diff --git a/bin/message.py b/bin/message.py
index b65a063..e72101b 100644
--- a/bin/message.py
+++ b/bin/message.py
@@ -1,6 +1,5 @@
 import sys
 import os
-import inspect
 from typing import Optional
 from path import BIN_PATH
 
@@ -11,11 +10,16 @@ def debug_print(message, prefix: Optional[str] = None, err = False):
     prefix: prefix to show; script path if None
     """
 
-    call_stack = inspect.stack()
-    script_path = call_stack[1].filename if len(call_stack) >= 2 else "??"
+    try:
+        frame = sys._getframe(1)
+        script_path = frame.f_code.co_filename
+        script_line = frame.f_lineno
+    except ValueError:
+        script_path = "??"
+        script_line = 0
+
     script_path = os.path.relpath(script_path, start=BIN_PATH)
-    script_line = call_stack[1].lineno
 
     prefix_message = f"{script_path}:{script_line}" if prefix is None else str(prefix)
 
-    print(f"\x1b[{31 if err else 32}m\x1b[1m[{prefix_message}]\x1b[m {str(message)}", file=sys.stderr)
\ No newline at end of file
+    print(f"\x1b[{31 if err else 32}m\x1b[1m[{prefix_message}]\x1b[m {str(message)}", file=sys.stderr)
diff --git a/bin/run.py b/bin/run.py
index 70e5257..462ba25 100644
--- a/bin/run.py
+++ b/bin/run.py
@@ -1,7 +1,6 @@
 import os
 import subprocess
 
-from engine import rewrite
 from message import debug_print
 from path import BIN_PATH
 
@@ -31,6 +30,8 @@ def compile(source_path):
             base_cpp = fp.read()
         with open(source_path, "w", encoding="utf8") as fp:
             fp.write(base_cpp)
+        from engine import rewrite
+
         rewrite(source_path, [], [])
 
     debug_print(f"Compiling {source_path} ...")
@@ -89,4 +90,4 @@ def execute(source_path, just=False, force=False):
         compile_if_modified(source_path)
 
     if not just:
-        exec(exec_path_of(source_path))
\ No newline at end of file
+        exec(exec_path_of(source_path))
diff --git a/bin/ysn b/bin/ysn
index 93f7eda..7d42161 100755
--- a/bin/ysn
+++ b/bin/ysn
@@ -5,11 +5,44 @@ SCRIPT_DIR="$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)"
 PROJ_DIR="$(dirname "$SCRIPT_DIR")"
 VENV_ACTIVATE="$PROJ_DIR/.env/bin/activate"
 
+# Most paths still need Python, so fail early with a setup hint if the venv is missing.
 if [ ! -f "$VENV_ACTIVATE" ]; then
     printf 'Python environment is missing. Run ./setup.sh from %s first.\n' "$PROJ_DIR" >&2
     exit 1
 fi
 
+# Fast path for the hot loop: `ysn a` can directly run `a.exe` when it is newer
+# than `a.cpp`. Any option, dotted path, missing binary, or stale binary falls
+# through to the Python implementation.
+if [ "$#" -eq 1 ] && [[ "$1" != -* ]] && [[ "$1" != *.* ]]; then
+    SOURCE_PATH="$1.cpp"
+    EXEC_PATH="$1.exe"
+
+    if [ -f "$SOURCE_PATH" ] && [ -x "$EXEC_PATH" ] && [ "$EXEC_PATH" -nt "$SOURCE_PATH" ]; then
+        if [[ "$EXEC_PATH" = /* ]]; then
+            RUN_PATH="$EXEC_PATH"
+        else
+            RUN_PATH="./$EXEC_PATH"
+        fi
+
+        printf '\033[32m\033[1m[ysn]\033[m Fast path: Executing %s ...\n' "$EXEC_PATH" >&2
+        start_us="${EPOCHREALTIME/./}"
+        set +e
+        "$RUN_PATH"
+        status="$?"
+        set -e
+        end_us="${EPOCHREALTIME/./}"
+        duration_ms="$(((end_us - start_us) / 1000))"
+
+        if [ "$status" -eq 0 ]; then
+            printf '\033[32m\033[1m[ysn]\033[m Successfully exited in %s ms\n' "$duration_ms" >&2
+        else
+            printf '\033[31m\033[1m[ysn]\033[m Execution failed in return code %s after %s ms\n' "$status" "$duration_ms" >&2
+        fi
+        exit "$status"
+    fi
+fi
+
 # shellcheck disable=SC1090
 source "$VENV_ACTIVATE"
 exec python "$SCRIPT_DIR/ysn.py" "$@"
diff --git a/bin/ysn.py b/bin/ysn.py
index 8b0db1a..d0323f5 100644
--- a/bin/ysn.py
+++ b/bin/ysn.py
@@ -3,7 +3,6 @@
 import argparse
 import os
 
-from engine import rewrite
 from run import execute
 
 def normalize_source_path(source_path: str) -> str:
@@ -40,10 +39,12 @@ def normalize_module_name(module_name: str) -> str:
     if args.install is None and args.remove is None:
         execute(normalized_source_path, just=args.just, force=args.force)
     else:
+        from engine import rewrite
+
         installed_modules = args.install if args.install is not None else []
         removed_modules = args.remove if args.remove is not None else []
 
         installed_modules = list(map(normalize_module_name, installed_modules))
         removed_modules = list(map(normalize_module_name, removed_modules))
 
-        rewrite(normalized_source_path, installed_modules, removed_modules)
\ No newline at end of file
+        rewrite(normalized_source_path, installed_modules, removed_modules)