diff --git a/hermes_cli/dashboard_auth/audit.py b/hermes_cli/dashboard_auth/audit.py
index af63fc0648..21cbd0de3c 100644
--- a/hermes_cli/dashboard_auth/audit.py
+++ b/hermes_cli/dashboard_auth/audit.py
@@ -8,6 +8,7 @@ import datetime as _dt
 import enum
 import json
 import logging
+import os
 import threading
 from pathlib import Path
 from typing import Any
@@ -17,6 +18,15 @@ _write_lock = threading.Lock()
 _MAX_FIELD_LENGTH = 256
 _TRUNCATION_MARKER = "...[truncated]"
 
+# Size-based rotation matching hermes_logging's agent.log / errors.log convention
+# (maxBytes=5MB, backupCount=3, stdlib RotatingFileHandler ``.1``/``.2``/``.3`` naming).
+# Implemented locally (check-and-rename, stdlib only) rather than importing
+# hermes_logging/_ManagedRotatingFileHandler: that would pull in hermes_constants
+# (and lazily hermes_cli.config) and break this module's import-light guarantee —
+# it is imported by middleware that runs very early in startup.
+_MAX_BYTES = 5 * 1024 * 1024
+_BACKUP_COUNT = 3
+
 # Field names that must never appear in the log raw; matching kwargs are dropped.
 _REDACTED_FIELDS: frozenset = frozenset({
     "access_token", "refresh_token", "code", "code_verifier",
@@ -64,18 +74,41 @@ def _bounded_value(value: Any) -> Any:
     return value
 
 
+def _rotate_if_needed(path: Path, pending_bytes: int) -> None:
+    """Rotate ``path`` to ``.1``/…/``.<_BACKUP_COUNT>`` once appending *pending_bytes* would
+    reach ``_MAX_BYTES``. Mirrors stdlib ``RotatingFileHandler``'s naming; must be called
+    with ``_write_lock`` held. Rotation errors must not break the write path."""
+    try:
+        if path.stat().st_size + pending_bytes < _MAX_BYTES:
+            return
+    except OSError:
+        return  # missing/vanished mid-check; the append itself surfaces real errors
+    try:
+        for i in range(_BACKUP_COUNT - 1, 0, -1):
+            src = path.with_name(f"{path.name}.{i}")
+            if src.exists():
+                os.replace(src, path.with_name(f"{path.name}.{i + 1}"))
+        os.replace(path, path.with_name(f"{path.name}.1"))
+    except OSError:
+        pass  # a log that cannot be rotated is still a working (if overlong) log
+
+
 def audit_log(event: AuditEvent, **fields: Any) -> None:
-    """Append one event; token-like fields dropped, log dir created. Write failures are logged at
-    WARNING but never raise — auth must not fail because the audit logger broke."""
+    """Append one event; token-like fields dropped, log dir created, size-capped rotation
+    (``_MAX_BYTES`` with ``_BACKUP_COUNT`` backups). Write failures are logged at WARNING
+    but never raise — auth must not fail because the audit logger broke."""
     try:
         entry = {
             "ts": _dt.datetime.now(_dt.timezone.utc).isoformat(),
             "event": event.value,
             **{k: _bounded_value(v) for k, v in fields.items() if k not in _REDACTED_FIELDS}}
         line = json.dumps(entry, separators=(",", ":")) + "\n"
+        line_bytes = line.encode("utf-8")
         path = _resolve_log_path()
         path.parent.mkdir(parents=True, exist_ok=True)
-        with _write_lock, open(path, "a", encoding="utf-8") as f:
-            f.write(line)
+        with _write_lock:
+            _rotate_if_needed(path, len(line_bytes))
+            with open(path, "ab") as f:
+                f.write(line_bytes)
     except Exception as e:
         _log.warning("dashboard-auth audit log write failed: %s", e)
diff --git a/tests/hermes_cli/test_dashboard_auth_audit.py b/tests/hermes_cli/test_dashboard_auth_audit.py
index 97d8250f70..cf8df5c71b 100644
--- a/tests/hermes_cli/test_dashboard_auth_audit.py
+++ b/tests/hermes_cli/test_dashboard_auth_audit.py
@@ -67,6 +67,43 @@ def test_audit_writes_jsonlines(profile_home):
     assert "ts" in second  # ISO-8601 timestamp
 
 
+def test_audit_rotates_when_over_max_bytes(profile_home, monkeypatch):
+    """Rotation is bounded: at most ``_BACKUP_COUNT`` ``.N`` files, current log under the cap."""
+    import hermes_cli.dashboard_auth.audit as audit_module
+
+    monkeypatch.setattr(audit_module, "_MAX_BYTES", 2000)
+    monkeypatch.setattr(audit_module, "_BACKUP_COUNT", 3)
+
+    for i in range(500):
+        audit_log(AuditEvent.LOGIN_START, provider="x" * 50, i=i)
+
+    logs = profile_home / "logs"
+    assert (logs / "dashboard-auth.log").exists()
+    for i in (1, 2, 3):
+        assert (logs / f"dashboard-auth.log.{i}").exists()
+    assert not (logs / "dashboard-auth.log.4").exists()
+    assert (logs / "dashboard-auth.log").stat().st_size < 2000
+
+
+def test_audit_rotates_on_crossing_write(profile_home, monkeypatch):
+    """A write that pushes the file to/past the cap must rotate BEFORE appending (the pending
+    line's bytes count toward the threshold), not stay over the cap until the next event."""
+    import hermes_cli.dashboard_auth.audit as audit_module
+
+    monkeypatch.setattr(audit_module, "_MAX_BYTES", 200)
+    monkeypatch.setattr(audit_module, "_BACKUP_COUNT", 3)
+
+    logs = profile_home / "logs"
+    logs.mkdir()
+    prior = logs / "dashboard-auth.log"
+    prior.write_bytes(b"x" * 190)
+
+    audit_log(AuditEvent.LOGIN_START, provider="nous")
+
+    assert (logs / "dashboard-auth.log.1").read_bytes() == b"x" * 190
+    assert prior.stat().st_size < 200
+
+
 def test_audit_redacts_token_like_fields(profile_home):
     audit_log(
         AuditEvent.LOGIN_SUCCESS,