diff --git a/start.py b/start.py
index a47da397..1e95cecd 100644
--- a/start.py
+++ b/start.py
@@ -231,7 +231,9 @@ def run_spec_creation(project_dir: Path) -> bool:
             check=False,  # Don't raise on non-zero exit
             cwd=str(Path(__file__).parent),  # Run from project root
             stderr=subprocess.PIPE,
-            text=True
+            text=True,
+            encoding="utf-8",  # Fix Windows CP1252 encoding issue (#138)
+            errors="replace",
         )
 
         # Check for authentication errors in stderr
@@ -403,7 +405,9 @@ def run_agent(project_name: str, project_dir: Path) -> None:
             cmd,
             check=False,
             stderr=subprocess.PIPE,
-            text=True
+            text=True,
+            encoding="utf-8",  # Fix Windows CP1252 encoding issue (#138)
+            errors="replace",
         )
 
         # Check for authentication errors
diff --git a/start_ui.py b/start_ui.py
index e9b08be4..a1555730 100644
--- a/start_ui.py
+++ b/start_ui.py
@@ -127,7 +127,9 @@ def check_node() -> bool:
         result = subprocess.run(
             ["node", "--version"],
             capture_output=True,
-            text=True
+            text=True,
+            encoding="utf-8",  # Fix Windows CP1252 encoding issue (#138)
+            errors="replace",
         )
         print(f"  Node.js version: {result.stdout.strip()}")
     except Exception: