diff --git a/frontend/src/utils/__tests__/invariants.test.js b/frontend/src/utils/__tests__/invariants.test.js
new file mode 100644
index 00000000..75233519
--- /dev/null
+++ b/frontend/src/utils/__tests__/invariants.test.js
@@ -0,0 +1,49 @@
+// Executable regression guards for the prod-breaking invariants documented in
+// CLAUDE.md. Prose + the pre-PR review hook only catch NEW violations in a
+// diff; these assertions are a deterministic, permanent guard against the
+// same bug classes recurring silently.
+import { readFileSync } from 'node:fs'
+import path from 'node:path'
+import { fileURLToPath } from 'node:url'
+import { describe, expect, it } from 'vitest'
+import { AFTER_MIDNIGHT_THRESHOLD_HOUR } from '../festivalDays'
+import { AFTER_MIDNIGHT_THRESHOLD_MINUTES } from '../../admin/utils/timeUtils'
+
+describe('after-midnight threshold invariant (#550, CLAUDE.md "After-midnight band sorting")', () => {
+  it('AFTER_MIDNIGHT_THRESHOLD_HOUR stays 6 — never remove or lower', () => {
+    // Bands starting before 6 AM belong to the previous evening's lineup.
+    // Lowering/removing this threshold resurrects the recurring bug where
+    // after-midnight sets sort to the TOP of the schedule instead of the end.
+    expect(AFTER_MIDNIGHT_THRESHOLD_HOUR).toBe(6)
+  })
+
+  it('AFTER_MIDNIGHT_THRESHOLD_MINUTES stays derived from the canonical hour, never a separate literal', () => {
+    // frontend/src/admin/utils/timeUtils.js derives its minutes-based
+    // threshold from festivalDays.js's AFTER_MIDNIGHT_THRESHOLD_HOUR rather
+    // than re-encoding its own literal. Pins the derivation so the two
+    // constants can never silently drift apart if one file is edited without
+    // the other.
+    expect(AFTER_MIDNIGHT_THRESHOLD_MINUTES).toBe(AFTER_MIDNIGHT_THRESHOLD_HOUR * 60)
+  })
+
+  it('bandUtils.js imports the threshold from festivalDays.js rather than re-encoding a literal 6', () => {
+    // CLAUDE.md: "every consumer imports it rather than re-encoding `6`" —
+    // festivalDays.js is the single canonical location (#550). A source-read
+    // check (rather than a value check) is the only way to catch a future
+    // edit that keeps the numeric behavior correct today but re-hardcodes the
+    // literal instead of importing, silently reopening the drift risk.
+    // Resolved via node:path rather than `new URL(relative, import.meta.url)`:
+    // jsdom's test environment overrides the global `URL` with its own WHATWG
+    // implementation, which Node's `fileURLToPath` rejects ("must be of
+    // scheme file") since it isn't `instanceof` Node's own URL class.
+    const currentFile = fileURLToPath(import.meta.url)
+    const bandUtilsPath = path.join(path.dirname(currentFile), '../bandUtils.js')
+    const source = readFileSync(bandUtilsPath, 'utf8')
+
+    expect(source).toMatch(/import\s*{[^}]*AFTER_MIDNIGHT_THRESHOLD_HOUR[^}]*}\s*from\s*['"][^'"]*festivalDays['"]/)
+    // Guards against shadowing the import with a locally re-encoded literal,
+    // e.g. `const AFTER_MIDNIGHT_THRESHOLD_HOUR = 6`, which would silently
+    // defeat the import assertion above while still "importing" the name.
+    expect(source).not.toMatch(/(?:const|let|var)\s+AFTER_MIDNIGHT_THRESHOLD_HOUR\s*=\s*6\b/)
+  })
+})
diff --git a/functions/event/__tests__/__snapshots__/slug.jsonld-golden.test.js.snap b/functions/event/__tests__/__snapshots__/slug.jsonld-golden.test.js.snap
new file mode 100644
index 00000000..ae421f3e
--- /dev/null
+++ b/functions/event/__tests__/__snapshots__/slug.jsonld-golden.test.js.snap
@@ -0,0 +1,97 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`JSON-LD golden snapshot — /event/[slug] > MusicEvent + BreadcrumbList match the pinned golden shape 1`] = `
+{
+  "@context": "https://schema.org",
+  "@type": "MusicEvent",
+  "description": "Six venues, one legendary night of live music in Waterloo Region.",
+  "endDate": "2026-08-02",
+  "eventAttendanceMode": "https://schema.org/OfflineEventAttendanceMode",
+  "eventStatus": "https://schema.org/EventScheduled",
+  "image": [
+    "https://band-photos.settimes.ca/event-posters/17-vol17.jpg",
+  ],
+  "location": [
+    {
+      "@type": "MusicVenue",
+      "address": {
+        "@type": "PostalAddress",
+        "addressCountry": "CA",
+        "addressLocality": "Waterloo",
+        "addressRegion": "ON",
+        "postalCode": "N2J 2W7",
+        "streetAddress": "28 King St N",
+      },
+      "name": "Blue Room (Inside Revive Karaoke)",
+    },
+    {
+      "@type": "MusicVenue",
+      "address": {
+        "@type": "PostalAddress",
+        "addressCountry": "CA",
+        "addressLocality": "Waterloo",
+        "addressRegion": "ON",
+        "postalCode": "N2J 2W9",
+        "streetAddress": "47 King St N",
+      },
+      "name": "Room 47",
+    },
+  ],
+  "name": "Long Weekend Band Crawl - Vol. 17",
+  "offers": {
+    "@type": "Offer",
+    "availability": "https://schema.org/InStock",
+    "priceCurrency": "CAD",
+    "url": "https://tickets.example.com/vol-17",
+    "validFrom": "2026-07-01",
+  },
+  "organizer": {
+    "@type": "Organization",
+    "name": "SetTimes",
+    "sameAs": [
+      "https://www.instagram.com/settimes.ca",
+    ],
+    "url": "https://settimes.ca",
+  },
+  "performer": [
+    {
+      "@type": "MusicGroup",
+      "name": "The Anti-Queens",
+      "url": "https://settimes.ca/band/2",
+    },
+    {
+      "@type": "MusicGroup",
+      "name": "Midtown Static",
+      "url": "https://settimes.ca/band/1",
+    },
+    {
+      "@type": "MusicGroup",
+      "name": "Zebra Sound",
+      "url": "https://settimes.ca/band/3",
+    },
+  ],
+  "startDate": "2026-08-02T18:45:00-04:00",
+  "url": "https://settimes.ca/event/golden-lwbc17",
+}
+`;
+
+exports[`JSON-LD golden snapshot — /event/[slug] > MusicEvent + BreadcrumbList match the pinned golden shape 2`] = `
+{
+  "@context": "https://schema.org",
+  "@type": "BreadcrumbList",
+  "itemListElement": [
+    {
+      "@type": "ListItem",
+      "item": "https://settimes.ca/",
+      "name": "Events",
+      "position": 1,
+    },
+    {
+      "@type": "ListItem",
+      "item": "https://settimes.ca/event/golden-lwbc17",
+      "name": "Long Weekend Band Crawl - Vol. 17",
+      "position": 2,
+    },
+  ],
+}
+`;
diff --git a/functions/event/__tests__/slug.jsonld-golden.test.js b/functions/event/__tests__/slug.jsonld-golden.test.js
new file mode 100644
index 00000000..289ebe52
--- /dev/null
+++ b/functions/event/__tests__/slug.jsonld-golden.test.js
@@ -0,0 +1,106 @@
+/**
+ * JSON-LD golden snapshot — /event/[slug] MusicEvent + BreadcrumbList
+ *
+ * This is a golden-master test: it seeds ONE deterministic event and snapshots
+ * the full parsed MusicEvent + BreadcrumbList JSON-LD objects the SSR route
+ * emits. This structured data is crawler-facing and SEO-critical, and it
+ * churned across 3 PRs this week (#504, #615, #616) with every existing test
+ * asserting only individual fields — none asserted the FULL shape. A golden
+ * snapshot makes every future change to this output visible as a diff in
+ * code review, not just a passing/failing assertion on one field.
+ *
+ * The committed .snap file reflects MAIN's current output: organizer,
+ * offers+validFrom, image, location, performer — deliberately NO subEvent
+ * (#634's per-venue MusicEvent nesting is not merged as of this test's
+ * creation).
+ *
+ * When a change to the JSON-LD shape is INTENTIONAL (e.g. #634 landing
+ * subEvent), regenerate with:
+ *   npx vitest run -u functions/event/__tests__/slug.jsonld-golden.test.js
+ * then REVIEW the resulting snapshot diff before committing — the diff IS
+ * the change under review. Never accept a snapshot update blindly.
+ */
+import { describe, expect, test } from "vitest";
+import { onRequest } from "../[slug].js";
+import { createTestEnv, insertEvent, insertBand, insertVenue } from "../../api/test-utils.js";
+
+const STUB_HTML = `<!doctype html><html><head>
+    <meta name="description" content="Homepage description" />
+    <title>SetTimes</title>
+  </head><body><div id="root"></div></body></html>`;
+
+function makeContext({ env, slug }) {
+  env.ASSETS = {
+    fetch: async () => new Response(STUB_HTML, { status: 200, headers: { "content-type": "text/html" } }),
+  };
+  return {
+    request: new Request(`https://settimes.ca/event/${slug}`),
+    env,
+    params: { slug },
+  };
+}
+
+function extractJsonLd(html) {
+  const matches = [...html.matchAll(/<script type="application\/ld\+json">([\s\S]*?)<\/script>/g)];
+  return matches.map((m) => JSON.parse(m[1]));
+}
+
+describe("JSON-LD golden snapshot — /event/[slug]", () => {
+  test("MusicEvent + BreadcrumbList match the pinned golden shape", async () => {
+    const { env, rawDb } = createTestEnv();
+    env.PUBLIC_DATA_PUBLISH_ENABLED = "true";
+
+    // Two venues, ordered by v.name in the route's query: Blue Room sorts
+    // before Room 47 (B < R).
+    const blueRoom = insertVenue(rawDb, {
+      name: "Blue Room (Inside Revive Karaoke)",
+      address_line1: "28 King St N",
+      city: "Waterloo",
+      region: "ON",
+      postal_code: "N2J 2W7",
+    });
+    const room47 = insertVenue(rawDb, {
+      name: "Room 47",
+      address_line1: "47 King St N",
+      city: "Waterloo",
+      region: "ON",
+      postal_code: "N2J 2W9",
+    });
+
+    const event = insertEvent(rawDb, {
+      name: "Long Weekend Band Crawl - Vol. 17",
+      slug: "golden-lwbc17",
+      date: "2026-08-02",
+    });
+    rawDb.prepare("UPDATE events SET is_published = 1 WHERE id = ?").run(event.id);
+    // Pin created_at: offers.validFrom is derived from it (see [slug].js),
+    // so leaving the schema default (datetime('now')) would make the
+    // snapshot re-run non-deterministic — a new validFrom every day.
+    rawDb
+      .prepare("UPDATE events SET description = ?, ticket_url = ?, poster_url = ?, created_at = ? WHERE id = ?")
+      .run(
+        "Six venues, one legendary night of live music in Waterloo Region.",
+        "https://tickets.example.com/vol-17",
+        "https://band-photos.settimes.ca/event-posters/17-vol17.jpg",
+        "2026-07-01 12:00:00",
+        event.id,
+      );
+
+    // Band names + insertion order deliberately exercise the article-strip
+    // sort (#587): raw SQL `ORDER BY bp.name` would place these
+    // "Midtown Static", "The Anti-Queens", "Zebra Sound" (M, T, Z) — but the
+    // route re-sorts by the article-stripped key, so "The Anti-Queens" (key
+    // "anti-queens") must move to FIRST place, not stay second.
+    insertBand(rawDb, { name: "Midtown Static", event_id: event.id, venue_id: room47.id });
+    insertBand(rawDb, { name: "The Anti-Queens", event_id: event.id, venue_id: blueRoom.id });
+    insertBand(rawDb, { name: "Zebra Sound", event_id: event.id, venue_id: blueRoom.id });
+
+    const response = await onRequest(makeContext({ env, slug: "golden-lwbc17" }));
+    expect(response.status).toBe(200);
+    const html = await response.text();
+
+    const [musicEvent, breadcrumb] = extractJsonLd(html);
+    expect(musicEvent).toMatchSnapshot();
+    expect(breadcrumb).toMatchSnapshot();
+  });
+});
diff --git a/functions/utils/__tests__/invariants.test.js b/functions/utils/__tests__/invariants.test.js
new file mode 100644
index 00000000..32e4e08d
--- /dev/null
+++ b/functions/utils/__tests__/invariants.test.js
@@ -0,0 +1,52 @@
+// Executable regression guards for the prod-breaking invariants documented in
+// CLAUDE.md. Prose + the pre-PR review hook only catch NEW violations in a
+// diff; these assertions are a deterministic, permanent guard against the
+// same bug classes recurring silently.
+import { describe, expect, it } from "vitest";
+import { checkAuthRateLimit } from "../authAttempts.js";
+
+// toSqliteDateTime() is NOT exported from authAttempts.js — it's a private
+// helper only reachable through checkAuthRateLimit's `windowStart` binding.
+// Rather than exporting it solely for this test (a production-code change
+// outside this task's scope) or reimplementing its logic in the test (which
+// would test the copy, not the real code), this stub captures the exact
+// value the real, unexported toSqliteDateTime() produces when
+// checkAuthRateLimit binds it into the rate-limit query.
+function createCapturingDB(queryResult = { count: 0, earliest_attempt: null }) {
+  let boundArgs = null;
+  const DB = {
+    prepare() {
+      return {
+        bind(...args) {
+          boundArgs = args;
+          return { first: async () => queryResult };
+        },
+      };
+    },
+  };
+  return { DB, getBoundArgs: () => boundArgs };
+}
+
+describe('SQLite datetime format invariant (CLAUDE.md "SQLite datetime format — do NOT use ISO 8601 T-separator")', () => {
+  it("toSqliteDateTime()'s output has a space separator, no T/Z/ms — the exact SEC-F1 bug shape", async () => {
+    // D1's datetime('now') returns `YYYY-MM-DD HH:MM:SS` (space separator).
+    // JS's toISOString() returns `YYYY-MM-DDTHH:MM:SS.mmmZ` (T separator,
+    // ms, Z suffix). A stored/compared value with a `T` silently breaks
+    // string comparison against datetime('now') — this exact bug caused a
+    // production invite-code expiry bypass (SEC-F1). checkAuthRateLimit's
+    // scope: "ip" binding order is [ipAddress, attemptType, windowStart]
+    // (see getRateLimitBindings in authAttempts.js), so windowStart —
+    // produced by toSqliteDateTime() — is the 3rd bound argument.
+    const { DB, getBoundArgs } = createCapturingDB();
+
+    await checkAuthRateLimit(DB, {
+      attemptType: "login",
+      ipAddress: "127.0.0.1",
+      scope: "ip",
+    });
+
+    const [, , windowStart] = getBoundArgs();
+    expect(windowStart).toMatch(/^\d{4}-\d{2}-\d{2} \d{2}:\d{2}:\d{2}$/);
+    expect(windowStart).not.toContain("T");
+  });
+});