diff --git a/bin/mosaic b/bin/mosaic
index 4afdedc..9a773f9 100755
--- a/bin/mosaic
+++ b/bin/mosaic
@@ -247,6 +247,47 @@ _detect_mission_prompt() {
   fi
 }
 
+# Write a session lock if an active mission exists in the current directory.
+# Called before exec so $$ captures the PID that will become the agent process.
+_write_launcher_session_lock() {
+  local runtime="$1"
+  local mission_file=".mosaic/orchestrator/mission.json"
+  local lock_file=".mosaic/orchestrator/session.lock"
+
+  # Only write lock if mission exists and is active
+  [[ -f "$mission_file" ]] || return 0
+  command -v jq &>/dev/null || return 0
+
+  local m_status
+  m_status="$(jq -r '.status // "inactive"' "$mission_file" 2>/dev/null)"
+  [[ "$m_status" == "active" || "$m_status" == "paused" ]] || return 0
+
+  local session_id
+  session_id="${runtime}-$(date +%Y%m%d-%H%M%S)-$$"
+
+  jq -n \
+    --arg sid "$session_id" \
+    --arg rt "$runtime" \
+    --arg pid "$$" \
+    --arg ts "$(date -u +%Y-%m-%dT%H:%M:%SZ)" \
+    --arg pp "$(pwd)" \
+    --arg mid "" \
+    '{
+      session_id: $sid,
+      runtime: $rt,
+      pid: ($pid | tonumber),
+      started_at: $ts,
+      project_path: $pp,
+      milestone_id: $mid
+    }' > "$lock_file"
+}
+
+# Clean up session lock on exit (covers normal exit + signals).
+# Registered via trap after _write_launcher_session_lock succeeds.
+_cleanup_session_lock() {
+  rm -f ".mosaic/orchestrator/session.lock" 2>/dev/null
+}
+
 # Launcher functions
 launch_claude() {
   check_mosaic_home
@@ -263,6 +304,8 @@ launch_claude() {
 
   # If active mission exists and no user prompt was given, inject initial prompt
   _detect_mission_prompt
+  _write_launcher_session_lock "claude"
+  trap _cleanup_session_lock EXIT INT TERM
   if [[ -n "$MOSAIC_MISSION_PROMPT" && $# -eq 0 ]]; then
     echo "[mosaic] Launching Claude Code (active mission detected)..."
     exec claude --append-system-prompt "$runtime_prompt" "$MOSAIC_MISSION_PROMPT"
@@ -283,6 +326,8 @@ launch_opencode() {
 
   # OpenCode reads from ~/.config/opencode/AGENTS.md
   ensure_runtime_config "opencode" "$HOME/.config/opencode/AGENTS.md"
+  _write_launcher_session_lock "opencode"
+  trap _cleanup_session_lock EXIT INT TERM
   echo "[mosaic] Launching OpenCode..."
   exec opencode "$@"
 }
@@ -298,6 +343,8 @@ launch_codex() {
 
   # Codex reads from ~/.codex/instructions.md
   ensure_runtime_config "codex" "$HOME/.codex/instructions.md"
+  _write_launcher_session_lock "codex"
+  trap _cleanup_session_lock EXIT INT TERM
   echo "[mosaic] Launching Codex..."
   exec codex "$@"
 }
@@ -325,6 +372,8 @@ launch_yolo() {
       runtime_prompt="$(build_runtime_prompt "claude")"
 
       _detect_mission_prompt
+      _write_launcher_session_lock "claude"
+      trap _cleanup_session_lock EXIT INT TERM
       if [[ -n "$MOSAIC_MISSION_PROMPT" && $# -eq 0 ]]; then
         echo "[mosaic] Launching Claude Code in YOLO mode (active mission detected)..."
         exec claude --dangerously-skip-permissions --append-system-prompt "$runtime_prompt" "$MOSAIC_MISSION_PROMPT"
@@ -342,6 +391,8 @@ launch_yolo() {
 
       # Codex reads instructions.md from ~/.codex and supports a direct dangerous flag.
       ensure_runtime_config "codex" "$HOME/.codex/instructions.md"
+      _write_launcher_session_lock "codex"
+      trap _cleanup_session_lock EXIT INT TERM
       echo "[mosaic] Launching Codex in YOLO mode (dangerous permissions enabled)..."
       exec codex --dangerously-bypass-approvals-and-sandbox "$@"
       ;;
@@ -354,6 +405,8 @@ launch_yolo() {
 
       # OpenCode defaults to allow-all permissions unless user config restricts them.
       ensure_runtime_config "opencode" "$HOME/.config/opencode/AGENTS.md"
+      _write_launcher_session_lock "opencode"
+      trap _cleanup_session_lock EXIT INT TERM
       echo "[mosaic] Launching OpenCode in YOLO mode..."
       exec opencode "$@"
       ;;
@@ -469,8 +522,9 @@ _check_resumable_session() {
     local pid
     pid="$(jq -r '.pid // 0' "$lock_file" 2>/dev/null)"
     if [[ -n "$pid" ]] && [[ "$pid" != "0" ]] && ! kill -0 "$pid" 2>/dev/null; then
-      echo "[mosaic] Previous orchestration session detected (crashed)."
-      echo "[mosaic] Run: mosaic coord resume"
+      # Stale lock from a dead session — clean it up
+      rm -f "$lock_file"
+      echo "[mosaic] Cleaned up stale session lock (PID $pid no longer running)."
       echo ""
     fi
   elif [[ -f "$mission_file" ]]; then