# Adding a new feature flag in chrome://flags This document describes how to add your new feature behind a flag. See also [Configuration](configuration.md), which gives more explanation about flags and other options for configuring Chrome. ## Step 1: Adding a new `base::Feature` This step would be different between where you want to use the flag. For example, if you want to use the flag in src/content, you should add a base::Feature to the following files: * [content/public/common/content_features.cc](https://cs.chromium.org/chromium/src/content/public/common/content_features.cc) * [content/public/common/content_features.h](https://cs.chromium.org/chromium/src/content/public/common/content_features.h) If you want to use the flag in blink, you should also read [Runtime Enable Features](https://chromium.googlesource.com/chromium/src/+/main/third_party/blink/renderer/platform/RuntimeEnabledFeatures.md). You can refer to [this CL](https://chromium-review.googlesource.com/c/554510/) and [this document](initialize_blink_features.md) to see 1. where to add the base::Feature [[1](https://chromium-review.googlesource.com/c/554510/8/content/public/common/content_features.cc#253)] [[2](https://chromium-review.googlesource.com/c/554510/8/content/public/common/content_features.h)] 2. how to use it [[1](https://chromium-review.googlesource.com/c/554510/8/content/common/service_worker/service_worker_utils.cc#153)] 3. how to wire your new base::Feature to a blink runtime feature[[1](https://chromium.googlesource.com/chromium/src/+/main/docs/initialize_blink_features.md)] 4. how to use it in blink [[1](https://chromium-review.googlesource.com/c/554510/8/third_party/blnk/renderere/core/workers/worker_thread.cc)] Also, this patch added a virtual test for running web tests with the flag. When you add a flag, you can consider to use that. ## Step 2: Adding the feature flag to the chrome://flags UI. *** promo **Tip:** Android WebView has its own flag UI. The WebView team recommends adding your features there too if they are supported on WebView. Follow [these steps](/android_webview/docs/developer-ui.md#Adding-your-flags-and-features-to-the-UI) for WebView flags. *** You have to modify these five files in total. * [chrome/browser/about_flags.cc](https://cs.chromium.org/chromium/src/chrome/browser/about_flags.cc) * [chrome/browser/flag_descriptions.cc](https://cs.chromium.org/chromium/src/chrome/browser/flag_descriptions.cc) * [chrome/browser/flag_descriptions.h](https://cs.chromium.org/chromium/src/chrome/browser/flag_descriptions.h) * [tools/metrics/histograms/enums.xml](https://cs.chromium.org/chromium/src/tools/metrics/histograms/enums.xml) * [chrome/browser/flag-metadata.json](https://cs.chromium.org/chromium/src/chrome/browser/flag-metadata.json) At first you need to add an entry to __about_flags.cc__, __flag_descriptions.cc__ and __flag_descriptions.h__. After that, try running the following test. ```bash # Build unit_tests autoninja -C out/Default unit_tests # Run AboutFlagsHistogramTest.CheckHistograms ./out/Default/unit_tests --gtest_filter=AboutFlagsHistogramTest.CheckHistograms # Run AboutFlagsHistogramTest.CheckHistograms on Android ./out/Default/bin/run_unit_tests --gtest_filter=AboutFlagsHistogramTest.CheckHistograms ``` That test will ask you to add several entries to enums.xml. After doing so, run `git cl format` which will insert the entries in enums.xml in the correct order and run the tests again. You can refer to [this CL](https://chromium-review.googlesource.com/c/593707) as an example. Finally, run the following test. ```bash ./out/Default/unit_tests --gtest_filter=AboutFlagsTest.EveryFlagHasMetadata ``` That test will ask you to update the flag expiry metadata in [flag-metadata.json](https://cs.chromium.org/chromium/src/chrome/browser/flag-metadata.json).