Risk Sizing Not Blocking

Experiment Overview

| Item | Details | |------|---------| | Date | 2024-12-24 | | Goal | Refactor risk/capital managers from blocking trades to sizing them correctly | | Environment | scripts/live_trader.py, alpaca_trading/risk/ | | Status | Success |

Context

The trading system had multiple components that could BLOCK trades:

1. Capital Manager: check_trade_allowed() returned False if trade exceeded limits
2. Portfolio Risk Manager: check_position_limits() returned False for VaR/concentration violations

This caused problems:

• Trades were rejected after signal generation
• Components didn't work in harmony
• Orders sized upstream were blocked downstream

Architectural Principle: All trading system components should work in harmony. There should never be an order that is sized too large for capital management.

Verified Workflow

The Wrong Pattern (Blocking)

# WRONG: Check if arbitrary size is allowed, block if not
estimated_qty = cash_available * alloc / price
allowed, reason = capital_mgr.check_trade_allowed(allocation, estimated_qty * price, symbol)
if not allowed:
    logger.warning(f"{symbol}: Capital Manager BLOCKED trade: {reason}")
    return state, {"skip_reason": "capital_limit"}  # Trade rejected!

The Correct Pattern (Sizing)

# CORRECT: Get max allowed size, use it as constraint
max_qty_from_capital = capital_mgr.get_max_trade_size(
    allocation=capital_allocation,
    symbol=symbol,
    current_price=price,
    positions=positions_dict
)

# Later, during GARCH sizing:
qty = garch_mgr.calculate_position_size(...)
qty = min(qty, max_qty_from_capital)  # Apply constraint
# Order executes with correct size - no blocking needed

Complete Sizing Flow

# 1. Capital Manager provides max qty from allocation
if capital_allocation is not None and capital_mgr is not None:
    max_qty_from_capital = capital_mgr.get_max_trade_size(
        allocation=capital_allocation,
        symbol=symbol,
        current_price=price,
        positions=positions_dict
    )
    if max_qty_from_capital <= 0:
        return state, {"skip_reason": "no_capacity"}  # Only skip if truly zero
else:
    max_qty_from_capital = float('inf')

# 2. Risk Manager provides max qty from position limits
if portfolio_risk_mgr is not None:
    max_value_from_risk = portfolio_metrics.total_value * 0.20  # 20% max
    max_qty_from_risk = max_value_from_risk / price
    max_qty_from_capital = min(max_qty_from_capital, max_qty_from_risk)

# 3. GARCH calculates initial size
qty = garch_mgr.calculate_position_size(symbol, account_value, signal_strength, price, returns)

# 4. Apply all constraints
qty = min(qty, max_qty_from_alloc)      # Allocation limit
qty = min(qty, max_qty_from_capital)     # Capital + risk limits
qty = qty * position_scale               # Drawdown scaling

# 5. Execute with correct size
executor.submit(symbol, qty=qty, side=entry_side, type="limit")

Failed Attempts (Critical)

| Attempt | Why it Failed | Lesson Learned | |---------|---------------|----------------| | check_trade_allowed() blocking | Rejected trades after signal generation | Use get_max_trade_size() for constraints | | check_position_limits() blocking | Portfolio risk rejected sized trades | Apply limits as max position value constraint | | Estimate size then check | Estimated size != final size, caused false blocks | Get max size first, use as constraint | | Hard-coded $1000 min_cash_buffer | Blocked trades on small accounts | Use percentage-based limits only |

Key Insights

Harmony Principle

All trading system components should work together:

Signal Generation → Sizing → Risk Constraints → Execution
      ↓               ↓            ↓              ↓
   Direction      Initial qty   Apply limits   Correct size

NOT:

Signal Generation → Sizing → Risk Check → BLOCKED!

When to Skip vs When to Size

| Situation | Action | |-----------|--------| | No capacity at all (max_qty = 0) | Skip with "no_capacity" | | Low confidence signal | Reject signal (not a blocking issue) | | Size exceeds limit | Reduce size to limit (don't block) | | Risk limit exceeded | Reduce size to risk limit (don't block) |

Methods That Size vs Methods That Block

| Method | Purpose | Use For | |--------|---------|---------| | get_max_trade_size() | Returns max qty | Sizing constraints | | calculate_allocation() | Returns capacity info | Sizing constraints | | check_trade_allowed() | Returns bool | AVOID - causes blocking | | check_position_limits() | Returns bool | AVOID - causes blocking |

Files Modified

scripts/live_trader.py:
  - Lines 1375-1391: Capital manager sizing (removed check_trade_allowed)
  - Lines 1393-1411: Risk manager sizing (removed check_position_limits)
  - Lines 1466-1467: Apply max_qty_from_capital constraint

References

• alpaca_trading/risk/capital_manager.py: get_max_trade_size() at lines 284-350
• alpaca_trading/risk/portfolio_risk.py: RiskLimits at lines 20-35
• scripts/live_trader.py: Sizing flow at lines 1375-1505