diff --git a/scripts/s/b b/scripts/s/b
index d442e49..f3eba75 100755
--- a/scripts/s/b
+++ b/scripts/s/b
@@ -77,13 +77,17 @@ fn_run_one() {
     local id=$1
     local B="$MOUNT/s/$id"
 
-    {
-        printf 'name=%s\n' "$id"
-        printf 'cwd=%s\n'  "$cwd"
-        [ -n "$agent"   ] && printf 'agent=%s\n'   "$agent"
-        [ -n "$backend" ] && printf 'backend=%s\n' "$backend"
-        [ -n "$model"   ] && printf 'model=%s\n'   "$model"
-    } > "$MOUNT/s/new" || die "failed to create session"
+    # Build as a single string: bash's printf builtin issues one write() per call;
+    # the server processes each write independently, so partial KV would fail.
+    local kv="name=${id}
+cwd=${cwd}"
+    [ -n "$agent"   ] && kv="${kv}
+agent=${agent}"
+    [ -n "$backend" ] && kv="${kv}
+backend=${backend}"
+    [ -n "$model"   ] && kv="${kv}
+model=${model}"
+    printf '%s\n' "$kv" > "$MOUNT/s/new" || die "failed to create session"
 
     printf '%s\n' "$prompt" > "$B/prompt" || die "failed to submit prompt"