diff --git a/lib/stack-lock.sh b/lib/stack-lock.sh
new file mode 100644
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--- /dev/null
+++ b/lib/stack-lock.sh
@@ -0,0 +1,197 @@
+#!/usr/bin/env bash
+# stack-lock.sh — File-based lock protocol for singleton project stack access
+#
+# Prevents CI pipelines and the reproduce-agent from stepping on each other
+# when sharing a single project stack (e.g. harb docker compose).
+#
+# Lock file: /home/agent/data/locks/<project>-stack.lock
+# Contents:  {"holder": "reproduce-agent-42", "since": "...", "heartbeat": "..."}
+#
+# Protocol:
+#   1. stack_lock_check   — inspect current lock state
+#   2. stack_lock_acquire — wait until lock is free, then claim it
+#   3. stack_lock_release — delete lock file when done
+#
+# Heartbeat: callers must update the heartbeat every 2 minutes while holding
+# the lock by calling stack_lock_heartbeat. A heartbeat older than 10 minutes
+# is considered stale — the next acquire will break it.
+#
+# Usage:
+#   source "$(dirname "$0")/../lib/stack-lock.sh"
+#   stack_lock_acquire "ci-pipeline-$BUILD_NUMBER" "myproject"
+#   trap 'stack_lock_release "myproject"' EXIT
+#   # ... do work ...
+#   stack_lock_release "myproject"
+
+set -euo pipefail
+
+STACK_LOCK_DIR="${HOME}/data/locks"
+STACK_LOCK_POLL_INTERVAL=30   # seconds between retry polls
+STACK_LOCK_STALE_SECONDS=600  # 10 minutes — heartbeat older than this = stale
+STACK_LOCK_MAX_WAIT=3600      # 1 hour — give up after this many seconds
+
+# _stack_lock_path <project>
+#   Print the path of the lock file for the given project.
+_stack_lock_path() {
+  local project="$1"
+  echo "${STACK_LOCK_DIR}/${project}-stack.lock"
+}
+
+# _stack_lock_now
+#   Print current UTC timestamp in ISO-8601 format.
+_stack_lock_now() {
+  date -u +"%Y-%m-%dT%H:%M:%SZ"
+}
+
+# _stack_lock_epoch <iso_timestamp>
+#   Convert an ISO-8601 UTC timestamp to a Unix epoch integer.
+_stack_lock_epoch() {
+  local ts="$1"
+  # Strip trailing Z, replace T with space for `date -d`
+  date -u -d "${ts%Z}" +%s 2>/dev/null || date -u -j -f "%Y-%m-%dT%H:%M:%S" "${ts%Z}" +%s 2>/dev/null
+}
+
+# stack_lock_check <project>
+#   Print lock status to stdout: "free", "held:<holder>", or "stale:<holder>".
+#   Returns 0 in all cases (status is in stdout).
+stack_lock_check() {
+  local project="$1"
+  local lock_file
+  lock_file="$(_stack_lock_path "$project")"
+
+  if [ ! -f "$lock_file" ]; then
+    echo "free"
+    return 0
+  fi
+
+  local holder heartbeat
+  holder=$(python3 -c "import sys,json; d=json.load(open('$lock_file')); print(d.get('holder','unknown'))" 2>/dev/null || echo "unknown")
+  heartbeat=$(python3 -c "import sys,json; d=json.load(open('$lock_file')); print(d.get('heartbeat',''))" 2>/dev/null || echo "")
+
+  if [ -z "$heartbeat" ]; then
+    echo "stale:${holder}"
+    return 0
+  fi
+
+  local hb_epoch now_epoch age
+  hb_epoch=$(_stack_lock_epoch "$heartbeat" 2>/dev/null || echo "0")
+  now_epoch=$(date -u +%s)
+  age=$(( now_epoch - hb_epoch ))
+
+  if [ "$age" -gt "$STACK_LOCK_STALE_SECONDS" ]; then
+    echo "stale:${holder}"
+  else
+    echo "held:${holder}"
+  fi
+}
+
+# stack_lock_acquire <holder_id> <project> [max_wait_seconds]
+#   Acquire the lock for <project> on behalf of <holder_id>.
+#   Polls every STACK_LOCK_POLL_INTERVAL seconds.
+#   Breaks stale locks automatically.
+#   Exits non-zero if the lock cannot be acquired within max_wait_seconds.
+stack_lock_acquire() {
+  local holder="$1"
+  local project="$2"
+  local max_wait="${3:-$STACK_LOCK_MAX_WAIT}"
+  local lock_file
+  lock_file="$(_stack_lock_path "$project")"
+  local deadline
+  deadline=$(( $(date -u +%s) + max_wait ))
+
+  mkdir -p "$STACK_LOCK_DIR"
+
+  while true; do
+    local status
+    status=$(stack_lock_check "$project")
+
+    case "$status" in
+      free)
+        # Attempt atomic write using a temp file + mv
+        local tmp_lock
+        tmp_lock=$(mktemp "${STACK_LOCK_DIR}/.lock-tmp-XXXXXX")
+        local now
+        now=$(_stack_lock_now)
+        printf '{"holder": "%s", "since": "%s", "heartbeat": "%s"}\n' \
+          "$holder" "$now" "$now" > "$tmp_lock"
+        mv "$tmp_lock" "$lock_file"
+        echo "[stack-lock] acquired lock for ${project} as ${holder}" >&2
+        return 0
+        ;;
+      stale:*)
+        local stale_holder="${status#stale:}"
+        echo "[stack-lock] breaking stale lock held by ${stale_holder} for ${project}" >&2
+        rm -f "$lock_file"
+        # Loop back immediately to re-check and claim
+        ;;
+      held:*)
+        local cur_holder="${status#held:}"
+        local remaining
+        remaining=$(( deadline - $(date -u +%s) ))
+        if [ "$remaining" -le 0 ]; then
+          echo "[stack-lock] timed out waiting for lock on ${project} (held by ${cur_holder})" >&2
+          return 1
+        fi
+        echo "[stack-lock] ${project} locked by ${cur_holder}, waiting ${STACK_LOCK_POLL_INTERVAL}s (${remaining}s left)..." >&2
+        sleep "$STACK_LOCK_POLL_INTERVAL"
+        ;;
+      *)
+        echo "[stack-lock] unexpected status '${status}' for ${project}" >&2
+        return 1
+        ;;
+    esac
+  done
+}
+
+# stack_lock_heartbeat <holder_id> <project>
+#   Update the heartbeat timestamp in the lock file.
+#   Should be called every 2 minutes while holding the lock.
+#   No-op if the lock file is absent or held by a different holder.
+stack_lock_heartbeat() {
+  local holder="$1"
+  local project="$2"
+  local lock_file
+  lock_file="$(_stack_lock_path "$project")"
+
+  [ -f "$lock_file" ] || return 0
+
+  local current_holder
+  current_holder=$(python3 -c "import sys,json; d=json.load(open('$lock_file')); print(d.get('holder',''))" 2>/dev/null || echo "")
+  [ "$current_holder" = "$holder" ] || return 0
+
+  local since
+  since=$(python3 -c "import sys,json; d=json.load(open('$lock_file')); print(d.get('since',''))" 2>/dev/null || echo "")
+  local now
+  now=$(_stack_lock_now)
+
+  local tmp_lock
+  tmp_lock=$(mktemp "${STACK_LOCK_DIR}/.lock-tmp-XXXXXX")
+  printf '{"holder": "%s", "since": "%s", "heartbeat": "%s"}\n' \
+    "$holder" "$since" "$now" > "$tmp_lock"
+  mv "$tmp_lock" "$lock_file"
+}
+
+# stack_lock_release <project> [holder_id]
+#   Release the lock for <project>.
+#   If holder_id is provided, only releases if the lock is held by that holder
+#   (prevents accidentally releasing someone else's lock).
+stack_lock_release() {
+  local project="$1"
+  local holder="${2:-}"
+  local lock_file
+  lock_file="$(_stack_lock_path "$project")"
+
+  [ -f "$lock_file" ] || return 0
+
+  if [ -n "$holder" ]; then
+    local current_holder
+    current_holder=$(python3 -c "import sys,json; d=json.load(open('$lock_file')); print(d.get('holder',''))" 2>/dev/null || echo "")
+    if [ "$current_holder" != "$holder" ]; then
+      echo "[stack-lock] refusing to release: lock held by '${current_holder}', not '${holder}'" >&2
+      return 1
+    fi
+  fi
+
+  rm -f "$lock_file"
+  echo "[stack-lock] released lock for ${project}" >&2
+}