diff --git a/internal/flags/memory.go b/internal/flags/memory.go
index 6c66823..2228a05 100644
--- a/internal/flags/memory.go
+++ b/internal/flags/memory.go
@@ -7,16 +7,16 @@ import (
 
 type MemoryRepository struct {
 	mu    sync.RWMutex
-	flags map[string]Flag
+	flags map[FlagKey]Flag
 }
 
 func NewMemoryRepository() *MemoryRepository {
 	return &MemoryRepository{
-		flags: make(map[string]Flag),
+		flags: make(map[FlagKey]Flag),
 	}
 }
 
-func (r *MemoryRepository) Get(_ context.Context, key string) (Flag, error) {
+func (r *MemoryRepository) Get(_ context.Context, key FlagKey) (Flag, error) {
 	r.mu.RLock()
 	defer r.mu.RUnlock()
 
diff --git a/internal/flags/repository.go b/internal/flags/repository.go
index 3e26d68..b50cfa6 100644
--- a/internal/flags/repository.go
+++ b/internal/flags/repository.go
@@ -11,6 +11,6 @@ var (
 )
 
 type Repository interface {
-	Get(ctx context.Context, key string) (Flag, error)
+	Get(ctx context.Context, key FlagKey) (Flag, error)
 	Create(ctx context.Context, flag Flag) error
 }
diff --git a/internal/flags/service.go b/internal/flags/service.go
index 42c95af..fbd006f 100644
--- a/internal/flags/service.go
+++ b/internal/flags/service.go
@@ -31,7 +31,7 @@ func (s *Service) Create(ctx context.Context, flag Flag) (Flag, error) {
 	return flag, nil
 }
 
-func (s *Service) Evaluate(ctx context.Context, key string, evalCtx EvalContext) (EvalResult, error) {
+func (s *Service) Evaluate(ctx context.Context, key FlagKey, evalCtx EvalContext) (EvalResult, error) {
 	flag, err := s.repo.Get(ctx, key)
 	if err != nil {
 		return EvalResult{}, err
diff --git a/internal/flags/service_test.go b/internal/flags/service_test.go
index 9cd29e7..60afb6a 100644
--- a/internal/flags/service_test.go
+++ b/internal/flags/service_test.go
@@ -26,7 +26,7 @@ func TestService_Create(t *testing.T) {
 	created, err := svc.Create(ctx, input)
 	require.NoError(t, err)
 
-	assert.Equal(t, "new-feature", created.Key)
+	assert.Equal(t, flags.FlagKey("new-feature"), created.Key)
 	assert.False(t, created.UpdatedAt.IsZero())
 }
 
@@ -69,7 +69,7 @@ func TestService_Evaluate_ReturnsDefault(t *testing.T) {
 	result, err := svc.Evaluate(ctx, "my-flag", flags.EvalContext{})
 	require.NoError(t, err)
 
-	assert.Equal(t, "my-flag", result.FlagKey)
+	assert.Equal(t, flags.FlagKey("my-flag"), result.FlagKey)
 	assert.Equal(t, flags.ReasonDefault, result.Reason)
 	assert.Equal(t, flags.FlagBool, result.Value.Kind)
 	assert.True(t, *result.Value.Bool)
diff --git a/internal/flags/types.go b/internal/flags/types.go
index e89dced..9533096 100644
--- a/internal/flags/types.go
+++ b/internal/flags/types.go
@@ -2,6 +2,8 @@ package flags
 
 import "time"
 
+type FlagKey string
+
 type FlagType string
 
 const (
@@ -11,7 +13,7 @@ const (
 )
 
 type Flag struct {
-	Key          string
+	Key          FlagKey
 	Type         FlagType
 	Enabled      bool // global kill switch
 	DefaultValue Value
@@ -76,7 +78,7 @@ const (
 )
 
 type EvalResult struct {
-	FlagKey     string
+	FlagKey     FlagKey
 	Value       Value
 	Reason      EvalReason
 	RuleID      string
diff --git a/internal/handler/handler.go b/internal/handler/handler.go
index 8868597..3586dc4 100644
--- a/internal/handler/handler.go
+++ b/internal/handler/handler.go
@@ -12,7 +12,7 @@ import (
 
 type FlagService interface {
 	Create(ctx context.Context, flag flags.Flag) (flags.Flag, error)
-	Evaluate(ctx context.Context, key string, evalCtx flags.EvalContext) (flags.EvalResult, error)
+	Evaluate(ctx context.Context, key flags.FlagKey, evalCtx flags.EvalContext) (flags.EvalResult, error)
 }
 
 type Handler struct {
@@ -44,7 +44,7 @@ func (h *Handler) CreateFlag(ctx context.Context, req *CreateFlagRequest) (*Crea
 func (h *Handler) EvaluateFlag(ctx context.Context, req *EvaluateFlagRequest) (*EvaluateFlagResponse, error) {
 	evalCtx := ToEvalContext(req.Body)
 
-	result, err := h.service.Evaluate(ctx, req.Key, evalCtx)
+	result, err := h.service.Evaluate(ctx, flags.FlagKey(req.Key), evalCtx)
 	if err != nil {
 		if errors.Is(err, flags.ErrFlagNotFound) {
 			return nil, huma.Error404NotFound("flag not found")
diff --git a/internal/handler/handler_test.go b/internal/handler/handler_test.go
index 7cd2c5a..690bbb4 100644
--- a/internal/handler/handler_test.go
+++ b/internal/handler/handler_test.go
@@ -45,7 +45,7 @@ func TestHandler_CreateFlag(t *testing.T) {
 	resp, err := h.CreateFlag(ctx, req)
 	require.NoError(t, err)
 
-	assert.Equal(t, "test-flag", resp.Body.Key)
+	assert.Equal(t, flags.FlagKey("test-flag"), resp.Body.Key)
 	assert.False(t, resp.Body.CreatedAt.IsZero())
 }
 
@@ -109,7 +109,7 @@ func TestHandler_EvaluateFlag(t *testing.T) {
 
 	boolVal := true
 	mockService.EXPECT().
-		Evaluate(gomock.Any(), "my-flag", gomock.Any()).
+		Evaluate(gomock.Any(), flags.FlagKey("my-flag"), gomock.Any()).
 		Return(flags.EvalResult{
 			FlagKey:     "my-flag",
 			Value:       flags.Value{Kind: flags.FlagBool, Bool: &boolVal},
@@ -128,7 +128,7 @@ func TestHandler_EvaluateFlag(t *testing.T) {
 	resp, err := h.EvaluateFlag(ctx, req)
 	require.NoError(t, err)
 
-	assert.Equal(t, "my-flag", resp.Body.FlagKey)
+	assert.Equal(t, flags.FlagKey("my-flag"), resp.Body.FlagKey)
 	assert.Equal(t, "default", resp.Body.Reason)
 	assert.Equal(t, "bool", resp.Body.Value.Kind)
 	assert.True(t, *resp.Body.Value.Bool)
@@ -143,7 +143,7 @@ func TestHandler_EvaluateFlag_NotFound(t *testing.T) {
 	ctx := context.Background()
 
 	mockService.EXPECT().
-		Evaluate(gomock.Any(), "nonexistent", gomock.Any()).
+		Evaluate(gomock.Any(), flags.FlagKey("nonexistent"), gomock.Any()).
 		Return(flags.EvalResult{}, flags.ErrFlagNotFound)
 
 	req := &handler.EvaluateFlagRequest{
@@ -166,7 +166,7 @@ func TestHandler_EvaluateFlag_RuleMatch(t *testing.T) {
 
 	boolVal := true
 	mockService.EXPECT().
-		Evaluate(gomock.Any(), "premium-feature", gomock.Any()).
+		Evaluate(gomock.Any(), flags.FlagKey("premium-feature"), gomock.Any()).
 		Return(flags.EvalResult{
 			FlagKey:     "premium-feature",
 			Value:       flags.Value{Kind: flags.FlagBool, Bool: &boolVal},
@@ -200,7 +200,7 @@ func TestHandler_EvaluateFlag_Disabled(t *testing.T) {
 
 	boolVal := false
 	mockService.EXPECT().
-		Evaluate(gomock.Any(), "disabled-flag", gomock.Any()).
+		Evaluate(gomock.Any(), flags.FlagKey("disabled-flag"), gomock.Any()).
 		Return(flags.EvalResult{
 			FlagKey:     "disabled-flag",
 			Value:       flags.Value{Kind: flags.FlagBool, Bool: &boolVal},
@@ -228,7 +228,7 @@ func TestHandler_EvaluateFlag_InternalError(t *testing.T) {
 	ctx := context.Background()
 
 	mockService.EXPECT().
-		Evaluate(gomock.Any(), "test-flag", gomock.Any()).
+		Evaluate(gomock.Any(), flags.FlagKey("test-flag"), gomock.Any()).
 		Return(flags.EvalResult{}, errors.New("database connection failed"))
 
 	req := &handler.EvaluateFlagRequest{
diff --git a/internal/handler/mapper.go b/internal/handler/mapper.go
index 4b7b8bf..d073fca 100644
--- a/internal/handler/mapper.go
+++ b/internal/handler/mapper.go
@@ -6,7 +6,7 @@ import (
 
 func ToFlag(body CreateFlagBody) flags.Flag {
 	return flags.Flag{
-		Key:          body.Key,
+		Key:          flags.FlagKey(body.Key),
 		Type:         flags.FlagType(body.Type),
 		Enabled:      body.Enabled,
 		DefaultValue: toValue(body.DefaultValue),
diff --git a/internal/handler/mapper_test.go b/internal/handler/mapper_test.go
index fc97e02..aec6ff8 100644
--- a/internal/handler/mapper_test.go
+++ b/internal/handler/mapper_test.go
@@ -37,7 +37,7 @@ func TestToFlag(t *testing.T) {
 
 	flag := handler.ToFlag(body)
 
-	assert.Equal(t, "test-flag", flag.Key)
+	assert.Equal(t, flags.FlagKey("test-flag"), flag.Key)
 	assert.Equal(t, flags.FlagBool, flag.Type)
 	assert.True(t, flag.Enabled)
 	assert.Equal(t, flags.FlagBool, flag.DefaultValue.Kind)
@@ -60,7 +60,7 @@ func TestToFlag_EmptyRules(t *testing.T) {
 
 	flag := handler.ToFlag(body)
 
-	assert.Equal(t, "simple-flag", flag.Key)
+	assert.Equal(t, flags.FlagKey("simple-flag"), flag.Key)
 	assert.Nil(t, flag.Rules)
 }
 
@@ -102,7 +102,7 @@ func TestToEvalResultBody(t *testing.T) {
 
 	body := handler.ToEvalResultBody(result)
 
-	assert.Equal(t, "my-flag", body.FlagKey)
+	assert.Equal(t, flags.FlagKey("my-flag"), body.FlagKey)
 	assert.Equal(t, "bool", body.Value.Kind)
 	assert.True(t, *body.Value.Bool)
 	assert.Equal(t, "rule_match", body.Reason)
diff --git a/internal/handler/mock_service_test.go b/internal/handler/mock_service_test.go
index 9d0c1e1..78ef28d 100644
--- a/internal/handler/mock_service_test.go
+++ b/internal/handler/mock_service_test.go
@@ -57,7 +57,7 @@ func (mr *MockFlagServiceMockRecorder) Create(ctx, flag any) *gomock.Call {
 }
 
 // Evaluate mocks base method.
-func (m *MockFlagService) Evaluate(ctx context.Context, key string, evalCtx flags.EvalContext) (flags.EvalResult, error) {
+func (m *MockFlagService) Evaluate(ctx context.Context, key flags.FlagKey, evalCtx flags.EvalContext) (flags.EvalResult, error) {
 	m.ctrl.T.Helper()
 	ret := m.ctrl.Call(m, "Evaluate", ctx, key, evalCtx)
 	ret0, _ := ret[0].(flags.EvalResult)
diff --git a/internal/handler/models.go b/internal/handler/models.go
index 5a1135e..39369ae 100644
--- a/internal/handler/models.go
+++ b/internal/handler/models.go
@@ -2,6 +2,8 @@ package handler
 
 import (
 	"time"
+
+	"github.com/serroba/features/internal/flags"
 )
 
 // Request/Response models for Create Flag
@@ -11,27 +13,27 @@ type CreateFlagRequest struct {
 }
 
 type CreateFlagBody struct {
-	Key          string     `json:"key"`
-	Type         string     `json:"type"`
+	Key          string     `json:"key"                maxLength:"128" minLength:"1" pattern:"^[a-z][a-z0-9-]*$"`
+	Type         string     `enum:"bool,string,number" json:"type"`
 	Enabled      bool       `json:"enabled"`
 	DefaultValue ValueBody  `json:"defaultValue"`
 	Rules        []RuleBody `json:"rules,omitempty"`
 }
 
 type RuleBody struct {
-	ID         string          `json:"id"`
-	Conditions []ConditionBody `json:"conditions"`
+	ID         string          `json:"id"         maxLength:"64" minLength:"1"`
+	Conditions []ConditionBody `json:"conditions" minItems:"1"`
 	Value      ValueBody       `json:"value"`
 }
 
 type ConditionBody struct {
-	Attr  string `json:"attr"`
-	Op    string `json:"op"`
+	Attr  string `json:"attr"                                maxLength:"64" minLength:"1"`
+	Op    string `enum:"eq,neq,in,not_in,exists,starts_with" json:"op"`
 	Value any    `json:"value"`
 }
 
 type ValueBody struct {
-	Kind   string   `json:"kind"`
+	Kind   string   `enum:"bool,string,number" json:"kind"`
 	Bool   *bool    `json:"bool,omitempty"`
 	String *string  `json:"string,omitempty"`
 	Number *float64 `json:"number,omitempty"`
@@ -42,20 +44,20 @@ type CreateFlagResponse struct {
 }
 
 type CreateFlagResponseBody struct {
-	Key       string    `json:"key"`
-	CreatedAt time.Time `json:"createdAt"`
+	Key       flags.FlagKey `json:"key"`
+	CreatedAt time.Time     `json:"createdAt"`
 }
 
 // Request/Response models for Evaluate Flag
 
 type EvaluateFlagRequest struct {
-	Key  string `path:"key"`
+	Key  string `maxLength:"128" minLength:"1" path:"key" pattern:"^[a-z][a-z0-9-]*$"`
 	Body EvaluateFlagBody
 }
 
 type EvaluateFlagBody struct {
-	TenantID string         `json:"tenantId,omitempty"`
-	UserID   string         `json:"userId,omitempty"`
+	TenantID string         `json:"tenantId,omitempty" maxLength:"128"`
+	UserID   string         `json:"userId,omitempty"   maxLength:"128"`
 	Attrs    map[string]any `json:"attrs,omitempty"`
 }
 
@@ -64,9 +66,9 @@ type EvaluateFlagResponse struct {
 }
 
 type EvalResultBody struct {
-	FlagKey     string    `json:"flagKey"`
-	Value       ValueBody `json:"value"`
-	Reason      string    `json:"reason"`
-	RuleID      string    `json:"ruleId,omitempty"`
-	EvaluatedAt time.Time `json:"evaluatedAt"`
+	FlagKey     flags.FlagKey `json:"flagKey"`
+	Value       ValueBody     `json:"value"`
+	Reason      string        `enum:"disabled,rule_match,default" json:"reason"`
+	RuleID      string        `json:"ruleId,omitempty"`
+	EvaluatedAt time.Time     `json:"evaluatedAt"`
 }