diff --git a/src/DiffEngine.Tests/AddToolBasedOnTests.cs b/src/DiffEngine.Tests/AddToolBasedOnTests.cs
new file mode 100644
index 00000000..eedc01ca
--- /dev/null
+++ b/src/DiffEngine.Tests/AddToolBasedOnTests.cs
@@ -0,0 +1,48 @@
+/// <summary>
+/// What a derived tool inherits.
+/// <para>
+/// Every flag on AddToolBasedOn is a nullable that falls back to the tool being derived from, and
+/// useShellExecute defaulted to true rather than null - so its `?? existing` was unreachable, and a
+/// caller who said nothing about it got true rather than what they derived from. For the five
+/// definitions that set it false (the bundled viewer, VS Code, Cursor, MsWordDiff, MsExcelDiff)
+/// that silently changed how the tool launches, and took the inherited CreateNoWindow with it,
+/// which is the console flash the viewer definition exists to prevent.
+/// </para>
+/// <para>
+/// Asserted on the defaults rather than on a derived tool, because AddToolBasedOn resolves through
+/// ToolLookup and so only works for a tool installed on the machine running the test. A behavioural
+/// test would quietly do nothing wherever the tool it names is absent - which, for the viewer, is
+/// this machine and CI both.
+/// </para>
+/// </summary>
+public class AddToolBasedOnTests
+{
+    [Test]
+    public async Task EveryOptionalFlagFallsBackToTheToolItIsBasedOn()
+    {
+        var method = typeof(DiffTools).GetMethod(nameof(DiffTools.AddToolBasedOn))!;
+
+        var withDefaults = method
+            .GetParameters()
+            .Where(_ => _.ParameterType == typeof(bool?))
+            .ToList();
+
+        // The flags, so a new one cannot be added without this noticing
+        await Assert.That(withDefaults.Select(_ => _.Name!).ToList()).IsEquivalentTo(
+        [
+            "autoRefresh",
+            "isMdi",
+            "supportsText",
+            "requiresTarget",
+            "useShellExecute",
+            "createNoWindow",
+            "killLockingProcess"
+        ]);
+
+        foreach (var parameter in withDefaults)
+        {
+            await Assert.That(parameter.HasDefaultValue).IsTrue();
+            await Assert.That(parameter.DefaultValue).IsNull();
+        }
+    }
+}
diff --git a/src/DiffEngine/DiffTools_Add.cs b/src/DiffEngine/DiffTools_Add.cs
index abf07fda..9b9af4db 100644
--- a/src/DiffEngine/DiffTools_Add.cs
+++ b/src/DiffEngine/DiffTools_Add.cs
@@ -8,7 +8,11 @@ public static partial class DiffTools
         bool? isMdi = null,
         bool? supportsText = null,
         bool? requiresTarget = null,
-        bool? useShellExecute = true,
+        // Null like every other flag here. Defaulting to true made the `?? existing` below dead
+        // code for this one alone, so a tool based on one of the five definitions that set it
+        // false - the bundled viewer, VS Code, Cursor, MsWordDiff, MsExcelDiff - silently became
+        // a ShellExecute launch, and took the inherited CreateNoWindow with it
+        bool? useShellExecute = null,
         bool? createNoWindow = null,
         bool? killLockingProcess = null,
         LaunchArguments? launchArguments = null,