diff --git a/wizard-work/beam/sliding_window.py b/wizard-work/beam/sliding_window.py
new file mode 100644
index 0000000..09ae228
--- /dev/null
+++ b/wizard-work/beam/sliding_window.py
@@ -0,0 +1,41 @@
+"""Sliding-window rate limiter.
+
+Public API:
+    SlidingWindow(limit, window) with .allow(key, now=None) -> bool
+Counts events per key within a rolling window of `window` seconds.
+Thread-safe? No (single-threaded). Deterministic given `now`.
+"""
+from __future__ import annotations
+
+from collections import defaultdict, deque
+from typing import Deque, Dict, Optional
+
+
+class SlidingWindow:
+    def __init__(self, limit: int, window: float):
+        if limit < 1:
+            raise ValueError("limit must be >= 1")
+        if window <= 0:
+            raise ValueError("window must be > 0")
+        self.limit = limit
+        self.window = window
+        self._events: Dict[object, Deque[float]] = defaultdict(deque)
+
+    def allow(self, key, now: Optional[float] = None) -> bool:
+        if now is None:
+            import time
+            now = time.monotonic()
+        q = self._events[key]
+        cutoff = now - self.window
+        while q and q[0] <= cutoff:
+            q.popleft()
+        if len(q) < self.limit:
+            q.append(now)
+            return True
+        return False
+
+    def reset(self, key=None):
+        if key is None:
+            self._events.clear()
+        else:
+            self._events.pop(key, None)
diff --git a/wizard-work/beam/test_sliding_window.py b/wizard-work/beam/test_sliding_window.py
new file mode 100644
index 0000000..2b3b936
--- /dev/null
+++ b/wizard-work/beam/test_sliding_window.py
@@ -0,0 +1,40 @@
+import sys
+sys.path.insert(0, "/root/.hermes/wizard-outputs/beam")
+from sliding_window import SlidingWindow
+
+def test_basic():
+    w = SlidingWindow(2, 10.0)
+    assert w.allow("k", 0) is True
+    assert w.allow("k", 1) is True
+    assert w.allow("k", 2) is False
+    assert w.allow("k", 11) is True  # first expired
+
+def test_window_boundary():
+    w = SlidingWindow(1, 5.0)
+    assert w.allow("k", 0) is True
+    assert w.allow("k", 5) is True  # exactly window: old event cutoff 0 <= 0 expired
+
+def test_per_key():
+    w = SlidingWindow(1, 10.0)
+    assert w.allow("a", 0) is True
+    assert w.allow("b", 0) is True
+    assert w.allow("a", 1) is False
+
+def test_invalid():
+    for kw in ({"limit":0,"window":1}, {"limit":1,"window":0}, {"limit":1,"window":-1}):
+        try:
+            SlidingWindow(**kw); raise SystemExit("expected ValueError")
+        except ValueError:
+            pass
+
+def test_reset():
+    w = SlidingWindow(1, 10.0)
+    assert w.allow("k",0) is True
+    assert w.allow("k",1) is False
+    w.reset("k")
+    assert w.allow("k",1) is True
+    w.allow("a",0); w.allow("b",0)
+    w.reset()
+    assert w.allow("a",0) is True
+
+print("ALL TESTS PASSED")