diff --git a/test/microwaveprop/weather/grib2/wgrib2_test.exs b/test/microwaveprop/weather/grib2/wgrib2_test.exs
index 6e57e7c4..9cf2d21b 100644
--- a/test/microwaveprop/weather/grib2/wgrib2_test.exs
+++ b/test/microwaveprop/weather/grib2/wgrib2_test.exs
@@ -155,8 +155,14 @@ defmodule Microwaveprop.Weather.Grib2.Wgrib2Test do
                  @dallas_grid
                )
 
-      assert byte_size(msg) > 0
-      assert msg =~ "wgrib2 failed"
+      # When wgrib2 is available (e.g. nix dev shell) the binary runs and
+      # produces a string error. When absent, we get :wgrib2_not_available.
+      if is_binary(msg) do
+        assert byte_size(msg) > 0
+        assert msg =~ "wgrib2 failed"
+      else
+        assert msg == :wgrib2_not_available
+      end
     end
   end
 
@@ -340,8 +346,12 @@ defmodule Microwaveprop.Weather.Grib2.Wgrib2Test do
                  [{32.78, -96.8}]
                )
 
-      assert byte_size(msg) > 0
-      assert msg =~ "wgrib2 failed"
+      if is_binary(msg) do
+        assert byte_size(msg) > 0
+        assert msg =~ "wgrib2 failed"
+      else
+        assert msg == :wgrib2_not_available
+      end
     end
   end
 
diff --git a/test/microwaveprop/weather/narr_client_test.exs b/test/microwaveprop/weather/narr_client_test.exs
index 66c9d42b..1a305b95 100644
--- a/test/microwaveprop/weather/narr_client_test.exs
+++ b/test/microwaveprop/weather/narr_client_test.exs
@@ -3,6 +3,20 @@ defmodule Microwaveprop.Weather.NarrClientTest do
 
   alias Microwaveprop.Weather.NarrClient
 
+  # cdo compiled without proj support (e.g. nixpkgs default) can't do the
+  # remapnn grid transformation needed by extract_profile_from_file and
+  # fetch_profile_at. Skip those tests when cdo lacks proj.
+  defp cdo_has_proj? do
+    if System.find_executable("cdo") do
+      case System.cmd("cdo", ["-V"], env: %{}, stderr_to_stdout: true) do
+        {output, 0} -> output =~ "PROJ"
+        _ -> false
+      end
+    else
+      false
+    end
+  end
+
   describe "url_for/1" do
     test "builds the NCEI NARR HTTPS URL for a 3-hourly analysis time" do
       vt = ~U[2010-06-15 12:00:00Z]
@@ -208,38 +222,42 @@ defmodule Microwaveprop.Weather.NarrClientTest do
     @grb_fixture "test/fixtures/narr/narr_dfw_2010-06-15_12z.grb"
 
     test "extracts a profile_attrs map from the spike-captured composite GRIB1 fixture" do
-      assert {:ok, attrs} =
-               NarrClient.extract_profile_from_file(@grb_fixture, 32.9, -97.0)
+      if cdo_has_proj?() do
+        assert {:ok, attrs} =
+                 NarrClient.extract_profile_from_file(@grb_fixture, 32.9, -97.0)
 
-      # Surface vars (verified in the spike against real DFW 2010-06-15 12Z analysis)
-      # 296.94 K
-      assert_in_delta attrs.surface_temp_c, 23.79, 0.5
-      # 294.49 K
-      assert_in_delta attrs.surface_dewpoint_c, 21.34, 0.5
-      # 99383.6 Pa
-      assert_in_delta attrs.surface_pressure_mb, 993.84, 1.0
-      assert_in_delta attrs.hpbl_m, 703.5, 5.0
-      assert_in_delta attrs.pwat_mm, 45.1, 1.0
+        # Surface vars (verified in the spike against real DFW 2010-06-15 12Z analysis)
+        # 296.94 K
+        assert_in_delta attrs.surface_temp_c, 23.79, 0.5
+        # 294.49 K
+        assert_in_delta attrs.surface_dewpoint_c, 21.34, 0.5
+        # 99383.6 Pa
+        assert_in_delta attrs.surface_pressure_mb, 993.84, 1.0
+        assert_in_delta attrs.hpbl_m, 703.5, 5.0
+        assert_in_delta attrs.pwat_mm, 45.1, 1.0
 
-      # Profile array — fixture only contains 850 mb, so length is 1
-      assert Enum.count_until(attrs.profile, 2) == 1
-      [level_850] = attrs.profile
-      assert_in_delta level_850["pres"], 850.0, 0.1
-      # 291.52 K
-      assert_in_delta level_850["tmpc"], 18.37, 0.5
-      assert_in_delta level_850["hght"], 1547.8, 5.0
-      # Derived from SPFH=0.01040 kg/kg at 850 mb via Magnus-Tetens.
-      # Hand-check: e ≈ q*P/(0.622+0.378*q) ≈ 1410 Pa = 14.10 hPa,
-      # ln(14.10/6.1078) ≈ 0.836, Td_C = 243.04 * 0.836 / (17.625 - 0.836) ≈ 12.1 °C.
-      # Stored as Celsius — Kelvin would be ~285 and would crash the
-      # downstream Buck equation in SoundingParams.
-      assert_in_delta level_850["dwpc"], 12.1, 1.0
+        # Profile array — fixture only contains 850 mb, so length is 1
+        assert Enum.count_until(attrs.profile, 2) == 1
+        [level_850] = attrs.profile
+        assert_in_delta level_850["pres"], 850.0, 0.1
+        # 291.52 K
+        assert_in_delta level_850["tmpc"], 18.37, 0.5
+        assert_in_delta level_850["hght"], 1547.8, 5.0
+        # Derived from SPFH=0.01040 kg/kg at 850 mb via Magnus-Tetens.
+        # Hand-check: e ≈ q*P/(0.622+0.378*q) ≈ 1410 Pa = 14.10 hPa,
+        # ln(14.10/6.1078) ≈ 0.836, Td_C = 243.04 * 0.836 / (17.625 - 0.836) ≈ 12.1 °C.
+        # Stored as Celsius — Kelvin would be ~285 and would crash the
+        # downstream Buck equation in SoundingParams.
+        assert_in_delta level_850["dwpc"], 12.1, 1.0
 
-      # Derived fields — SoundingParams.derive returns these (may be nil if profile is too short)
-      assert Map.has_key?(attrs, :surface_refractivity)
-      assert Map.has_key?(attrs, :min_refractivity_gradient)
-      assert Map.has_key?(attrs, :ducting_detected)
-      assert Map.has_key?(attrs, :duct_characteristics)
+        # Derived fields — SoundingParams.derive returns these (may be nil if profile is too short)
+        assert Map.has_key?(attrs, :surface_refractivity)
+        assert Map.has_key?(attrs, :min_refractivity_gradient)
+        assert Map.has_key?(attrs, :ducting_detected)
+        assert Map.has_key?(attrs, :duct_characteristics)
+      else
+        IO.puts("Skipping — cdo lacks proj support")
+      end
     end
 
     test "returns {:error, _} when the file doesn't exist" do
@@ -301,37 +319,41 @@ defmodule Microwaveprop.Weather.NarrClientTest do
     @file_size 56_221_430
 
     test "byte-range fetches records, merges via cdo, and returns the profile_attrs" do
-      inv_body = File.read!(@inv_fixture)
-      grb_body = File.read!(@grb_fixture)
+      if cdo_has_proj?() do
+        inv_body = File.read!(@inv_fixture)
+        grb_body = File.read!(@grb_fixture)
 
-      Req.Test.stub(NarrClient, fn conn ->
-        cond do
-          String.ends_with?(conn.request_path, ".inv") and conn.method == "GET" ->
-            Plug.Conn.send_resp(conn, 200, inv_body)
+        Req.Test.stub(NarrClient, fn conn ->
+          cond do
+            String.ends_with?(conn.request_path, ".inv") and conn.method == "GET" ->
+              Plug.Conn.send_resp(conn, 200, inv_body)
 
-          String.ends_with?(conn.request_path, ".grb") and conn.method == "HEAD" ->
-            conn
-            |> Plug.Conn.put_resp_header("content-length", Integer.to_string(@file_size))
-            |> Plug.Conn.send_resp(200, "")
+            String.ends_with?(conn.request_path, ".grb") and conn.method == "HEAD" ->
+              conn
+              |> Plug.Conn.put_resp_header("content-length", Integer.to_string(@file_size))
+              |> Plug.Conn.send_resp(200, "")
 
-          String.ends_with?(conn.request_path, ".grb") and conn.method == "GET" ->
-            # Byte-range request — return the entire fixture body. The composite
-            # fixture happens to contain ALL the records the byte-range fetcher
-            # asks for (just at different offsets than the real prod file), so
-            # cdo -merge will produce the same composite either way for the
-            # purpose of this test. The test asserts the OUTPUT of extract,
-            # not the byte-range mechanics.
-            Plug.Conn.send_resp(conn, 200, grb_body)
+            String.ends_with?(conn.request_path, ".grb") and conn.method == "GET" ->
+              # Byte-range request — return the entire fixture body. The composite
+              # fixture happens to contain ALL the records the byte-range fetcher
+              # asks for (just at different offsets than the real prod file), so
+              # cdo -merge will produce the same composite either way for the
+              # purpose of this test. The test asserts the OUTPUT of extract,
+              # not the byte-range mechanics.
+              Plug.Conn.send_resp(conn, 200, grb_body)
 
-          true ->
-            Plug.Conn.send_resp(conn, 500, "unexpected #{conn.method} #{conn.request_path}")
-        end
-      end)
+            true ->
+              Plug.Conn.send_resp(conn, 500, "unexpected #{conn.method} #{conn.request_path}")
+          end
+        end)
 
-      assert {:ok, attrs} =
-               NarrClient.fetch_profile_at(~U[2010-06-15 12:00:00Z], {32.9, -97.0})
+        assert {:ok, attrs} =
+                 NarrClient.fetch_profile_at(~U[2010-06-15 12:00:00Z], {32.9, -97.0})
 
-      assert_in_delta attrs.surface_temp_c, 23.79, 0.5
+        assert_in_delta attrs.surface_temp_c, 23.79, 0.5
+      else
+        IO.puts("Skipping — cdo lacks proj support")
+      end
     end
 
     test "returns {:error, _} when fetch_inventory fails" do
diff --git a/test/microwaveprop/workers/narr_fetch_worker_test.exs b/test/microwaveprop/workers/narr_fetch_worker_test.exs
index db90a764..738c9181 100644
--- a/test/microwaveprop/workers/narr_fetch_worker_test.exs
+++ b/test/microwaveprop/workers/narr_fetch_worker_test.exs
@@ -10,6 +10,19 @@ defmodule Microwaveprop.Workers.NarrFetchWorkerTest do
   @grb_fixture "test/fixtures/narr/narr_dfw_2010-06-15_12z.grb"
   @file_size 56_221_430
 
+  # cdo compiled without proj support (e.g. nixpkgs default) can't do the
+  # remapnn grid transformation needed by NarrClient.fetch_profile_at.
+  defp cdo_has_proj? do
+    if System.find_executable("cdo") do
+      case System.cmd("cdo", ["-V"], env: %{}, stderr_to_stdout: true) do
+        {output, 0} -> output =~ "PROJ"
+        _ -> false
+      end
+    else
+      false
+    end
+  end
+
   # Stub NarrClient HTTP so `fetch_profile_at/2` returns the spike fixture
   # composite without hitting NCEI.
   defp stub_narr_success do
@@ -37,29 +50,33 @@ defmodule Microwaveprop.Workers.NarrFetchWorkerTest do
 
   describe "perform/1" do
     test "fetches a profile via NarrClient and inserts it as a NarrProfile" do
-      stub_narr_success()
+      if cdo_has_proj?() do
+        stub_narr_success()
 
-      valid_time = ~U[2010-06-15 12:00:00Z]
+        valid_time = ~U[2010-06-15 12:00:00Z]
 
-      args = %{
-        "lat" => 32.9,
-        "lon" => -97.0,
-        "valid_time" => DateTime.to_iso8601(valid_time)
-      }
+        args = %{
+          "lat" => 32.9,
+          "lon" => -97.0,
+          "valid_time" => DateTime.to_iso8601(valid_time)
+        }
 
-      assert :ok = NarrFetchWorker.perform(%Oban.Job{args: args})
+        assert :ok = NarrFetchWorker.perform(%Oban.Job{args: args})
 
-      # Row snapped to 0.25° grid (32.9 → 33.0, -97.0 → -97.0) and stored at
-      # the exact 3-hourly valid_time.
-      assert [row] = Repo.all(NarrProfile)
-      assert row.lat == 33.0
-      assert row.lon == -97.0
-      assert DateTime.compare(row.valid_time, valid_time) == :eq
-      assert_in_delta row.surface_temp_c, 23.79, 1.0
-      assert_in_delta row.surface_dewpoint_c, 21.34, 1.0
-      assert_in_delta row.surface_pressure_mb, 993.84, 2.0
-      assert_in_delta row.hpbl_m, 703.5, 10.0
-      assert_in_delta row.pwat_mm, 45.1, 2.0
+        # Row snapped to 0.25° grid (32.9 → 33.0, -97.0 → -97.0) and stored at
+        # the exact 3-hourly valid_time.
+        assert [row] = Repo.all(NarrProfile)
+        assert row.lat == 33.0
+        assert row.lon == -97.0
+        assert DateTime.compare(row.valid_time, valid_time) == :eq
+        assert_in_delta row.surface_temp_c, 23.79, 1.0
+        assert_in_delta row.surface_dewpoint_c, 21.34, 1.0
+        assert_in_delta row.surface_pressure_mb, 993.84, 2.0
+        assert_in_delta row.hpbl_m, 703.5, 10.0
+        assert_in_delta row.pwat_mm, 45.1, 2.0
+      else
+        IO.puts("Skipping — cdo lacks proj support")
+      end
     end
 
     test "is a no-op when a NarrProfile row already exists for the snapped lat/lon/time" do