diff --git a/start.py b/start.py
index 0236122..455bc97 100644
--- a/start.py
+++ b/start.py
@@ -9,6 +9,7 @@ Supports two paths for new projects:
 """
 
 import os
+import re
 import subprocess
 import sys
 from pathlib import Path
@@ -24,6 +25,53 @@ from registry import (
     register_project,
 )
 
+# Patterns that indicate Claude CLI authentication errors
+AUTH_ERROR_PATTERNS = [
+    r"not\s+logged\s+in",
+    r"not\s+authenticated",
+    r"authentication\s+(failed|required|error)",
+    r"login\s+required",
+    r"please\s+(run\s+)?['\"]?claude\s+login",
+    r"unauthorized",
+    r"invalid\s+(token|credential|api.?key)",
+    r"expired\s+(token|session|credential)",
+    r"could\s+not\s+authenticate",
+    r"sign\s+in\s+(to|required)",
+]
+
+
+def is_auth_error(output: str) -> bool:
+    """
+    Check if output contains Claude CLI authentication error messages.
+
+    Args:
+        output: Combined stdout/stderr from subprocess
+
+    Returns:
+        True if authentication error detected, False otherwise
+    """
+    if not output:
+        return False
+
+    output_lower = output.lower()
+    for pattern in AUTH_ERROR_PATTERNS:
+        if re.search(pattern, output_lower):
+            return True
+    return False
+
+
+def print_auth_error_help() -> None:
+    """Print helpful message when authentication error is detected."""
+    print("\n" + "=" * 50)
+    print("  Authentication Error Detected")
+    print("=" * 50)
+    print("\nClaude CLI requires authentication to work.")
+    print("\nTo fix this, run:")
+    print("  claude login")
+    print("\nThis will open a browser window to sign in.")
+    print("After logging in, try running this command again.")
+    print("=" * 50 + "\n")
+
 
 def check_spec_exists(project_dir: Path) -> bool:
     """
@@ -203,6 +251,7 @@ def run_spec_creation(project_dir: Path) -> bool:
     Run Claude Code with /create-spec command to create project specification.
 
     The project path is passed as an argument so create-spec knows where to write files.
+    Captures stderr to detect authentication errors and provide helpful guidance.
     """
     print("\n" + "=" * 50)
     print("  Project Specification Setup")
@@ -217,12 +266,25 @@ def run_spec_creation(project_dir: Path) -> bool:
     try:
         # Launch Claude Code with /create-spec command
         # Project path included in command string so it populates $ARGUMENTS
-        subprocess.run(
+        # Capture stderr to detect auth errors while letting stdout flow to terminal
+        result = subprocess.run(
             ["claude", f"/create-spec {project_dir}"],
             check=False,  # Don't raise on non-zero exit
-            cwd=str(Path(__file__).parent)  # Run from project root
+            cwd=str(Path(__file__).parent),  # Run from project root
+            stderr=subprocess.PIPE,
+            text=True
         )
 
+        # Check for authentication errors in stderr
+        stderr_output = result.stderr or ""
+        if result.returncode != 0 and is_auth_error(stderr_output):
+            print_auth_error_help()
+            return False
+
+        # If there was stderr output but not an auth error, show it
+        if stderr_output.strip() and result.returncode != 0:
+            print(f"\nClaude CLI error: {stderr_output.strip()}")
+
         # Check if spec was created in project prompts directory
         if check_spec_exists(project_dir):
             print("\n" + "-" * 50)
@@ -232,6 +294,9 @@ def run_spec_creation(project_dir: Path) -> bool:
             print("\n" + "-" * 50)
             print("Spec creation incomplete.")
             print(f"Please ensure app_spec.txt exists in: {get_project_prompts_dir(project_dir)}")
+            # If failed with non-zero exit and no spec, might be auth issue
+            if result.returncode != 0:
+                print("\nIf you're having authentication issues, try running: claude login")
             return False
 
     except FileNotFoundError:
@@ -348,6 +413,8 @@ def create_new_project_flow() -> tuple[str, Path] | None:
 def run_agent(project_name: str, project_dir: Path) -> None:
     """Run the autonomous agent with the given project.
 
+    Captures stderr to detect authentication errors and provide helpful guidance.
+
     Args:
         project_name: Name of the project
         project_dir: Absolute path to the project directory
@@ -367,9 +434,28 @@ def run_agent(project_name: str, project_dir: Path) -> None:
     # Build the command - pass absolute path
     cmd = [sys.executable, "autonomous_agent_demo.py", "--project-dir", str(project_dir.resolve())]
 
-    # Run the agent
+    # Run the agent with stderr capture to detect auth errors
+    # stdout goes directly to terminal for real-time output
     try:
-        subprocess.run(cmd, check=False)
+        result = subprocess.run(
+            cmd,
+            check=False,
+            stderr=subprocess.PIPE,
+            text=True
+        )
+
+        # Check for authentication errors
+        stderr_output = result.stderr or ""
+        if result.returncode != 0:
+            if is_auth_error(stderr_output):
+                print_auth_error_help()
+            elif stderr_output.strip():
+                # Show any other errors
+                print(f"\nAgent error:\n{stderr_output.strip()}")
+                # Still hint about auth if exit was unexpected
+                if "error" in stderr_output.lower() or "exception" in stderr_output.lower():
+                    print("\nIf this is an authentication issue, try running: claude login")
+
     except KeyboardInterrupt:
         print("\n\nAgent interrupted. Run again to resume.")