feat: slop-diff shows only NEW findings introduced on this branch

Runs slop-scan on HEAD and the merge-base, diffs results with
line-number-insensitive fingerprinting so shifted code doesn't create
false positives. Uses git worktree for clean base comparison. Shows
net new vs removed findings. Runs automatically after bun test.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -20,7 +20,8 @@ bun run dev:skill    # watch mode: auto-regen + validate on change
 bun run eval:list    # list all eval runs from ~/.gstack-dev/evals/
 bun run eval:compare # compare two eval runs (auto-picks most recent)
 bun run eval:summary # aggregate stats across all eval runs
-bun run slop          # slop-scan diagnostic (not a gate, just a number)
+bun run slop          # full slop-scan report (all files)
+bun run slop:diff     # slop findings in files changed on this branch only
 ```
 
 `test:evals` requires `ANTHROPIC_API_KEY`. Codex E2E tests (`test/codex-e2e.test.ts`)

package.json

@@ -13,7 +13,7 @@
     "gen:skill-docs": "bun run scripts/gen-skill-docs.ts",
     "dev": "bun run browse/src/cli.ts",
     "server": "bun run browse/src/server.ts",
-    "test": "bun test browse/test/ test/ --ignore 'test/skill-e2e-*.test.ts' --ignore test/skill-llm-eval.test.ts --ignore test/skill-routing-e2e.test.ts --ignore test/codex-e2e.test.ts --ignore test/gemini-e2e.test.ts && echo '── slop-scan (diagnostic) ──' && bun run slop 2>/dev/null | tail -8 || true",
+    "test": "bun test browse/test/ test/ --ignore 'test/skill-e2e-*.test.ts' --ignore test/skill-llm-eval.test.ts --ignore test/skill-routing-e2e.test.ts --ignore test/codex-e2e.test.ts --ignore test/gemini-e2e.test.ts && bun run slop:diff 2>/dev/null || true",
     "test:evals": "EVALS=1 bun test --retry 2 --concurrent --max-concurrency ${EVALS_CONCURRENCY:-15} test/skill-llm-eval.test.ts test/skill-e2e-*.test.ts test/skill-routing-e2e.test.ts test/codex-e2e.test.ts test/gemini-e2e.test.ts",
     "test:evals:all": "EVALS=1 EVALS_ALL=1 bun test --retry 2 --concurrent --max-concurrency ${EVALS_CONCURRENCY:-15} test/skill-llm-eval.test.ts test/skill-e2e-*.test.ts test/skill-routing-e2e.test.ts test/codex-e2e.test.ts test/gemini-e2e.test.ts",
     "test:e2e": "EVALS=1 bun test --retry 2 --concurrent --max-concurrency ${EVALS_CONCURRENCY:-15} test/skill-e2e-*.test.ts test/skill-routing-e2e.test.ts test/codex-e2e.test.ts test/gemini-e2e.test.ts",
@@ -34,7 +34,8 @@
     "eval:select": "bun run scripts/eval-select.ts",
     "analytics": "bun run scripts/analytics.ts",
     "test:audit": "bun test test/audit-compliance.test.ts",
-    "slop": "npx slop-scan scan . 2>/dev/null || echo 'slop-scan not available (install with: npm i -g slop-scan)'"
+    "slop": "npx slop-scan scan . 2>/dev/null || echo 'slop-scan not available (install with: npm i -g slop-scan)'",
+    "slop:diff": "bun run scripts/slop-diff.ts"
   },
   "dependencies": {
     "@ngrok/ngrok": "^1.7.0",

scripts/slop-diff.ts

@@ -0,0 +1,170 @@
+#!/usr/bin/env bun
+/**
+ * slop-diff: show NEW slop-scan findings introduced on this branch.
+ *
+ * Runs slop-scan on HEAD and on the merge-base, then diffs the results
+ * to show only findings that were added. Line-number-insensitive comparison
+ * so shifting code doesn't create false positives.
+ *
+ * Usage:
+ *   bun run slop:diff              # diff against main
+ *   bun run slop:diff origin/release  # diff against another base
+ */
+
+import { spawnSync } from 'child_process';
+import * as fs from 'fs';
+import * as os from 'os';
+import * as path from 'path';
+
+const base = process.argv[2] || 'main';
+
+// 1. Find changed files
+const diffResult = spawnSync('git', ['diff', '--name-only', `${base}...HEAD`], {
+  encoding: 'utf-8', timeout: 10000,
+});
+const changedFiles = new Set(
+  (diffResult.stdout || '').trim().split('\n').filter(Boolean)
+);
+if (changedFiles.size === 0) {
+  console.log('No files changed vs', base, '— nothing to check.');
+  process.exit(0);
+}
+
+// 2. Run slop-scan on HEAD
+const scanHead = spawnSync('npx', ['slop-scan', 'scan', '.', '--json'], {
+  encoding: 'utf-8', timeout: 120000, shell: true,
+});
+if (!scanHead.stdout) {
+  console.log('slop-scan not available. Install: npm i -g slop-scan');
+  process.exit(0);
+}
+let headReport: any;
+try { headReport = JSON.parse(scanHead.stdout); } catch {
+  console.log('slop-scan returned invalid JSON.'); process.exit(0);
+}
+
+// 3. Get base branch findings using git stash approach
+//    Check out base versions of changed files, scan, then restore
+const mergeBase = spawnSync('git', ['merge-base', base, 'HEAD'], {
+  encoding: 'utf-8', timeout: 5000,
+}).stdout?.trim();
+
+// Fingerprint: strip line numbers so shifting code doesn't create false positives
+// "line 142: empty catch, boundary=none" -> "empty catch, boundary=none"
+function stripLineNum(evidence: string): string {
+  return evidence.replace(/^line \d+: /, '').replace(/ at line \d+ /, ' ');
+}
+
+// Count evidence items per (rule, file, stripped-evidence) for the base
+const baseCounts = new Map<string, number>();
+
+if (mergeBase) {
+  // Create temp worktree for base scan
+  const tmpWorktree = path.join(os.tmpdir(), `slop-base-${Date.now()}`);
+  const wtResult = spawnSync('git', ['worktree', 'add', '--detach', tmpWorktree, mergeBase], {
+    encoding: 'utf-8', timeout: 30000,
+  });
+
+  if (wtResult.status === 0) {
+    // Copy slop-scan config if it exists
+    const configFile = 'slop-scan.config.json';
+    if (fs.existsSync(configFile)) {
+      try { fs.copyFileSync(configFile, path.join(tmpWorktree, configFile)); } catch {}
+    }
+
+    const scanBase = spawnSync('npx', ['slop-scan', 'scan', tmpWorktree, '--json'], {
+      encoding: 'utf-8', timeout: 120000, shell: true,
+    });
+
+    if (scanBase.stdout) {
+      try {
+        const baseReport = JSON.parse(scanBase.stdout);
+        for (const f of baseReport.findings) {
+          // Remap worktree paths back to repo-relative
+          const realPath = f.path.replace(tmpWorktree + '/', '');
+          if (!changedFiles.has(realPath)) continue;
+          for (const ev of f.evidence || []) {
+            const key = `${f.ruleId}|${realPath}|${stripLineNum(ev)}`;
+            baseCounts.set(key, (baseCounts.get(key) || 0) + 1);
+          }
+        }
+      } catch {}
+    }
+
+    // Clean up worktree
+    spawnSync('git', ['worktree', 'remove', '--force', tmpWorktree], {
+      timeout: 10000,
+    });
+  }
+}
+
+// 4. Find genuinely new findings
+//    For each evidence item on HEAD, check if the base had the same (rule, file, stripped-evidence).
+//    Use counts to handle duplicates: if base had 2 and HEAD has 3, that's 1 new.
+const headCounts = new Map<string, { count: number; evidence: string[] }>();
+const headFindings = headReport.findings.filter((f: any) => changedFiles.has(f.path));
+
+for (const f of headFindings) {
+  for (const ev of f.evidence || []) {
+    const key = `${f.ruleId}|${f.path}|${stripLineNum(ev)}`;
+    const entry = headCounts.get(key) || { count: 0, evidence: [] };
+    entry.count++;
+    entry.evidence.push(ev);
+    headCounts.set(key, entry);
+  }
+}
+
+// Compute net new
+type NewFinding = { ruleId: string; filePath: string; evidence: string };
+const newFindings: NewFinding[] = [];
+let removedCount = 0;
+
+for (const [key, entry] of headCounts) {
+  const baseCount = baseCounts.get(key) || 0;
+  const netNew = entry.count - baseCount;
+  if (netNew > 0) {
+    const [ruleId, filePath] = key.split('|');
+    // Take the last N evidence items as the "new" ones
+    for (const ev of entry.evidence.slice(-netNew)) {
+      newFindings.push({ ruleId, filePath, evidence: ev });
+    }
+  }
+}
+
+for (const [key, baseCount] of baseCounts) {
+  const headCount = headCounts.get(key)?.count || 0;
+  if (headCount < baseCount) removedCount += baseCount - headCount;
+}
+
+// 5. Print results
+if (newFindings.length === 0) {
+  if (removedCount > 0) {
+    console.log(`\n  slop-scan: no new findings. Removed ${removedCount} pre-existing findings.\n`);
+  } else {
+    console.log(`\n  slop-scan: no new findings in ${changedFiles.size} changed files.\n`);
+  }
+  process.exit(0);
+}
+
+console.log(`\n── slop-scan: ${newFindings.length} new findings (+${newFindings.length} / -${removedCount}) ──\n`);
+
+// Group by file, then by rule
+const grouped = new Map<string, Map<string, string[]>>();
+for (const { ruleId, filePath, evidence } of newFindings) {
+  if (!grouped.has(filePath)) grouped.set(filePath, new Map());
+  const rules = grouped.get(filePath)!;
+  if (!rules.has(ruleId)) rules.set(ruleId, []);
+  rules.get(ruleId)!.push(evidence);
+}
+
+for (const [filePath, rules] of grouped) {
+  console.log(`  ${filePath}`);
+  for (const [ruleId, evidence] of rules) {
+    console.log(`    ${ruleId}:`);
+    for (const ev of evidence) {
+      console.log(`      ${ev}`);
+    }
+  }
+}
+
+console.log(`\n  Net: +${newFindings.length} new, -${removedCount} removed\n`);