diff --git a/tasks/sud.py b/tasks/sud.py
index 3cb3502..e89cd82 100644
--- a/tasks/sud.py
+++ b/tasks/sud.py
@@ -7,6 +7,34 @@ from components import APid, AStirrer
 from components.plant import APlant
 from components.sud import Sud, SudState
 
+# Upper bound on how many (t, theta) points the forecast is thinned to
+# before going out over the wire (see _send_forecast()) - a fine enough
+# dt over a multi-hour brew can otherwise produce a single JSON message
+# of several MB, large enough to exceed the websockets library's default
+# 1 MiB max_size and get the connection closed outright (code 1009). The
+# GUI's forecast plot is a few hundred pixels wide, so this many points
+# is already far more resolution than it can show; self.forecast_t/
+# forecast_theta themselves stay at full simulated resolution - this
+# only thins the copy actually sent to clients.
+MAX_FORECAST_POINTS = 1000
+
+
+def _downsample(t, theta, max_points=MAX_FORECAST_POINTS):
+	"""Returns (t, theta) thinned to at most max_points entries by simple
+	decimation, always keeping the first and last point - losing a few
+	intermediate samples doesn't matter for a plot this size, but losing
+	the endpoints would visibly truncate the curve or its final value."""
+	n = len(t)
+	if n <= max_points:
+		return t, theta
+	step = -(-n // max_points)  # ceil(n / max_points)
+	t_ds = t[::step]
+	theta_ds = theta[::step]
+	if t_ds[-1] != t[-1]:
+		t_ds = t_ds + [t[-1]]
+		theta_ds = theta_ds + [theta[-1]]
+	return t_ds, theta_ds
+
 
 class SudTask(ATask):
 	def __init__(self, sud: Sud, tc: APid, stirrer: AStirrer, pot: APlant, dt, interval, msg_handler: MsgIo,
@@ -166,9 +194,10 @@ class SudTask(ATask):
 		await self._send_forecast()
 
 	async def _send_forecast(self):
+		t, theta = _downsample(self.forecast_t, self.forecast_theta)
 		await self.send({'Forecast': {
-			'T': self.forecast_t,
-			'Theta': self.forecast_theta,
+			'T': t,
+			'Theta': theta,
 			'Finished': self.forecast_finished,
 		}})