diff --git a/tests/copilot_usage/test_render_detail.py b/tests/copilot_usage/test_render_detail.py
index 186c7498..a60fe4eb 100644
--- a/tests/copilot_usage/test_render_detail.py
+++ b/tests/copilot_usage/test_render_detail.py
@@ -22,6 +22,7 @@
     ToolTelemetry,
 )
 from copilot_usage.render_detail import (
+    _build_event_details,
     _extract_tool_name,
     _render_code_changes,
     _render_recent_events,
@@ -608,3 +609,41 @@ def test_multi_model_shutdown_via_full_render(self) -> None:
         row = next(line for line in output.splitlines() if "2025-01-01 01:00" in line)
         assert re.search(r"\b7\b", row)  # total model calls = 3 + 4
         assert re.search(r"\b800\b", row)  # total output tokens = 500 + 300
+
+
+# ---------------------------------------------------------------------------
+# Issue #860 — untested branch in _build_event_details
+# ---------------------------------------------------------------------------
+
+
+class TestBuildEventDetailsUntestedBranches:
+    """Cover untested branches in _build_event_details:
+
+    1. SESSION_SHUTDOWN with falsy shutdownType → returns "".
+    2. TOOL_EXECUTION_COMPLETE with truthy model → includes model=<value>.
+    """
+
+    def test_session_shutdown_with_falsy_shutdown_type(self) -> None:
+        """SESSION_SHUTDOWN with shutdownType='' returns empty string."""
+        ev = SessionEvent(
+            type=EventType.SESSION_SHUTDOWN,
+            data={"totalPremiumRequests": 5},
+        )
+        detail = _build_event_details(ev)
+        assert detail == ""
+
+    def test_tool_execution_complete_with_model(self) -> None:
+        """TOOL_EXECUTION_COMPLETE with model field includes model=<value> in detail."""
+        ev = SessionEvent(
+            type=EventType.TOOL_EXECUTION_COMPLETE,
+            data={
+                "toolCallId": "t1",
+                "success": True,
+                "model": "claude-sonnet-4",
+                "toolTelemetry": {"properties": {"tool_name": "bash"}},
+            },
+        )
+        detail = _build_event_details(ev)
+        assert "model=claude-sonnet-4" in detail
+        assert "bash" in detail
+        assert "✓" in detail