diff --git a/src/parser.rs b/src/parser.rs
index c82444a82..b9a6fb2d0 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -274,8 +274,14 @@ pub struct LibraryData {
     /// the command line.
     pub transient_commandline: ScopedRefCell<Option<WString>>,
 
-    /// A file descriptor holding the current working directory, for use in openat().
-    /// This is never null and never invalid.
+    /// A file descriptor holding the current working directory.
+    /// Note that the long-term design of fish will implement subshells using threads;
+    /// these subshells necessarily may have distinct CWDs. The process-wide CWD is only
+    /// used for transitory checks, such as surrounding fork().
+    /// fish code should generally NOT assume that the process-wide CWD matches this.
+    /// Either resolve full paths, or use the _at() family of syscalls with this fd.
+    /// This is never null and never invalid except in crazy error conditions
+    /// (e.g. starting fish in a chmod 000 directory) which have yet to be rationalized.
     pub cwd_fd: Option<Arc<OwnedFd>>,
 
     /// Variables supporting the "status" builtin.
diff --git a/tests/checks/init-unreadable-cwd.fish b/tests/checks/init-unreadable-cwd.fish
index 3710498bd..8c9e69dc1 100644
--- a/tests/checks/init-unreadable-cwd.fish
+++ b/tests/checks/init-unreadable-cwd.fish
@@ -12,6 +12,11 @@ chmod 000 .
 $fish -c 'echo Look Ma! No crashing!' 2>/dev/null
 #CHECK: Look Ma! No crashing!
 
+# Verify fish reports the unreadable cwd on stderr instead of silently leaving cwd_fd unset.
+$fish -c 'true' 2>&1 1>/dev/null | string match -q '*Unable to open the current working directory*'
+and echo "cwd error reported"
+#CHECK: cwd error reported
+
 # Careful here, Solaris' rm tests if the directory is in $PWD, so we need to cd back
 cd $oldpwd
 rmdir $tmpdir