diff --git a/packages/mosaic/framework/tools/fleet/test-start-agent-session.sh b/packages/mosaic/framework/tools/fleet/test-start-agent-session.sh
index df73747..11a4c3f 100755
--- a/packages/mosaic/framework/tools/fleet/test-start-agent-session.sh
+++ b/packages/mosaic/framework/tools/fleet/test-start-agent-session.sh
@@ -50,8 +50,10 @@ grep -qF 'already running' /tmp/mosaic-start-agent-idempotent.out || fail "dupli
 #   - Intercepts 'new-session' calls and records its arguments to a file.
 #   - For 'has-session' calls, exits 1 (session does not exist) so the script
 #     proceeds to launch instead of printing "already running".
-#   - For 'list-panes' calls, returns a synthetic PID so the heartbeat sidecar
-#     path is exercised without needing a real tmux session.
+#   - For 'list-panes' calls, returns empty so PANE_PID stays unset and the
+#     heartbeat sidecar is NOT spawned (heartbeat is not the focus of this test;
+#     test 6 and 7 cover that path).  This prevents any real-filesystem side
+#     effects or leaked background processes.
 #   - For all other subcommands, exits 0.
 #
 # Assertions:
@@ -62,7 +64,8 @@ grep -qF 'already running' /tmp/mosaic-start-agent-idempotent.out || fail "dupli
 FAKE_BIN=$(mktemp -d)
 FAKE_RUNTIME_BIN=$(mktemp -d)
 TMUX_ARGS_FILE=$(mktemp)
-CLEANUP_DIRS+=("$FAKE_BIN" "$FAKE_RUNTIME_BIN")
+HB_RUN_DIR3=$(mktemp -d)
+CLEANUP_DIRS+=("$FAKE_BIN" "$FAKE_RUNTIME_BIN" "$HB_RUN_DIR3")
 
 # Write the fake tmux shim (uses only positional args, no sourced vars).
 cat > "$FAKE_BIN/tmux" <<SHIM
@@ -77,7 +80,8 @@ if [ "\$subcmd" = "new-session" ]; then
   exit 0
 fi
 if [ "\$subcmd" = "list-panes" ]; then
-  echo "99999"   # synthetic pane PID for heartbeat path
+  # Return empty: no sidecar spawned (heartbeat is not the focus of this test).
+  echo ""
   exit 0
 fi
 exit 0
@@ -95,6 +99,7 @@ MOSAIC_AGENT_WORKDIR="$WORKDIR3" \
 MOSAIC_AGENT_RUNTIME="pi" \
 MOSAIC_RUNTIME_BIN="$FAKE_RUNTIME_BIN" \
 MOSAIC_AGENT_COMMAND="mosaic yolo pi --model openai-codex/gpt-5.5:high" \
+MOSAIC_HEARTBEAT_RUN_DIR="$HB_RUN_DIR3" \
   "$START" "$AGENT3"
 
 all_args=$(cat "$TMUX_ARGS_FILE" 2>/dev/null || true)
@@ -118,7 +123,8 @@ echo "$all_args" | grep -qF "mosaic yolo pi --model openai-codex/gpt-5.5:high" |
 # ── Test 4: when no extra runtime-bin dirs exist, exec still appears ───────────
 TMUX_ARGS_FILE2=$(mktemp)
 FAKE_BIN2=$(mktemp -d)
-CLEANUP_DIRS+=("$FAKE_BIN2")
+HB_RUN_DIR4=$(mktemp -d)
+CLEANUP_DIRS+=("$FAKE_BIN2" "$HB_RUN_DIR4")
 
 cat > "$FAKE_BIN2/tmux" <<SHIM2
 #!/usr/bin/env bash
@@ -129,7 +135,8 @@ if [ "\$subcmd" = "new-session" ]; then
   exit 0
 fi
 if [ "\$subcmd" = "list-panes" ]; then
-  echo "99998"
+  # Return empty: no sidecar spawned (heartbeat is not the focus of this test).
+  echo ""
   exit 0
 fi
 exit 0
@@ -149,6 +156,7 @@ MOSAIC_AGENT_WORKDIR="$WORKDIR4" \
 MOSAIC_AGENT_RUNTIME="pi" \
 MOSAIC_RUNTIME_BIN="/nonexistent-dir-$$" \
 MOSAIC_AGENT_COMMAND="mosaic yolo pi" \
+MOSAIC_HEARTBEAT_RUN_DIR="$HB_RUN_DIR4" \
   "$START" "$AGENT4"
 
 all_args4=$(cat "$TMUX_ARGS_FILE2" 2>/dev/null || true)
@@ -171,7 +179,8 @@ echo "$all_args4" | grep -qF "mosaic yolo pi" || fail "pane command does not inc
 TMUX_ARGS_FILE5=$(mktemp)
 FAKE_BIN5=$(mktemp -d)
 FAKE_RUNTIME_BIN5=$(mktemp -d)  # this dir IS on the launcher's PATH below
-CLEANUP_DIRS+=("$FAKE_BIN5" "$FAKE_RUNTIME_BIN5")
+HB_RUN_DIR5=$(mktemp -d)
+CLEANUP_DIRS+=("$FAKE_BIN5" "$FAKE_RUNTIME_BIN5" "$HB_RUN_DIR5")
 
 cat > "$FAKE_BIN5/tmux" <<SHIM5
 #!/usr/bin/env bash
@@ -182,7 +191,8 @@ if [ "\$subcmd" = "new-session" ]; then
   exit 0
 fi
 if [ "\$subcmd" = "list-panes" ]; then
-  echo "99997"
+  # Return empty: no sidecar spawned (heartbeat is not the focus of this test).
+  echo ""
   exit 0
 fi
 exit 0
@@ -204,6 +214,7 @@ MOSAIC_AGENT_WORKDIR="$WORKDIR5" \
 MOSAIC_AGENT_RUNTIME="pi" \
 MOSAIC_RUNTIME_BIN="$FAKE_RUNTIME_BIN5" \
 MOSAIC_AGENT_COMMAND="mosaic yolo pi" \
+MOSAIC_HEARTBEAT_RUN_DIR="$HB_RUN_DIR5" \
   "$START" "$AGENT5"
 
 all_args5=$(cat "$TMUX_ARGS_FILE5" 2>/dev/null || true)