diff --git a/opsm_ex/lib/opsm/runtime/manager.ex b/opsm_ex/lib/opsm/runtime/manager.ex
index 0b5a627..fe544e3 100644
--- a/opsm_ex/lib/opsm/runtime/manager.ex
+++ b/opsm_ex/lib/opsm/runtime/manager.ex
@@ -292,31 +292,55 @@ defmodule Opsm.Runtime.Manager do
         nil ->
           {:error, {:no_plugin, tool}}
         path ->
-          case System.cmd("nickel", ["export", "--format", "json", path], stderr_to_stdout: true) do
-            {json_str, 0} ->
-              case Jason.decode(json_str) do
-                {:ok, plugin} ->
-                  # Cache for next time
-                  File.mkdir_p!(@plugins_dir)
-                  File.write!(cached, json_str)
-                  {:ok, plugin}
-                err -> {:error, {:json_decode, err}}
+          # System.cmd/3 raises ErlangError (:enoent) when the nickel binary
+          # is not installed — surface that as a structured error instead.
+          try do
+            case System.cmd("nickel", ["export", "--format", "json", path], stderr_to_stdout: true) do
+              {json_str, 0} ->
+                case Jason.decode(json_str) do
+                  {:ok, plugin} ->
+                    # Cache for next time
+                    File.mkdir_p!(@plugins_dir)
+                    File.write!(cached, json_str)
+                    {:ok, plugin}
+                  err -> {:error, {:json_decode, err}}
+                end
+              {err_str, _} ->
+                {:error, {:nickel_eval, err_str}}
+            end
+          rescue
+            e in ErlangError ->
+              case e do
+                %ErlangError{original: :enoent} -> {:error, {:nickel_not_installed, tool}}
+                _ -> reraise e, __STACKTRACE__
               end
-            {err_str, _} ->
-              {:error, {:nickel_eval, err_str}}
           end
       end
     end
   end
 
-  @plugin_search_dirs [
-    # Relative to the OPSM install — resolved at runtime
-    Path.expand("../runtime/core", :code.priv_dir(:opsm)),
-    Path.expand("~/.opsm/plugins/core"),
-  ]
+  # Computed at call time, NOT a module attribute: an attribute freezes
+  # :code.priv_dir/1 at compile time (pointing inside whatever _build the
+  # module was compiled in), and the repo-checkout fallbacks depend on cwd.
+  defp plugin_search_dirs do
+    priv_relative =
+      case :code.priv_dir(:opsm) do
+        {:error, _} -> []
+        priv -> [Path.expand("../runtime/core", priv)]
+      end
+
+    priv_relative ++
+      [
+        Path.expand("~/.opsm/plugins/core"),
+        # Repo checkout: cwd is opsm_ex/ under mix, the repo root as escript
+        Path.expand("../runtime/core", File.cwd!()),
+        Path.expand("runtime/core", File.cwd!())
+      ]
+  end
 
-  defp find_plugin_ncl(tool) do
-    Enum.find_value(@plugin_search_dirs, fn dir ->
+  @doc false
+  def find_plugin_ncl(tool) do
+    Enum.find_value(plugin_search_dirs(), fn dir ->
       path = Path.join(dir, "#{tool}.ncl")
       if File.exists?(path), do: path
     end)
diff --git a/opsm_ex/test/opsm/runtime/integration_test.exs b/opsm_ex/test/opsm/runtime/integration_test.exs
index 414ec40..a798468 100644
--- a/opsm_ex/test/opsm/runtime/integration_test.exs
+++ b/opsm_ex/test/opsm/runtime/integration_test.exs
@@ -140,7 +140,6 @@ defmodule Opsm.Runtime.IntegrationTest do
   # ---------------------------------------------------------------------------
 
   describe "Manager.install/2 — live download (zig 0.13.0)" do
-    @tag :external_api
     @tag :live_download
     test "installs zig 0.13.0 and creates expected directory layout" do
       install_result = Manager.install("zig", @zig_stable)
@@ -158,7 +157,6 @@ defmodule Opsm.Runtime.IntegrationTest do
       assert File.dir?(install_dir), "expected install_dir #{install_dir} to exist"
     end
 
-    @tag :external_api
     @tag :live_download
     test "which/1 returns a path after successful install" do
       Manager.install("zig", @zig_stable)
@@ -175,7 +173,6 @@ defmodule Opsm.Runtime.IntegrationTest do
       end
     end
 
-    @tag :external_api
     @tag :live_download
     test "current_version/1 returns the installed version after install" do
       Manager.install("zig", @zig_stable)
@@ -185,7 +182,6 @@ defmodule Opsm.Runtime.IntegrationTest do
       assert version == @zig_stable or version == "none"
     end
 
-    @tag :external_api
     @tag :live_download
     test "remove/1 cleans up installed tool" do
       Manager.install("zig", @zig_stable)
@@ -202,7 +198,6 @@ defmodule Opsm.Runtime.IntegrationTest do
   # ---------------------------------------------------------------------------
 
   describe "Manager.install/2 — idempotency" do
-    @tag :external_api
     @tag :live_download
     test "installing the same version twice returns :ok both times" do
       on_exit(fn -> Manager.remove("zig") end)
@@ -229,7 +224,6 @@ defmodule Opsm.Runtime.IntegrationTest do
       {:ok, dir: dir}
     end
 
-    @tag :external_api
     @tag :live_download
     test "installs tools declared in [runtime] section", %{dir: dir} do
       manifest = Path.join(dir, "opsm.toml")
diff --git a/opsm_ex/test/opsm/runtime/manager_test.exs b/opsm_ex/test/opsm/runtime/manager_test.exs
index 82a7564..f36c833 100644
--- a/opsm_ex/test/opsm/runtime/manager_test.exs
+++ b/opsm_ex/test/opsm/runtime/manager_test.exs
@@ -105,6 +105,24 @@ defmodule Opsm.Runtime.ManagerTest do
     end
   end
 
+  # ---------------------------------------------------------------------------
+  # find_plugin_ncl/1 — plugin search path resolution
+  # ---------------------------------------------------------------------------
+
+  describe "find_plugin_ncl/1" do
+    test "resolves a core plugin from the repo checkout (runtime/core)" do
+      # Under mix, cwd is opsm_ex/ — the sibling runtime/core/ dir must be
+      # searched at call time (a compile-time priv_dir attribute cannot see it)
+      path = Manager.find_plugin_ncl("zig")
+      assert is_binary(path), "expected zig.ncl to resolve from the repo checkout"
+      assert String.ends_with?(path, "/zig.ncl")
+    end
+
+    test "returns nil for a tool with no plugin definition" do
+      assert Manager.find_plugin_ncl("definitely-not-a-real-tool-xyz") == nil
+    end
+  end
+
   # ---------------------------------------------------------------------------
   # list_installed/0 — filesystem scan
   # ---------------------------------------------------------------------------