diff --git a/CLAUDE.md b/CLAUDE.md
index c9fbe7c..3f2e03d 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -95,8 +95,9 @@ Two conventions hold across the real file/stdout handoffs:
   HTML by `split().join()` on comment markers (`/*__GRAPH_DATA__*/`, `/*__LCP_DATA__*/`,
   `/*__RUN_DATA__*/`, `/*__FORCE_GRAPH_LIB__*/`) in `assets/*.template.html`, deliberately not regex
   (the 1.3 MB vendored `assets/3d-force-graph.min.js` could match one). Renaming a marker without
-  updating its builder fails loudly — keep it that way. Only `build_run_report.mjs` escapes
-  `</script>` in injected data; the other two inject raw JSON/text into a `<script>` block.
+  updating its builder fails loudly — keep it that way. All three replace every `<` in the
+  injected data with `\u003c` (blocks `</script` and `<!--<script` breakout); `check_invariants.mjs`
+  builds each one from a hostile payload to keep it that way.
 
 ## Cross-file invariants
 
diff --git a/skills/browsergnome/scripts/build_lcp_map.mjs b/skills/browsergnome/scripts/build_lcp_map.mjs
index 184248e..60c0ba2 100644
--- a/skills/browsergnome/scripts/build_lcp_map.mjs
+++ b/skills/browsergnome/scripts/build_lcp_map.mjs
@@ -152,9 +152,11 @@ async function main() {
     process.exit(1);
   }
 
+  // Data lands inside a <script> block: a raw `<` in any value (bundle module/chunk names, URLs) can form `</script`
+  // or `<!--<script` and end or derail it. `\u003c` parses back to the same character.
   const html = template
     .split(LIB_MARKER).join(lib)
-    .split(DATA_MARKER).join(dataJson);
+    .split(DATA_MARKER).join(dataJson.replace(/</g, '\\u003c'));
 
   fs.writeFileSync(outFile, html);
   const outAbs = path.resolve(outFile);
diff --git a/skills/browsergnome/scripts/build_perf_map.mjs b/skills/browsergnome/scripts/build_perf_map.mjs
index 86204a8..0b3b31d 100644
--- a/skills/browsergnome/scripts/build_perf_map.mjs
+++ b/skills/browsergnome/scripts/build_perf_map.mjs
@@ -65,9 +65,12 @@ function main() {
 
   // Literal split/join (not regex) so the 1.3MB minified lib can't be
   // misinterpreted as a replacement pattern.
+  //
+  // Data lands inside a <script> block: a raw `<` in any value (scanned file paths) can form `</script`
+  // or `<!--<script` and end or derail it. `\u003c` parses back to the same character.
   const html = template
     .split(LIB_MARKER).join(lib)
-    .split(DATA_MARKER).join(graphRaw.trim());
+    .split(DATA_MARKER).join(graphRaw.trim().replace(/</g, '\\u003c'));
 
   fs.writeFileSync(outFile, html);
   const outAbs = path.resolve(outFile);
diff --git a/skills/browsergnome/scripts/build_run_report.mjs b/skills/browsergnome/scripts/build_run_report.mjs
index 7dc8586..e8fcc3f 100644
--- a/skills/browsergnome/scripts/build_run_report.mjs
+++ b/skills/browsergnome/scripts/build_run_report.mjs
@@ -51,8 +51,9 @@ function main() {
     process.exit(1);
   }
 
-  // Escape </script> so it can't close the script block if it appears in data values
-  const safeData = rawData.replace(/<\/script>/gi, '<\\/script>');
+  // Data lands inside a <script> block: a raw `<` in any value (agent-written run state) can form `</script`
+  // or `<!--<script` and end or derail it. `\u003c` parses back to the same character.
+  const safeData = rawData.replace(/</g, '\\u003c');
   const [before, after] = template.split(DATA_MARKER);
   const out = before + safeData + after;
 
diff --git a/skills/browsergnome/scripts/check_invariants.mjs b/skills/browsergnome/scripts/check_invariants.mjs
index 8e3fe29..a900405 100644
--- a/skills/browsergnome/scripts/check_invariants.mjs
+++ b/skills/browsergnome/scripts/check_invariants.mjs
@@ -21,9 +21,11 @@
  */
 
 import fs from 'node:fs';
+import os from 'node:os';
 import path from 'node:path';
 import process from 'node:process';
 import { fileURLToPath } from 'node:url';
+import { spawnSync } from 'node:child_process';
 import { checkVersionPin, checkProviderPin, checkThirdPartyFlag } from './doctor.mjs';
 
 const repoRoot = path.resolve(path.dirname(fileURLToPath(import.meta.url)), '../../..');
@@ -166,6 +168,26 @@ function main() {
   report('Perf Map calibration SHA (pages.yml <-> perf-map.md)', calibration.ok,
     calibration.ok ? calibration.workflowSha.slice(0, 8) : `workflow=${calibration.workflowSha ?? '?'} docs=${calibration.docsSha ?? '?'}`);
 
+  // 5. HTML builders escape every `<` in the data they inject into a <script> block
+  const HOSTILE = '</script><!--<script>';
+  const ESCAPED = HOSTILE.replace(/</g, '\\u003c');
+  const tmp = fs.mkdtempSync(path.join(os.tmpdir(), 'bgn-escape-'));
+  const builders = [
+    ['build_perf_map.mjs', { nodes: [{ id: HOSTILE, label: HOSTILE }], links: [] }],
+    ['build_lcp_map.mjs', { url: HOSTILE, nodes: [{ class: 'module', label: HOSTILE, ms: 1, confidence: 'measured' }] }],
+    ['build_run_report.mjs', { target: HOSTILE, iterations: [] }],
+  ];
+  for (const [script, data] of builders) {
+    const inFile = path.join(tmp, 'in.json');
+    const outFile = path.join(tmp, 'out.html');
+    fs.writeFileSync(inFile, JSON.stringify(data));
+    // cwd = tmp: no package.json/.bgn there, so build_lcp_map resolves no prod URL and stays offline
+    const run = spawnSync(process.execPath, [path.join(repoRoot, 'skills/browsergnome/scripts', script), inFile, '--out', outFile], { cwd: tmp, encoding: 'utf8' });
+    const ok = run.status === 0 && fs.readFileSync(outFile, 'utf8').includes(ESCAPED);
+    report(`${script} escapes \`<\` in injected data`, ok, ok ? undefined : run.stderr.trim() || 'hostile payload not escaped');
+  }
+  fs.rmSync(tmp, { recursive: true, force: true });
+
   console.log(`\n  ${pass} passed, ${fail} failed`);
   process.exit(fail ? 1 : 0);
 }