diff --git a/internal/cache/result_cache.go b/internal/cache/result_cache.go
new file mode 100644
index 0000000..5c39a01
--- /dev/null
+++ b/internal/cache/result_cache.go
@@ -0,0 +1,319 @@
+package cache
+
+import (
+	"crypto/md5"
+	"encoding/json"
+	"fmt"
+	"os"
+	"path/filepath"
+	"sync"
+	"time"
+)
+
+// CacheKey represents a cache key for an activity result
+type CacheKey struct {
+	ActivityType string // "implementer", "judge", "planner"
+	TaskID       string
+	InputHash    string // MD5 hash of input
+	ModelID      string // LLM model used
+}
+
+// String returns a string representation of the cache key
+func (ck *CacheKey) String() string {
+	return fmt.Sprintf("%s:%s:%s:%s", ck.ActivityType, ck.TaskID, ck.InputHash, ck.ModelID)
+}
+
+// CacheEntry represents a cached activity result
+type CacheEntry struct {
+	Key       CacheKey              `json:"key"`
+	Result    map[string]interface{} `json:"result"`
+	CreatedAt time.Time             `json:"created_at"`
+	HitCount  int                   `json:"hit_count"`
+	Metadata  map[string]interface{} `json:"metadata,omitempty"`
+}
+
+// ResultCache caches activity results to avoid redundant computations
+type ResultCache struct {
+	mu        sync.RWMutex
+	basePath  string
+	cache     map[string]*CacheEntry
+	maxSize   int
+	ttl       time.Duration
+}
+
+// NewResultCache creates a new result cache
+func NewResultCache(basePath string, maxSize int, ttl time.Duration) *ResultCache {
+	return &ResultCache{
+		basePath: basePath,
+		cache:    make(map[string]*CacheEntry),
+		maxSize:  maxSize,
+		ttl:      ttl,
+	}
+}
+
+// ComputeHash computes a hash of the input data
+func ComputeHash(data interface{}) (string, error) {
+	jsonData, err := json.Marshal(data)
+	if err != nil {
+		return "", err
+	}
+
+	hash := md5.Sum(jsonData)
+	return fmt.Sprintf("%x", hash), nil
+}
+
+// Set stores a result in the cache
+func (rc *ResultCache) Set(key *CacheKey, result map[string]interface{}) error {
+	if key == nil {
+		return fmt.Errorf("cache key cannot be nil")
+	}
+
+	rc.mu.Lock()
+	defer rc.mu.Unlock()
+
+	keyStr := key.String()
+
+	entry := &CacheEntry{
+		Key:       *key,
+		Result:    result,
+		CreatedAt: time.Now(),
+		Metadata:  make(map[string]interface{}),
+	}
+
+	// Check size limit
+	if len(rc.cache) >= rc.maxSize && rc.cache[keyStr] == nil {
+		// Evict oldest entry (simple FIFO)
+		var oldestKey string
+		var oldestTime time.Time
+
+		for k, v := range rc.cache {
+			if oldestTime.IsZero() || v.CreatedAt.Before(oldestTime) {
+				oldestKey = k
+				oldestTime = v.CreatedAt
+			}
+		}
+
+		if oldestKey != "" {
+			delete(rc.cache, oldestKey)
+		}
+	}
+
+	rc.cache[keyStr] = entry
+	return rc.persistLocked(keyStr, entry)
+}
+
+// Get retrieves a result from the cache
+func (rc *ResultCache) Get(key *CacheKey) (map[string]interface{}, bool, error) {
+	if key == nil {
+		return nil, false, fmt.Errorf("cache key cannot be nil")
+	}
+
+	rc.mu.Lock()
+	defer rc.mu.Unlock()
+
+	keyStr := key.String()
+	entry, exists := rc.cache[keyStr]
+
+	if !exists {
+		return nil, false, nil
+	}
+
+	// Check TTL
+	if rc.ttl > 0 && time.Since(entry.CreatedAt) > rc.ttl {
+		delete(rc.cache, keyStr)
+		return nil, false, nil
+	}
+
+	// Increment hit count
+	entry.HitCount++
+	_ = rc.persistLocked(keyStr, entry)
+
+	return entry.Result, true, nil
+}
+
+// Invalidate removes a cache entry
+func (rc *ResultCache) Invalidate(key *CacheKey) error {
+	if key == nil {
+		return fmt.Errorf("cache key cannot be nil")
+	}
+
+	rc.mu.Lock()
+	defer rc.mu.Unlock()
+
+	keyStr := key.String()
+	delete(rc.cache, keyStr)
+
+	// Delete from disk
+	cacheFile := filepath.Join(rc.basePath, "cache", fmt.Sprintf("%s.json", keyStr))
+	_ = os.Remove(cacheFile)
+
+	return nil
+}
+
+// Clear clears all cache entries
+func (rc *ResultCache) Clear() error {
+	rc.mu.Lock()
+	defer rc.mu.Unlock()
+
+	rc.cache = make(map[string]*CacheEntry)
+
+	// Clear disk cache
+	cacheDir := filepath.Join(rc.basePath, "cache")
+	_ = os.RemoveAll(cacheDir)
+
+	return nil
+}
+
+// GetStats returns cache statistics
+func (rc *ResultCache) GetStats() map[string]interface{} {
+	rc.mu.RLock()
+	defer rc.mu.RUnlock()
+
+	totalHits := 0
+	for _, entry := range rc.cache {
+		totalHits += entry.HitCount
+	}
+
+	return map[string]interface{}{
+		"size":        len(rc.cache),
+		"max_size":    rc.maxSize,
+		"total_hits":  totalHits,
+		"usage_ratio": float64(len(rc.cache)) / float64(rc.maxSize),
+	}
+}
+
+// GetSize returns the current cache size
+func (rc *ResultCache) GetSize() int {
+	rc.mu.RLock()
+	defer rc.mu.RUnlock()
+
+	return len(rc.cache)
+}
+
+// persistLocked saves a cache entry to disk (must be called with lock held)
+func (rc *ResultCache) persistLocked(keyStr string, entry *CacheEntry) error {
+	cacheDir := filepath.Join(rc.basePath, "cache")
+
+	// Create directory if it doesn't exist
+	if err := os.MkdirAll(cacheDir, 0755); err != nil {
+		return err
+	}
+
+	cacheFile := filepath.Join(cacheDir, fmt.Sprintf("%s.json", keyStr))
+
+	data, err := json.MarshalIndent(entry, "", "  ")
+	if err != nil {
+		return err
+	}
+
+	return os.WriteFile(cacheFile, data, 0644)
+}
+
+// Load loads cache from disk
+func (rc *ResultCache) Load() error {
+	rc.mu.Lock()
+	defer rc.mu.Unlock()
+
+	cacheDir := filepath.Join(rc.basePath, "cache")
+	entries, err := os.ReadDir(cacheDir)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return nil // Cache doesn't exist yet
+		}
+		return err
+	}
+
+	for _, entry := range entries {
+		if entry.IsDir() {
+			continue
+		}
+
+		filePath := filepath.Join(cacheDir, entry.Name())
+		data, err := os.ReadFile(filePath)
+		if err != nil {
+			continue
+		}
+
+		var cacheEntry CacheEntry
+		if err := json.Unmarshal(data, &cacheEntry); err != nil {
+			continue
+		}
+
+		// Skip expired entries
+		if rc.ttl > 0 && time.Since(cacheEntry.CreatedAt) > rc.ttl {
+			continue
+		}
+
+		keyStr := cacheEntry.Key.String()
+		rc.cache[keyStr] = &cacheEntry
+	}
+
+	return nil
+}
+
+// InvalidateByActivity invalidates all cache entries for an activity type
+func (rc *ResultCache) InvalidateByActivity(activityType string) error {
+	rc.mu.Lock()
+	defer rc.mu.Unlock()
+
+	keysToDelete := make([]string, 0)
+	for keyStr, entry := range rc.cache {
+		if entry.Key.ActivityType == activityType {
+			keysToDelete = append(keysToDelete, keyStr)
+		}
+	}
+
+	for _, keyStr := range keysToDelete {
+		delete(rc.cache, keyStr)
+
+		// Delete from disk
+		cacheFile := filepath.Join(rc.basePath, "cache", fmt.Sprintf("%s.json", keyStr))
+		_ = os.Remove(cacheFile)
+	}
+
+	return nil
+}
+
+// InvalidateByTask invalidates all cache entries for a task
+func (rc *ResultCache) InvalidateByTask(taskID string) error {
+	rc.mu.Lock()
+	defer rc.mu.Unlock()
+
+	keysToDelete := make([]string, 0)
+	for keyStr, entry := range rc.cache {
+		if entry.Key.TaskID == taskID {
+			keysToDelete = append(keysToDelete, keyStr)
+		}
+	}
+
+	for _, keyStr := range keysToDelete {
+		delete(rc.cache, keyStr)
+
+		// Delete from disk
+		cacheFile := filepath.Join(rc.basePath, "cache", fmt.Sprintf("%s.json", keyStr))
+		_ = os.Remove(cacheFile)
+	}
+
+	return nil
+}
+
+// GetHitRate returns the cache hit rate
+func (rc *ResultCache) GetHitRate() (float64, int) {
+	rc.mu.RLock()
+	defer rc.mu.RUnlock()
+
+	if len(rc.cache) == 0 {
+		return 0, 0
+	}
+
+	totalHits := 0
+	for _, entry := range rc.cache {
+		totalHits += entry.HitCount
+	}
+
+	if totalHits == 0 {
+		return 0, len(rc.cache)
+	}
+
+	return float64(totalHits) / float64(len(rc.cache)), len(rc.cache)
+}
diff --git a/internal/cache/result_cache_test.go b/internal/cache/result_cache_test.go
new file mode 100644
index 0000000..3387f3d
--- /dev/null
+++ b/internal/cache/result_cache_test.go
@@ -0,0 +1,316 @@
+package cache
+
+import (
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestCacheKeyString(t *testing.T) {
+	key := &CacheKey{
+		ActivityType: "implementer",
+		TaskID:       "T1.1",
+		InputHash:    "abc123",
+		ModelID:      "claude-opus",
+	}
+
+	keyStr := key.String()
+	assert.Contains(t, keyStr, "implementer")
+	assert.Contains(t, keyStr, "T1.1")
+	assert.Contains(t, keyStr, "abc123")
+	assert.Contains(t, keyStr, "claude-opus")
+}
+
+func TestComputeHash(t *testing.T) {
+	data := map[string]interface{}{
+		"task": "T1.1",
+		"code": "package main",
+	}
+
+	hash1, err := ComputeHash(data)
+	assert.NoError(t, err)
+	assert.NotEmpty(t, hash1)
+
+	hash2, err := ComputeHash(data)
+	assert.NoError(t, err)
+	assert.Equal(t, hash1, hash2)
+}
+
+func TestSetAndGet(t *testing.T) {
+	tmpDir := t.TempDir()
+	cache := NewResultCache(tmpDir, 100, 0)
+
+	key := &CacheKey{
+		ActivityType: "implementer",
+		TaskID:       "T1.1",
+		InputHash:    "abc123",
+		ModelID:      "claude-opus",
+	}
+
+	result := map[string]interface{}{
+		"output": "implementation code",
+		"files": []string{"file1.go", "file2.go"},
+	}
+
+	err := cache.Set(key, result)
+	assert.NoError(t, err)
+
+	retrieved, found, err := cache.Get(key)
+	assert.NoError(t, err)
+	assert.True(t, found)
+	assert.Equal(t, "implementation code", retrieved["output"])
+}
+
+func TestCacheMiss(t *testing.T) {
+	tmpDir := t.TempDir()
+	cache := NewResultCache(tmpDir, 100, 0)
+
+	key := &CacheKey{
+		ActivityType: "implementer",
+		TaskID:       "T1.1",
+		InputHash:    "abc123",
+		ModelID:      "claude-opus",
+	}
+
+	retrieved, found, err := cache.Get(key)
+	assert.NoError(t, err)
+	assert.False(t, found)
+	assert.Nil(t, retrieved)
+}
+
+func TestInvalidate(t *testing.T) {
+	tmpDir := t.TempDir()
+	cache := NewResultCache(tmpDir, 100, 0)
+
+	key := &CacheKey{
+		ActivityType: "implementer",
+		TaskID:       "T1.1",
+		InputHash:    "abc123",
+		ModelID:      "claude-opus",
+	}
+
+	cache.Set(key, map[string]interface{}{"output": "code"})
+	assert.Equal(t, 1, cache.GetSize())
+
+	cache.Invalidate(key)
+	assert.Equal(t, 0, cache.GetSize())
+
+	_, found, _ := cache.Get(key)
+	assert.False(t, found)
+}
+
+func TestClear(t *testing.T) {
+	tmpDir := t.TempDir()
+	cache := NewResultCache(tmpDir, 100, 0)
+
+	for i := 0; i < 10; i++ {
+		key := &CacheKey{
+			ActivityType: "implementer",
+			TaskID:       "T1.1",
+			InputHash:    string(rune(48 + i)),
+			ModelID:      "claude-opus",
+		}
+		cache.Set(key, map[string]interface{}{"output": "code"})
+	}
+
+	assert.Equal(t, 10, cache.GetSize())
+
+	cache.Clear()
+	assert.Equal(t, 0, cache.GetSize())
+}
+
+func TestGetStats(t *testing.T) {
+	tmpDir := t.TempDir()
+	cache := NewResultCache(tmpDir, 100, 0)
+
+	key := &CacheKey{
+		ActivityType: "implementer",
+		TaskID:       "T1.1",
+		InputHash:    "abc123",
+		ModelID:      "claude-opus",
+	}
+
+	cache.Set(key, map[string]interface{}{"output": "code"})
+	cache.Get(key) // Hit
+
+	stats := cache.GetStats()
+	assert.Equal(t, 1, stats["size"])
+	assert.Equal(t, 100, stats["max_size"])
+	assert.Equal(t, 1, stats["total_hits"])
+}
+
+func TestTTLExpiration(t *testing.T) {
+	tmpDir := t.TempDir()
+	cache := NewResultCache(tmpDir, 100, 100*time.Millisecond)
+
+	key := &CacheKey{
+		ActivityType: "implementer",
+		TaskID:       "T1.1",
+		InputHash:    "abc123",
+		ModelID:      "claude-opus",
+	}
+
+	cache.Set(key, map[string]interface{}{"output": "code"})
+
+	// Should find immediately
+	_, found, _ := cache.Get(key)
+	assert.True(t, found)
+
+	// Wait for TTL to expire
+	time.Sleep(150 * time.Millisecond)
+
+	// Should not find after TTL
+	_, found, _ = cache.Get(key)
+	assert.False(t, found)
+}
+
+func TestMaxSizeEviction(t *testing.T) {
+	tmpDir := t.TempDir()
+	cache := NewResultCache(tmpDir, 3, 0)
+
+	// Add 3 entries
+	for i := 0; i < 3; i++ {
+		key := &CacheKey{
+			ActivityType: "implementer",
+			TaskID:       "T1.1",
+			InputHash:    string(rune(48 + i)),
+			ModelID:      "claude-opus",
+		}
+		cache.Set(key, map[string]interface{}{"output": "code"})
+	}
+
+	assert.Equal(t, 3, cache.GetSize())
+
+	// Add 4th entry (should evict oldest)
+	key4 := &CacheKey{
+		ActivityType: "implementer",
+		TaskID:       "T1.1",
+		InputHash:    "3",
+		ModelID:      "claude-opus",
+	}
+	cache.Set(key4, map[string]interface{}{"output": "code"})
+
+	// Size should still be 3
+	assert.Equal(t, 3, cache.GetSize())
+}
+
+func TestInvalidateByActivity(t *testing.T) {
+	tmpDir := t.TempDir()
+	cache := NewResultCache(tmpDir, 100, 0)
+
+	// Add implementer entries
+	for i := 0; i < 2; i++ {
+		key := &CacheKey{
+			ActivityType: "implementer",
+			TaskID:       "T1.1",
+			InputHash:    string(rune(48 + i)),
+			ModelID:      "claude-opus",
+		}
+		cache.Set(key, map[string]interface{}{"output": "code"})
+	}
+
+	// Add judge entries
+	for i := 0; i < 2; i++ {
+		key := &CacheKey{
+			ActivityType: "judge",
+			TaskID:       "T1.1",
+			InputHash:    string(rune(48 + i)),
+			ModelID:      "claude-opus",
+		}
+		cache.Set(key, map[string]interface{}{"output": "verdict"})
+	}
+
+	assert.Equal(t, 4, cache.GetSize())
+
+	// Invalidate implementer entries
+	cache.InvalidateByActivity("implementer")
+
+	assert.Equal(t, 2, cache.GetSize())
+}
+
+func TestInvalidateByTask(t *testing.T) {
+	tmpDir := t.TempDir()
+	cache := NewResultCache(tmpDir, 100, 0)
+
+	// Add entries for T1.1
+	for i := 0; i < 2; i++ {
+		key := &CacheKey{
+			ActivityType: "implementer",
+			TaskID:       "T1.1",
+			InputHash:    string(rune(48 + i)),
+			ModelID:      "claude-opus",
+		}
+		cache.Set(key, map[string]interface{}{"output": "code"})
+	}
+
+	// Add entries for T1.2
+	for i := 0; i < 2; i++ {
+		key := &CacheKey{
+			ActivityType: "implementer",
+			TaskID:       "T1.2",
+			InputHash:    string(rune(48 + i)),
+			ModelID:      "claude-opus",
+		}
+		cache.Set(key, map[string]interface{}{"output": "code"})
+	}
+
+	assert.Equal(t, 4, cache.GetSize())
+
+	// Invalidate T1.1 entries
+	cache.InvalidateByTask("T1.1")
+
+	assert.Equal(t, 2, cache.GetSize())
+}
+
+func TestGetHitRate(t *testing.T) {
+	tmpDir := t.TempDir()
+	cache := NewResultCache(tmpDir, 100, 0)
+
+	key1 := &CacheKey{
+		ActivityType: "implementer",
+		TaskID:       "T1.1",
+		InputHash:    "1",
+		ModelID:      "claude-opus",
+	}
+
+	key2 := &CacheKey{
+		ActivityType: "implementer",
+		TaskID:       "T1.1",
+		InputHash:    "2",
+		ModelID:      "claude-opus",
+	}
+
+	cache.Set(key1, map[string]interface{}{"output": "code"})
+	cache.Set(key2, map[string]interface{}{"output": "code"})
+
+	cache.Get(key1)
+	cache.Get(key1)
+	cache.Get(key2)
+
+	hitRate, count := cache.GetHitRate()
+	assert.Equal(t, 2, count)
+	assert.GreaterOrEqual(t, hitRate, 1.0)
+}
+
+func TestPersistence(t *testing.T) {
+	tmpDir := t.TempDir()
+	cache1 := NewResultCache(tmpDir, 100, 0)
+
+	key := &CacheKey{
+		ActivityType: "implementer",
+		TaskID:       "T1.1",
+		InputHash:    "abc123",
+		ModelID:      "claude-opus",
+	}
+
+	cache1.Set(key, map[string]interface{}{"output": "code"})
+
+	// Create new cache and load
+	cache2 := NewResultCache(tmpDir, 100, 0)
+	cache2.Load()
+
+	retrieved, found, _ := cache2.Get(key)
+	assert.True(t, found)
+	assert.Equal(t, "code", retrieved["output"])
+}
diff --git a/tasks/board-T2.md b/tasks/board-T2.md
index f3899d2..70dc2ad 100644
--- a/tasks/board-T2.md
+++ b/tasks/board-T2.md
@@ -4,7 +4,7 @@
 
 | ID | Scope | Status | Branch | Verification |
 |----|-------|--------|--------|--------------|
-| T2.1 | Activity result caching: deduplicate repeated LLM calls for same task state | [ ] | `task/T2.1` | Implementer called 2x on same code → second call returns cached Implementer output |
+| T2.1 | Activity result caching: deduplicate repeated LLM calls for same task state | [x] | `task/T2.1` | Implementer called 2x on same code → second call returns cached Implementer output |
 | T2.2 | Parallel task dispatch: multiple T0.x tasks execute truly concurrently (not sequential) | [ ] | `task/T2.2` | 9 tasks complete in ~1/9 total time (wall-clock speedup measured) |
 | T2.3 | Prompt template caching: pre-compile Go templates on worker startup | [ ] | `task/T2.3` | Template render latency < 100ms (vs parse+render each time) |
 | T2.4 | Lessons file indexing: fast lookup of past failures without full file scan | [ ] | `task/T2.4` | Query lessons by task type → return in < 10ms for 1000s of entries |