diff --git a/src/libutil/args.cc b/src/libutil/args.cc
index d8d004e6f..2c27ed862 100644
--- a/src/libutil/args.cc
+++ b/src/libutil/args.cc
@@ -236,7 +236,8 @@ void ParseQuoted::operator()(std::shared_ptr<Parser> &state, Strings & r) {
     assert(false);
 }
 
-Strings parseShebangContent(std::string_view s) {
+Strings parseShebangContent(std::string_view s)
+{
     Strings result;
     std::shared_ptr<Parser> parserState(std::make_shared<ParseUnquoted>(ParseUnquoted(s)));
 
diff --git a/src/nix/main.cc b/src/nix/main.cc
index 6144f746f..13d669795 100644
--- a/src/nix/main.cc
+++ b/src/nix/main.cc
@@ -84,6 +84,19 @@ static bool haveInternet()
 #endif
 }
 
+static void disableNet()
+{
+    // FIXME: should check for command line overrides only.
+    if (!settings.useSubstitutes.overridden)
+        settings.useSubstitutes = false;
+    if (!settings.tarballTtl.overridden)
+        settings.tarballTtl = std::numeric_limits<unsigned int>::max();
+    if (!fileTransferSettings.tries.overridden)
+        fileTransferSettings.tries = 0;
+    if (!fileTransferSettings.connectTimeout.overridden)
+        fileTransferSettings.connectTimeout = 1;
+}
+
 std::string programPath;
 
 struct NixArgs : virtual MultiCommand, virtual MixCommonArgs, virtual RootArgs
@@ -452,10 +465,16 @@ void mainWrapped(int argc, char * * argv)
         }
     });
 
+    if (getEnv("NIX_GET_COMPLETIONS"))
+        /* Avoid fetching stuff during tab completion. We have to this
+           early because we haven't checked `haveInternet()` yet
+           (below). */
+        disableNet();
+
     try {
         auto isNixCommand = std::regex_search(programName, std::regex("nix$"));
         auto allowShebang = isNixCommand && argc > 1;
-        args.parseCmdline(argvToStrings(argc, argv),allowShebang);
+        args.parseCmdline(argvToStrings(argc, argv), allowShebang);
     } catch (UsageError &) {
         if (!args.helpRequested && !args.completions) throw;
     }
@@ -494,17 +513,8 @@ void mainWrapped(int argc, char * * argv)
         args.useNet = false;
     }
 
-    if (!args.useNet) {
-        // FIXME: should check for command line overrides only.
-        if (!settings.useSubstitutes.overridden)
-            settings.useSubstitutes = false;
-        if (!settings.tarballTtl.overridden)
-            settings.tarballTtl = std::numeric_limits<unsigned int>::max();
-        if (!fileTransferSettings.tries.overridden)
-            fileTransferSettings.tries = 0;
-        if (!fileTransferSettings.connectTimeout.overridden)
-            fileTransferSettings.connectTimeout = 1;
-    }
+    if (!args.useNet)
+        disableNet();
 
     if (args.refresh) {
         settings.tarballTtl = 0;