diff --git a/lib/microwaveprop_web/grid_bounds.ex b/lib/microwaveprop_web/grid_bounds.ex
index b809a8c9..b6fd215d 100644
--- a/lib/microwaveprop_web/grid_bounds.ex
+++ b/lib/microwaveprop_web/grid_bounds.ex
@@ -22,11 +22,22 @@ defmodule MicrowavepropWeb.GridBounds do
   @lon_min -150.0
   @lon_max -50.0
 
-  # Comfortably above the HRDPS+HRRR union (~3150 sq deg), but tight
-  # enough that the fully-clamped extent (~4500 sq deg) still trips
-  # the cap — a "give me everything" request is exactly the DoS
-  # shape we want to reject.
-  @max_viewport_area_sq_deg 4000.0
+  # Sits just above the largest area the clamp above can produce
+  # (45° lat × 100° lon = 4500 sq deg), so a fully zoomed-out client
+  # gets the whole grid rather than a 413.
+  #
+  # The clamp — not this cap — is what bounds the work: after clamping,
+  # no request can ask for more than the full grid (~92k cells, ~1.4 MB),
+  # which the pipeline already materializes once per valid_time and
+  # serves from cache. An earlier 4000.0 was tuned to reject the
+  # full-extent request as a DoS shape, but that is also exactly what
+  # the map requests at its own `minZoom: 4`, so it silently blanked
+  # the overlay whenever a user zoomed all the way out.
+  #
+  # Kept as defense-in-depth: if the clamp window is ever widened (e.g.
+  # extending coverage to Alaska), this catches the resulting blow-up
+  # instead of letting response size grow unbounded.
+  @max_viewport_area_sq_deg 4600.0
 
   @type bounds :: %{optional(String.t()) => float()} | nil
 
diff --git a/test/microwaveprop_web/controllers/scores_controller_test.exs b/test/microwaveprop_web/controllers/scores_controller_test.exs
index 1a9981a7..af97da7f 100644
--- a/test/microwaveprop_web/controllers/scores_controller_test.exs
+++ b/test/microwaveprop_web/controllers/scores_controller_test.exs
@@ -101,10 +101,16 @@ defmodule MicrowavepropWeb.ScoresControllerTest do
       assert json_response(conn, 400) == %{"error" => "invalid params"}
     end
 
-    test "returns 413 for global / oversized viewport bounds", %{conn: conn} do
+    # Mirrors /weather/cells: a fully zoomed-out viewport clamps to the
+    # supported extent and is served rather than rejected.
+    test "serves global / oversized viewport bounds by clamping them", %{conn: conn} do
+      valid_time = ~U[2026-04-28 12:00:00Z]
+      ScoresFile.write!(10_000, valid_time, [%{lat: 33.0, lon: -97.0, score: 75}])
+
       conn = get(conn, ~p"/scores/cells?band=10000&south=-90&north=90&west=-180&east=180")
-      body = json_response(conn, 413)
-      assert body["error"] == "viewport_too_large"
+
+      assert conn.status == 200
+      assert <<"PSCR", 1::8, _reserved::24, 1::little-32, _rest::binary>> = conn.resp_body
     end
   end
 end
diff --git a/test/microwaveprop_web/controllers/weather_tile_controller_test.exs b/test/microwaveprop_web/controllers/weather_tile_controller_test.exs
index c1dbefeb..b0dd59df 100644
--- a/test/microwaveprop_web/controllers/weather_tile_controller_test.exs
+++ b/test/microwaveprop_web/controllers/weather_tile_controller_test.exs
@@ -169,7 +169,10 @@ defmodule MicrowavepropWeb.WeatherTileControllerTest do
       File.rm_rf!(hrdps_dir)
     end
 
-    test "returns 413 for global / oversized viewport bounds", %{conn: conn} do
+    # A fully zoomed-out map (minZoom: 4) sends bounds well past the grid
+    # extent. Those clamp down to the supported region and are served —
+    # rejecting them left the overlay blank with no feedback.
+    test "serves global / oversized viewport bounds by clamping them", %{conn: conn} do
       time = DateTime.to_iso8601(~U[2026-04-28 12:00:00Z])
 
       conn =
@@ -178,8 +181,8 @@ defmodule MicrowavepropWeb.WeatherTileControllerTest do
           ~p"/weather/cells?time=#{time}&south=-90&north=90&west=-180&east=180"
         )
 
-      body = json_response(conn, 413)
-      assert body["error"] == "viewport_too_large"
+      assert conn.status == 200
+      assert <<"WCEL", 1::8, _rest::binary>> = conn.resp_body
     end
   end
 end
diff --git a/test/microwaveprop_web/grid_bounds_test.exs b/test/microwaveprop_web/grid_bounds_test.exs
index ba922a07..5b7c6430 100644
--- a/test/microwaveprop_web/grid_bounds_test.exs
+++ b/test/microwaveprop_web/grid_bounds_test.exs
@@ -15,12 +15,25 @@ defmodule MicrowavepropWeb.GridBoundsTest do
 
     test "clamps south/north/west/east to the supported extent" do
       bounds = %{"south" => -50.0, "north" => 90.0, "west" => -200.0, "east" => 50.0}
-      assert {:error, :viewport_too_large} = GridBounds.clamp(bounds)
+
+      assert {:ok, %{"south" => 20.0, "north" => 65.0, "west" => -150.0, "east" => -50.0}} =
+               GridBounds.clamp(bounds)
     end
 
-    test "rejects global bounds with 413-equivalent error" do
+    # The map's own `minZoom: 4` produces a viewport that clamps to the
+    # full extent. Rejecting it blanked the overlay whenever a user
+    # zoomed all the way out, so the clamped full grid must be served.
+    test "serves global bounds by clamping them to the supported extent" do
       global = %{"south" => -90.0, "north" => 90.0, "west" => -180.0, "east" => 180.0}
-      assert GridBounds.clamp(global) == {:error, :viewport_too_large}
+
+      assert {:ok, %{"south" => 20.0, "north" => 65.0, "west" => -150.0, "east" => -50.0}} =
+               GridBounds.clamp(global)
+    end
+
+    # The cap is defense-in-depth against the clamp window being widened
+    # later; nothing the clamp can emit today exceeds it.
+    test "the clamped extent stays inside the area cap" do
+      assert (65.0 - 20.0) * (-50.0 - -150.0) <= GridBounds.max_viewport_area_sq_deg()
     end
 
     test "rejects inverted bounds" do