diff --git a/src/Tests/ToolLauncherTests.cs b/src/Tests/ToolLauncherTests.cs
index 7320f23..999e18e 100644
--- a/src/Tests/ToolLauncherTests.cs
+++ b/src/Tests/ToolLauncherTests.cs
@@ -56,5 +56,6 @@ public void ToStartInfo_preserves_no_shell_and_no_redirect()
         Assert.False(start.RedirectStandardOutput);
         Assert.False(start.RedirectStandardError);
         Assert.Equal(["--flag"], start.ArgumentList.ToArray());
+        Assert.Equal("1", start.Environment["NDX_LAUNCHED"]);
     }
 }
diff --git a/src/ndx/ProcessStartSettings.cs b/src/ndx/ProcessStartSettings.cs
index edbaf53..9a27a7c 100644
--- a/src/ndx/ProcessStartSettings.cs
+++ b/src/ndx/ProcessStartSettings.cs
@@ -4,7 +4,8 @@ namespace ndx;
 
 /// <summary>
 /// Process start settings for the downloaded tool. ndx always starts the child
-/// without a shell and with inherited standard in/out/error.
+/// without a shell, with inherited standard in/out/error, and with
+/// <c>NDX_LAUNCHED=1</c> so the child can tell it was started by ndx.
 /// </summary>
 public sealed record ProcessStartSettings
 {
@@ -33,6 +34,8 @@ public ProcessStartInfo ToStartInfo()
         foreach (var argument in Arguments)
             start.ArgumentList.Add(argument);
 
+        start.Environment["NDX_LAUNCHED"] = "1";
+
         return start;
     }
 }