diff --git a/docs/index.md b/docs/index.md
index 5d527b60e7..ab00c3f098 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -947,6 +947,12 @@ Enforce a rule that tests may not be exclusive (use of e.g., `describe.only()` o
 
 `--forbid-only` causes Mocha to fail when an exclusive ("only'd") test or suite is encountered, and it will abort further test execution.
 
+Defaults:
+
+1. Before v12: false
+2. After v12: false, unless an environment variable called `CI` is set. Many popular
+   CI providers, like github actions or gitlab, do this automatically.
+
 ### `--forbid-pending`
 
 Enforce a rule that tests may not be skipped (use of e.g., `describe.skip()`, `it.skip()`, or `this.skip()` anywhere is disallowed).
diff --git a/lib/cli/run.js b/lib/cli/run.js
index 561592b916..5984b561f1 100644
--- a/lib/cli/run.js
+++ b/lib/cli/run.js
@@ -26,6 +26,7 @@ const {ONE_AND_DONES, ONE_AND_DONE_ARGS} = require('./one-and-dones');
 const debug = require('debug')('mocha:cli:run');
 const defaults = require('../mocharc.json');
 const {types, aliases} = require('./run-option-metadata');
+const {isCI} = require('../utils');
 
 /**
  * Logical option groups
@@ -123,7 +124,8 @@ exports.builder = yargs =>
       },
       'forbid-only': {
         description: 'Fail if exclusive test(s) encountered',
-        group: GROUPS.RULES
+        group: GROUPS.RULES,
+        default: isCI()
       },
       'forbid-pending': {
         description: 'Fail if pending test(s) encountered',
diff --git a/lib/errors.js b/lib/errors.js
index 79786c24c1..bcf842fa51 100644
--- a/lib/errors.js
+++ b/lib/errors.js
@@ -9,6 +9,7 @@
 
 const {format} = require('node:util');
 const { constants } = require('./error-constants.js');
+const { isCI } = require('./utils');
 
 /**
  * Contains error codes, factory functions to create throwable error objects,
@@ -329,11 +330,17 @@ function createMultipleDoneError(runnable, originalErr) {
  * @returns {Error} Error with code {@link constants.FORBIDDEN_EXCLUSIVITY}
  */
 function createForbiddenExclusivityError(mocha) {
-  var err = new Error(
-    mocha.isWorker
-      ? '`.only` is not supported in parallel mode'
-      : '`.only` forbidden by --forbid-only'
-  );
+  var message;
+  if (mocha.isWorker) {
+    message = '`.only` is not supported in parallel mode';
+  } else {
+    message = '`.only` forbidden by --forbid-only';
+    if (isCI()) {
+      message += ' (default in CI, add `--no-forbid-only` to allow `.only`)';
+    }
+  }
+
+  var err = new Error(message);
   err.code = constants.FORBIDDEN_EXCLUSIVITY;
   return err;
 }
diff --git a/lib/utils.js b/lib/utils.js
index 4971b8de46..eaaa6afe89 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -696,3 +696,17 @@ exports.breakCircularDeps = inputObj => {
 exports.isNumeric = input => {
   return !isNaN(parseFloat(input));
 };
+
+/**
+ * Checks if being ran in a CI environment.
+ *
+ * This uses the CI env variable, which is set by most popular CI providers. Some
+ * examples include:
+ * Github:     https://docs.github.com/en/actions/reference/workflows-and-actions/variables
+ * Gitlab:     https://docs.gitlab.com/ci/variables/predefined_variables/
+ * CircleCI:   https://circleci.com/docs/reference/variables/#built-in-environment-variables
+ * Bitbucket:  https://support.atlassian.com/bitbucket-cloud/docs/variables-and-secrets/
+ */
+exports.isCI = () => {
+  return !!process.env.CI;
+};
diff --git a/package.json b/package.json
index 819f7d5040..22a757c847 100644
--- a/package.json
+++ b/package.json
@@ -71,7 +71,7 @@
     "test-browser": "run-s clean build test-browser:*",
     "test-coverage-clean": "rimraf .nyc_output coverage",
     "test-coverage-generate": "nyc report --reporter=lcov --reporter=text",
-    "test-node-run-only": "nyc --no-clean --reporter=json node bin/mocha.js",
+    "test-node-run-only": "nyc --no-clean --reporter=json node bin/mocha.js --no-forbid-only",
     "test-node-run": "nyc --no-clean --reporter=json node bin/mocha.js --forbid-only",
     "test-node:integration": "run-s clean build && npm run -s test-node-run -- --parallel --timeout 10000 --slow 3750 \"test/integration/**/*.spec.js\"",
     "test-node:integration:watch": "run-s clean build && npm run -s test-node-run -- --parallel --timeout 10000 --slow 3750 \"test/integration/options/watch.spec.js\"",
diff --git a/test/integration/common-js-require.spec.js b/test/integration/common-js-require.spec.js
index 43a59d6532..5418b31c7f 100644
--- a/test/integration/common-js-require.spec.js
+++ b/test/integration/common-js-require.spec.js
@@ -5,7 +5,8 @@ const {runMochaAsync} = require('./helpers');
 describe('common js require', () => {
   it('should be able to run a test where all mocha exports are used', async () => {
     const result = await runMochaAsync('common-js-require.fixture.js', [
-      '--delay'
+      '--delay',
+      '--no-forbid-only'
     ]);
     expect(result.output, 'to contain', 'running before');
     expect(result.output, 'to contain', 'running suiteSetup');
diff --git a/test/integration/only.spec.js b/test/integration/only.spec.js
index f7a317a054..65d5a05af2 100644
--- a/test/integration/only.spec.js
+++ b/test/integration/only.spec.js
@@ -6,7 +6,7 @@ var assert = require('node:assert');
 describe('.only()', function () {
   describe('bdd', function () {
     it('should run only tests that marked as `only`', function (done) {
-      run('options/only/bdd.fixture.js', ['--ui', 'bdd'], function (err, res) {
+      run('options/only/bdd.fixture.js', ['--ui', 'bdd', '--no-forbid-only'], function (err, res) {
         if (err) {
           done(err);
           return;
@@ -22,7 +22,7 @@ describe('.only()', function () {
 
   describe('tdd', function () {
     it('should run only tests that marked as `only`', function (done) {
-      run('options/only/tdd.fixture.js', ['--ui', 'tdd'], function (err, res) {
+      run('options/only/tdd.fixture.js', ['--ui', 'tdd', '--no-forbid-only'], function (err, res) {
         if (err) {
           done(err);
           return;
@@ -40,7 +40,7 @@ describe('.only()', function () {
     it('should run only tests that marked as `only`', function (done) {
       run(
         'options/only/qunit.fixture.js',
-        ['--ui', 'qunit'],
+        ['--ui', 'qunit', '--no-forbid-only'],
         function (err, res) {
           if (err) {
             done(err);
diff --git a/test/integration/options/delay.spec.js b/test/integration/options/delay.spec.js
index c1da3afa02..0ee0c53f4e 100644
--- a/test/integration/options/delay.spec.js
+++ b/test/integration/options/delay.spec.js
@@ -5,11 +5,7 @@ var helpers = require('../helpers');
 var runMochaJSON = helpers.runMochaJSON;
 
 describe('--delay', function () {
-  var args = [];
-
-  before(function () {
-    args = ['--delay'];
-  });
+  var args = ['--delay', '--no-forbid-only'];
 
   it('should run the generated test suite', function (done) {
     var fixture = path.join('options', 'delay');
diff --git a/test/integration/options/dryRun.spec.js b/test/integration/options/dryRun.spec.js
index 759cc0bc27..25a216ad29 100644
--- a/test/integration/options/dryRun.spec.js
+++ b/test/integration/options/dryRun.spec.js
@@ -5,7 +5,7 @@ var helpers = require('../helpers');
 var runMochaJSON = helpers.runMochaJSON;
 
 describe('--dry-run', function () {
-  var args = ['--dry-run'];
+  var args = ['--dry-run', '--no-forbid-only'];
 
   it('should only report, but not execute any test', function (done) {
     var fixture = path.join('options/dry-run', 'dry-run');
diff --git a/test/integration/regression.spec.js b/test/integration/regression.spec.js
index f6ae4c1c33..2dad065184 100644
--- a/test/integration/regression.spec.js
+++ b/test/integration/regression.spec.js
@@ -51,7 +51,7 @@ describe('regressions', function () {
   });
 
   it('issue-2406: should run nested describe.only suites', function (done) {
-    runJSON('regression/issue-2406.fixture.js', [], function (err, res) {
+    runJSON('regression/issue-2406.fixture.js', ['--no-forbid-only'], function (err, res) {
       if (err) {
         done(err);
         return;
@@ -64,7 +64,7 @@ describe('regressions', function () {
   });
 
   it('issue-2417: should not recurse infinitely with .only suites nested within each other', function (done) {
-    runJSON('regression/issue-2417.fixture.js', [], function (err, res) {
+    runJSON('regression/issue-2417.fixture.js', ['--no-forbid-only'], function (err, res) {
       if (err) {
         done(err);
         return;