diff --git a/litebox/src/fs/in_mem.rs b/litebox/src/fs/in_mem.rs
index 68efcfe673..3b5b1fe6c9 100644
--- a/litebox/src/fs/in_mem.rs
+++ b/litebox/src/fs/in_mem.rs
@@ -298,11 +298,6 @@ impl<Platform: sync::RawSyncPrimitivesProvider> super::backend::Backend for InMe
         flags: super::OFlags,
     ) -> Result<super::backend::DirHandle, OpenError> {
         assert_supported_oflags(flags);
-        if flags.intersects(super::OFlags::WRONLY | super::OFlags::RDWR) {
-            // TODO(jayb): POSIX requires `EISDIR` when write access is requested on a directory,
-            // but `OpenError` has no such variant yet.
-            unimplemented!()
-        }
         Ok(super::backend::DirHandle::from_typed::<Self>(
             InMemDirHandle {
                 flags,
diff --git a/litebox/src/fs/nine_p/mod.rs b/litebox/src/fs/nine_p/mod.rs
index 721c488a06..a1c13f6315 100644
--- a/litebox/src/fs/nine_p/mod.rs
+++ b/litebox/src/fs/nine_p/mod.rs
@@ -352,11 +352,6 @@ where
         flags: OFlags,
     ) -> Result<DirHandle, OpenError> {
         assert_supported_oflags(flags);
-        if flags.intersects(OFlags::WRONLY | OFlags::RDWR) {
-            // TODO(jayb): POSIX requires `EISDIR` when write access is requested on a directory,
-            // but `OpenError` has no such variant yet.
-            unimplemented!()
-        }
         let (fid, is_backend_root) = dir.into_typed::<Self>().into_dir();
         if flags.contains(OFlags::PATH) {
             // An `O_PATH` handle is never opened server-side, so the walked fid can be handed over
diff --git a/litebox_shim_linux/src/syscalls/file.rs b/litebox_shim_linux/src/syscalls/file.rs
index 8bf5bafdb6..98c5ed5ef3 100644
--- a/litebox_shim_linux/src/syscalls/file.rs
+++ b/litebox_shim_linux/src/syscalls/file.rs
@@ -365,10 +365,26 @@ impl<Platform: ShimPlatform> Task<Platform> {
         let files = self.files.borrow();
         let fs = self.fs.borrow();
         let context = fs.context.read();
-        files
+        let file = files
             .fs
             .open(&context, path, flags - OFlags::CLOEXEC, mode)
-            .map_err(Errno::from)
+            .map_err(Errno::from)?;
+        if flags.intersects(OFlags::WRONLY | OFlags::RDWR)
+            && !flags.contains(OFlags::PATH)
+            && files
+                .fs
+                .fd_file_status(&file)
+                .map_err(|error| {
+                    files.fs.close(&file).unwrap();
+                    Errno::from(error)
+                })?
+                .file_type
+                == litebox::fs::FileType::Directory
+        {
+            files.fs.close(&file).unwrap();
+            return Err(Errno::EISDIR);
+        }
+        Ok(file)
     }
 
     fn do_openat(