An exec line over 1024 bytes fails the write, naming the limit

A command line longer than the 1024 bytes a command pane takes was said on
the message row after the write had already succeeded. An exec write (the
root's, a pane's, tagexec, a column's) now refuses it before anything runs,
EINVAL with "a command line is at most 1024 bytes" and an err record; a
builtin's line, a long Msg or an Edit block, is not limited.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

1 files changed, 4 insertions, 1 deletions

diff --git a/docs/fs.md b/docs/fs.md
index 5836215f..e64cd6b3 100644
--- a/docs/fs.md
+++ b/docs/fs.md
@@ -326,7 +326,10 @@ same tree without leaving the process.
   the screen and its scrollback (`clear`, ED3) erases the history above it. A command pane's own exec starts
   the next command there too. The log says `run <serial> <line>` and `exit
   <serial> <N|?>`; `exec` reads back the command pane's serial; Kill stops
-  its whole line, `&` jobs included; a line is at most 1 KB. To run a command again, execute
+  its whole line, `&` jobs included; a command line is at most 1024 bytes, and a
+  longer one written to an exec fails the write (EINVAL, `a command line is
+  at most 1024 bytes`) before anything in it runs; a builtin's line (a long
+  `Msg`, an Edit block) may be longer. To run a command again, execute
   its line again from its directory: `echo 'make test' > pane/<n>/exec` on
   the command pane runs it there, below the last run. A misspelled word is a
   command that says so and ends `exit 127`. `echo Tty > pane/<n>/ctl` makes