diff --git a/lib/microwaveprop/workers/era5_submit_worker.ex b/lib/microwaveprop/workers/era5_submit_worker.ex
index 3748d1cc..45619606 100644
--- a/lib/microwaveprop/workers/era5_submit_worker.ex
+++ b/lib/microwaveprop/workers/era5_submit_worker.ex
@@ -38,6 +38,16 @@ defmodule Microwaveprop.Workers.Era5SubmitWorker do
 
   @poll_delay_seconds 5 * 60
 
+  # CDS enforces a server-side per-user cap of 150 in-flight jobs. Each
+  # successful submit adds 2 rows to era5_cds_jobs (single-level +
+  # pressure-level), so we stop submitting once the in-flight count is
+  # within `@cap_headroom` of the ceiling. Snoozing (instead of failing
+  # or sleeping) releases the Oban slot immediately and lets the poll
+  # workers drain in-flight jobs before we push more in.
+  @cds_ceiling 150
+  @cap_headroom 10
+  @snooze_seconds 5 * 60
+
   @impl Oban.Worker
   def backoff(%Oban.Job{attempt: attempt}) do
     # The submit itself is cheap; retry fast so transient CDS/HTTP blips
@@ -65,11 +75,24 @@ defmodule Microwaveprop.Workers.Era5SubmitWorker do
 
         {:ok, :already_submitted}
 
+      cds_queue_full?() ->
+        in_flight = Repo.aggregate(Era5CdsJob, :count, :id)
+
+        Logger.info(
+          "Era5Submit: CDS in-flight cap reached (#{in_flight}/#{@cds_ceiling}) — snoozing #{year}-#{pad(month)} tile #{tile_lat},#{tile_lon}"
+        )
+
+        {:snooze, @snooze_seconds}
+
       true ->
         submit_and_persist(year, month, tile_lat, tile_lon)
     end
   end
 
+  defp cds_queue_full? do
+    Repo.aggregate(Era5CdsJob, :count, :id) + 2 > @cds_ceiling - @cap_headroom
+  end
+
   defp submit_and_persist(year, month, tile_lat, tile_lon) do
     days = Calendar.ISO.days_in_month(year, month)
     area = tile_area(tile_lat, tile_lon)
diff --git a/test/microwaveprop/workers/era5_submit_worker_test.exs b/test/microwaveprop/workers/era5_submit_worker_test.exs
index 4a91c762..493f530c 100644
--- a/test/microwaveprop/workers/era5_submit_worker_test.exs
+++ b/test/microwaveprop/workers/era5_submit_worker_test.exs
@@ -124,6 +124,88 @@ defmodule Microwaveprop.Workers.Era5SubmitWorkerTest do
     end
   end
 
+  describe "CDS in-flight cap" do
+    # CDS rejects with "Number of queued requests is limited to 150" when
+    # a user has too many pending jobs. Each submit adds 2 rows to
+    # era5_cds_jobs (single-level + pressure-level), so we snooze when
+    # the in-flight count is close to the ceiling.
+    @cap_headroom 10
+    @cds_ceiling 150
+
+    test "snoozes when era5_cds_jobs count is at or above the ceiling - headroom" do
+      # Seed enough in-flight rows to push us over the snooze threshold.
+      seed_count = @cds_ceiling - @cap_headroom
+
+      rows =
+        for i <- 1..seed_count do
+          %{
+            id: Ecto.UUID.generate(),
+            year: 2014,
+            month: 3,
+            tile_lat: 32,
+            tile_lon: -98 - i,
+            single_job_id: "seed-single-#{i}",
+            pressure_job_id: "seed-pressure-#{i}",
+            submitted_at: DateTime.truncate(DateTime.utc_now(), :second),
+            inserted_at: DateTime.truncate(DateTime.utc_now(), :second),
+            updated_at: DateTime.truncate(DateTime.utc_now(), :second),
+            poll_count: 0
+          }
+        end
+
+      Repo.insert_all(Era5CdsJob, rows)
+
+      Req.Test.stub(Era5Client, fn _ -> raise "CDS should not be contacted when capped" end)
+
+      Oban.Testing.with_testing_mode(:manual, fn ->
+        assert {:snooze, seconds} = Era5SubmitWorker.perform(%Oban.Job{args: @default_args})
+        assert is_integer(seconds) and seconds > 0
+      end)
+
+      # No new era5_cds_jobs row inserted for the requested tile.
+      refute Repo.one(
+               from j in Era5CdsJob,
+                 where: j.year == 2014 and j.month == 3 and j.tile_lat == 32 and j.tile_lon == -98
+             )
+    end
+
+    test "submits normally when in-flight count is well below the ceiling" do
+      # 10 in-flight — comfortably under the cap.
+      rows =
+        for i <- 1..10 do
+          %{
+            id: Ecto.UUID.generate(),
+            year: 2013,
+            month: i,
+            tile_lat: 40,
+            tile_lon: -90,
+            single_job_id: "warm-single-#{i}",
+            pressure_job_id: "warm-pressure-#{i}",
+            submitted_at: DateTime.truncate(DateTime.utc_now(), :second),
+            inserted_at: DateTime.truncate(DateTime.utc_now(), :second),
+            updated_at: DateTime.truncate(DateTime.utc_now(), :second),
+            poll_count: 0
+          }
+        end
+
+      Repo.insert_all(Era5CdsJob, rows)
+
+      Req.Test.stub(Era5Client, fn conn ->
+        dataset_id =
+          cond do
+            String.contains?(conn.request_path, "single-levels") -> "fresh-single"
+            String.contains?(conn.request_path, "pressure-levels") -> "fresh-pressure"
+          end
+
+        Req.Test.json(conn, %{"jobID" => dataset_id})
+      end)
+
+      Oban.Testing.with_testing_mode(:manual, fn ->
+        assert {:ok, %Era5CdsJob{}} = Era5SubmitWorker.perform(%Oban.Job{args: @default_args})
+      end)
+    end
+  end
+
   describe "unique constraint" do
     test "collapses duplicate (year, month, tile_lat, tile_lon) Oban enqueues" do
       Oban.Testing.with_testing_mode(:manual, fn ->