fix: weak bots now actually play weak — MultiPV suboptimal move selection

Before: even amina (skill 1) played near-optimal moves because Stockfish
at depth 3 still finds good moves. Blunders just reduced depth to 1 which
barely changes anything in simple positions.

Now:
- Blunder rolls: analyze 5 lines and pick from the worst (3rd-5th best)
- Weak bots (skill <= 5): use MultiPV and randomly pick suboptimal moves
  - Skill 1-2 (amina): NEVER plays the best move, picks from 2nd-5th line
  - Skill 3-5 (tarek): 40% best move, 60% picks from worse options
- Strong bots (skill > 5): unchanged, use standard depth/skill search
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/api/handlers.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"log"
+	"math/rand"
 	"net/http"
 	"time"
 
@@ -49,32 +50,101 @@ func HandleGetMove(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
+	start := time.Now()
+	isBlunder := bot.ShouldBlunder()
+
 	depth := bot.Depth
 	skillLevel := bot.SkillLevel
 	contempt := bot.Contempt
 
-	// For blunder-prone bots, reduce depth dramatically on blunder rolls
-	if bot.ShouldBlunder() {
-		depth = 1
-		skillLevel = 0
-	}
-
 	timeLimitMs := body.TimeLimitMs
 	if timeLimitMs <= 0 {
-		timeLimitMs = 0 // use depth-based search
+		timeLimitMs = 0
 	}
 
-	req := engine.MoveRequest{
-		FEN:         body.FEN,
-		Depth:       depth,
-		SkillLevel:  skillLevel,
-		Contempt:    contempt,
-		TimeLimitMs: timeLimitMs,
-		MultiPV:     1,
+	var resp *engine.MoveResponse
+	var err error
+
+	if isBlunder {
+		// On blunder: get multiple lines and pick a bad one
+		ctx, cancel := context.WithTimeout(r.Context(), 10*time.Second)
+		defer cancel()
+		lines, analyzeErr := engine.Analyze(ctx, body.FEN, 6, 5)
+		if analyzeErr == nil && len(lines) > 1 {
+			// Pick from the worst options (index 2-4 if available)
+			pickIdx := len(lines) - 1
+			if pickIdx > 1 {
+				pickIdx = 2 + rand.Intn(len(lines)-2)
+				if pickIdx >= len(lines) {
+					pickIdx = len(lines) - 1
+				}
+			}
+			resp = &engine.MoveResponse{
+				BestMove:   lines[pickIdx].Move,
+				Evaluation: lines[pickIdx].Evaluation,
+				Depth:      lines[pickIdx].Depth,
+			}
+			log.Printf("BLUNDER bot=%s picked line %d/%d move=%s eval=%.2f (best was %s eval=%.2f)",
+				body.BotID, pickIdx+1, len(lines), resp.BestMove, resp.Evaluation, lines[0].Move, lines[0].Evaluation)
+		} else {
+			// Fallback: depth 1 search
+			req := engine.MoveRequest{FEN: body.FEN, Depth: 1, SkillLevel: 0, MultiPV: 1}
+			resp, err = engine.GetMove(r.Context(), req)
+		}
+	} else if bot.SkillLevel <= 5 {
+		// Weak bots: use MultiPV and randomly pick a suboptimal move
+		ctx, cancel := context.WithTimeout(r.Context(), 10*time.Second)
+		defer cancel()
+		numLines := 4
+		if bot.SkillLevel <= 2 {
+			numLines = 5
+		}
+		lines, analyzeErr := engine.Analyze(ctx, body.FEN, depth, numLines)
+		if analyzeErr == nil && len(lines) > 1 {
+			// Weight toward worse moves for weaker bots
+			// skill 1: pick from index 1-4, skill 5: pick from index 0-2
+			maxPick := len(lines) - 1
+			minPick := 0
+			if bot.SkillLevel <= 2 {
+				minPick = 1 // never play the best move
+				if maxPick < 1 {
+					maxPick = 1
+				}
+			} else if bot.SkillLevel <= 5 {
+				// 40% chance of best, 60% suboptimal
+				if rand.Float64() > 0.4 && len(lines) > 1 {
+					minPick = 1
+				}
+			}
+			pickIdx := minPick
+			if maxPick > minPick {
+				pickIdx = minPick + rand.Intn(maxPick-minPick+1)
+			}
+			resp = &engine.MoveResponse{
+				BestMove:   lines[pickIdx].Move,
+				Evaluation: lines[pickIdx].Evaluation,
+				Depth:      lines[pickIdx].Depth,
+			}
+			log.Printf("WEAK bot=%s skill=%d picked line %d/%d move=%s",
+				body.BotID, bot.SkillLevel, pickIdx+1, len(lines), resp.BestMove)
+		} else {
+			// Fallback to normal search
+			req := engine.MoveRequest{FEN: body.FEN, Depth: depth, SkillLevel: skillLevel, Contempt: contempt, MultiPV: 1}
+			resp, err = engine.GetMove(r.Context(), req)
+		}
+	} else {
+		// Normal strength bots: use standard search
+		req := engine.MoveRequest{
+			FEN:         body.FEN,
+			Depth:       depth,
+			SkillLevel:  skillLevel,
+			Contempt:    contempt,
+			TimeLimitMs: timeLimitMs,
+			MultiPV:     1,
+		}
+		resp, err = engine.GetMove(r.Context(), req)
 	}
 
-	start := time.Now()
-	resp, err := engine.GetMove(r.Context(), req)
 	if err != nil {
 		log.Printf("ERROR get_move bot=%s fen=%s err=%v", body.BotID, body.FEN, err)
 		writeJSON(w, http.StatusInternalServerError, ErrorResponse{Error: "engine error: " + err.Error()})
@@ -95,8 +165,8 @@ func HandleGetMove(w http.ResponseWriter, r *http.Request) {
 
 	resp.ThinkTimeMs = time.Since(start).Milliseconds()
 
-	log.Printf("MOVE bot=%s depth=%d skill=%d fen=%.40s move=%s eval=%.2f time=%dms",
-		body.BotID, depth, skillLevel, body.FEN, resp.BestMove, resp.Evaluation, resp.ThinkTimeMs)
+	log.Printf("MOVE bot=%s depth=%d skill=%d blunder=%v fen=%.40s move=%s eval=%.2f time=%dms",
+		body.BotID, depth, skillLevel, isBlunder, body.FEN, resp.BestMove, resp.Evaluation, resp.ThinkTimeMs)
 
 	writeJSON(w, http.StatusOK, resp)
 }