fix(spawn): target session not active window when creating crewmate windows

README.md

@@ -260,6 +260,8 @@ tests/fm-bootstrap.test.sh                # bootstrap dependency and feature-pro
 tests/fm-update.test.sh                   # fast-forward-only self-update, reread, nudge, dedup, and skip-safety tests
 tests/fm-secondmate.test.sh               # persistent secondmate routing, seeding, idle charter, backlog handoff, spawn, recovery, teardown, and FM_HOME tests
 tests/fm-teardown.test.sh                 # fm-teardown.sh safety and reminder checks: local-only fork-remote allow, truly-unpushed refuse, merged-to-main allow, no-mistakes regression, tasks-axi reminder, --force override
+tests/fm-spawn-batch.test.sh              # fm-spawn.sh batch dispatch argument routing: id=repo pair parsing, batch-vs-single-task detection, partial-failure reporting, and FM_HOME/FM_PROJECTS path scoping
+tests/fm-spawn-window-index.test.sh       # fm-spawn.sh new-window session-targeting fix: next-free-index placement and the occupied-index failure it avoids
 [ "$(readlink CLAUDE.md)" = "AGENTS.md" ]
 [ "$(readlink .claude/skills)" = "../.agents/skills" ]
 FM_HEARTBEAT=2 FM_POLL=1 bin/fm-watch-arm.sh  # watcher re-arm smoke test (prints "heartbeat")

bin/fm-spawn.sh

@@ -327,7 +327,7 @@ if tmux list-windows -t "$SES" -F '#{window_name}' | grep -qx "$W"; then
   exit 1
 fi
 
-tmux new-window -d -t "$SES" -n "$W" -c "$PROJ_ABS"
+tmux new-window -d -t "$SES:" -n "$W" -c "$PROJ_ABS"
 if [ "$KIND" != secondmate ]; then
   tmux send-keys -t "$T" 'treehouse get' Enter
 

tests/fm-spawn-window-index.test.sh

@@ -0,0 +1,93 @@
+#!/usr/bin/env bash
+# Regression tests for the fm-spawn.sh window-targeting fix.
+#
+# fm-spawn creates each crewmate window with `tmux new-window`. The pre-fix command
+# targeted the bare session name (`-t "$SES"`), which on some tmux versions resolves to
+# the session's ACTIVE window and tries to create the new window at that already-occupied
+# index, failing "create window failed: index N in use" when low indices are taken.
+# The fix appends a colon (`-t "$SES:"`) to force session-only targeting so tmux always
+# picks the next free index.
+#
+# These tests run entirely on a PRIVATE tmux socket (-L) with $TMUX unset, so they never
+# touch any live firstmate session. They skip cleanly when tmux is unavailable.
+set -u
+
+ROOT="$(cd "$(dirname "${BASH_SOURCE[0]}")/.." && pwd)"
+SPAWN="$ROOT/bin/fm-spawn.sh"
+
+fail() {
+  printf 'not ok - %s\n' "$1" >&2
+  exit 1
+}
+
+pass() {
+  printf 'ok - %s\n' "$1"
+}
+
+if ! command -v tmux >/dev/null 2>&1; then
+  pass "tmux unavailable; window-index behavior tests skipped"
+  exit 0
+fi
+
+SOCK="fm-spawn-winidx-$$"
+unset TMUX  # detach from any ambient tmux server so probes use only our private socket
+cleanup() { tmux -L "$SOCK" kill-server 2>/dev/null || true; }
+trap cleanup EXIT
+tmux -L "$SOCK" start-server 2>/dev/null || true
+
+# Build the realistic firstmate-inside-tmux condition: a session whose low window
+# indices (0,1,2) are occupied and whose ACTIVE window is a low occupied index.
+build_session() {
+  local ses=$1
+  tmux -L "$SOCK" kill-session -t "$ses" 2>/dev/null || true
+  tmux -L "$SOCK" set -g base-index 0 2>/dev/null || true  # deterministic regardless of host tmux.conf
+  tmux -L "$SOCK" new-session -d -s "$ses" -n w0
+  tmux -L "$SOCK" new-window -d -t "$ses:1" -n w1
+  tmux -L "$SOCK" new-window -d -t "$ses:2" -n w2
+  tmux -L "$SOCK" select-window -t "$ses:0"  # active = occupied low index
+}
+
+# 1. The source carries the session-targeting fix (guards the literal one-line change).
+test_source_uses_session_target() {
+  # shellcheck disable=SC2016  # single quotes are deliberate: this greps fm-spawn.sh's literal source, not an expansion.
+  grep -Fq 'tmux new-window -d -t "$SES:"' "$SPAWN" \
+    || fail "fm-spawn.sh must target the session ('\$SES:'), not the bare session/active window"
+  # shellcheck disable=SC2016  # single quotes are deliberate: this greps fm-spawn.sh's literal source, not an expansion.
+  if grep -E 'tmux new-window -d -t "\$SES"[^:]' "$SPAWN" >/dev/null 2>&1; then
+    fail "fm-spawn.sh still uses the bare-name new-window target"
+  fi
+  pass "fm-spawn.sh issues new-window against the session target (\$SES:)"
+}
+
+# 2. The fixed form lands the crewmate window at the next free index and never errors,
+#    even with low indices occupied and the active window on a low occupied index.
+test_session_target_picks_next_free() {
+  local ses=FIX out st idx
+  build_session "$ses"
+  # Exactly the command fm-spawn.sh issues, with the same flags.
+  out=$(tmux -L "$SOCK" new-window -d -t "$ses:" -n fm-crew-test -c "$ROOT" 2>&1)
+  st=$?
+  [ "$st" -eq 0 ] || fail "session-target new-window failed (exit $st): $out"
+  idx=$(tmux -L "$SOCK" list-windows -t "$ses" -F '#{window_index} #{window_name}' \
+    | awk '$2=="fm-crew-test"{print $1}')
+  [ -n "$idx" ] || fail "crewmate window was not created"
+  [ "$idx" -ge 3 ] || fail "crewmate window landed at occupied index $idx instead of next free (>=3)"
+  pass "session-target form creates the crewmate window at the next free index ($idx)"
+}
+
+# 3. The failure the fix avoids is real: targeting an occupied window index fails with the
+#    exact "index N in use" error from the bug report, which session-only targeting sidesteps.
+test_occupied_index_target_is_rejected() {
+  local ses=BUG out st
+  build_session "$ses"
+  out=$(tmux -L "$SOCK" new-window -d -t "$ses:0" -n fm-crew-test -c "$ROOT" 2>&1)
+  st=$?
+  [ "$st" -ne 0 ] || fail "targeting occupied index 0 should fail, but it succeeded"
+  printf '%s\n' "$out" | grep -Fq 'index 0 in use' \
+    || fail "expected 'index N in use' failure, got: $out"
+  pass "targeting an occupied window index fails 'index 0 in use' (the bug the fix avoids)"
+}
+
+test_source_uses_session_target
+test_session_target_picks_next_free
+test_occupied_index_target_is_rejected