diff --git a/cross_eval/runtime_env.py b/cross_eval/runtime_env.py
index caa5a77..6e5cc62 100644
--- a/cross_eval/runtime_env.py
+++ b/cross_eval/runtime_env.py
@@ -94,6 +94,9 @@ def build_runtime_environment(
 ) -> tuple[dict[str, str], list[Path], dict[str, str]]:
     """Build subprocess env plus metadata about loaded files and names."""
     env = os.environ.copy() if execution.inherit_env else {}
+    # Remove CLAUDECODE to avoid "nested session" errors when spawning
+    # Claude Code as a subprocess from within a Claude Code session.
+    env.pop("CLAUDECODE", None)
     loaded_files = resolve_env_files(execution, project_root)
     loaded_values: dict[str, str] = {}
     for path in loaded_files: