🎨 Palette: Add friendly CLI input validation

.Jules/palette.md

@@ -5,3 +5,7 @@
 ## 2025-02-28 - Structured CLI Reports
 **Learning:** Dense numerical data in CLI output is hard to parse. Using ASCII box-drawing characters and alignment to create a "dashboard" or "invoice" style summary significantly improves readability and perceived quality.
 **Action:** When summarizing simulation or batch job results, always format the final report as a structured table or box rather than a list of print statements.
+
+## 2025-06-15 - CLI Input Hygiene
+**Learning:** Default validation errors (stack traces, unhandled exceptions) break the "tool" illusion and feel amateur. Friendly, upfront validation with color-coded errors (e.g. "Error: Days must be positive") establishes trust and guides the user.
+**Action:** Always validate CLI arguments explicitly before execution and use `sys.exit(1)` with a clear message.

bitcoin_trading_simulation.py

@@ -142,6 +142,20 @@ def countdown(quiet=False):
     if args.no_color:
         Colors.disable()
 
+    # Input validation
+    if args.days <= 0:
+        print(f"{Colors.FAIL}Error: Days must be a positive integer.{Colors.ENDC}", file=sys.stderr)
+        sys.exit(1)
+    if args.initial_cash <= 0:
+        print(f"{Colors.FAIL}Error: Initial cash must be positive.{Colors.ENDC}", file=sys.stderr)
+        sys.exit(1)
+    if args.initial_price <= 0:
+        print(f"{Colors.FAIL}Error: Initial price must be positive.{Colors.ENDC}", file=sys.stderr)
+        sys.exit(1)
+    if args.volatility < 0:
+        print(f"{Colors.FAIL}Error: Volatility must be non-negative.{Colors.ENDC}", file=sys.stderr)
+        sys.exit(1)
+
     # Simulate prices
     prices = simulate_bitcoin_prices(days=args.days, initial_price=args.initial_price, volatility=args.volatility)
 

test_cli_args.py

@@ -0,0 +1,33 @@
+import subprocess
+import sys
+import pytest
+
+def run_simulation(args):
+    """Runs the simulation script with the given arguments."""
+    cmd = [sys.executable, "bitcoin_trading_simulation.py"] + args
+    return subprocess.run(cmd, capture_output=True, text=True)
+
+def test_invalid_days():
+    result = run_simulation(["--days", "-5"])
+    assert result.returncode != 0, "Simulation should fail with negative days"
+    assert "days must be a positive integer" in result.stderr.lower() or "days must be a positive integer" in result.stdout.lower()
+
+def test_invalid_initial_cash():
+    result = run_simulation(["--initial-cash", "-1000"])
+    assert result.returncode != 0, "Simulation should fail with negative cash"
+    assert "initial cash must be positive" in result.stderr.lower() or "initial cash must be positive" in result.stdout.lower()
+
+def test_invalid_initial_price():
+    result = run_simulation(["--initial-price", "-500"])
+    assert result.returncode != 0, "Simulation should fail with negative price"
+    assert "initial price must be positive" in result.stderr.lower() or "initial price must be positive" in result.stdout.lower()
+
+def test_invalid_volatility():
+    result = run_simulation(["--volatility", "-0.1"])
+    assert result.returncode != 0, "Simulation should fail with negative volatility"
+    assert "volatility must be non-negative" in result.stderr.lower() or "volatility must be non-negative" in result.stdout.lower()
+
+def test_valid_run():
+    # Run with quiet mode to speed up test
+    result = run_simulation(["--days", "10", "--quiet"])
+    assert result.returncode == 0, "Valid simulation should succeed"