diff --git a/src/session_analytics/storage.py b/src/session_analytics/storage.py
index 1c8d1af..02f9514 100644
--- a/src/session_analytics/storage.py
+++ b/src/session_analytics/storage.py
@@ -166,7 +166,7 @@ class BusEvent:
 DEFAULT_DB_PATH = Path.home() / ".claude" / "contrib" / "analytics" / "data.db"
 
 # Schema version for migrations
-SCHEMA_VERSION = 6
+SCHEMA_VERSION = 7
 
 # Migration functions: dict of version -> (migration_name, migration_func)
 # Each migration upgrades FROM version-1 TO version
@@ -379,6 +379,17 @@ def migrate_v6(conn):
     conn.execute("CREATE INDEX IF NOT EXISTS idx_bus_events_repo ON bus_events(repo)")
 
 
+@migration(7, "add_tool_id_index")
+def migrate_v7(conn):
+    """Add index on tool_id for faster self-joins.
+
+    The query_error_details() function joins events to itself on tool_id
+    to correlate tool_result errors with their tool_use parameters.
+    Without this index, queries took ~25s on 160K events.
+    """
+    conn.execute("CREATE INDEX IF NOT EXISTS idx_events_tool_id ON events(tool_id)")
+
+
 class SQLiteStorage:
     """SQLite-backed storage for session analytics."""
 
@@ -705,6 +716,9 @@ def _init_db(self):
             conn.execute("CREATE INDEX IF NOT EXISTS idx_events_parent_uuid ON events(parent_uuid)")
             conn.execute("CREATE INDEX IF NOT EXISTS idx_events_agent_id ON events(agent_id)")
 
+            # Performance index for tool_use ↔ tool_result self-joins (migration v7)
+            conn.execute("CREATE INDEX IF NOT EXISTS idx_events_tool_id ON events(tool_id)")
+
     # Event operations
 
     def add_event(self, event: Event) -> Event:
diff --git a/tests/test_storage.py b/tests/test_storage.py
index f2cc54c..18e80c5 100644
--- a/tests/test_storage.py
+++ b/tests/test_storage.py
@@ -982,3 +982,9 @@ def test_index_on_agent_id(self, storage):
         rows = storage.execute_query("PRAGMA index_list(events)")
         indexes = {row[1] for row in rows}
         assert "idx_events_agent_id" in indexes
+
+    def test_index_on_tool_id(self, storage):
+        """Verify that idx_events_tool_id index exists for self-join performance."""
+        rows = storage.execute_query("PRAGMA index_list(events)")
+        indexes = {row[1] for row in rows}
+        assert "idx_events_tool_id" in indexes