diff --git a/pkg/mining/simulated_miner.go b/pkg/mining/simulated_miner.go
index 46ec044..0374afa 100644
--- a/pkg/mining/simulated_miner.go
+++ b/pkg/mining/simulated_miner.go
@@ -2,9 +2,9 @@ package mining
 
 import (
 	"context"
-	"fmt"
 	"math"
 	"math/rand"
+	"strconv"
 	"sync"
 	"time"
 )
@@ -103,7 +103,7 @@ func (m *SimulatedMiner) Start(config *Config) error {
 	m.mutex.Lock()
 	if m.Running {
 		m.mutex.Unlock()
-		return fmt.Errorf("simulated miner %s is already running", m.Name)
+		return ErrMinerExists(m.Name)
 	}
 
 	m.Running = true
@@ -113,10 +113,11 @@ func (m *SimulatedMiner) Start(config *Config) error {
 	m.stopChan = make(chan struct{})
 	m.HashrateHistory = make([]HashratePoint, 0)
 	m.LowResHistory = make([]HashratePoint, 0)
+	timestamp := time.Now().Format("15:04:05")
 	m.logs = []string{
-		fmt.Sprintf("[%s] Simulated miner starting...", time.Now().Format("15:04:05")),
-		fmt.Sprintf("[%s] Connecting to %s", time.Now().Format("15:04:05"), m.poolName),
-		fmt.Sprintf("[%s] Pool connected, algorithm: %s", time.Now().Format("15:04:05"), m.Algorithm),
+		"[" + timestamp + "] Simulated miner starting...",
+		"[" + timestamp + "] Connecting to " + m.poolName,
+		"[" + timestamp + "] Pool connected, algorithm: " + m.Algorithm,
 	}
 	m.mutex.Unlock()
 
@@ -132,12 +133,12 @@ func (m *SimulatedMiner) Stop() error {
 	defer m.mutex.Unlock()
 
 	if !m.Running {
-		return fmt.Errorf("simulated miner %s is not running", m.Name)
+		return ErrMinerNotRunning(m.Name)
 	}
 
 	close(m.stopChan)
 	m.Running = false
-	m.logs = append(m.logs, fmt.Sprintf("[%s] Miner stopped", time.Now().Format("15:04:05")))
+	m.logs = append(m.logs, "["+time.Now().Format("15:04:05")+"] Miner stopped")
 
 	return nil
 }
@@ -249,11 +250,11 @@ func (m *SimulatedMiner) simulateShare() {
 	// 2% chance of rejected share
 	if rand.Float64() < 0.02 {
 		m.rejected++
-		m.logs = append(m.logs, fmt.Sprintf("[%s] Share rejected (stale)", time.Now().Format("15:04:05")))
+		m.logs = append(m.logs, "["+time.Now().Format("15:04:05")+"] Share rejected (stale)")
 	} else {
 		m.shares++
 		diff := m.difficultyBase + rand.Intn(m.difficultyBase/2)
-		m.logs = append(m.logs, fmt.Sprintf("[%s] Share accepted (%d/%d) diff %d", time.Now().Format("15:04:05"), m.shares, m.rejected, diff))
+		m.logs = append(m.logs, "["+time.Now().Format("15:04:05")+"] Share accepted ("+strconv.Itoa(m.shares)+"/"+strconv.Itoa(m.rejected)+") diff "+strconv.Itoa(diff))
 	}
 
 	// Keep last 100 log lines
@@ -263,13 +264,14 @@ func (m *SimulatedMiner) simulateShare() {
 }
 
 // metrics, err := miner.GetStats(ctx)
-// fmt.Printf("hashrate: %d H/s, shares: %d\n", metrics.Hashrate, metrics.Shares)
+// _ = metrics.Hashrate  // current H/s
+// _ = metrics.Shares    // accepted share count
 func (m *SimulatedMiner) GetStats(ctx context.Context) (*PerformanceMetrics, error) {
 	m.mutex.RLock()
 	defer m.mutex.RUnlock()
 
 	if !m.Running {
-		return nil, fmt.Errorf("simulated miner %s is not running", m.Name)
+		return nil, ErrMinerNotRunning(m.Name)
 	}
 
 	// Calculate current hashrate from recent history
@@ -409,10 +411,10 @@ func (m *SimulatedMiner) WriteStdin(input string) error {
 	defer m.mutex.Unlock()
 
 	if !m.Running {
-		return fmt.Errorf("simulated miner %s is not running", m.Name)
+		return ErrMinerNotRunning(m.Name)
 	}
 
-	m.logs = append(m.logs, fmt.Sprintf("[%s] stdin: %s", time.Now().Format("15:04:05"), input))
+	m.logs = append(m.logs, "["+time.Now().Format("15:04:05")+"] stdin: "+input)
 	return nil
 }