notes(bf-413): document completion of AI Code Battle mechanics iteration

All backlog items completed:
- Combat kill scoring in engine (turn.go:274)
- Fitness formula blends win rate + kill rate (run.go:608)
- Flee thresholds reduced with outnumber logic
- CombatDeaths tracked through arena MatchOutcome
- Aggression derived from actual kill rate in behavior vector

This Genesis bead tracked the full mechanics iteration to make combat
economically necessary and reward aggression in the evolver.

notes/bf-413.md

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+# Bead bf-413: Genesis - AI Code Battle Mechanics Iteration - COMPLETE
+
+## Status: ALL BACKLOG ITEMS COMPLETED
+
+This genesis bead tracked ongoing work to improve ai-code-battle game mechanics so the evolutionary loop produces genuinely aggressive, combat-seeking bots.
+
+## Core Problem
+
+Most bots flee before they enter attack range. Combat only happens between two non-fleeing bots (e.g. hunter+swarm). The evolver has no gradient toward aggression because fitness was pure win rate.
+
+## All Backlog Items Completed
+
+### ✅ bf-z9m: Award score for combat kills in engine (turn.go) — P0
+- Commit: `c1acd83` (2024-06-16)
+- Engine change: `gs.Players[e.Owner].Score += gs.Config.KillScore` (turn.go:272-275)
+- Combat kills now award points to the killer
+
+### ✅ Reduce flee thresholds so bots only flee when outnumbered, not preemptively
+- Commit: `d5515e0` (2024-06-17)
+- Changed flee threshold from `AttackRadius2 + 4` to `AttackRadius2` across all strategy bots
+- Added outnumber logic: only flee when nearbyAllies < nearbyEnemies
+- Modified files:
+  - bots/farmer/strategy.go: new shouldFlee() method with outnumber check
+  - bots/gatherer/strategy.go: shouldFlee() updated to check local counts
+  - bots/siege/strategy.go: shouldFlee() updated to check local counts
+- Bots now only flee when enemies are actually in attack range AND they are outnumbered
+
+### ✅ Surface CombatDeaths through arena MatchOutcome into fitness formula
+- Commit: `d42d1a5` (2024-06-17)
+- arena.go: CombatDeaths []int field in MatchOutcome
+- arena.go:204-221: Kill tracking and kill rate computation
+- turn.go:269-271: gs.CombatDeaths[e.Owner]++ tracks kills per player
+
+### ✅ Derive aggression behavior vector from actual kill rate (not LLM self-report)
+- Commit: `d5515e0` (2024-06-17)
+- cmd/acb-evolver/run.go:610-628: compute behavior vector from arena results
+- Formula: `BehaviorVector[0] = min(killRate, 1.0)` - aggression derived from actual kill data
+- Preserves existing economy value (BehaviorVector[1]) or defaults to 0.5 if not available
+- Enhanced logging to show derived aggression value in arena output
+
+### ✅ Evolver fitness: blend win rate + kill rate (bf-59h)
+- Commit: `d42d1a5` (2024-06-17)
+- run.go:601-608: `fitness = 0.7*winRate + 0.3*killRate`
+- cmd/acb-evolver/internal/prompt/builder.go:154-159: System prompt explains the fitness formula
+- Kill rate is now 30% of fitness, encouraging combat aggression
+
+## Design Principle Applied
+
+Aggression must be economically necessary, not just rewarded. Reference: Google AI Challenge 2011 Ants — expansion forced encounters, combat was unavoidable.
+
+In ACB, the flee logic previously created a false safe option that broke this loop. Now bots must fight when:
+1. They are in attack range of enemies (no preemptive flee)
+2. They are NOT outnumbered (flee is only for disadvantageous fights)
+
+## Recent Aggressive Seed Bots (commit d42d1a5)
+
+Three new aggressive seed bots were added as generation-0:
+- **Phalanx**: Defensive cluster that moves as a group
+- **Assassin**: High-damage single-target eliminator
+- **Opportunist**: Waits for weakened enemies then attacks
+
+These provide better initial aggression diversity for the evolver.
+
+## Verification Steps
+
+To verify these changes produce more aggressive bots:
+1. Run the evolver for several generations
+2. Check that kill rates increase across generations
+3. Check that bots no longer flee preemptively
+4. Verify that behavior vector aggression values align with actual kill rates
+
+## Related Beads
+
+- bf-59h: Evolver fitness weight combat kill rate alongside win rate (COMPLETED)
+- bf-z9m: Award score for combat kills in engine (COMPLETED)