diff --git a/docs/runbook_propagation_pipeline.md b/docs/runbook_propagation_pipeline.md
index a62727d2..789d0902 100644
--- a/docs/runbook_propagation_pipeline.md
+++ b/docs/runbook_propagation_pipeline.md
@@ -58,7 +58,7 @@ unsubscribed for several seconds while Rust writes land.
 
 ### FM3 — NFS stale read racing Rust write
 
-**Symptom.** `ScoresFile.read_bounds/2` raises `File.Error` when
+**Symptom.** `ScoresFile.read/2` returns `{:error, reason}` when
 the reader opens the file exactly between Rust's `rename(2)` of the
 `.tmp.<uniq>` file and the `fsync` landing on the NFS server.
 
@@ -66,11 +66,13 @@ the reader opens the file exactly between Rust's `rename(2)` of the
 NFS client cache can briefly serve a stale directory listing or a
 no-longer-existing path.
 
-**Recovery.** Reconciler's `rescue File.Error` returns `:error` and
-moves on; the next tick re-reads the file after NFS consistency
-settles. Lazy-miss path (`Propagation.scores_at_fetch/3`) has
-historically surfaced this as a LiveView error — the reconciler
-warms the cache before that miss happens.
+**Recovery.** Reconciler calls `ScoresFile.read/2` directly and
+pattern-matches `{:error, _} -> :error`, logging at debug and
+skipping the key. The next tick re-reads the file after NFS
+consistency settles. Lazy-miss path
+(`Propagation.scores_at_fetch/3`) has historically surfaced this as
+a LiveView error — the reconciler warms the cache before that miss
+happens.
 
 ### FM4 — Cluster partition splits ScoreCache broadcast
 
@@ -99,7 +101,12 @@ scores.
 **Recovery.** Manual. `kubectl -n prop logs deploy/prop-grid-rs` for
 cause. The Elixir side is read-only against the grid — it does not
 regenerate scores from within the BEAM since the extraction in
-`65693ed`.
+`65693ed`. `lib/microwaveprop/workers/propagation_grid_worker.ex`
+still exists as a seed worker: the hourly cron fires it with empty
+args and it inserts 19 `grid_tasks` rows (f00 + f01..f18) for Rust
+to drain. All fetch/decode/score helpers moved to
+`rust/prop_grid_rs/src/pipeline.rs`; the Elixir module is retained
+only as the cron entry point, not as a fallback compute path.
 
 ## Monitoring signals
 
diff --git a/lib/microwaveprop/propagation.ex b/lib/microwaveprop/propagation.ex
index 03dcf2e3..f4fed9dd 100644
--- a/lib/microwaveprop/propagation.ex
+++ b/lib/microwaveprop/propagation.ex
@@ -330,12 +330,23 @@ defmodule Microwaveprop.Propagation do
   cluster. Called from `PropagationGridWorker` after each forecast
   hour so all pods have a warm cache by the time clients begin
   requesting the new hour.
+
+  Returns `{:error, reason}` when the score file is missing or
+  corrupt — callers distinguish those from the successful empty-grid
+  case to avoid poisoning the cache with `[]` on a bad read.
   """
-  @spec warm_cache_and_broadcast(non_neg_integer(), DateTime.t()) :: :ok
+  @spec warm_cache_and_broadcast(non_neg_integer(), DateTime.t()) ::
+          :ok | {:error, :enoent | :invalid_format}
   def warm_cache_and_broadcast(band_mhz, valid_time) do
-    scores = ScoresFile.read_bounds(band_mhz, valid_time)
-    ScoreCache.broadcast_put(band_mhz, valid_time, scores)
-    :ok
+    case ScoresFile.read(band_mhz, valid_time) do
+      {:ok, payload} ->
+        scores = ScoresFile.extract_points(payload, nil)
+        ScoreCache.broadcast_put(band_mhz, valid_time, scores)
+        :ok
+
+      {:error, reason} ->
+        {:error, reason}
+    end
   end
 
   defp filter_bounds(scores, nil), do: scores
diff --git a/lib/microwaveprop/propagation/notify_listener.ex b/lib/microwaveprop/propagation/notify_listener.ex
index b2a35241..30c8f339 100644
--- a/lib/microwaveprop/propagation/notify_listener.ex
+++ b/lib/microwaveprop/propagation/notify_listener.ex
@@ -107,11 +107,18 @@ defmodule Microwaveprop.Propagation.NotifyListener do
   end
 
   defp warm_band(band_mhz, valid_time) do
-    Propagation.warm_cache_and_broadcast(band_mhz, valid_time)
-    :ok
+    case Propagation.warm_cache_and_broadcast(band_mhz, valid_time) do
+      :ok ->
+        :ok
+
+      {:error, reason} ->
+        Logger.warning("NotifyListener: warm skipped band=#{band_mhz} valid_time=#{valid_time}: #{inspect(reason)}")
+
+        :error
+    end
   rescue
     e ->
-      Logger.warning("NotifyListener: warm failed for band=#{band_mhz} valid_time=#{valid_time}: #{inspect(e)}")
+      Logger.warning("NotifyListener: warm crashed band=#{band_mhz} valid_time=#{valid_time}: #{inspect(e)}")
 
       :error
   end
diff --git a/lib/microwaveprop/propagation/score_cache_reconciler.ex b/lib/microwaveprop/propagation/score_cache_reconciler.ex
index 12a092ab..2bab079a 100644
--- a/lib/microwaveprop/propagation/score_cache_reconciler.ex
+++ b/lib/microwaveprop/propagation/score_cache_reconciler.ex
@@ -90,15 +90,21 @@ defmodule Microwaveprop.Propagation.ScoreCacheReconciler do
     end)
   end
 
+  # File may vanish between list and read if a prune runs concurrently,
+  # or be half-written during an atomic-rename race. `ScoresFile.read/2`
+  # reports both as `{:error, _}`; we skip and self-heal on the next
+  # sweep rather than caching an empty grid under that key.
   defp warm_one(band_mhz, valid_time) do
-    scores = ScoresFile.read_bounds(band_mhz, valid_time)
-    ScoreCache.put(band_mhz, valid_time, scores)
-    :ok
-  rescue
-    # File may vanish between list and read if a prune runs concurrently.
-    # Missing files are self-healing on the next sweep — no point logging.
-    e in [File.Error] ->
-      Logger.debug("ScoreCacheReconciler: skipped #{band_mhz}/#{valid_time}: #{inspect(e)}")
-      :error
+    case ScoresFile.read(band_mhz, valid_time) do
+      {:ok, payload} ->
+        scores = ScoresFile.extract_points(payload, nil)
+        ScoreCache.put(band_mhz, valid_time, scores)
+        :ok
+
+      {:error, reason} ->
+        Logger.debug("ScoreCacheReconciler: skipped #{band_mhz}/#{valid_time}: #{inspect(reason)}")
+
+        :error
+    end
   end
 end
diff --git a/lib/microwaveprop/propagation/scores_file.ex b/lib/microwaveprop/propagation/scores_file.ex
index 3e5f6fe2..f450b5a2 100644
--- a/lib/microwaveprop/propagation/scores_file.ex
+++ b/lib/microwaveprop/propagation/scores_file.ex
@@ -361,7 +361,15 @@ defmodule Microwaveprop.Propagation.ScoresFile do
 
   # ── Read helpers ────────────────────────────────────────────────
 
-  defp extract_points(%{scores: body} = payload, bounds) do
+  @doc """
+  Decode a `read/2` payload into `[%{lat, lon, score}]`. Exposed so
+  callers that need to distinguish "missing file" from "empty grid"
+  can invoke `read/2` directly and feed the payload here after their
+  own error-path handling.
+  """
+  @spec extract_points(map(), map() | nil) ::
+          [%{lat: float(), lon: float(), score: non_neg_integer()}]
+  def extract_points(%{scores: body} = payload, bounds) do
     %{lat_min: lat_min, lon_min: lon_min, step: step, n_rows: n_rows, n_cols: n_cols} = payload
     {row_lo, row_hi, col_lo, col_hi} = bounds_to_index_range(payload, bounds)
 
diff --git a/lib/microwaveprop/weather.ex b/lib/microwaveprop/weather.ex
index 2638d167..75a314c4 100644
--- a/lib/microwaveprop/weather.ex
+++ b/lib/microwaveprop/weather.ex
@@ -5,6 +5,7 @@ defmodule Microwaveprop.Weather do
 
   alias Microwaveprop.Propagation.ProfilesFile
   alias Microwaveprop.Radio
+  alias Microwaveprop.Radio.Contact
   alias Microwaveprop.Repo
   alias Microwaveprop.Weather.GefsProfile
   alias Microwaveprop.Weather.GridCache
@@ -627,7 +628,9 @@ defmodule Microwaveprop.Weather do
 
       # Prefer explicit values from the profile map (Rust-written ProfilesFile
       # persists derived sounding params alongside the raw profile) and fall
-      # back to deriving from the profile list when absent.
+      # back to deriving from the profile list when absent. `prefer/3` uses
+      # `Map.fetch/2` so a legitimate `false` / `0` persisted value is not
+      # silently clobbered by a `||` fallback.
       row = %{
         lat: lat,
         lon: lon,
@@ -636,20 +639,31 @@ defmodule Microwaveprop.Weather do
         dewpoint_depression: depression(temp_c, profile[:surface_dewpoint_c]),
         bl_height: profile[:hpbl_m],
         pwat: profile[:pwat_mm],
-        refractivity_gradient: profile[:min_refractivity_gradient] || sounding[:min_refractivity_gradient],
-        ducting: profile[:ducting_detected] || sounding[:ducting_detected],
+        refractivity_gradient: prefer(profile, :min_refractivity_gradient, sounding[:min_refractivity_gradient]),
+        ducting: prefer(profile, :ducting_detected, sounding[:ducting_detected]),
         surface_pressure_mb: profile[:surface_pressure_mb],
         surface_temp_c: temp_c,
         surface_dewpoint_c: profile[:surface_dewpoint_c],
-        surface_refractivity: profile[:surface_refractivity] || sounding[:surface_refractivity],
+        surface_refractivity: prefer(profile, :surface_refractivity, sounding[:surface_refractivity]),
         profile: profile[:profile] || [],
-        duct_characteristics: profile[:duct_characteristics] || sounding[:duct_characteristics]
+        duct_characteristics: prefer(profile, :duct_characteristics, sounding[:duct_characteristics])
       }
 
       [derive_and_clean(row)]
     end
   end
 
+  # Fetch `key` from `profile` verbatim when present (including `false`,
+  # `0`, or `[]`); only fall through to the derived value when the key is
+  # absent. `profile[key] || default` would discard legitimate `false` /
+  # `0` as if they weren't set.
+  defp prefer(profile, key, default) do
+    case Map.fetch(profile, key) do
+      {:ok, value} -> value
+      :error -> default
+    end
+  end
+
   defp derive_sounding(profile) when is_list(profile) and length(profile) >= 3 do
     SoundingParams.derive(profile) || %{}
   end
@@ -848,11 +862,19 @@ defmodule Microwaveprop.Weather do
   nearest HRRR hour. Lets the enrichment enqueuer skip :queued → :queued
   churn when the backing data is already present.
 
-  Returns `false` if `pos1` is nil — a contact with no position can't be
-  looked up.
+  Returns `false` if `pos1` or `qso_timestamp` is nil — a contact with no
+  position or no timestamp can't be looked up.
   """
-  @spec hrrr_data_fully_present?(map()) :: boolean()
+  @spec hrrr_data_fully_present?(
+          Contact.t()
+          | %{
+              required(:pos1) => term(),
+              required(:qso_timestamp) => DateTime.t() | nil,
+              optional(:pos2) => term()
+            }
+        ) :: boolean()
   def hrrr_data_fully_present?(%{pos1: nil}), do: false
+  def hrrr_data_fully_present?(%{qso_timestamp: nil}), do: false
 
   def hrrr_data_fully_present?(contact) do
     rounded = HrrrClient.nearest_hrrr_hour(contact.qso_timestamp)
diff --git a/lib/microwaveprop/workers/admin_task_worker.ex b/lib/microwaveprop/workers/admin_task_worker.ex
index e9cfcba2..408dff1d 100644
--- a/lib/microwaveprop/workers/admin_task_worker.ex
+++ b/lib/microwaveprop/workers/admin_task_worker.ex
@@ -198,9 +198,11 @@ defmodule Microwaveprop.Workers.AdminTaskWorker do
   end
 
   # Bulk-update via one `UPDATE ... FROM unnest(...)` statement per batch:
-  # one Postgres round-trip and one fsync instead of N, which was the
-  # dominant cost for runs of 10k profiles against the remote DB host.
-  @bulk_update_chunk 2000
+  # one Postgres round-trip and one fsync instead of N. Chunk size is
+  # conservative to contain the blast radius of a single bad row
+  # (deleted UUID, bad duct payload) — the chunk re-runs row-by-row on
+  # failure so the remaining rows in the job still make progress.
+  @bulk_update_chunk 500
 
   defp derive_native_row(profile) do
     p = %{
@@ -246,7 +248,33 @@ defmodule Microwaveprop.Workers.AdminTaskWorker do
   defp bulk_update_native_derivations(rows) do
     rows
     |> Enum.chunk_every(@bulk_update_chunk)
-    |> Enum.reduce(0, fn chunk, acc -> acc + execute_bulk_update(chunk) end)
+    |> Enum.reduce(0, fn chunk, acc -> acc + safe_execute_chunk(chunk) end)
+  end
+
+  # Wraps the chunk UPDATE so one bad row doesn't kill the whole job.
+  # On Postgrex failure, fall back to per-row updates for just that
+  # chunk — rows that still succeed contribute to the count; rows that
+  # re-raise are logged and skipped.
+  defp safe_execute_chunk(chunk) do
+    execute_bulk_update(chunk)
+  rescue
+    e in [Postgrex.Error, DBConnection.EncodeError] ->
+      Logger.warning("AdminTask.native_derive: bulk chunk failed (#{inspect(e)}); falling back row-by-row")
+
+      per_row_fallback(chunk)
+  end
+
+  defp per_row_fallback(chunk) do
+    Enum.reduce(chunk, 0, fn row, acc ->
+      try do
+        execute_bulk_update([row])
+        acc + 1
+      rescue
+        e ->
+          Logger.warning("AdminTask.native_derive: skip id=#{row.id}: #{inspect(e)}")
+          acc
+      end
+    end)
   end
 
   defp execute_bulk_update(chunk) do
diff --git a/mix.exs b/mix.exs
index 3ecc79f5..66353b9c 100644
--- a/mix.exs
+++ b/mix.exs
@@ -14,7 +14,14 @@ defmodule Microwaveprop.MixProject do
       listeners: [Phoenix.CodeReloader],
       dialyzer: [
         plt_add_apps: [:mix, :ex_unit],
-        plt_file: {:no_warn, "priv/plts/project.plt"}
+        plt_file: {:no_warn, "priv/plts/project.plt"},
+        flags: [
+          :error_handling,
+          :unknown,
+          :unmatched_returns,
+          :extra_return,
+          :missing_return
+        ]
       ]
     ]
   end
diff --git a/test/microwaveprop/propagation/score_cache_reconciler_test.exs b/test/microwaveprop/propagation/score_cache_reconciler_test.exs
index 02100e8a..5cc82fae 100644
--- a/test/microwaveprop/propagation/score_cache_reconciler_test.exs
+++ b/test/microwaveprop/propagation/score_cache_reconciler_test.exs
@@ -3,6 +3,8 @@ defmodule Microwaveprop.Propagation.ScoreCacheReconcilerTest do
   # Application env and mutates the shared ScoreCache ETS table.
   use ExUnit.Case, async: false
 
+  import ExUnit.CaptureLog
+
   alias Microwaveprop.Propagation.BandConfig
   alias Microwaveprop.Propagation.Grid
   alias Microwaveprop.Propagation.ScoreCache
@@ -74,4 +76,109 @@ defmodule Microwaveprop.Propagation.ScoreCacheReconcilerTest do
       assert 0 = ScoreCacheReconciler.sweep_once()
     end
   end
+
+  describe "GenServer lifecycle" do
+    # The gate in start_link/1 defaults :start_score_cache_reconciler to
+    # false in the test env. Flip it on for this block so start_supervised!
+    # gets a live pid rather than :ignore.
+    setup do
+      Application.put_env(:microwaveprop, :start_score_cache_reconciler, true)
+
+      on_exit(fn ->
+        Application.put_env(:microwaveprop, :start_score_cache_reconciler, false)
+      end)
+
+      :ok
+    end
+
+    defp wait_for_cache(band_mhz, valid_time, timeout_ms) do
+      deadline = System.monotonic_time(:millisecond) + timeout_ms
+      poll_cache(band_mhz, valid_time, deadline)
+    end
+
+    defp poll_cache(band_mhz, valid_time, deadline) do
+      case ScoreCache.fetch(band_mhz, valid_time) do
+        {:ok, _} = ok ->
+          ok
+
+        :miss ->
+          if System.monotonic_time(:millisecond) >= deadline do
+            :timeout
+          else
+            Process.sleep(25)
+            poll_cache(band_mhz, valid_time, deadline)
+          end
+      end
+    end
+
+    test "run_on_start: true warms cache from disk via handle_info(:sweep)",
+         %{band_mhz: band_mhz} do
+      valid_time = ~U[2026-04-21 14:00:00Z]
+      ScoresFile.write!(band_mhz, valid_time, sample_scores())
+
+      assert :miss = ScoreCache.fetch(band_mhz, valid_time)
+
+      start_supervised!({ScoreCacheReconciler, run_on_start: true, interval_ms: 100})
+
+      # The boot timer is 500ms, so the first sweep lands somewhere
+      # around t+500ms. Give it generous headroom to avoid flakes on
+      # loaded CI workers.
+      assert {:ok, _scores} = wait_for_cache(band_mhz, valid_time, 2_000)
+    end
+
+    test "run_on_start: false does not sweep within the wait window",
+         %{band_mhz: band_mhz} do
+      valid_time = ~U[2026-04-21 15:00:00Z]
+      ScoresFile.write!(band_mhz, valid_time, sample_scores())
+
+      start_supervised!({ScoreCacheReconciler, run_on_start: false, interval_ms: 100})
+
+      # Wait past the 500ms boot delay the other branch would have used.
+      Process.sleep(750)
+
+      assert :miss = ScoreCache.fetch(band_mhz, valid_time)
+    end
+
+    test "interval_ms rescheduling picks up files written after the first sweep",
+         %{band_mhz: band_mhz} do
+      first = ~U[2026-04-21 16:00:00Z]
+      second = ~U[2026-04-21 17:00:00Z]
+
+      ScoresFile.write!(band_mhz, first, sample_scores())
+
+      start_supervised!({ScoreCacheReconciler, run_on_start: true, interval_ms: 150})
+
+      # First sweep warms `first` but knows nothing about `second`.
+      assert {:ok, _} = wait_for_cache(band_mhz, first, 2_000)
+      assert :miss = ScoreCache.fetch(band_mhz, second)
+
+      # Drop the second file and wait for the next scheduled tick.
+      ScoresFile.write!(band_mhz, second, sample_scores())
+
+      assert {:ok, _} = wait_for_cache(band_mhz, second, 2_000)
+    end
+
+    test "logs an info line when warmed > 0", %{band_mhz: band_mhz} do
+      valid_time = ~U[2026-04-21 18:00:00Z]
+      ScoresFile.write!(band_mhz, valid_time, sample_scores())
+
+      # config/test.exs pins the global logger to :warning, which drops the
+      # :info line before capture_log sees it. Raise the level for the
+      # duration of the test and restore it on exit.
+      prev_level = Logger.level()
+      Logger.configure(level: :info)
+      on_exit(fn -> Logger.configure(level: prev_level) end)
+
+      log =
+        capture_log([level: :info], fn ->
+          start_supervised!({ScoreCacheReconciler, run_on_start: true, interval_ms: 100})
+
+          assert {:ok, _} = wait_for_cache(band_mhz, valid_time, 2_000)
+          # Give the logger a beat to flush before we snapshot the buffer.
+          Process.sleep(50)
+        end)
+
+      assert log =~ "ScoreCacheReconciler: warmed"
+    end
+  end
 end
diff --git a/test/microwaveprop/weather_grid_test.exs b/test/microwaveprop/weather_grid_test.exs
index 10af499c..8b4231bf 100644
--- a/test/microwaveprop/weather_grid_test.exs
+++ b/test/microwaveprop/weather_grid_test.exs
@@ -4,6 +4,7 @@ defmodule Microwaveprop.WeatherGridTest do
   alias Microwaveprop.Propagation.ProfilesFile
   alias Microwaveprop.Weather
   alias Microwaveprop.Weather.GridCache
+  alias Microwaveprop.Weather.SoundingParams
 
   setup do
     # GridCache ETS state leaks between tests (it's a named table outside the
@@ -362,6 +363,46 @@ defmodule Microwaveprop.WeatherGridTest do
       assert length(Weather.build_grid_cache_rows(grid_data, now, nil)) == 2
     end
 
+    test "explicit ducting_detected: false on the profile wins over derived true from sounding params" do
+      # Regression guard: the `||` fallback used to clobber legitimate `false`
+      # values written by the Rust ProfilesFile with whatever
+      # `SoundingParams.derive/1` produced. The profile below has a steep
+      # near-surface N drop (moist sfc → dry aloft) that derives ducts
+      # = [_], so `sounding[:ducting_detected]` would be `true`. With the
+      # explicit `false` on the profile map, the row must carry `false`.
+      now = DateTime.truncate(DateTime.utc_now(), :second)
+
+      strong_inversion_profile = [
+        %{"pres" => 1013.0, "tmpc" => 30.0, "dwpc" => 25.0, "hght" => 100.0},
+        %{"pres" => 1000.0, "tmpc" => 35.0, "dwpc" => -5.0, "hght" => 200.0},
+        %{"pres" => 925.0, "tmpc" => 20.0, "dwpc" => 0.0, "hght" => 800.0},
+        %{"pres" => 850.0, "tmpc" => 15.0, "dwpc" => -5.0, "hght" => 1500.0},
+        %{"pres" => 500.0, "tmpc" => -15.0, "dwpc" => -25.0, "hght" => 5600.0}
+      ]
+
+      # Sanity check: this profile does derive a duct from SoundingParams.
+      derived = SoundingParams.derive(strong_inversion_profile)
+      assert derived.ducting_detected == true
+
+      grid_data = %{
+        {32.125, -97.375} => %{
+          profile: strong_inversion_profile,
+          surface_temp_c: 30.0,
+          surface_dewpoint_c: 25.0,
+          surface_pressure_mb: 1013.0,
+          hpbl_m: 500.0,
+          pwat_mm: 30.0,
+          # Explicit false — persisted by the Rust worker. Must NOT be
+          # clobbered by the derived-from-sounding true above.
+          ducting_detected: false
+        }
+      }
+
+      [row] = Weather.build_grid_cache_rows(grid_data, now)
+
+      assert row.ducting == false
+    end
+
     test "strips raw profile/duct_characteristics/surface_temp_c/surface_dewpoint_c fields" do
       now = DateTime.truncate(DateTime.utc_now(), :second)
 
diff --git a/test/microwaveprop/weather_test.exs b/test/microwaveprop/weather_test.exs
index b4c84cc7..0aa9565c 100644
--- a/test/microwaveprop/weather_test.exs
+++ b/test/microwaveprop/weather_test.exs
@@ -720,6 +720,16 @@ defmodule Microwaveprop.WeatherTest do
                qso_timestamp: @contact_time
              })
     end
+
+    test "returns false when qso_timestamp is nil" do
+      # A Contact with no timestamp can't be mapped to an HRRR hour; guard
+      # against HrrrClient.nearest_hrrr_hour(nil) crashing here.
+      refute Weather.hrrr_data_fully_present?(%{
+               pos1: %{"lat" => 32.9, "lon" => -97.04},
+               pos2: nil,
+               qso_timestamp: nil
+             })
+    end
   end
 
   describe "round_to_hrrr_grid/2" do
diff --git a/test/microwaveprop/workers/radar_frame_worker_test.exs b/test/microwaveprop/workers/radar_frame_worker_test.exs
index e1719a98..4f31e12a 100644
--- a/test/microwaveprop/workers/radar_frame_worker_test.exs
+++ b/test/microwaveprop/workers/radar_frame_worker_test.exs
@@ -168,6 +168,25 @@ defmodule Microwaveprop.Workers.RadarFrameWorkerTest do
     end
   end
 
+  describe "NexradClient error contract" do
+    # Pins the exact "NEXRAD n0q HTTP <code>" format emitted by
+    # NexradClient.fetch_decoded_frame/1 on a non-200 response. RadarFrameWorker
+    # pattern-matches on this literal prefix in `permanent_error?/1` to
+    # classify 4xx as permanent (stop retrying) and everything else as
+    # transient (let Oban retry). A silent format change would quietly turn
+    # permanent 404s into infinite retries — fail loudly here instead.
+    test "fetch_decoded_frame/1 returns error starting with \"NEXRAD n0q HTTP \" on 4xx" do
+      Req.Test.stub(NexradClient, fn conn ->
+        Plug.Conn.send_resp(conn, 404, "not found")
+      end)
+
+      assert {:error, reason} = NexradClient.fetch_decoded_frame(~U[2024-09-15 18:30:00Z])
+      assert is_binary(reason)
+      assert String.starts_with?(reason, "NEXRAD n0q HTTP ")
+      assert reason == "NEXRAD n0q HTTP 404"
+    end
+  end
+
   describe "unique constraint" do
     test "rejects a second insert with the same frame_ts while one is pending" do
       frame_ts = "2024-09-15T18:30:00Z"