diff --git a/cache/redis/redis.go b/cache/redis/redis.go
index 9fcc24a..a4f27b1 100644
--- a/cache/redis/redis.go
+++ b/cache/redis/redis.go
@@ -83,7 +83,15 @@ type RedisClientConfig struct {
 // entry.
 const defaultTTL = 60 * time.Second
 
+// DefaultKeyPrefix namespaces cache keys when no prefix is configured, so Clear
+// and CountQueries never touch keys of other applications on a shared Redis.
+const DefaultKeyPrefix = "gmp:"
+
 func New(redisClientConfig *RedisClientConfig) (*RedisConfig, error) {
+	prefix := redisClientConfig.Prefix
+	if prefix == "" {
+		prefix = DefaultKeyPrefix
+	}
 	c := &RedisConfig{
 		client: redis.NewClient(&redis.Options{
 			Addr:     redisClientConfig.RedisServer,
@@ -91,7 +99,7 @@ func New(redisClientConfig *RedisClientConfig) (*RedisConfig, error) {
 			DB:       redisClientConfig.RedisDB,
 		}),
 		ctx:    context.Background(),
-		prefix: redisClientConfig.Prefix,
+		prefix: prefix,
 		builderPool: &sync.Pool{
 			New: func() any {
 				return &strings.Builder{}
diff --git a/cache/redis/redis_default_prefix_test.go b/cache/redis/redis_default_prefix_test.go
new file mode 100644
index 0000000..4b82eda
--- /dev/null
+++ b/cache/redis/redis_default_prefix_test.go
@@ -0,0 +1,30 @@
+package libpack_cache_redis
+
+import (
+	"testing"
+
+	"github.com/alicebob/miniredis/v2"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// Pins theorems/Theorems/RedisClear.lean: with no configured prefix, Clear must still spare foreign keys.
+func TestClear_DefaultPrefixSparesForeignKeys(t *testing.T) {
+	t.Parallel()
+	s, err := miniredis.Run()
+	require.NoError(t, err)
+	t.Cleanup(s.Close)
+
+	require.NoError(t, s.Set("other-app:session", "keep"))
+
+	c, err := New(&RedisClientConfig{RedisServer: s.Addr()})
+	require.NoError(t, err)
+	require.NoError(t, c.Set("q1", []byte("v"), 0))
+
+	require.NoError(t, c.Clear())
+
+	assert.True(t, s.Exists("other-app:session"), "foreign key was deleted")
+	_, found, err := c.Get("q1")
+	require.NoError(t, err)
+	assert.False(t, found, "own key must be cleared")
+}
diff --git a/cache/redis/redis_test.go b/cache/redis/redis_test.go
index 59b5c81..95c8f45 100644
--- a/cache/redis/redis_test.go
+++ b/cache/redis/redis_test.go
@@ -131,11 +131,11 @@ func (suite *RedisConfigSuite) TestGetKeys() {
 	err = suite.redisConfig.Set("otherkey", []byte("othervalue"), ttl)
 	assert.NoError(suite.T(), err)
 
-	keys, _ := suite.redisConfig.client.Keys(suite.redisConfig.ctx, "testkey*").Result()
-	expectedKeys := []string{"testkey1", "testkey2"}
+	keys, _ := suite.redisConfig.client.Keys(suite.redisConfig.ctx, DefaultKeyPrefix+"testkey*").Result()
+	expectedKeys := []string{DefaultKeyPrefix + "testkey1", DefaultKeyPrefix + "testkey2"}
 	assert.ElementsMatch(suite.T(), expectedKeys, keys)
 
-	suite.redisConfig.client.Del(suite.redisConfig.ctx, "testkey1", "testkey2", "otherkey")
+	suite.redisConfig.client.Del(suite.redisConfig.ctx, DefaultKeyPrefix+"testkey1", DefaultKeyPrefix+"testkey2", DefaultKeyPrefix+"otherkey")
 }
 
 func (suite *RedisConfigSuite) TestGetKeysCount() {
@@ -154,5 +154,5 @@ func (suite *RedisConfigSuite) TestGetKeysCount() {
 	assert.NoError(suite.T(), err)
 	assert.Equal(suite.T(), int64(3), count3)
 
-	suite.redisConfig.client.Del(suite.redisConfig.ctx, "testkey1", "testkey2", "otherkey")
+	suite.redisConfig.client.Del(suite.redisConfig.ctx, DefaultKeyPrefix+"testkey1", DefaultKeyPrefix+"testkey2", DefaultKeyPrefix+"otherkey")
 }