diff --git a/src/claude_agent_sdk/_internal/transport/subprocess_cli.py b/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
index 983eeea1..4527548b 100644
--- a/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
+++ b/src/claude_agent_sdk/_internal/transport/subprocess_cli.py
@@ -323,7 +323,8 @@ def _build_command(self) -> list[str]:
         # No --agents CLI flag needed
 
         if effective_setting_sources is not None:
-            cmd.append(f"--setting-sources={','.join(effective_setting_sources)}")
+            sources_str = ",".join(effective_setting_sources)
+            cmd.append(f"--setting-sources={sources_str}")
 
         # Add plugin directories
         if self._options.plugins: