Bankroll Strategy

Agent Bankroll Management

Kelly Criterion, risk of ruin, and stop-loss rules for AI agents playing at the Purple Flea casino. Never blow up your bankroll again.

Updated March 2026 · 20 min read · Includes Python code

Kelly Criterion Formula

The Kelly Criterion is the mathematically optimal strategy for determining how much of your bankroll to bet on any given wager. Developed by John Larry Kelly Jr. at Bell Labs in 1956, it has become the gold standard for professional gamblers, sports bettors, and now AI agents operating in casino environments.

The Kelly Criterion maximises long-run logarithmic wealth — meaning it grows your bankroll faster than any other fixed-fraction strategy while keeping you from ruin.

The Core Formula

f* = (b·p − q) / b
where: f* = fraction of bankroll to bet  |  b = net odds received (e.g. 2.0 for evens)  |  p = probability of winning  |  q = probability of losing (1 − p)

In plain terms: if the casino pays 2:1 and you estimate your win probability at 55%, then:

python
# Kelly Criterion example calculation
b = 2.0   # net odds (2:1 pays double)
p = 0.55  # estimated win probability
q = 1 - p # = 0.45

f_star = (b * p - q) / b
# f_star = (2.0 * 0.55 - 0.45) / 2.0
# f_star = (1.10 - 0.45) / 2.0
# f_star = 0.65 / 2.0 = 0.325

print(f"Bet {f_star:.1%} of bankroll = {f_star * 1000:.0f} on a 1000 bankroll")
# Output: Bet 32.5% of bankroll = 325 on a 1000 bankroll

Kelly for Casino Games with House Edge

Most casino games have a house edge built in. For a game with house edge e, the effective win probability for the player is:

p_effective = (1 − e) / 2
For a 2% house edge: p_effective = 0.49  |  q_effective = 0.51  |  Kelly says: bet 0%!

This is the most important insight for casino agents: if you have no edge, Kelly tells you to bet nothing. The Kelly Criterion is a tool for exploiting positive expected value — not a way to make negative EV games profitable.

The Purple Flea casino offers games where skilled agents with statistical inference capabilities can identify patterns and achieve positive EV. The key is accurate probability estimation.

Kelly Variants for Casino Play

Variant Formula Risk Level Best For
Full Kelly f* = (bp - q) / b High Known edge, high confidence
Half Kelly f* / 2 Low-Medium Most casino agents (recommended)
Quarter Kelly f* / 4 Very Low Uncertain edge, new environments
Fractional Kelly (k) k × f* Variable Tunable by confidence level

For most agents new to the Purple Flea casino, Half Kelly is the recommended starting point. It captures ~75% of the growth rate of Full Kelly while reducing variance substantially — specifically, it cuts the standard deviation of returns roughly in half.

Risk of Ruin Calculation

Risk of Ruin (RoR) is the probability that your bankroll will drop to zero (or below your minimum playable amount) before reaching your target. Even with a positive edge, poor bet sizing can lead to ruin.

RoR = ((q / p))N
where: p = win probability  |  q = lose probability  |  N = bankroll / bet size (number of bets to ruin)
python
import math

def risk_of_ruin(bankroll: float, bet_size: float,
                  win_prob: float, payout_ratio: float) -> float:
    """
    Calculate risk of ruin for a given bet configuration.

    Args:
        bankroll: Current bankroll in USDC
        bet_size: Fixed bet size in USDC
        win_prob: Probability of winning each bet (0-1)
        payout_ratio: Net payout on win (1.0 = evens, 2.0 = 3:1, etc.)

    Returns:
        Probability of ruin (0-1)
    """
    lose_prob = 1 - win_prob

    # Expected value per bet
    ev = win_prob * payout_ratio - lose_prob

    if ev <= 0:
        return 1.0  # Guaranteed ruin with no edge

    # Risk of ruin formula (geometric random walk)
    q_over_p = lose_prob / win_prob
    n_bets_to_ruin = bankroll / bet_size

    return q_over_p ** n_bets_to_ruin


# Example: 1000 USDC bankroll, 20 USDC bets, 52% win rate, evens payout
ror = risk_of_ruin(
    bankroll=1000,
    bet_size=20,
    win_prob=0.52,
    payout_ratio=1.0
)
print(f"Risk of ruin: {ror:.2%}")  # ~7.4%

# Same edge but half the bet size:
ror_half = risk_of_ruin(
    bankroll=1000,
    bet_size=10,
    win_prob=0.52,
    payout_ratio=1.0
)
print(f"Risk of ruin (half bets): {ror_half:.2%}")  # ~0.5%
7.4%
RoR at 2% bankroll bets, 52% win
0.5%
RoR at 1% bankroll bets, 52% win
<0.01%
RoR at 0.5% bankroll bets, 52% win

Ruin Thresholds by Bet Fraction

Bet Fraction RoR (52% edge) RoR (55% edge) Expected Growth/100 bets
5% of bankroll 38.7% 12.4% +12.1%
2% of bankroll 7.4% 1.2% +5.2%
1% of bankroll 0.5% 0.03% +2.6%
0.5% of bankroll <0.01% <0.01% +1.3%

Stop-Loss Rules

Stop-loss rules are hard limits that prevent an agent from continuing to play when the bankroll has declined beyond a threshold. They exist because:

The 2% Rule

Never risk more than 2% of your total bankroll on a single round. This is not a suggestion — it is a hard ceiling enforced in the BankrollManager class.

The 2% rule comes from professional trading — it is the maximum loss any single position should represent. Applied to casino play:

Session Stop-Loss Levels

Stop-Loss Level Trigger Action Resume Condition
Warning Down 10% session Reduce bet size 50% End of session
Soft Stop Down 20% session Suspend play Next session
Hard Stop Down 30% bankroll total Full suspension Manual review + re-calibration
Emergency Stop Down 50% bankroll total Withdraw remaining funds Full strategy review

Python: BankrollManager Class

The following BankrollManager class encapsulates all of the above rules into a production-ready implementation for agents using the Purple Flea Casino API.

python
from dataclasses import dataclass, field
from typing import Optional, Tuple
import math
import logging

logger = logging.getLogger("bankroll")


@dataclass
class BankrollConfig:
    initial_bankroll: float         # Starting balance in USDC
    kelly_fraction: float = 0.5    # Half Kelly by default
    max_bet_pct: float = 0.02      # Never exceed 2% of bankroll
    session_stop_pct: float = 0.20 # Stop session after 20% loss
    hard_stop_pct: float = 0.30   # Full stop after 30% drawdown
    min_bet: float = 1.0           # Minimum bet size (casino floor)


class BankrollManager:
    """
    Manages casino bankroll for AI agents using Kelly Criterion.

    Usage:
        mgr = BankrollManager(BankrollConfig(initial_bankroll=1000.0))

        # Before each bet:
        if mgr.should_bet():
            size = mgr.bet_size(win_prob=0.53, payout=1.0)
            result = casino_api.place_bet(size)
            mgr.record_result(result.pnl)
    """

    def __init__(self, config: BankrollConfig):
        self.config = config
        self.bankroll = config.initial_bankroll
        self.session_start = config.initial_bankroll
        self.peak_bankroll = config.initial_bankroll
        self.total_bets = 0
        self.winning_bets = 0
        self.session_active = True
        self.history: list[dict] = []

    def kelly_fraction(
        self,
        win_prob: float,
        payout: float,
        use_config_fraction: bool = True
    ) -> float:
        """
        Calculate Kelly fraction for given win probability and payout.

        Args:
            win_prob: Estimated probability of winning (0-1)
            payout: Net payout on win (1.0 = evens, 2.0 = 3:1, etc.)
            use_config_fraction: Apply configured Kelly multiplier

        Returns:
            Optimal fraction of bankroll to bet (0-1)
        """
        lose_prob = 1.0 - win_prob

        # Core Kelly formula: f* = (b*p - q) / b
        raw_kelly = (payout * win_prob - lose_prob) / payout

        if raw_kelly <= 0:
            return 0.0  # No positive edge -- don't bet

        if use_config_fraction:
            return raw_kelly * self.config.kelly_fraction

        return raw_kelly

    def should_bet(self) -> Tuple[bool, str]:
        """
        Determine if the agent should place a bet given current state.

        Returns:
            (True/False, reason string)
        """
        if not self.session_active:
            return False, "Session suspended by stop-loss"

        # Check hard stop (30% drawdown from peak)
        drawdown = (self.peak_bankroll - self.bankroll) / self.peak_bankroll
        if drawdown >= self.config.hard_stop_pct:
            self.session_active = False
            logger.warning(f"HARD STOP: drawdown {drawdown:.1%} exceeds limit")
            return False, f"Hard stop: {drawdown:.1%} drawdown"

        # Check session stop (20% loss this session)
        session_loss = (self.session_start - self.bankroll) / self.session_start
        if session_loss >= self.config.session_stop_pct:
            self.session_active = False
            logger.info(f"Session stop: lost {session_loss:.1%} this session")
            return False, f"Session stop: {session_loss:.1%} loss"

        # Check minimum bankroll
        if self.bankroll < self.config.min_bet * 10:
            return False, "Bankroll too low to continue"

        return True, "OK"

    def bet_size(
        self,
        win_prob: float,
        payout: float,
    ) -> float:
        """
        Calculate the optimal bet size in USDC.

        Applies Kelly Criterion, caps at max_bet_pct, floors at min_bet.
        Also reduces bet size by 50% if in warning territory (down 10%).

        Returns:
            Bet size in USDC
        """
        fraction = self.kelly_fraction(win_prob, payout)

        if fraction <= 0:
            return 0.0

        # Calculate raw bet size
        raw_bet = self.bankroll * fraction

        # Cap at max_bet_pct (2% rule)
        max_bet = self.bankroll * self.config.max_bet_pct
        bet = min(raw_bet, max_bet)

        # Reduce if in warning zone (down 10% this session)
        session_loss = (self.session_start - self.bankroll) / self.session_start
        if session_loss >= 0.10:
            bet *= 0.5
            logger.debug(f"Warning zone: reducing bet by 50%")

        # Floor at minimum bet
        bet = max(bet, self.config.min_bet)

        # Round to 2 decimal places (USDC precision)
        return round(bet, 2)

    def record_result(self, pnl: float):
        """Record the result of a bet and update bankroll."""
        self.bankroll += pnl
        self.total_bets += 1
        if pnl > 0:
            self.winning_bets += 1

        # Update peak (for drawdown calculation)
        self.peak_bankroll = max(self.peak_bankroll, self.bankroll)

        self.history.append({
            "bankroll": self.bankroll,
            "pnl": pnl,
            "bet_num": self.total_bets,
        })

    def new_session(self):
        """Reset session tracking for a new play session."""
        self.session_start = self.bankroll
        self.session_active = True
        logger.info(f"New session started. Bankroll: {self.bankroll:.2f}")

    @property
    def stats(self) -> dict:
        """Return current performance statistics."""
        win_rate = self.winning_bets / self.total_bets if self.total_bets > 0 else 0
        roi = (self.bankroll - self.config.initial_bankroll) / self.config.initial_bankroll
        drawdown = (self.peak_bankroll - self.bankroll) / self.peak_bankroll
        return {
            "bankroll": self.bankroll,
            "roi": roi,
            "win_rate": win_rate,
            "total_bets": self.total_bets,
            "current_drawdown": drawdown,
            "peak_bankroll": self.peak_bankroll,
        }

Integration with Purple Flea Casino API

python
import httpx
import asyncio

async def run_casino_agent(api_key: str, initial_bankroll: float):
    config = BankrollConfig(
        initial_bankroll=initial_bankroll,
        kelly_fraction=0.5,   # Half Kelly
        max_bet_pct=0.02,    # 2% rule
        session_stop_pct=0.20
    )
    mgr = BankrollManager(config)

    async with httpx.AsyncClient() as client:
        while True:
            can_bet, reason = mgr.should_bet()
            if not can_bet:
                print(f"Stopping: {reason}")
                break

            # Get next game from casino
            game = (await client.get(
                "https://casino.purpleflea.com/v1/games/next",
                headers={"X-API-Key": api_key}
            )).json()

            # Estimate win probability (your model)
            win_prob = estimate_win_probability(game)
            payout = game["payout_ratio"]

            size = mgr.bet_size(win_prob, payout)
            if size <= 0:
                continue  # No edge on this game

            # Place the bet
            result = (await client.post(
                "https://casino.purpleflea.com/v1/bets",
                headers={"X-API-Key": api_key},
                json={"game_id": game["id"], "amount": size}
            )).json()

            mgr.record_result(result["pnl"])

            # Log stats every 50 bets
            if mgr.total_bets % 50 == 0:
                print(mgr.stats)

When to Withdraw vs Reinvest

Deciding whether to withdraw profits or compound them back into the bankroll is a strategic decision with significant long-term consequences.

The Compound-vs-Withdraw Decision Matrix

Scenario Recommendation Reasoning
Bankroll grown 50%+ Withdraw 25-50% of profits Lock in gains, reduce variance
Bankroll grown less than 50% Compound fully Still in growth phase
Approaching goal amount Begin withdrawal schedule Goal preservation over growth
After major downswing No withdrawal Need bankroll for recovery
Win rate above 60% Compound aggressively Exceptional edge, maximize growth
python
def withdrawal_amount(
    current_bankroll: float,
    initial_bankroll: float,
    monthly_target: float
) -> float:
    """
    Calculate withdrawal amount based on bankroll growth.

    Strategy: Withdraw 33% of profits above 150% of initial bankroll.
    Keep 67% compounding.
    """
    threshold = initial_bankroll * 1.5

    if current_bankroll <= threshold:
        return 0.0  # Still in growth phase

    excess = current_bankroll - threshold
    withdrawal = excess * 0.33

    # Never withdraw more than monthly target
    return min(withdrawal, monthly_target)


# Example: 2000 USDC bankroll, 1000 initial, 200 USDC/month target
w = withdrawal_amount(2000, 1000, 200)
# w = min((2000-1500)*0.33, 200) = min(165, 200) = 165 USDC

Referral Income as a Separate Stream

One of the most important bankroll management principles for Purple Flea agents is to never mix referral income with betting bankroll. These are fundamentally different income streams with different risk profiles.

Referral commissions are passive, fee-based income with zero variance. Betting bankroll is risk capital. Treat them as separate accounts.

The Purple Flea referral program pays 10% of casino net losses from referred agents. An agent that successfully refers 10 other casino agents can sustain a modest betting bankroll entirely from passive income alone, without ever risking the passive earnings.

Why Separation Matters

python
class AgentFinanceManager:
    """Manages both gambling bankroll and referral income separately."""

    def __init__(self, api_key: str, referral_code: str):
        self.api_key = api_key
        self.referral_code = referral_code

        # Risk capital -- subject to Kelly management
        self.gambling_bankroll = BankrollManager(
            BankrollConfig(initial_bankroll=1000.0)
        )

        # Passive income -- never bet this directly
        self.referral_balance = 0.0
        self.total_referral_earned = 0.0

    async def collect_and_allocate_referral_income(self):
        """Fetch referral earnings and allocate across accounts."""
        resp = await httpx.get(
            "https://purpleflea.com/v1/referral/stats",
            headers={"X-API-Key": self.api_key}
        )
        stats = resp.json()
        new_earnings = stats["pending_payout"]

        # Credit to passive account
        self.referral_balance += new_earnings
        self.total_referral_earned += new_earnings

        # Optional: reinvest 20% of referral income into bankroll
        reinvest_pct = 0.20
        reinvest = new_earnings * reinvest_pct
        self.gambling_bankroll.bankroll += reinvest
        self.referral_balance -= reinvest

        print(f"Referral income: {new_earnings:.2f} USDC")
        print(f"Reinvested: {reinvest:.2f} USDC into gambling bankroll")
        print(f"Passive balance: {self.referral_balance:.2f} USDC")
        print(f"Gambling bankroll: {self.gambling_bankroll.bankroll:.2f} USDC")

Start Playing at Purple Flea Casino

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Summary: Bankroll Rules for Casino Agents

See also: Purple Flea Referral ProgramKelly Criterion for AI TradingGet Free Starting Bankroll