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Add feature flag

agents/prompts/eval/feature_flags_add/prompt.md

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Add feature flag

Context

Your task is to add a new base::Feature flag to the Chromium codebase. This involves defining the feature, and optionally exposing it in about:flags for manual testing.

You must read @//docs/how_to_add_your_feature_flag.md to understand the standard procedures, which will inform the files you need to modify and the tests you need to run.

Feature flags are component-specific. For example:

  • //content features: Defined in @//content/public/common/content_features.h and .cc.
  • Android //chrome features: Defined in @//chrome/browser/flags/android/chrome_feature_list.h, .cc, and @//chrome/browser/flags/android/java/src/org/chromium/chrome/browser/flags/ChromeFeatureList.java.

Requirement

  • This task requires changing multiple files. Before you perform any code modification, you MUST state your plan and ask for confirmation before editing the code.

  • You should try to keep the change set as minimal. Focus only on adding the new flag the user specified. Avoid changing code around the new lines.

Instruction

0. Understand the Standard Process First, read @//docs/how_to_add_your_feature_flag.md to load the official instructions into your context. This will help you identify the correct files and testing procedures.

1. Determine Flag Location If the user hasn't specified where the flag should live, analyze their request to infer the most logical component (e.g., content, blink, browser). Propose the file locations to the user for confirmation.

  • Plan Example: "Based on your request, I believe this is a //content feature. I will add the flag definition to @//content/public/common/content_features.h and .cc. Is this correct?"
  • If the location is unclear, search the codebase for existing *_features.cc files in relevant directories to find the established convention.

2. Add the Feature Flag Definition Once the location is confirmed, read the relevant C++ and/or Java files. Modify them to add the new feature flag.

  • Follow Patterns: Strictly adhere to existing code patterns, especially alphabetical ordering of flags.
  • Default State: Assume the feature is DISABLED_BY_DEFAULT unless the user specifies otherwise.
  • OS Specification: If a flag is only meant to be used for one platform but not the other, make sure it is wrapped with platform build flags (e.g. #if BUILDFLAG(IS_ANDROID), or #if BUILDFLAG(IS_WIN)). Ask the user if you are not sure.
  • Do not perform Android Caching: For Android flags in @//chrome/browser/flags/android/java/src/org/chromium/chrome/browser/flags/ChromeFeatureList.java, some of the feature flags are cached. YOU SHOULD NEVER add CachedFlag or MutableFlag for this task.

3. Expose in about:flags Most feature flags should be exposed in about:flags for testing. Propose this as the default next step.

  • Plan Example: "Next, I will add the flag to the about:flags page. If you do not want this, please let me know."
  • If the user objects, skip to Verifications.

4. Implement about:flags Entry If the user agrees, modify the necessary files to add the flag to the UI.

NOTE: The files that requires changes here are large. Follow the steps, and you should ALWAYS only read the files one at a time.

  1. flag_descriptions: Declare the user-visible name in @//chrome/browser/flag_descriptions.h and define the strings in //chrome/browser/flag_descriptions.cc.

  2. about_flags.cc: Append the new entry to the kFeatureEntries array in @//chrome/browser/about_flags.cc. You do not need to read the entire file; find the array and add the entry near the end.

  3. flag-metadata.json: Add a new entry to @//chrome/browser/flag-metadata.json. For the owners field, stop and ask the user for confirmation.

  4. Generate entries in enums.xml: Generate the entries in enums.xml. Please refer to @//docs/how_to_add_your_feature_flag.md for the testing procedures.

Verification

After completing the task, the final set of modified files should be consistent with the work you've done.

Example file set for an Android flag added to about:flags:

chrome/browser/about_flags.cc
chrome/browser/flag_descriptions.h
chrome/browser/flag_descriptions.cc
chrome/browser/flag-metadata.json
chrome/browser/flags/android/chrome_feature_list.h
chrome/browser/flags/android/chrome_feature_list.cc
chrome/browser/flags/android/java/src/org/chromium/chrome/browser/flags/ChromeFeatureList.java
tools/metrics/histograms/enums.xml