diff --git a/backend/app/api/agent.py b/backend/app/api/agent.py
index 8d149795..2a826cad 100644
--- a/backend/app/api/agent.py
+++ b/backend/app/api/agent.py
@@ -373,10 +373,14 @@ async def list_boards(
         # Main agents (board_id=None) must be scoped to their organization
         # via their gateway to prevent cross-tenant board leakage.
         gateway = await Gateway.objects.by_id(agent_ctx.agent.gateway_id).first(session)
-        if gateway is not None:
-            statement = statement.where(
-                col(Board.organization_id) == gateway.organization_id,
+        if gateway is None:
+            raise HTTPException(
+                status_code=status.HTTP_403_FORBIDDEN,
+                detail="Agent gateway not found; cannot determine organization scope.",
             )
+        statement = statement.where(
+            col(Board.organization_id) == gateway.organization_id,
+        )
     statement = statement.order_by(col(Board.created_at).desc())
     return await paginate(session, statement)
 
diff --git a/backend/app/api/board_webhooks.py b/backend/app/api/board_webhooks.py
index e1d2dbf5..eb7dd870 100644
--- a/backend/app/api/board_webhooks.py
+++ b/backend/app/api/board_webhooks.py
@@ -501,6 +501,8 @@ async def ingest_board_webhook(
         )
 
     # Enforce a 1 MB payload size limit to prevent memory exhaustion.
+    # Read the body in chunks via request.stream() so an attacker cannot
+    # cause OOM by sending a huge body with a missing/spoofed Content-Length.
     max_payload_bytes = 1_048_576
     content_length = request.headers.get("content-length")
     if content_length and int(content_length) > max_payload_bytes:
@@ -508,12 +510,17 @@ async def ingest_board_webhook(
             status_code=status.HTTP_413_CONTENT_TOO_LARGE,
             detail=f"Payload exceeds maximum size of {max_payload_bytes} bytes.",
         )
-    raw_body = await request.body()
-    if len(raw_body) > max_payload_bytes:
-        raise HTTPException(
-            status_code=status.HTTP_413_CONTENT_TOO_LARGE,
-            detail=f"Payload exceeds maximum size of {max_payload_bytes} bytes.",
-        )
+    chunks: list[bytes] = []
+    total_size = 0
+    async for chunk in request.stream():
+        total_size += len(chunk)
+        if total_size > max_payload_bytes:
+            raise HTTPException(
+                status_code=status.HTTP_413_CONTENT_TOO_LARGE,
+                detail=f"Payload exceeds maximum size of {max_payload_bytes} bytes.",
+            )
+        chunks.append(chunk)
+    raw_body = b"".join(chunks)
     _verify_webhook_signature(webhook, raw_body, request)
 
     content_type = request.headers.get("content-type")
diff --git a/backend/app/core/rate_limit.py b/backend/app/core/rate_limit.py
index 2487474b..6b14b0da 100644
--- a/backend/app/core/rate_limit.py
+++ b/backend/app/core/rate_limit.py
@@ -11,6 +11,9 @@ import time
 from collections import defaultdict
 from threading import Lock
 
+# Run a full sweep of all keys every 128 calls to is_allowed.
+_CLEANUP_INTERVAL = 128
+
 
 class InMemoryRateLimiter:
     """Token-bucket rate limiter keyed by arbitrary string (typically client IP)."""
@@ -20,14 +23,30 @@ class InMemoryRateLimiter:
         self._window_seconds = window_seconds
         self._buckets: dict[str, list[float]] = defaultdict(list)
         self._lock = Lock()
+        self._call_count = 0
+
+    def _sweep_expired(self, cutoff: float) -> None:
+        """Remove keys whose timestamps have all expired."""
+        expired_keys = [
+            k for k, ts_list in self._buckets.items()
+            if not ts_list or ts_list[-1] <= cutoff
+        ]
+        for k in expired_keys:
+            del self._buckets[k]
 
     def is_allowed(self, key: str) -> bool:
         """Return True if the request should be allowed, False if rate-limited."""
         now = time.monotonic()
         cutoff = now - self._window_seconds
         with self._lock:
+            self._call_count += 1
+            # Periodically sweep all keys to evict stale entries from
+            # clients that have stopped making requests.
+            if self._call_count % _CLEANUP_INTERVAL == 0:
+                self._sweep_expired(cutoff)
+
             timestamps = self._buckets[key]
-            # Prune expired entries
+            # Prune expired entries for the current key
             self._buckets[key] = [ts for ts in timestamps if ts > cutoff]
             if len(self._buckets[key]) >= self._max_requests:
                 return False
diff --git a/backend/tests/test_rate_limit.py b/backend/tests/test_rate_limit.py
index 5d2f025a..66fe00a2 100644
--- a/backend/tests/test_rate_limit.py
+++ b/backend/tests/test_rate_limit.py
@@ -43,3 +43,29 @@ def test_window_expiry_resets_limit() -> None:
     future = time.monotonic() + 2.0
     with patch("time.monotonic", return_value=future):
         assert limiter.is_allowed("client-a") is True
+
+
+def test_sweep_removes_expired_keys() -> None:
+    """Keys whose timestamps have all expired should be evicted during periodic sweep."""
+    from app.core.rate_limit import _CLEANUP_INTERVAL
+
+    limiter = InMemoryRateLimiter(max_requests=100, window_seconds=1.0)
+
+    # Fill with many unique IPs
+    for i in range(10):
+        limiter.is_allowed(f"stale-{i}")
+
+    assert len(limiter._buckets) == 10
+
+    # Advance time so all timestamps expire, then trigger enough calls to
+    # hit the cleanup interval.
+    future = time.monotonic() + 2.0
+    with patch("time.monotonic", return_value=future):
+        # Drive the call count up to a multiple of _CLEANUP_INTERVAL
+        remaining = _CLEANUP_INTERVAL - (limiter._call_count % _CLEANUP_INTERVAL)
+        for i in range(remaining):
+            limiter.is_allowed("trigger-sweep")
+
+    # Stale keys should have been swept; only "trigger-sweep" should remain
+    assert "stale-0" not in limiter._buckets
+    assert "trigger-sweep" in limiter._buckets