diff --git a/Partnership/partnership_manager.sh b/Partnership/partnership_manager.sh
index fa6f159..8d88a75 100755
--- a/Partnership/partnership_manager.sh
+++ b/Partnership/partnership_manager.sh
@@ -79,8 +79,25 @@
 # SSH_TIMEOUT on all remote calls
 #   Every ssh/scp call is timeout-protected. No operation hangs on an unreachable peer.
 #
+# No acquire_lock — Deliberate
+#   This file is dual-role: an executable dispatcher AND a library that
+#   partnership_offboard.sh and partnership_transfer.sh source with
+#   PARTNERSHIP_LIB_MODE=1. A top-level acquire_lock would fire for every sourcing
+#   script, and --check can re-invoke this file as itself (bash "$0" --offboard),
+#   which a strict lock would deadlock. Concurrency is handled per-write with flock
+#   instead. Do not "fix" this to match the single-role scripts.
+#
 # flock on state writes
-#   Prevents concurrent state file corruption from overlapping --check cycles.
+#   write_state_file() serialises full state-file rewrites, and the --check counter
+#   updates are flocked separately on their own lock file. The offline counter is a
+#   read-modify-write: without the lock two overlapping --check cycles both read N and
+#   both write N+1, silently losing an increment and pushing the auto-offboard threshold
+#   past its configured window. The last_seen_remote sed is inside the same lock because
+#   it edits a file write_state_file() rewrites wholesale from other paths.
+#
+#   Note the redirection must sit INSIDE a command substitution — "$( ... ) 201>file"
+#   attaches the descriptor to the assignment rather than the subshell, and flock then
+#   fails with "Bad file descriptor" while the unlocked write proceeds anyway.
 #
 # SIGTERM trap on grace period sleep
 #   Offboard grace period is interruptible — Ctrl-C aborts cleanly.
@@ -1028,17 +1045,33 @@ fi
 # ==============================================================================================
 if [[ "$MODE" == "check" ]]; then
 
-    # Update last_seen_remote and offline counter based on rsync outcome
+    # Update last_seen_remote and offline counter based on rsync outcome.
+    #
+    # Both branches are flocked. The counter is a read-modify-write, so two overlapping
+    # --check cycles would otherwise both read N and both write N+1 — silently losing an
+    # increment and pushing the auto-offboard threshold further out than configured. The
+    # sed on the state file is included because write_state_file() flocks the same file
+    # from other code paths, and an unsynchronised sed -i can land mid-rewrite.
     if [[ "$REMOTE_SEEN" == true ]]; then
-        echo "0" > "$OFFLINE_COUNTER"
-        if [[ -f "$LOCAL_STATE_FILE" ]]; then
-            sed -i "s|^last_seen_remote=.*|last_seen_remote=$(date '+%Y-%m-%d %H:%M:%S')|" \
-                "$LOCAL_STATE_FILE" 2>/dev/null
-        fi
+        (
+            flock -x 201
+            echo "0" > "$OFFLINE_COUNTER"
+            if [[ -f "$LOCAL_STATE_FILE" ]]; then
+                sed -i "s|^last_seen_remote=.*|last_seen_remote=$(date '+%Y-%m-%d %H:%M:%S')|" \
+                    "$LOCAL_STATE_FILE" 2>/dev/null
+            fi
+        ) 201>"${OFFLINE_COUNTER}.lock"
     elif [[ "$REMOTE_UNSEEN" == true ]]; then
-        OFFLINE_COUNT=$(cat "$OFFLINE_COUNTER" 2>/dev/null || echo 0)
-        OFFLINE_COUNT=$(( OFFLINE_COUNT + 1 ))
-        echo "$OFFLINE_COUNT" > "$OFFLINE_COUNTER"
+        # Redirection must live INSIDE the substitution — "$( ... ) 201>file" attaches the fd
+        # to the assignment, not to the subshell doing the work, and flock then fails with
+        # "Bad file descriptor" while the increment silently proceeds unlocked.
+        OFFLINE_COUNT=$( {
+            flock -x 201
+            _c=$(cat "$OFFLINE_COUNTER" 2>/dev/null || echo 0)
+            _c=$(( _c + 1 ))
+            echo "$_c" > "$OFFLINE_COUNTER"
+            echo "$_c"
+        } 201>"${OFFLINE_COUNTER}.lock" )
 
         # Auto-offboard threshold: threshold_days × 48 intervals/day (every 30min)
         THRESHOLD_INTERVALS=$(( ${PARTNERSHIP_OFFLINE_THRESHOLD:-30} * 48 ))