diff --git a/AGENTS.md b/AGENTS.md
index 7fb2520..f879334 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -39,7 +39,8 @@ Runtime: Node.js (ESM, `.mjs` throughout). No build step. No bundler. `server.mj
 - `lib/fallback/` — fallback engine (trigger detection, chain advancement, idempotent-failure safety, header annotation). Governed by ADR 0004.
 - `lib/keys.mjs` — multi-key auth, per-key namespacing, identity layer. Carries OCP's per-key isolation model into OLP. **✅ Phase 2 — D44 core + D45 server integration + D46 owner gating shipped (validateKey on every /v1/* + /health; chain filtered by providers_enabled; touchLastUsed fires post-response; /health payload trimmed for non-owner; X-OLP-Fallback-Detail gated by fallback_detail_header_policy).**
 - `bin/olp-keys.mjs` — keygen CLI bootstrap surface per ADR 0007 § 9.1. **✅ Shipped at D47.** Subcommands: `keygen [--owner|--name=X|--providers=csv|--force]`, `list [--owner-only|--include-revoked]`, `revoke --id=X`. Plaintext token printed once on keygen. Installed via `package.json bin` so `npx olp-keys ...` works (also `npm run olp-keys ...`).
-- `lib/audit.mjs` — append-only ndjson audit per ADR 0007 § 6.2 + § 8. **🟡 D45 — appendAuditEvent + getAuditDropCount shipped. Fires per /v1/chat/completions + /v1/models request including 401/403/5xx paths. Warn+1-retry on append failure; no memory buffer at Phase 2 (forward path).** **D52 will extend with daily rotation per ADR 0008 § 5.**
+- `lib/audit.mjs` — append-only ndjson audit per ADR 0007 § 6.2 + § 8 + daily rotation per ADR 0008 § 5. **✅ D45 (append) + D52 (rotation) shipped. `appendAuditEvent` fires per /v1/chat/completions + /v1/models + /v0/management/* request (warn + 1 retry; no memory buffer). `_maybeRotateAudit` (sync) is called BEFORE the append when the UTC date changes; renames live → `audit-YYYY-MM-DD.ndjson`. Optional external cron tool `bin/olp-audit-rotate.mjs` for exact-at-midnight rotation.**
+- `bin/olp-audit-rotate.mjs` — external audit rotation cron tool per ADR 0008 § 5.2. **✅ D52 — `runCli(argv, { out, err })` invocable + main-guard for direct execution. Installed via `package.json bin` so `npx olp-audit-rotate` works (also `npm run olp-audit-rotate`). Idempotent + safe alongside in-server first-append trigger.**
 - `lib/audit-query.mjs` — audit ndjson aggregate query layer per ADR 0008 § 4. **🟡 D49 — discoverAuditFiles + readAuditWindow + aggregateRequests + topFallbackChains + spendTrendDaily + cacheHitRateWindow shipped. Cross-file walk over `audit.ndjson` (live) + `audit-YYYY-MM-DD.ndjson` (rotated). PII guard: aggregate shapes never include message content. In-memory scan per request (ADR 0008 Lane 2 = A; SQLite hybrid deferred to ADR 0007 § 13 trigger).**
 - `dashboard.html` — owner-only multi-provider dashboard (4 panels: per-provider quota / 24h request+cache+fallback / 30d spend trend SVG sparkline / top-10 fallback chains). **✅ D51 — full UI shipped at repo root per ADR 0008 § 6. Vanilla HTML + JS + fetch (no build step, no framework, no CDN). 30s page poll with `document.visibilityState` pause/resume. Served by `/dashboard` route in server.mjs owner-only_block. Cached in memory at first request via `_loadDashboardHtml`.**
 - `models-registry.json` — single source of truth for `(provider, model) → metadata`. SPOT.
diff --git a/CHANGELOG.md b/CHANGELOG.md
index eed1fb8..7ec53b4 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -4,6 +4,29 @@ All notable changes to OLP land here. Per `CLAUDE.md` release_kit overlay, this
 
 ## Unreleased
 
+### D52 — Daily audit rotation (`lib/audit.mjs` extension + `bin/olp-audit-rotate.mjs`)
+
+Fifth Phase 3 D-day. Adds daily UTC-aware rotation to `lib/audit.mjs` per ADR 0008 § 5 + ships an external cron tool. Rotation is **synchronous** at v0.3.0 (Lane 3 = B daily rotation; synchronous design eliminates the race that an async wrapper would create between date-change-detection and the append).
+
+- **`lib/audit.mjs` extended**:
+  - New `_maybeRotateAudit({ olpHome, logEvent })` (synchronous): probes the live `audit.ndjson`; if it holds events from a past UTC date, renames it to `audit-YYYY-MM-DD.ndjson`. Idempotent. If the target file already exists (cron beat the in-server check), logs warn + skips per ADR 0008 § 5.3 race safety.
+  - `appendAuditEvent` extended: cheap fast-path date check via module-cached `_lastSeenUtcDate`. On date change, calls `_maybeRotateAudit` synchronously BEFORE `appendFileSync` — so old-date events land in the rotated file and new-date events land in the fresh live file. No event straddles the boundary.
+  - Why synchronous instead of async: an async wrapper would let the sync `appendFileSync` race the not-yet-completed `renameSync`, landing today's event in the about-to-be-renamed file. Sync rotation is the only correct ordering at the append-fired-from-many-routes scale OLP runs.
+  - New exports: `_maybeRotateAudit` (sync), `getAuditRotateCount`, `getAuditRotateFailCount`, `__resetAuditRotateState`, `__setLastSeenUtcDateForTesting`.
+  - First-event-date discovery: when probing the live file's date, reads only the first ndjson line + parses its `ts`. Falls back to file mtime if events absent (corrupt/empty edge).
+- **`bin/olp-audit-rotate.mjs`** (~95 lines): external cron tool per ADR 0008 § 5.2. Calls `_maybeRotateAudit` once + reports outcome. Exit codes 0 (success or no-op), 1 (bad usage), 2 (rotation failed). Installed via `package.json bin` so `npx olp-audit-rotate [--olp-home=<path>]` works. Example cron line documented in the file header.
+- **Concurrent-safety semantics** (ADR 0008 § 5.3): in-process sequential appends after the first date-change detection short-circuit via the updated `_lastSeenUtcDate` cache → exactly 1 rename even under N sequential appends. Cross-process (cron + server) coexistence handled by the "target already exists → skip + warn" branch.
+- **Test surface (Suite 26, +12 tests — 588 → 600):**
+  - 26a-1..5: `_maybeRotateAudit` (no live file / today already / yesterday→rotate / idempotent re-call / cron-race target-exists warn)
+  - 26b-1: `appendAuditEvent` past UTC date change triggers sync rotation + append lands in fresh live file
+  - 26c-1: 10 sequential `appendAuditEvent` across date change → exactly 1 rotation + all 10 events in new live file
+  - 26d-1..4: `bin/olp-audit-rotate.mjs` CLI (--help / no-live-file / yesterday-file-rotates / unknown-flag exit 1)
+  - 26e-1: rotated files queryable via `lib/audit-query.mjs` `discoverAuditFiles` + `readAuditWindow` cross-file read
+- **`package.json`**: `bin.olp-audit-rotate` + `scripts.olp-audit-rotate` entries added.
+- **Documentation:** AGENTS.md `lib/audit.mjs` marker promoted to ✅ (D45 append + D52 rotation both shipped); new `bin/olp-audit-rotate.mjs` entry.
+- **Test count:** 588 → 600 (+12 D52 tests in Suite 26).
+- **Authority:** ADR 0008 § 5.1 (first-append-after-UTC-midnight trigger), § 5.2 (external cron alternative), § 5.3 (concurrent-rotation safety + cron-coexistence semantics), § 5.4 (renamed-file query path consumed by D49 lib/audit-query.mjs); CLAUDE.md `release_kit overlay phase_rolling_mode` — under Unreleased; standing autopilot grant.
+
 ### D51 — `dashboard.html` full multi-panel UI (Phase 3)
 
 Fourth Phase 3 D-day. Replaces the D50 `dashboard.html` placeholder with the full 4-panel UI per ADR 0008 § 6. Vanilla HTML + JS + fetch — no build step, no framework, no CDN (Lane 1 = A). 30s page poll with `document.visibilityState` pause/resume (Lane 4 = A).
diff --git a/bin/olp-audit-rotate.mjs b/bin/olp-audit-rotate.mjs
new file mode 100755
index 0000000..cc4a84a
--- /dev/null
+++ b/bin/olp-audit-rotate.mjs
@@ -0,0 +1,94 @@
+#!/usr/bin/env node
+/**
+ * bin/olp-audit-rotate.mjs — External audit rotation cron tool (Phase 3 / D52)
+ *
+ * Authority: ADR 0008 § 5.2 (external cron alternative to in-server first-
+ * append-after-UTC-midnight trigger).
+ *
+ * Use case: operators who want exact-at-UTC-midnight rotation rather than
+ * "first request after midnight". Invoke from a host cron / launchd job.
+ *
+ * Idempotent + safe to run alongside the in-server check (both detect the
+ * date condition; whichever fires first does the rename; the other no-ops).
+ *
+ * Usage:
+ *   olp-audit-rotate [--olp-home=<path>]
+ *
+ * Exit codes:
+ *   0 = success (rotation performed OR no rotation needed)
+ *   1 = bad usage (unknown flag)
+ *   2 = rotation attempted + failed (e.g., EACCES)
+ *
+ * Example cron line (UTC midnight):
+ *   1 0 * * * /usr/local/bin/node /path/to/bin/olp-audit-rotate.mjs >> /var/log/olp-audit-rotate.log 2>&1
+ */
+
+import { _maybeRotateAudit } from '../lib/audit.mjs';
+
+function parseArgv(argv) {
+  const flags = {};
+  for (const arg of argv) {
+    if (arg.startsWith('--')) {
+      const eq = arg.indexOf('=');
+      if (eq > 0) flags[arg.slice(2, eq)] = arg.slice(eq + 1);
+      else flags[arg.slice(2)] = true;
+    }
+  }
+  return flags;
+}
+
+const USAGE = `OLP audit rotation cron tool
+
+Usage:
+  olp-audit-rotate [--olp-home=<path>]
+
+Triggers a rotation check: if the live audit.ndjson holds events from a
+past UTC date, rename it to audit-YYYY-MM-DD.ndjson. Idempotent; safe to
+run alongside the in-server first-append-after-UTC-midnight trigger.
+
+Authority: ADR 0008 § 5.2.`;
+
+export async function runCli(argv, opts = {}) {
+  const ioOut = opts.out ?? (s => process.stdout.write(s));
+  const ioErr = opts.err ?? (s => process.stderr.write(s));
+
+  if (argv.includes('--help') || argv.includes('-h')) {
+    ioOut(USAGE + '\n');
+    return 0;
+  }
+
+  const flags = parseArgv(argv);
+  const allowed = new Set(['olp-home', 'help', 'h']);
+  for (const k of Object.keys(flags)) {
+    if (!allowed.has(k)) {
+      ioErr(`Error: unknown flag --${k}\n${USAGE}\n`);
+      return 1;
+    }
+  }
+
+  const olpHome = typeof flags['olp-home'] === 'string' ? flags['olp-home'] : undefined;
+
+  try {
+    // _maybeRotateAudit is synchronous at v0.3.0 (D52) — rotation must
+    // complete BEFORE the next append so no event straddles the boundary.
+    const result = _maybeRotateAudit({ olpHome });
+    if (result.rotated) {
+      ioOut(`Rotated ${result.fromPath} -> ${result.toPath} (dateUsed=${result.dateUsed}).\n`);
+    } else {
+      ioOut('No rotation needed (live audit is current or absent).\n');
+    }
+    return 0;
+  } catch (err) {
+    ioErr(`Error: rotation failed: ${err?.message ?? err}\n`);
+    return 2;
+  }
+}
+
+// Main guard
+const isMain = (() => {
+  try { return import.meta.url === `file://${process.argv[1]}`; }
+  catch { return false; }
+})();
+if (isMain) {
+  runCli(process.argv.slice(2)).then(code => process.exit(code));
+}
diff --git a/lib/audit.mjs b/lib/audit.mjs
index ec23bf8..ea961e2 100644
--- a/lib/audit.mjs
+++ b/lib/audit.mjs
@@ -23,15 +23,23 @@
  * message content. Hash + shape only.
  */
 
-import { appendFileSync, mkdirSync, chmodSync } from 'node:fs';
+import { appendFileSync, mkdirSync, chmodSync, renameSync, existsSync, statSync, readFileSync } from 'node:fs';
 import { join } from 'node:path';
 import { homedir } from 'node:os';
 
 const DEFAULT_OLP_HOME = join(homedir(), '.olp');
 const OLP_HOME_ENV = 'OLP_HOME';
 const RETRY_COUNT = 1; // § 6.2: warn + 1 retry
+const LIVE_AUDIT_FILE = 'audit.ndjson';
+const ROTATED_FILE_PREFIX = 'audit-';
+const ROTATED_FILE_SUFFIX = '.ndjson';
 
 let _dropCounter = 0;
+let _rotateCounter = 0;       // observability + test assertion target
+let _rotateFailCounter = 0;
+// Module-cached "last UTC date we saw at append time" so we don't read
+// disk metadata on every append just to check for rotation.
+let _lastSeenUtcDate = _utcDateNow();
 
 /**
  * Resolve OLP home dir (matches lib/keys.mjs precedence): opts.olpHome →
@@ -44,6 +52,120 @@ function _resolveOlpHome(opts) {
   return DEFAULT_OLP_HOME;
 }
 
+/**
+ * Current UTC date as YYYY-MM-DD. Module-private; reused by rotation logic.
+ */
+function _utcDateNow() {
+  return new Date().toISOString().slice(0, 10);
+}
+
+/**
+ * Read the first event's `ts` from the live audit file to discover the
+ * date it was opened on (for stale-cache recovery when the process is
+ * restarted and the in-memory `_lastSeenUtcDate` doesn't match). Returns
+ * the YYYY-MM-DD string, or null if the file is missing / empty /
+ * malformed.
+ */
+function _firstEventDateInLiveFile(livePath) {
+  try {
+    if (!existsSync(livePath)) return null;
+    // Read the file + slice off the first ndjson line. For audit ndjson the
+    // first line is at most a few hundred bytes; this is fine for the rare
+    // "process restart with a stale live file" recovery path. (Family-scale
+    // single-file audit is bounded; multi-MB scans are not a real concern
+    // until Phase 4+ when Option 3 SQLite migration would kick in anyway.)
+    const raw = readFileSync(livePath, 'utf-8');
+    const nl = raw.indexOf('\n');
+    const firstLine = nl === -1 ? raw : raw.slice(0, nl);
+    if (!firstLine) return null;
+    const obj = JSON.parse(firstLine);
+    if (typeof obj?.ts !== 'string') return null;
+    return obj.ts.slice(0, 10);
+  } catch {
+    return null;
+  }
+}
+
+/**
+ * Rotate the live audit file (if any) to its UTC-date suffix. Idempotent:
+ * if a rotated file with that name already exists (e.g., cron beat us to
+ * it), this skips the rename.
+ *
+ * Per ADR 0008 § 5.1 + § 5.3. SYNCHRONOUS so callers (appendAuditEvent +
+ * external cron) can rely on it completing before the next IO operation.
+ * Sync rotation eliminates the race that an async wrapper would create
+ * between the date-change-detection and the append (where the append could
+ * land in the old un-rotated file).
+ *
+ * Concurrent in-process invocations: Node's single-threaded event loop
+ * serializes synchronous calls within a tick. Cross-tick concurrency
+ * uses the `_lastSeenUtcDate` cache as the gate — once set, subsequent
+ * appends short-circuit the rotation check.
+ *
+ * @param {object} args - { olpHome, logEvent, _nowFn (test injection) }
+ * @returns {{ rotated: boolean, fromPath?: string, toPath?: string, dateUsed?: string }}
+ */
+export function _maybeRotateAudit(args = {}) {
+  const olpHome = _resolveOlpHome(args);
+  const logEvent = args.logEvent ?? ((level, ev, data) => {
+    const entry = { ts: new Date().toISOString(), level, event: ev, ...(data ?? {}) };
+    process.stderr.write(JSON.stringify(entry) + '\n');
+  });
+  const nowFn = args._nowFn ?? (() => new Date());
+
+  const logsDir = join(olpHome, 'logs');
+  const livePath = join(logsDir, LIVE_AUDIT_FILE);
+  if (!existsSync(livePath)) {
+    // No live file yet (first append ever); no rotation needed.
+    _lastSeenUtcDate = nowFn().toISOString().slice(0, 10);
+    return { rotated: false };
+  }
+  // Determine the "date the live file holds" — use the first event's ts.
+  // Fall back to file mtime if events absent (corrupt / empty file edge).
+  let fileDate = _firstEventDateInLiveFile(livePath);
+  if (fileDate === null) {
+    try {
+      fileDate = statSync(livePath).mtime.toISOString().slice(0, 10);
+    } catch {
+      return { rotated: false };
+    }
+  }
+  const today = nowFn().toISOString().slice(0, 10);
+  if (fileDate === today) {
+    _lastSeenUtcDate = today;
+    return { rotated: false };
+  }
+
+  // Rotate: rename live → audit-<fileDate>.ndjson.
+  const rotatedName = `${ROTATED_FILE_PREFIX}${fileDate}${ROTATED_FILE_SUFFIX}`;
+  const rotatedPath = join(logsDir, rotatedName);
+  if (existsSync(rotatedPath)) {
+    // Cron or another writer beat us; the live file holding fileDate's
+    // events must be merged manually. Per ADR 0008 § 5.3 we log + skip.
+    logEvent('warn', 'audit_rotate_target_exists', {
+      livePath, rotatedPath,
+      message: 'rotation target exists — concurrent rotator beat in-process check; manual merge required if events overlap',
+    });
+    _lastSeenUtcDate = today;
+    return { rotated: false };
+  }
+  try {
+    renameSync(livePath, rotatedPath);
+    _rotateCounter++;
+    _lastSeenUtcDate = today;
+    logEvent('info', 'audit_rotated', { fromPath: livePath, toPath: rotatedPath, dateUsed: fileDate });
+    return { rotated: true, fromPath: livePath, toPath: rotatedPath, dateUsed: fileDate };
+  } catch (err) {
+    _rotateFailCounter++;
+    logEvent('warn', 'audit_rotate_failed', {
+      livePath, rotatedPath,
+      error: err?.message ?? String(err),
+    });
+    // Don't update _lastSeenUtcDate so the next append retries.
+    return { rotated: false };
+  }
+}
+
 /**
  * Append a single audit event to ~/.olp/logs/audit.ndjson.
  *
@@ -60,13 +182,28 @@ function _resolveOlpHome(opts) {
 export function appendAuditEvent(event, opts = {}) {
   const olpHome = _resolveOlpHome(opts);
   const logsDir = join(olpHome, 'logs');
-  const path = join(logsDir, 'audit.ndjson');
+  const path = join(logsDir, LIVE_AUDIT_FILE);
   const line = JSON.stringify(event) + '\n';
   const logEvent = opts.logEvent ?? ((level, ev, data) => {
     const entry = { ts: new Date().toISOString(), level, event: ev, ...(data ?? {}) };
     process.stderr.write(JSON.stringify(entry) + '\n');
   });
 
+  // D52: cheap fast-path date check. If the module-cached date matches the
+  // current UTC date, skip the rotation probe entirely (no disk I/O). If
+  // the date has changed, synchronously rotate BEFORE the append so the
+  // append lands in the (post-rotation) new live file rather than the
+  // about-to-rotate-away old one.
+  // Per ADR 0008 § 5.1: rotation fires "on the first append after a UTC
+  // date change." Synchronous rotation ensures no append straddles the
+  // boundary — old-date events land in the rotated file; new-date events
+  // land in the fresh live file. The cache flip happens INSIDE
+  // _maybeRotateAudit so concurrent in-process re-triggers short-circuit.
+  const today = _utcDateNow();
+  if (today !== _lastSeenUtcDate) {
+    _maybeRotateAudit({ olpHome, logEvent });
+  }
+
   for (let attempt = 0; attempt <= RETRY_COUNT; attempt++) {
     try {
       mkdirSync(logsDir, { recursive: true, mode: 0o700 });
@@ -108,3 +245,38 @@ export function getAuditDropCount() {
 export function __resetAuditDropCount() {
   _dropCounter = 0;
 }
+
+/**
+ * Per-process count of successful rotations performed. Test + future
+ * observability surface.
+ */
+export function getAuditRotateCount() {
+  return _rotateCounter;
+}
+
+/**
+ * Per-process count of failed rotation attempts (rename threw, target
+ * existed, etc.). Test + future observability surface.
+ */
+export function getAuditRotateFailCount() {
+  return _rotateFailCounter;
+}
+
+/**
+ * Test-only: reset the rotation counters + the in-process "last seen UTC
+ * date" cache so each test exercises a fresh code path.
+ */
+export function __resetAuditRotateState() {
+  _rotateCounter = 0;
+  _rotateFailCounter = 0;
+  _lastSeenUtcDate = _utcDateNow();
+}
+
+/**
+ * Test-only: force the cached "last seen UTC date" to a specific value
+ * so the next appendAuditEvent observes a date change and triggers
+ * rotation deterministically.
+ */
+export function __setLastSeenUtcDateForTesting(dateStr) {
+  _lastSeenUtcDate = dateStr;
+}
diff --git a/package.json b/package.json
index b8786bf..04bc8f9 100644
--- a/package.json
+++ b/package.json
@@ -5,12 +5,14 @@
   "type": "module",
   "main": "server.mjs",
   "bin": {
-    "olp-keys": "./bin/olp-keys.mjs"
+    "olp-keys": "./bin/olp-keys.mjs",
+    "olp-audit-rotate": "./bin/olp-audit-rotate.mjs"
   },
   "scripts": {
     "start": "node server.mjs",
     "test": "node test-features.mjs",
-    "olp-keys": "node bin/olp-keys.mjs"
+    "olp-keys": "node bin/olp-keys.mjs",
+    "olp-audit-rotate": "node bin/olp-audit-rotate.mjs"
   },
   "engines": {
     "node": ">=18"
diff --git a/test-features.mjs b/test-features.mjs
index e36871c..030d7fa 100644
--- a/test-features.mjs
+++ b/test-features.mjs
@@ -11726,3 +11726,236 @@ describe('Suite 25 — D51 dashboard.html UI smoke (Phase 3, ADR 0008 § 6)', ()
     assert.match(r.body, /401.*owner-tier/i, 'dashboard error banner must mention owner-tier in 401 case');
   });
 });
+
+// ── Suite 26: D52 audit rotation (Phase 3, ADR 0008 § 5) ──────────────────
+//
+// Tests for daily audit rotation:
+//   - First append after UTC date change triggers rename
+//   - Concurrent appends during rotation produce exactly one rename
+//   - External cron tool (bin/olp-audit-rotate.mjs) coexists with in-server
+//   - Rotated files readable by lib/audit-query.mjs after rotation
+
+import {
+  appendAuditEvent as appendAuditEventS26,
+  _maybeRotateAudit,
+  getAuditRotateCount,
+  __resetAuditRotateState,
+  __setLastSeenUtcDateForTesting,
+} from './lib/audit.mjs';
+import { runCli as runAuditRotateCli } from './bin/olp-audit-rotate.mjs';
+
+describe('Suite 26 — D52 audit rotation (Phase 3, ADR 0008 § 5)', () => {
+
+  function writeAuditFile(tmp, filename, events) {
+    mkdirSync(_pathJoinForSetup(tmp, 'logs'), { recursive: true, mode: 0o700 });
+    const path = _pathJoinForSetup(tmp, 'logs', filename);
+    const lines = events.map(ev => JSON.stringify(ev)).join('\n') + '\n';
+    fsWriteFileSyncForS23(path, lines, { mode: 0o600 });
+  }
+
+  function makeEvent(ts) {
+    return {
+      ts,
+      key_id: 'k1',
+      owner_tier: 'owner',
+      method: 'POST',
+      path: '/v1/chat/completions',
+      provider: 'anthropic',
+      model: 'claude-sonnet-4-6',
+      status_code: 200,
+      latency_ms: 100,
+      cache_status: 'miss',
+      fallback_hops: 0,
+      tried_providers: ['anthropic'],
+    };
+  }
+
+  describe('26a — _maybeRotateAudit', () => {
+    let TMP;
+    before(() => { TMP = _mkdtempSyncForSetup(_pathJoinForSetup(_tmpdirForSetup(), 'olp-test-26a-')); });
+    after(() => { rmSync(TMP, { recursive: true, force: true }); __resetAuditRotateState(); });
+
+    it('26a-1: no live file → no rotation, returns { rotated: false }', () => {
+      __resetAuditRotateState();
+      const r = _maybeRotateAudit({ olpHome: TMP });
+      assert.equal(r.rotated, false);
+    });
+
+    it('26a-2: live file already on today → no rotation', () => {
+      __resetAuditRotateState();
+      const today = new Date().toISOString().slice(0, 10);
+      writeAuditFile(TMP, 'audit.ndjson', [makeEvent(`${today}T10:00:00Z`)]);
+      const r = _maybeRotateAudit({ olpHome: TMP });
+      assert.equal(r.rotated, false);
+    });
+
+    it('26a-3: live file holds yesterday\'s events → rotated to audit-YYYY-MM-DD.ndjson', () => {
+      __resetAuditRotateState();
+      const livePath = _pathJoinForSetup(TMP, 'logs', 'audit.ndjson');
+      rmSync(livePath, { force: true });
+      const yesterday = new Date(Date.now() - 86400 * 1000).toISOString().slice(0, 10);
+      writeAuditFile(TMP, 'audit.ndjson', [makeEvent(`${yesterday}T15:00:00Z`)]);
+      const r = _maybeRotateAudit({ olpHome: TMP });
+      assert.equal(r.rotated, true);
+      assert.equal(r.dateUsed, yesterday);
+      const rotatedPath = _pathJoinForSetup(TMP, 'logs', `audit-${yesterday}.ndjson`);
+      assert.ok(fsExistsSync(rotatedPath));
+      assert.ok(!fsExistsSync(livePath));
+      assert.equal(getAuditRotateCount(), 1);
+    });
+
+    it('26a-4: idempotent — second call after rotation is a no-op', () => {
+      const r = _maybeRotateAudit({ olpHome: TMP });
+      assert.equal(r.rotated, false);
+      assert.equal(getAuditRotateCount(), 1);
+    });
+
+    it('26a-5: rotation target already exists → skip + warn (cron-race safety)', () => {
+      __resetAuditRotateState();
+      const livePath = _pathJoinForSetup(TMP, 'logs', 'audit.ndjson');
+      rmSync(livePath, { force: true });
+      const yesterday = new Date(Date.now() - 86400 * 1000).toISOString().slice(0, 10);
+      writeAuditFile(TMP, `audit-${yesterday}.ndjson`, [makeEvent(`${yesterday}T08:00:00Z`)]);
+      writeAuditFile(TMP, 'audit.ndjson', [makeEvent(`${yesterday}T20:00:00Z`)]);
+
+      let warned = false;
+      const r = _maybeRotateAudit({
+        olpHome: TMP,
+        logEvent: (level, ev) => { if (level === 'warn' && ev === 'audit_rotate_target_exists') warned = true; },
+      });
+      assert.equal(r.rotated, false);
+      assert.ok(warned, 'must warn when rotation target already exists');
+      assert.ok(fsExistsSync(livePath));
+    });
+  });
+
+  describe('26b — appendAuditEvent triggers rotation on date change', () => {
+    let TMP;
+    before(() => { TMP = _mkdtempSyncForSetup(_pathJoinForSetup(_tmpdirForSetup(), 'olp-test-26b-')); });
+    after(() => { rmSync(TMP, { recursive: true, force: true }); __resetAuditRotateState(); });
+
+    it('26b-1: appending past a UTC date change triggers sync rotation + append lands in new file', () => {
+      __resetAuditRotateState();
+      const livePath = _pathJoinForSetup(TMP, 'logs', 'audit.ndjson');
+      rmSync(livePath, { force: true });
+      const yesterday = new Date(Date.now() - 86400 * 1000).toISOString().slice(0, 10);
+      writeAuditFile(TMP, 'audit.ndjson', [makeEvent(`${yesterday}T10:00:00Z`)]);
+      __setLastSeenUtcDateForTesting(yesterday);
+
+      const todayTs = new Date().toISOString();
+      appendAuditEventS26(makeEvent(todayTs), { olpHome: TMP });
+
+      const rotatedPath = _pathJoinForSetup(TMP, 'logs', `audit-${yesterday}.ndjson`);
+      assert.ok(fsExistsSync(rotatedPath), `yesterday's rotated file must exist (${rotatedPath})`);
+      assert.ok(fsExistsSync(livePath), 'live file must exist (with today\'s new event)');
+      const liveContent = fsReadFileSync(livePath, 'utf-8');
+      assert.ok(liveContent.includes(todayTs), 'live file must contain today\'s appended event');
+      assert.ok(!liveContent.includes(`${yesterday}T10:00:00Z`), 'live file must NOT contain yesterday\'s content (it rotated)');
+      assert.equal(getAuditRotateCount(), 1);
+    });
+  });
+
+  describe('26c — concurrent appends during rotation', () => {
+    let TMP;
+    before(() => { TMP = _mkdtempSyncForSetup(_pathJoinForSetup(_tmpdirForSetup(), 'olp-test-26c-')); });
+    after(() => { rmSync(TMP, { recursive: true, force: true }); __resetAuditRotateState(); });
+
+    it('26c-1: N sequential appendAuditEvent during date change → exactly 1 rotation + all events land in live', () => {
+      __resetAuditRotateState();
+      const livePath = _pathJoinForSetup(TMP, 'logs', 'audit.ndjson');
+      rmSync(livePath, { force: true });
+      const yesterday = new Date(Date.now() - 86400 * 1000).toISOString().slice(0, 10);
+      writeAuditFile(TMP, 'audit.ndjson', [makeEvent(`${yesterday}T10:00:00Z`)]);
+      __setLastSeenUtcDateForTesting(yesterday);
+
+      // 10 sync appends in quick succession. The first triggers rotation
+      // (date change detected); subsequent appends short-circuit the date
+      // check because _lastSeenUtcDate was updated inside the first call's
+      // _maybeRotateAudit. Result: exactly 1 rotation + all 10 events in
+      // the new live file.
+      const todayTs = new Date().toISOString();
+      for (let i = 0; i < 10; i++) {
+        appendAuditEventS26(makeEvent(todayTs), { olpHome: TMP });
+      }
+
+      assert.equal(getAuditRotateCount(), 1, 'exactly 1 rotation despite 10 sequential appends past date change');
+      const rotatedPath = _pathJoinForSetup(TMP, 'logs', `audit-${yesterday}.ndjson`);
+      assert.ok(fsExistsSync(rotatedPath));
+      const liveContent = fsReadFileSync(livePath, 'utf-8');
+      const liveLines = liveContent.split('\n').filter(Boolean);
+      assert.equal(liveLines.length, 10, 'live file has all 10 appended events');
+    });
+  });
+
+  describe('26d — bin/olp-audit-rotate.mjs CLI', () => {
+    let TMP;
+    before(() => { TMP = _mkdtempSyncForSetup(_pathJoinForSetup(_tmpdirForSetup(), 'olp-test-26d-')); });
+    after(() => { rmSync(TMP, { recursive: true, force: true }); __resetAuditRotateState(); });
+
+    it('26d-1: --help → exit 0 with usage', async () => {
+      let out = '';
+      const code = await runAuditRotateCli(['--help'], { out: s => { out += s; }, err: () => {} });
+      assert.equal(code, 0);
+      assert.match(out, /OLP audit rotation cron tool/);
+    });
+
+    it('26d-2: no live file → exit 0 "no rotation needed"', async () => {
+      __resetAuditRotateState();
+      let out = '';
+      const code = await runAuditRotateCli([`--olp-home=${TMP}`], { out: s => { out += s; }, err: () => {} });
+      assert.equal(code, 0);
+      assert.match(out, /No rotation needed/);
+    });
+
+    it('26d-3: live file with yesterday events → CLI rotates + exit 0', async () => {
+      __resetAuditRotateState();
+      const yesterday = new Date(Date.now() - 86400 * 1000).toISOString().slice(0, 10);
+      writeAuditFile(TMP, 'audit.ndjson', [makeEvent(`${yesterday}T10:00:00Z`)]);
+
+      let out = '';
+      const code = await runAuditRotateCli([`--olp-home=${TMP}`], { out: s => { out += s; }, err: () => {} });
+      assert.equal(code, 0);
+      assert.match(out, /Rotated.*dateUsed=/);
+      const rotatedPath = _pathJoinForSetup(TMP, 'logs', `audit-${yesterday}.ndjson`);
+      assert.ok(fsExistsSync(rotatedPath));
+    });
+
+    it('26d-4: unknown flag → exit 1', async () => {
+      let err = '';
+      const code = await runAuditRotateCli([`--olp-home=${TMP}`, '--unknown'], {
+        out: () => {}, err: s => { err += s; },
+      });
+      assert.equal(code, 1);
+      assert.match(err, /unknown flag --unknown/);
+    });
+  });
+
+  describe('26e — rotated files queryable via lib/audit-query.mjs', () => {
+    let TMP;
+    before(() => { TMP = _mkdtempSyncForSetup(_pathJoinForSetup(_tmpdirForSetup(), 'olp-test-26e-')); });
+    after(() => { rmSync(TMP, { recursive: true, force: true }); __resetAuditRotateState(); });
+
+    it('26e-1: after rotation, discoverAuditFiles + readAuditWindow see both live + rotated', () => {
+      __resetAuditRotateState();
+      const yesterday = new Date(Date.now() - 86400 * 1000).toISOString().slice(0, 10);
+      const today = new Date().toISOString().slice(0, 10);
+      writeAuditFile(TMP, 'audit.ndjson', [
+        makeEvent(`${yesterday}T10:00:00Z`),
+        makeEvent(`${yesterday}T11:00:00Z`),
+      ]);
+      __setLastSeenUtcDateForTesting(yesterday);
+
+      // Sync rotation: completes before append returns
+      appendAuditEventS26(makeEvent(`${today}T05:00:00Z`), { olpHome: TMP });
+
+      const files = discoverAuditFiles({ olpHome: TMP });
+      assert.ok(files.has('live'), 'live file present');
+      assert.ok(files.has(yesterday), `rotated file for ${yesterday} present`);
+
+      const startMs = Date.parse(`${yesterday}T00:00:00Z`);
+      const endMs = Date.parse(`${today}T23:59:59Z`);
+      const events = [...readAuditWindow({ startMs, endMs, olpHome: TMP })];
+      assert.equal(events.length, 3, 'cross-file read yields yesterday(2) + today(1) = 3 events');
+    });
+  });
+});