--- a/sys/vfs/ext2fs/ext2fs.h	2026-06-29 12:51:19.000000000 +0000
+++ b/sys/vfs/ext2fs/ext2fs.h	2026-10-26 03:28:45.669473983 +0000
@@ -329,9 +329,11 @@
 					 EXT2F_ROCOMPAT_DIR_NLINK | \
 					 EXT2F_ROCOMPAT_HUGE_FILE | \
 					 EXT2F_ROCOMPAT_EXTRA_ISIZE)
+/* NOTE: EXT2F_INCOMPAT_EXTENTS deliberately NOT supported: every extents
+ * I/O path in this driver is an EINVAL stub, so extents-flagged inodes must
+ * fail at mount instead of mounting and then erroring on each operation. */
 #define	EXT2F_INCOMPAT_SUPP		(EXT2F_INCOMPAT_FTYPE | \
 					 EXT2F_INCOMPAT_META_BG | \
-					 EXT2F_INCOMPAT_EXTENTS | \
 					 EXT2F_INCOMPAT_64BIT | \
 					 EXT2F_INCOMPAT_FLEX_BG | \
 					 EXT2F_INCOMPAT_CSUM_SEED)