeFile = path.relative(project.project.testDir, suite.location!.file);
+  const fileId = calculateSha1(toPosixPath(relativeFile)).slice(0, 20);
 
   // Clone suite.
   const result = suite._deepClone();
@@ -51,7 +52,8 @@ export function bindFileSuiteToProject(project: FullProjectInternal, suite: Suit
   result.forEachTest((test, suite) => {
     suite._fileId = fileId;
     // At the point of the query, suite is not yet attached to the project, so we only get file, describe and test titles.
-    const testIdExpression = `[project=${project.id}]${test.titlePath().join('\x1e')}`;
+    const [file, ...titles] = test.titlePath();
+    const testIdExpression = `[project=${project.id}]${toPosixPath(file)}\x1e${titles.join('\x1e')}`;
     const testId = fileId + '-' + calculateSha1(testIdExpression).slice(0, 20);
     test.id = testId;
     test._projectId = project.id;
diff --git a/packages/playwright/src/reporters/html.ts b/packages/playwright/src/reporters/html.ts
index dbebacc176..74bf628932 100644
--- a/packages/playwright/src/reporters/html.ts
+++ b/packages/playwright/src/reporters/html.ts
@@ -20,11 +20,10 @@ import fs from 'fs';
 import path from 'path';
 import type { TransformCallback } from 'stream';
 import { Transform } from 'stream';
-import { toPosixPath } from './json';
 import { codeFrameColumns } from '../transform/babelBundle';
 import type { FullResult, FullConfig, Location, Suite, TestCase as TestCasePublic, TestResult as TestResultPublic, TestStep as TestStepPublic, TestError } from '../../types/testReporter';
 import type { SuitePrivate } from '../../types/reporterPrivate';
-import { HttpServer, assert, calculateSha1, copyFileAndMakeWritable, gracefullyProcessExitDoNotHang, removeFolders, sanitizeForFilePath } from 'playwright-core/lib/utils';
+import { HttpServer, assert, calculateSha1, copyFileAndMakeWritable, gracefullyProcessExitDoNotHang, removeFolders, sanitizeForFilePath, toPosixPath } from 'playwright-core/lib/utils';
 import { colors, formatError, formatResultFailure, stripAnsiEscapes } from './base';
 import { resolveReporterOutputPath } from '../util';
 import type { Metadata } from '../../types/test';
diff --git a/packages/playwright/src/reporters/json.ts b/packages/playwright/src/reporters/json.ts
index d677079ab5..41c26b653c 100644
--- a/packages/playwright/src/reporters/json.ts
+++ b/packages/playwright/src/reporters/json.ts
@@ -18,15 +18,10 @@ import fs from 'fs';
 import path from 'path';
 import type { FullConfig, TestCase, Suite, TestResult, TestError, TestStep, FullResult, Location, JSONReport, JSONReportSuite, JSONReportSpec, JSONReportTest, JSONReportTestResult, JSONReportTestStep, JSONReportError } from '../../types/testReporter';
 import { formatError, prepareErrorStack } from './base';
-import { MultiMap } from 'playwright-core/lib/utils';
-import { assert } from 'playwright-core/lib/utils';
+import { MultiMap, assert, toPosixPath } from 'playwright-core/lib/utils';
 import { getProjectId } from '../common/config';
 import EmptyReporter from './empty';
 
-export function toPosixPath(aPath: string): string {
-  return aPath.split(path.sep).join(path.posix.sep);
-}
-
 class JSONReporter extends EmptyReporter {
   config!: FullConfig;
   suite!: Suite;
diff --git a/tests/playwright-test/reporter.spec.ts b/tests/playwright-test/reporter.spec.ts
index 4157310dd9..653728fd78 100644
--- a/tests/playwright-test/reporter.spec.ts
+++ b/tests/playwright-test/reporter.spec.ts
@@ -797,4 +797,30 @@ var import_test = __toModule(require("@playwright/test"));
   4 |         `);
     });
   });
-}
\ No newline at end of file
+}
+
+test('should report a stable test.id', async ({ runInlineTest }) => {
+  const result = await runInlineTest({
+    'reporter.ts': `
+      class Reporter {
+        onTestBegin(test) {
+          console.log('\\n%%testbegin-' + test.id);
+        }
+      }
+      export default Reporter;
+    `,
+    'playwright.config.ts': `
+      module.exports = { reporter: [[ './reporter.ts' ]] };
+    `,
+    'a.test.ts': `
+      import { test, expect } from '@playwright/test';
+      test('example test', async ({}) => {
+      });
+    `
+  }, { reporter: '', workers: 1 });
+
+  expect(result.exitCode).toBe(0);
+  expect(result.outputLines).toEqual([
+    'testbegin-20289bcdad95a5e18c38-8b63c3695b9c8bd62d98',
+  ]);
+});