Evaluate AI CSS Completion

Evaluate CSS Completion Skill

This skill allows you to temporarily expose an evaluation hook in DevTools to measure CSS code completion trigger rates using a Puppeteer script.

[!WARNING] NEVER commit the devtools_app.ts patch. It is only for local evaluation. Always revert it before uploading your CL.

Step 1: Apply the Temporary Patch

Modify front_end/entrypoints/devtools_app/devtools_app.ts to expose the global testCssCompletion hook.

Add the following code at the end of front_end/entrypoints/devtools_app/devtools_app.ts:

// --- TEMPORARY EVALUATION HOOK ---
// TEMPORARY PATCH - REMOVE BEFORE COMMIT

import * as Host from '../../core/host/host.js';
import * as AiCodeCompletion from '../../models/ai_code_completion/ai_code_completion.js';

(self as any).testCss = {
  getCases() {
    return [
      {
        name: 'Generic CSS Test Case',
        url: null,
        prefix: 'h1 { font-s',
        suffix: ' }',
      },
      // Add more test cases here
    ];
  },

  async evaluate(uiSourceCodeUrl: string | null, prefix: string, suffix: string, additionalFiles?: any[]) {
    const aidaClient = new Host.AidaClient.AidaClient();
    const completion = new AiCodeCompletion.AiCodeCompletion.AiCodeCompletion(
      {aidaClient},
      AiCodeCompletion.AiCodeCompletion.ContextFlavor.STYLES
    );

    const formattedAdditionalFiles = additionalFiles?.map(f => ({
      path: f.path || f.name,
      content: f.content || f.text,
      included_reason: f.included_reason ?? Host.AidaClient.Reason.RELATED_FILE,
    }));

    const result = await completion.completeCode(
      prefix,
      suffix,
      prefix.length,
      Host.AidaClient.AidaInferenceLanguage.CSS,
      formattedAdditionalFiles
    );

    return {
      hasSuggestion: result.response !== null && result.response.generatedSamples.length > 0,
      suggestions: result.response?.generatedSamples.map(s => s.generationString) ?? [],
      injectedFiles: formattedAdditionalFiles?.map(f => f.path) ?? [],
    };
  }
};
// ----------------------------------

Step 2: Launch Chrome with Local DevTools

Use the npm start script to build DevTools, launch Chrome Canary with remote debugging, and disable watch mode (so we build manually instead).

You must specify a persistent user data directory so you can log in once and reuse the session:

npm start -- --no-watch --browser=canary --remote-debugging-port=9222 --user-data-dir=/tmp/devtools-ai-evaluate-css-completion

[!IMPORTANT] Instructions for the Agent: After launching Chrome, you MUST print the following checklist to the user and wait for their explicit confirmation before running the evaluation script:

1. Sign in to Chrome with your corporate account in the new window.
2. Open DevTools, go to Settings (gear icon) > AI Innovations, and ensure Code Completions is enabled.
3. Focus the DevTools window and press Cmd + Option + I (on macOS) or Ctrl + Shift + I (on Windows/Linux) to open DevTools-on-DevTools (inspecting DevTools itself). Go to the Network tab of the second DevTools instance and check the Disable cache checkbox. This ensures your local builds are loaded instead of cached versions when reloaded.
4. Ask the user to reply when they are ready.

Step 3: Configure Test Cases

Before running the evaluation, configure your test cases directly in front_end/entrypoints/devtools_app/devtools_app.ts inside the getCases() method of the patch you applied in Step 1.

After modifying the test cases, manually run a build to compile the changes:

autoninja -C out/Default

For example, to test spacing sensitivity, update the returned array:

  getCases() {
    return [
      {
        name: 'With CSS Context',
        url: null,
        prefix: 'h1 { font-s',
        suffix: ' }',
        additionalFiles: [
          {
            path: 'other.css',
            content: 'body { color: red; }',
          }
        ]
      }
    ];
  }

Step 4: Run the Evaluation Script

Run the Puppeteer script to execute the evaluation:

node .agents/skills/evaluate-ai-css-completion/scripts/evaluate.js

The script will connect to the DevTools instance, trigger completions, and output the results.

Debugging and Diagnostics

If you get 0% trigger rate or unexpected errors, check the following:

1. Browser Logs: The evaluation script forwards browser console logs (prefixed with BROWSER LOG:). Look for these in your terminal.
2. Host Config: The script prints the DevTools Host Config at the start. Verify that "devToolsAiCodeCompletionStyles": { "enabled": true } is present.
3. GCA/AIDA Logs: The script automatically enables verbose AIDA logging. Look for GCA request succeeded: in the logs:
   • If the response has "usageMetadata": { "candidatesTokenCount": X } (where X > 0) but the candidates array is missing or empty, the backend generated tokens but they were filtered out (e.g., due to safety classifiers or recitation blocks).
4. Context Requirement: The CSS model is sensitive to context. A minimal prompt like h1 { font-s might not trigger AIDA. Ensure your test cases have enough context in the prefix (e.g., several preceding CSS rules) or pass them via additionalFiles to simulate a larger stylesheet.

Step 5: Revert the Patch

Once the evaluation is complete and you have recorded the results, revert the patch in devtools_app.ts:

git checkout front_end/entrypoints/devtools_app/devtools_app.ts