# Adding a new feature flag in chrome://flags

This document describes how to add a new Chrome feature flag visible to users
via chrome://flag UI.

*** note
**NOTE:** It's NOT required if you don't intend to make your feature appear in
chrome://flag UI.
***

See also the following for definitions:
*  [Configuration: Features](configuration.md#features)
*  [Configuration: Flags](configuration.md#flags)

## Step 1: Adding a new `base::Feature`

This step would be different depending on where you want to use the flag:

### To Use the Flag in `content/` Only

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)

### To Use the Flag in `third_party/blink/` (and Possibly in `content/`)

Add a `base::Feature` to the following files:

* [third_party/blink/common/features.cc](https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/common/features.cc)
* [third_party/blink/public/common/features.h](https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/public/common/features.h)

Historically, Blink also has its own runtime feature mechanism. So if you
feature needs to be runtime-enabled, read also Blink's
[Runtime Enable Features][blink-rte] doc and
[Initialization of Blink runtime features in content layer][blink-rte-init].

[blink-rte]: ../third_party/blink/renderer/platform/RuntimeEnabledFeatures.md

### Examples

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][blink-rte-init]]
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.

[blink-rte-init]: initialize_blink_features.md

## Step 2: Adding the feature flag to the chrome://flags UI.

*** promo
Googlers: Read also [Chrome Feature Flag in chrome://flags](http/go/finch-feature-api#chrome-feature-flag-in-chromeflags).
***

*** 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).

## Related Documents

* [Chromium Feature API & Finch (Googler-only)](http://go/finch-feature-api)
* [Configuration: Prefs, Settings, Features, Switches & Flags](configuration.md)
* [Runtime Enabled Features](../third_party/blink/renderer/platform/RuntimeEnabledFeatures.md)
* [Initialization of Blink runtime features in content layer](initialize_blink_features.md)