ci: post code coverage delta comment on PRs

.github/workflows/coverage-experiment.yml

@@ -0,0 +1,113 @@
+name: Coverage Delta
+
+on:
+  pull_request:
+    branches: [main]
+
+jobs:
+  compare-coverage:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/setup-node@v3
+        with:
+          node-version: 24
+
+      - name: Checkout Main
+        uses: actions/checkout@v4
+        with:
+          ref: main
+
+      - name: Install & Test (Main)
+        run: |
+          npm ci
+          npm run coverage
+          mv ./coverage/coverage-summary.json ./coverage-main-summary.json
+
+      - name: Checkout PR Branch
+        uses: actions/checkout@v4
+        with:
+          clean: true
+
+      - name: Install & Test (PR)
+        run: |
+          npm ci
+          npm run coverage
+
+      - name: Report Coverage Delta
+        uses: actions/github-script@v6
+        with:
+          script: |
+            const fs = require('fs');
+
+            const loadSummary = (path) => {
+              try {
+                return JSON.parse(fs.readFileSync(path, 'utf8'));
+              } catch (error) {
+                console.log(`Could not load ${path}: ${error.message}`);
+                return null;
+              }
+            };
+
+            const mainSummary = loadSummary('./coverage-main-summary.json');
+            const prSummary = loadSummary('./coverage/coverage-summary.json');
+
+            if (!mainSummary || !prSummary) {
+              core.setFailed('Could not load coverage summaries for comparison.');
+              return;
+            }
+
+            const metrics = ['lines', 'statements', 'functions', 'branches'];
+
+            const getPct = (summary, metric) => summary.total[metric].pct;
+
+            let markdown = `### 🧪 Code Coverage Delta\n\n`;
+            markdown += `| Metric | Main | PR | Delta |\n`;
+            markdown += `| :--- | :---: | :---: | :---: |\n`;
+
+            metrics.forEach(metric => {
+              const oldPct = getPct(mainSummary, metric);
+              const newPct = getPct(prSummary, metric);
+              const diff = (newPct - oldPct).toFixed(2);
+
+              let icon = '';
+              if (diff > 0) icon = '🟢';
+              else if (diff < 0) icon = '🔴';
+              else icon = '⚪️';
+
+              // Format with + sign for positive numbers
+              const diffStr = diff > 0 ? `+${diff}%` : `${diff}%`;
+
+              markdown += `| **${metric}** | ${oldPct}% | ${newPct}% | ${icon} ${diffStr} |\n`;
+            });
+
+            markdown += `\n_Generated by c8 and GitHub Actions_`;
+
+            const { owner, repo } = context.repo;
+            const issue_number = context.issue.number;
+
+            const comments = await github.rest.issues.listComments({
+              owner,
+              repo,
+              issue_number,
+            });
+
+            const botComment = comments.data.find(c => 
+              c.body.includes('### 🧪 Code Coverage Delta') && 
+              c.user.type === 'Bot'
+            );
+
+            if (botComment) {
+              await github.rest.issues.updateComment({
+                owner,
+                repo,
+                comment_id: botComment.id,
+                body: markdown
+              });
+            } else {
+              await github.rest.issues.createComment({
+                owner,
+                repo,
+                issue_number,
+                body: markdown
+              });
+            }

vitest.config.ts

@@ -7,7 +7,7 @@ export default defineConfig({
     include: ['test/**/*.spec.ts'],
     coverage: {
       provider: 'v8',
-      reporter: ['text', 'json', 'html'],
+      reporter: ['text', 'json', 'html', 'json-summary'],
     },
   },
 });