diff --git a/SECURITY.md b/SECURITY.md
index bda5689f..160e5314 100644
--- a/SECURITY.md
+++ b/SECURITY.md
@@ -92,6 +92,22 @@ single-worker deployment (see the Deployment Note below).
 | `AUTH_IP_WINDOW_MINUTES` | `15` | Rolling window for counting failures. |
 | `AUTH_IP_BLOCK_MINUTES` | `15` | How long a blocked IP stays blocked. |
 
+## Frontend HTML Sinks (XSS)
+
+Raw-HTML sinks (`dangerouslySetInnerHTML`, `innerHTML =`, `insertAdjacentHTML`,
+`new Function`, `eval`) must sit behind a sanitizer/escaper, and template output
+on the backend (`|safe`, `Markup(`, `render_template_string`) is avoided in favor
+of Jinja's default autoescaping. Today every sink is safe by construction:
+extension icons pass through `sanitizeSvgInner`, assistant markdown through the
+escape-then-allowlist `renderMarkdownToHtml`, and the SQL/file syntax tinting
+escapes input before inserting its own token spans.
+
+To keep the sweep swept, each sink must reference an allowlisted sanitizer **or**
+carry a `sink-safe: <sanitizer> — <why>` comment. `scripts/check-html-sinks.mjs`
+(run in `npm run lint`, and mirrored by `backend/tests/test_html_sink_sweep.py`)
+fails CI on any new unannotated sink, so a raw-HTML injection point can't land
+silently.
+
 ## Deployment Note
 
 The agent gateway keeps all connected-agent state in-memory in a single process.
diff --git a/backend/tests/test_html_sink_sweep.py b/backend/tests/test_html_sink_sweep.py
new file mode 100644
index 00000000..88f16377
--- /dev/null
+++ b/backend/tests/test_html_sink_sweep.py
@@ -0,0 +1,61 @@
+"""HTML-sink sweep guard — proving test (plan 55 Phase 1, D4).
+
+Mirrors scripts/check-html-sinks.mjs in Python so the sweep stays swept inside
+the backend suite (the authoritative gate) even where node isn't available: every
+raw-HTML sink in frontend/src or backend/app must reference an allowlisted
+sanitizer/escaper or carry a `sink-safe:` annotation. A new unannotated sink
+fails this test.
+"""
+import os
+import re
+
+_REPO_ROOT = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
+
+FRONTEND_SINK = re.compile(
+    r'dangerouslySetInnerHTML|\.innerHTML\s*=[^=]|insertAdjacentHTML'
+    r'|new\s+Function\s*\(|\beval\s*\(')
+BACKEND_SINK = re.compile(r'\|\s*safe\b|\bMarkup\s*\(|render_template_string')
+ALLOWLISTED_SANITIZERS = re.compile(r'sanitizeSvgInner|renderMarkdownToHtml|highlightLine|hlSql')
+ANNOTATION = re.compile(r'sink-safe:')
+_COMMENT_PREFIXES = ('//', '*', '/*', '#')
+
+
+def _walk(root, exts):
+    for dirpath, dirnames, filenames in os.walk(root):
+        dirnames[:] = [d for d in dirnames
+                       if d not in ('node_modules', 'dist', '__pycache__')]
+        for name in filenames:
+            if name.endswith(exts):
+                yield os.path.join(dirpath, name)
+
+
+def _scan(path, sink_re, allow_sanitizer):
+    with open(path, 'r', encoding='utf-8', errors='replace') as f:
+        lines = f.read().split('\n')
+    violations = []
+    for i, line in enumerate(lines):
+        if not sink_re.search(line):
+            continue
+        if line.strip().startswith(_COMMENT_PREFIXES):
+            continue  # a comment that merely mentions a sink keyword
+        window = '\n'.join(lines[max(0, i - 3):i + 1])
+        cleared = (allow_sanitizer and ALLOWLISTED_SANITIZERS.search(line)) \
+            or ANNOTATION.search(window)
+        if not cleared:
+            violations.append(f'{os.path.relpath(path, _REPO_ROOT)}:{i + 1}  {line.strip()}')
+    return violations
+
+
+def test_no_unannotated_html_sinks():
+    violations = []
+    fe = os.path.join(_REPO_ROOT, 'frontend', 'src')
+    for path in _walk(fe, ('.js', '.jsx', '.ts', '.tsx')):
+        violations += _scan(path, FRONTEND_SINK, allow_sanitizer=True)
+    be = os.path.join(_REPO_ROOT, 'backend', 'app')
+    for path in _walk(be, ('.py', '.html')):
+        violations += _scan(path, BACKEND_SINK, allow_sanitizer=False)
+
+    assert not violations, (
+        'Unannotated raw-HTML sink(s) — each must reference a sanitizer/escaper '
+        'or carry a `sink-safe:` comment (see scripts/check-html-sinks.mjs):\n'
+        + '\n'.join(violations))
diff --git a/frontend/package.json b/frontend/package.json
index ef11b398..6f32a060 100644
--- a/frontend/package.json
+++ b/frontend/package.json
@@ -6,8 +6,9 @@
     "scripts": {
         "dev": "vite",
         "build": "vite build",
-        "lint": "eslint . && node scripts/check-settings-index.mjs",
+        "lint": "eslint . && node scripts/check-settings-index.mjs && node ../scripts/check-html-sinks.mjs",
         "lint:settings-index": "node scripts/check-settings-index.mjs",
+        "lint:html-sinks": "node ../scripts/check-html-sinks.mjs",
         "capture:skeletons": "node scripts/capture-skeletons.mjs",
         "preview": "vite preview"
     },
diff --git a/frontend/src/components/ai/Markdown.jsx b/frontend/src/components/ai/Markdown.jsx
index 726d2862..125aeedd 100644
--- a/frontend/src/components/ai/Markdown.jsx
+++ b/frontend/src/components/ai/Markdown.jsx
@@ -3,6 +3,7 @@ import { renderMarkdownToHtml } from '../../lib/ai/markdown';
 
 // Renders assistant text. The renderer escapes all input before injecting its
 // own allowlisted tags, so dangerouslySetInnerHTML is safe here.
+// sink-safe: renderMarkdownToHtml (lib/ai/markdown.js) — escapes then allowlists.
 const Markdown = ({ text }) => {
     const html = useMemo(() => renderMarkdownToHtml(text || ''), [text]);
     return <div className="sk-ai-markdown" dangerouslySetInnerHTML={{ __html: html }} />;
diff --git a/frontend/src/components/databases/SqlEditor.jsx b/frontend/src/components/databases/SqlEditor.jsx
index 094e63bc..4505487f 100644
--- a/frontend/src/components/databases/SqlEditor.jsx
+++ b/frontend/src/components/databases/SqlEditor.jsx
@@ -46,6 +46,7 @@ const SqlEditor = forwardRef(function SqlEditor({ value, onChange, onKeyDown, pl
                     className="dbx-sqled__hl"
                     ref={preRef}
                     aria-hidden="true"
+                    /* sink-safe: hlSql() esc()-escapes each line before tinting */
                     dangerouslySetInnerHTML={{ __html: html }}
                 />
                 <textarea
diff --git a/frontend/src/pages/Marketplace.jsx b/frontend/src/pages/Marketplace.jsx
index bc656edd..c708419b 100644
--- a/frontend/src/pages/Marketplace.jsx
+++ b/frontend/src/pages/Marketplace.jsx
@@ -586,6 +586,7 @@ const ExtensionCover = ({ entry, category, brandSize = 34 }) => {
                 aria-hidden="true"
                 focusable="false"
                 className="extension-card__glyph"
+                /* sink-safe: iconSvg = sanitizeSvgInner(entry.icon) above */
                 dangerouslySetInnerHTML={{ __html: iconSvg }}
             />
         );
diff --git a/scripts/check-html-sinks.mjs b/scripts/check-html-sinks.mjs
new file mode 100644
index 00000000..09a680b5
--- /dev/null
+++ b/scripts/check-html-sinks.mjs
@@ -0,0 +1,112 @@
+/**
+ * HTML-sink guard (plan 55 Phase 1, Decision D4).
+ *
+ * Keeps the XSS-sink sweep swept: every raw-HTML sink must sit behind a known
+ * sanitizer/escaper OR carry an explicit `sink-safe:` annotation naming why it's
+ * safe. A new unannotated sink fails this check, so the sweep can't silently
+ * regress (the plan-41 completeness-script pattern).
+ *
+ *   node scripts/check-html-sinks.mjs          # exit 1 on any uncleared sink
+ *
+ * Scans:
+ *   - frontend/src   for  dangerouslySetInnerHTML | .innerHTML= | insertAdjacentHTML
+ *                          | new Function( | eval(
+ *   - backend/app    for  |safe | Markup( | render_template_string
+ *
+ * A sink LINE is cleared when it either references an allowlisted sanitizer
+ * (frontend only) or a `sink-safe:` comment appears on it or within the 3 lines
+ * above it. Allowlisted sanitizers live in frontend/src/utils + lib + the
+ * escape-first highlighters; extend ALLOWLISTED_SANITIZERS only alongside a real
+ * escaper/sanitizer.
+ */
+import { promises as fs } from 'fs';
+import path from 'path';
+import { fileURLToPath } from 'url';
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url));
+const ROOT = path.resolve(__dirname, '..');
+
+const FRONTEND_SINK = /dangerouslySetInnerHTML|\.innerHTML\s*=[^=]|insertAdjacentHTML|new\s+Function\s*\(|\beval\s*\(/;
+const BACKEND_SINK = /\|\s*safe\b|\bMarkup\s*\(|render_template_string/;
+// Escapers/sanitizers that make a raw-HTML sink safe by construction.
+const ALLOWLISTED_SANITIZERS = /sanitizeSvgInner|renderMarkdownToHtml|highlightLine|hlSql/;
+const ANNOTATION = /sink-safe:/;
+
+async function walk(dir, exts) {
+    const out = [];
+    let entries;
+    try {
+        entries = await fs.readdir(dir, { withFileTypes: true });
+    } catch {
+        return out;
+    }
+    for (const e of entries) {
+        if (e.name === 'node_modules' || e.name === 'dist' || e.name === '__pycache__') continue;
+        const full = path.join(dir, e.name);
+        if (e.isDirectory()) {
+            out.push(...await walk(full, exts));
+        } else if (exts.some((x) => e.name.endsWith(x))) {
+            out.push(full);
+        }
+    }
+    return out;
+}
+
+function scanFile(rel, text, sinkRe, allowSanitizer) {
+    const lines = text.split('\n');
+    const violations = [];
+    for (let i = 0; i < lines.length; i++) {
+        if (!sinkRe.test(lines[i])) continue;
+        // Skip comment-only lines: a doc comment that merely *mentions* a sink
+        // keyword isn't a sink (e.g. "// dangerouslySetInnerHTML is safe here").
+        const trimmed = lines[i].trim();
+        if (trimmed.startsWith('//') || trimmed.startsWith('*')
+            || trimmed.startsWith('/*') || trimmed.startsWith('#')) continue;
+        const window = lines.slice(Math.max(0, i - 3), i + 1).join('\n');
+        const cleared = (allowSanitizer && ALLOWLISTED_SANITIZERS.test(lines[i]))
+            || ANNOTATION.test(window);
+        if (!cleared) {
+            violations.push({ file: rel, line: i + 1, text: lines[i].trim() });
+        }
+    }
+    return violations;
+}
+
+async function main() {
+    const violations = [];
+    let sinkCount = 0;
+
+    const feFiles = await walk(path.join(ROOT, 'frontend', 'src'), ['.js', '.jsx', '.ts', '.tsx']);
+    for (const f of feFiles) {
+        const text = await fs.readFile(f, 'utf8');
+        const rel = path.relative(ROOT, f);
+        for (const line of text.split('\n')) if (FRONTEND_SINK.test(line)) sinkCount++;
+        violations.push(...scanFile(rel, text, FRONTEND_SINK, true));
+    }
+
+    const beFiles = await walk(path.join(ROOT, 'backend', 'app'), ['.py', '.html']);
+    for (const f of beFiles) {
+        const text = await fs.readFile(f, 'utf8');
+        const rel = path.relative(ROOT, f);
+        for (const line of text.split('\n')) if (BACKEND_SINK.test(line)) sinkCount++;
+        violations.push(...scanFile(rel, text, BACKEND_SINK, false));
+    }
+
+    if (violations.length) {
+        console.error('\n✖ HTML-sink guard: unannotated raw-HTML sink(s) found.\n');
+        for (const v of violations) {
+            console.error(`  ${v.file}:${v.line}  ${v.text}`);
+        }
+        console.error('\nEach raw-HTML sink must reference a sanitizer/escaper or carry a');
+        console.error('`sink-safe: <sanitizer> — <why>` comment (on the line or up to 3 above).');
+        console.error('See scripts/check-html-sinks.mjs and SECURITY.md.\n');
+        process.exit(1);
+    }
+
+    console.log(`✓ html-sinks: ${sinkCount} raw-HTML sink(s), all sanitized or annotated.`);
+}
+
+main().catch((e) => {
+    console.error(e);
+    process.exit(2);
+});