diff --git a/src/libmain/progress-bar.cc b/src/libmain/progress-bar.cc
index 22f890f7d..a3b86790b 100644
--- a/src/libmain/progress-bar.cc
+++ b/src/libmain/progress-bar.cc
@@ -543,7 +543,7 @@ public:
         auto state(state_.lock());
         if (!state->active) return {};
         std::cerr << fmt("\r\e[K%s ", msg);
-        auto s = trim(readLine(STDIN_FILENO));
+        auto s = trim(readLine(STDIN_FILENO, true));
         if (s.size() != 1) return {};
         draw(*state);
         return s[0];
diff --git a/src/libutil/file-descriptor.hh b/src/libutil/file-descriptor.hh
index b54bab83c..710efaf1d 100644
--- a/src/libutil/file-descriptor.hh
+++ b/src/libutil/file-descriptor.hh
@@ -77,8 +77,13 @@ void writeFull(Descriptor fd, std::string_view s, bool allowInterrupts = true);
 
 /**
  * Read a line from a file descriptor.
+ *
+ * @param fd The file descriptor to read from
+ * @param eofOk If true, return an unterminated line if EOF is reached. (e.g. the empty string)
+ *
+ * @return A line of text ending in `\n`, or a string without `\n` if `eofOk` is true and EOF is reached.
  */
-std::string readLine(Descriptor fd);
+std::string readLine(Descriptor fd, bool eofOk = false);
 
 /**
  * Write a line to a file descriptor.
diff --git a/src/libutil/unix/file-descriptor.cc b/src/libutil/unix/file-descriptor.cc
index 2c1126e09..ac7c086af 100644
--- a/src/libutil/unix/file-descriptor.cc
+++ b/src/libutil/unix/file-descriptor.cc
@@ -47,7 +47,7 @@ void writeFull(int fd, std::string_view s, bool allowInterrupts)
 }
 
 
-std::string readLine(int fd)
+std::string readLine(int fd, bool eofOk)
 {
     std::string s;
     while (1) {
@@ -58,8 +58,12 @@ std::string readLine(int fd)
         if (rd == -1) {
             if (errno != EINTR)
                 throw SysError("reading a line");
-        } else if (rd == 0)
-            throw EndOfFile("unexpected EOF reading a line");
+        } else if (rd == 0) {
+            if (eofOk)
+                return s;
+            else
+                throw EndOfFile("unexpected EOF reading a line");
+        }
         else {
             if (ch == '\n') return s;
             s += ch;
diff --git a/tests/functional/flakes/config.sh b/tests/functional/flakes/config.sh
index 256a595bc..48f1c7a85 100755
--- a/tests/functional/flakes/config.sh
+++ b/tests/functional/flakes/config.sh
@@ -27,7 +27,17 @@ cat <<EOF > flake.nix
 EOF
 
 # Without --accept-flake-config, the post hook should not run.
+# To test variations in stderr tty-ness, we run the command in different ways,
+# none of which should block on stdin or accept the `nixConfig`s.
 nix build < /dev/null
+nix build < /dev/null 2>&1 | cat
+# EOF counts as no, even when interactive (throw EOF error before)
+if type -p script >/dev/null && script -q -c true /dev/null; then
+    echo "script is available and GNU-like, so we can ensure a tty"
+    script -q -c 'nix build < /dev/null' /dev/null
+else
+    echo "script is not available or not GNU-like, so we skip testing with an added tty"
+fi
 (! [[ -f post-hook-ran ]])
 TODO_NixOS
 clearStore
diff --git a/tests/functional/package.nix b/tests/functional/package.nix
index 71029146b..d1582b05d 100644
--- a/tests/functional/package.nix
+++ b/tests/functional/package.nix
@@ -1,7 +1,6 @@
 { lib
 , stdenv
 , mkMesonDerivation
-, releaseTools
 
 , meson
 , ninja
@@ -16,10 +15,6 @@
 , nix-expr
 , nix-cli
 
-, rapidcheck
-, gtest
-, runCommand
-
 , busybox-sandbox-shell ? null
 
 # Configuration Options
@@ -60,6 +55,7 @@ mkMesonDerivation (finalAttrs: {
     # etc.
     busybox-sandbox-shell
     # For Overlay FS tests need `mount`, `umount`, and `unshare`.
+    # For `script` command (ensuring a TTY)
     # TODO use `unixtools` to be precise over which executables instead?
     util-linux
   ];