diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..c6210a2
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,7 @@
+.ONESHELL:
+
+generate: generate-proto
+
+generate-proto:
+	cd ./schemas/proto
+	buf generate
diff --git a/cmd/zen/main.go b/cmd/zen/main.go
index fb56211..866b01b 100644
--- a/cmd/zen/main.go
+++ b/cmd/zen/main.go
@@ -1,4 +1,8 @@
 package main
 
+import "gitlab.bareksa.com/backend/zen/cmd/zen/serve"
+
 // This is the main entry point for CLI interface.
-func main() {}
+func main() {
+	serve.Serve()
+}
diff --git a/cmd/zen/serve/serve.go b/cmd/zen/serve/serve.go
new file mode 100644
index 0000000..3659286
--- /dev/null
+++ b/cmd/zen/serve/serve.go
@@ -0,0 +1,33 @@
+package serve
+
+import (
+	"log/slog"
+	"net/http"
+
+	"connectrpc.com/grpcreflect"
+	"gitlab.bareksa.com/backend/zen/internal/gen/proto/notify/v1/notifyv1connect"
+	"gitlab.bareksa.com/backend/zen/internal/rpchandler"
+
+	"golang.org/x/net/http2"
+	"golang.org/x/net/http2/h2c"
+)
+
+func Serve() {
+	mux := http.NewServeMux()
+	reflector := grpcreflect.NewStaticReflector(
+		notifyv1connect.NotifyServiceName,
+	)
+
+	mux.Handle(grpcreflect.NewHandlerV1(reflector))
+	mux.Handle(grpcreflect.NewHandlerV1Alpha(reflector))
+	mux.Handle(notifyv1connect.NewNotifyServiceHandler(rpchandler.NotifyServiceHandler{}))
+
+	slog.Info("Starting server on :8080")
+	err := http.ListenAndServe(
+		":8080",
+		h2c.NewHandler(mux, &http2.Server{}),
+	)
+	if err != nil {
+		panic(err)
+	}
+}
diff --git a/core/zerr/error.go b/core/zerr/error.go
new file mode 100644
index 0000000..fd086a9
--- /dev/null
+++ b/core/zerr/error.go
@@ -0,0 +1,64 @@
+package zerr
+
+import (
+	"context"
+	"log/slog"
+	"time"
+
+	"connectrpc.com/connect"
+	"gitlab.bareksa.com/backend/zen/core/zoptions"
+)
+
+type Error interface {
+	error
+	// LogValue returns log fields to be consumed by logger.
+	LogValue() slog.Value
+
+	// Code sets error code.
+	Code(code connect.Code) Error
+	GetCode() connect.Code
+
+	// Message sets error message. It uses fmt.Sprintf to format the message.
+	Message(msg string, args ...any) Error
+	GetMessage() string
+
+	PublicMessage(msg string, args ...any) Error
+	GetPublicMessage() string
+
+	Caller(pc uintptr) Error
+	GetCaller() uintptr
+
+	// Details sets error details. It's a key-value pair where the odd index is the key and the even index is the value.
+	//
+	// Invalid key-value pair number and format will cause the misplaced value to be paired up with "!BADKEY".
+	//
+	// Example:
+	//
+	//    err.Details(
+	//      "key1", 12345,
+	//      "key2", float64(12345.67),
+	//    )
+	Details(fields ...any) Error
+	GetDetails() []any
+
+	// Time sets the error time.
+	//
+	// Time is already set to current when the error is created or wrapped,
+	// so this method is only useful when you want to set a different time.
+	Time(t time.Time) Error
+	GetTime() time.Time
+
+	// ID sets the error ID.
+	//
+	// ID is used to identify the error. Used by Zen to consider if an error is the same as another.
+	//
+	// Current implementation
+	ID(msg string, args ...any) Error
+	GetID() string
+
+	Log(ctx context.Context) Error
+	Notify(ctx context.Context, opts ...zoptions.NotifyOption) Error
+
+	// Sequence returns the state of the error sequence.
+	Sequence() *Sequence
+}
diff --git a/core/zerr/implementation.go b/core/zerr/implementation.go
new file mode 100644
index 0000000..9a75d95
--- /dev/null
+++ b/core/zerr/implementation.go
@@ -0,0 +1,212 @@
+package zerr
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"strconv"
+	"strings"
+	"time"
+
+	"connectrpc.com/connect"
+	"gitlab.bareksa.com/backend/zen/core/zoptions"
+)
+
+type Err struct {
+	message       string
+	publicMessage string
+	code          connect.Code
+	errs          []error
+	caller        uintptr
+	details       []any
+	time          time.Time
+	id            string
+	logger        Logger
+	notifier      Notifier
+	sequence      *Sequence
+}
+
+func (mu *Err) Error() string {
+	if len(mu.errs) == 0 {
+		return "[nil]"
+	}
+	s := strings.Builder{}
+	if len(mu.errs) == 1 {
+		str := getNestedErrorString(mu.errs[0])
+		str, found := strings.CutPrefix(str, mu.message)
+		if found {
+			str, _ = strings.CutPrefix(str, ": ")
+		}
+		s.WriteString(mu.message)
+		s.WriteString(": ")
+		s.WriteString(str)
+		return s.String()
+	}
+	s.WriteString(mu.message)
+	for i, e := range mu.errs {
+		if i > 0 {
+			s.WriteString("\n")
+		}
+		s.WriteString(strconv.Itoa(i + 1))
+		s.WriteString(". ")
+		s.WriteString(getNestedErrorString(e))
+	}
+	return s.String()
+}
+
+func getNestedErrorString(err error) string {
+	if err == nil {
+		return "[nil]"
+	}
+	s := strings.Builder{}
+	if e, ok := err.(interface{ Unwrap() error }); ok {
+		current := err.Error()
+		if err := e.Unwrap(); err != nil {
+			next := getNestedErrorString(err)
+			hasPrefix := strings.HasPrefix(next, current)
+			if !hasPrefix {
+				s.WriteString(current)
+				s.WriteString(": ")
+			}
+			s.WriteString(next)
+		}
+	}
+
+	if errs, ok := err.(interface{ Unwrap() []error }); ok {
+		for i, e := range errs.Unwrap() {
+			if i > 0 {
+				s.WriteString("\n")
+			}
+			s.WriteString(strconv.Itoa(i + 1))
+			s.WriteString(". ")
+			s.WriteString(getNestedErrorString(e))
+		}
+	}
+
+	if s.Len() == 0 {
+		s.WriteString(err.Error())
+	}
+
+	return s.String()
+}
+
+// LogValue returns log fields to be consumed by logger.
+func (mu *Err) LogValue() slog.Value {
+	attrs := make([]slog.Attr, 0, len(mu.errs))
+	i := 0
+	for _, err := range mu.errs {
+		if err == nil {
+			continue
+		}
+		i++
+		attrs = append(attrs, slog.Attr{
+			Key:   strconv.Itoa(i),
+			Value: errorValuer{err}.LogValue(),
+		})
+	}
+	return slog.GroupValue(attrs...)
+}
+
+// Code sets error code. Ignored in Multi.
+func (mu *Err) Code(code connect.Code) Error {
+	mu.code = code
+	return mu
+}
+
+// GetCode Always returns 0 in Multi.
+func (mu *Err) GetCode() connect.Code {
+	return mu.code
+}
+
+// Message sets error message. It uses fmt.Sprintf to format the message.
+func (mu *Err) Message(msg string, args ...any) Error {
+	mu.message = fmt.Sprintf(msg, args...)
+	return mu
+}
+
+func (mu *Err) GetMessage() string {
+	return mu.message
+}
+
+func (mu *Err) PublicMessage(msg string, args ...any) Error {
+	mu.publicMessage = fmt.Sprintf(msg, args...)
+	return mu
+}
+
+func (mu *Err) GetPublicMessage() string {
+	return mu.publicMessage
+}
+
+func (mu *Err) Caller(pc uintptr) Error {
+	mu.caller = pc
+	return mu
+}
+
+func (mu *Err) GetCaller() uintptr {
+	return mu.caller
+}
+
+// Details sets error details. It's a key-value pair where the odd index is the key and the even index is the value.
+//
+// Invalid key-value pair number and format will cause the misplaced value to be paired up with "!BADKEY".
+//
+// Example:
+//
+//	err.Details(
+//	  "key1", 12345,
+//	  "key2", float64(12345.67),
+//	)
+func (mu *Err) Details(fields ...any) Error {
+	mu.details = fields
+	return mu
+}
+
+func (mu *Err) GetDetails() []any {
+	return mu.details
+}
+
+// Time sets the error time.
+//
+// Time is already set to current when the error is created or wrapped,
+// so this method is only useful when you want to set a different time.
+func (mu *Err) Time(t time.Time) Error {
+	mu.time = t
+	return mu
+}
+
+func (mu *Err) GetTime() time.Time {
+	return mu.time
+}
+
+// ID sets the error ID.
+//
+// ID is used to identify the error. Used by Zen to consider if an error is the same as another.
+//
+// Current implementation
+func (mu *Err) ID(msg string, args ...any) Error {
+	mu.id = fmt.Sprintf(msg, args...)
+	return mu
+}
+
+func (mu *Err) GetID() string {
+	return mu.id
+}
+
+func (mu *Err) Log(ctx context.Context) Error {
+	mu.logger.Log(ctx, mu)
+	return mu
+}
+
+func (mu *Err) Notify(ctx context.Context, opts ...zoptions.NotifyOption) Error {
+	mu.notifier.Notify(ctx, mu, opts...)
+	return mu
+}
+
+// Sequence returns the state of the error sequence.
+func (mu *Err) Sequence() *Sequence {
+	return mu.sequence
+}
+
+func (mu *Err) Unwrap() []error {
+	return mu.errs
+}
diff --git a/core/zerr/sequence.go b/core/zerr/sequence.go
new file mode 100644
index 0000000..d6d4c38
--- /dev/null
+++ b/core/zerr/sequence.go
@@ -0,0 +1,98 @@
+package zerr
+
+import "iter"
+
+// Sequence is the implementation of zerr.Sequence.
+//
+// The implementation is reversed linked list.
+//
+// New Sequence value will be added to the start of the list.
+type Sequence struct {
+	prev  *Sequence
+	next  *Sequence
+	err   Error
+	index int
+}
+
+// Set prepends the next Sequence to the current Sequence.
+func (se *Sequence) Set(next *Sequence) {
+	next.next = se
+	se.prev = next
+	se.index = next.index + 1
+	if next.next != nil {
+		next.next.Set(se)
+	}
+}
+
+// Outer returns the error that wraps this Error.
+// Return nil if this Error is not wrapped
+// or wrapping outer is not a zerr.Error.
+func (se *Sequence) Outer() Error {
+	if se.prev != nil {
+		return se.prev.err
+	}
+	return nil
+}
+
+// Inner returns the inner zerr.Error in the sequence.
+//
+// if there is no inner zerr.Error, it returns nil.
+//
+// if the inner error is not a zerr.Error, this returns nil.
+func (se *Sequence) Inner() Error {
+	if se.next != nil {
+		return se.next.err
+	}
+	return nil
+}
+
+// Index returns the index of this Error in the sequence.
+func (se *Sequence) Index() int {
+	return se.index
+}
+
+// Root returns the outermost Error in the sequence.
+//
+// Returns self if there is no outermost.
+func (se *Sequence) Root() *Sequence {
+	root := se
+	for root.prev != nil {
+		root = root.prev
+	}
+	return root
+}
+
+func (se *Sequence) IsRoot() bool {
+	return se.prev == nil
+}
+
+// Iter returns an iterator to traverse the sequence.
+//
+// The iterator will start from outermost zerr.Error (root) to the last zerr.Error
+// in depth-first order.
+//
+// Iterator will skip any nil values in the sequence.
+//
+// Iterator does not guarantee idempotent behavior.
+// Next call to iterator may start from different root
+// because the parent in the tree may be wrapped
+// by another zerr.Error.
+//
+// Example:
+//
+//	for _, e := range err.Sequence().Iter() {
+//	   // e is zerr.Error
+//	   // do something with e
+//	}
+func (se *Sequence) Iter() iter.Seq[Error] {
+	root := se.Root()
+	return iter.Seq[Error](func(yield func(V Error) bool) {
+		for next := root; next != nil; next = next.next {
+			if next.err != nil {
+				if !yield(next.err) {
+					return
+				}
+			}
+		}
+	})
+}
diff --git a/core/zerr/valuer.go b/core/zerr/valuer.go
new file mode 100644
index 0000000..14d7314
--- /dev/null
+++ b/core/zerr/valuer.go
@@ -0,0 +1,31 @@
+package zerr
+
+import (
+	"log/slog"
+	"reflect"
+)
+
+type errorValuer struct {
+	error
+}
+
+func (ev errorValuer) LogValue() slog.Value {
+	if ev.error == nil {
+		return slog.AnyValue(nil)
+	}
+	if lv, ok := ev.error.(slog.LogValuer); ok {
+		return lv.LogValue()
+	}
+	if unwrap, ok := ev.error.(interface{ Unwrap() []error }); ok {
+		return (&Err{errs: unwrap.Unwrap()}).LogValue()
+	}
+	attrs := make([]slog.Attr, 0, 3)
+	typ := reflect.TypeOf(ev.error).String()
+	attrs = append(attrs, slog.String("type", typ))
+	attrs = append(attrs, slog.String("error", ev.error.Error()))
+	if typ == "*errors.errorString" {
+		return slog.GroupValue(attrs...)
+	}
+	attrs = append(attrs, slog.Any("details", ev.error))
+	return slog.GroupValue(attrs...)
+}
diff --git a/core/zerr/wrap.go b/core/zerr/wrap.go
new file mode 100644
index 0000000..d9306f2
--- /dev/null
+++ b/core/zerr/wrap.go
@@ -0,0 +1,41 @@
+package zerr
+
+import (
+	"context"
+	"time"
+
+	"gitlab.bareksa.com/backend/zen/core/zoptions"
+)
+
+type WrapInitialInput struct {
+	Error    error
+	Message  string
+	PC       uintptr
+	Time     time.Time
+	Logger   Logger
+	Notifier Notifier
+}
+
+type Wrapper interface {
+	// Wrap creates a zerr.Error with inputs
+	// generated by global entry points.
+	Wrap(input WrapInitialInput) Error
+}
+
+type Logger interface {
+	Log(ctx context.Context, err Error)
+}
+
+type Notifier interface {
+	Notify(ctx context.Context, err Error, opts ...zoptions.NotifyOption)
+}
+
+type WrapperFunc func(input WrapInitialInput) Error
+
+func (wr WrapperFunc) Wrap(input WrapInitialInput) Error {
+	return wr(input)
+}
+
+var DefaultWrapper Wrapper = WrapperFunc(func(input WrapInitialInput) Error {
+	panic("not implemented")
+})
diff --git a/core/zlog/zlog.go b/core/zlog/zlog.go
new file mode 100644
index 0000000..ade69da
--- /dev/null
+++ b/core/zlog/zlog.go
@@ -0,0 +1,7 @@
+package zlog
+
+import notifyv1 "gitlab.bareksa.com/backend/zen/internal/gen/proto/notify/v1"
+
+func log() {
+	notifyv1.Payload
+}
diff --git a/zcore/zoptions/notify.go b/core/zoptions/notify.go
similarity index 100%
rename from zcore/zoptions/notify.go
rename to core/zoptions/notify.go
diff --git a/go.mod b/go.mod
index 000d784..e1dd7b6 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,12 @@
 module gitlab.bareksa.com/backend/zen
 
-go 1.22.5
+go 1.23
+
+require (
+	connectrpc.com/connect v1.16.2
+	connectrpc.com/grpcreflect v1.2.0
+	golang.org/x/net v0.23.0
+	google.golang.org/protobuf v1.34.2
+)
+
+require golang.org/x/text v0.14.0 // indirect
diff --git a/go.sum b/go.sum
new file mode 100644
index 0000000..e0f11ee
--- /dev/null
+++ b/go.sum
@@ -0,0 +1,12 @@
+connectrpc.com/connect v1.16.2 h1:ybd6y+ls7GOlb7Bh5C8+ghA6SvCBajHwxssO2CGFjqE=
+connectrpc.com/connect v1.16.2/go.mod h1:n2kgwskMHXC+lVqb18wngEpF95ldBHXjZYJussz5FRc=
+connectrpc.com/grpcreflect v1.2.0 h1:Q6og1S7HinmtbEuBvARLNwYmTbhEGRpHDhqrPNlmK+U=
+connectrpc.com/grpcreflect v1.2.0/go.mod h1:nwSOKmE8nU5u/CidgHtPYk1PFI3U9ignz7iDMxOYkSY=
+github.com/google/go-cmp v0.5.9 h1:O2Tfq5qg4qc4AmwVlvv0oLiVAGB7enBSJ2x2DqQFi38=
+github.com/google/go-cmp v0.5.9/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
+golang.org/x/net v0.23.0 h1:7EYJ93RZ9vYSZAIb2x3lnuvqO5zneoD6IvWjuhfxjTs=
+golang.org/x/net v0.23.0/go.mod h1:JKghWKKOSdJwpW2GEx0Ja7fmaKnMsbu+MWVZTokSYmg=
+golang.org/x/text v0.14.0 h1:ScX5w1eTa3QqT8oi6+ziP7dTV1S2+ALU0bI+0zXKWiQ=
+golang.org/x/text v0.14.0/go.mod h1:18ZOQIKpY8NJVqYksKHtTdi31H5itFRjB5/qKTNYzSU=
+google.golang.org/protobuf v1.34.2 h1:6xV6lTsCfpGD21XK49h7MhtcApnLqkfYgPcdHftf6hg=
+google.golang.org/protobuf v1.34.2/go.mod h1:qYOHts0dSfpeUzUFpOMr/WGzszTmLH+DiWniOlNbLDw=
diff --git a/internal/rpchandler/rpc_handler.go b/internal/rpchandler/rpc_handler.go
new file mode 100644
index 0000000..a2b292f
--- /dev/null
+++ b/internal/rpchandler/rpc_handler.go
@@ -0,0 +1,17 @@
+package rpchandler
+
+import (
+	"gitlab.bareksa.com/backend/zen/internal/gen/proto/notify/v1/notifyv1connect"
+)
+
+type NotifyServiceHandler struct {
+	notifyv1connect.UnimplementedNotifyServiceHandler
+}
+
+// func (no *NotifyServiceHandler) SendNotification(_ context.Context, _ *connect.Request[notifyv1.SendNotificationRequest]) (*connect.Response[notifyv1.SendNotificationResponse], error) {
+// 	panic("not implemented") // TODO: Implement
+// }
+//
+// func (no *NotifyServiceHandler) SendAttachment(_ context.Context, _ *connect.ClientStream[notifyv1.SendAttachmentRequest]) (*connect.Response[notifyv1.SendAttachmentResponse], error) {
+// 	panic("not implemented") // TODO: Implement
+// }
diff --git a/schemas/proto/buf.gen.yaml b/schemas/proto/buf.gen.yaml
index 4e69c53..c61e81e 100644
--- a/schemas/proto/buf.gen.yaml
+++ b/schemas/proto/buf.gen.yaml
@@ -7,18 +7,18 @@ managed:
 
   override:
     - file_option: go_package_prefix
-      value: gitlab.bareksa.com/backend/zen/zcore/internal/gen/proto
+      value: gitlab.bareksa.com/backend/zen/internal/gen/proto
 
 plugins:
   - remote: buf.build/protocolbuffers/go:v1.34.2
-    out: ../../zcore/internal/gen/proto
+    out: ../../internal/gen/proto
     opt:
       - paths=source_relative
 
   - remote: buf.build/connectrpc/go
-    out: ../../zcore/internal/gen/proto
+    out: ../../internal/gen/proto
     opt:
       - paths=source_relative
 
   - remote: buf.build/community/pseudomuto-doc:v1.5.1
-    out: ../../zcore/internal/gen/proto
+    out: ../../internal/gen/proto
diff --git a/schemas/proto/notify/v1/send_notification.proto b/schemas/proto/notify/v1/send_notification.proto
index df2957f..20b3d49 100644
--- a/schemas/proto/notify/v1/send_notification.proto
+++ b/schemas/proto/notify/v1/send_notification.proto
@@ -47,6 +47,7 @@ message Payload {
     bytes e_json = 6;
     string e_text = 7;
   }
+  string id = 8;
 }
 
 message Service {
diff --git a/zcore/internal/server/server.go b/zcore/internal/server/server.go
deleted file mode 100644
index abb4e43..0000000
--- a/zcore/internal/server/server.go
+++ /dev/null
@@ -1 +0,0 @@
-package server
diff --git a/zcore/zerr/zerr.go b/zcore/zerr/zerr.go
deleted file mode 100644
index 07b86ba..0000000
--- a/zcore/zerr/zerr.go
+++ /dev/null
@@ -1,33 +0,0 @@
-package zerr
-
-import (
-	"context"
-	"time"
-
-	"gitlab.bareksa.com/backend/zen/zcore/zoptions"
-)
-
-type Error interface {
-	error
-
-	Code(code int) Error
-	GetCode() int
-
-	Message(msg string, args ...any) Error
-	GetMessage() string
-
-	PublicMessage(msg string, args ...any) Error
-	GetPublicMessage() string
-
-	Caller(pc uintptr) Error
-	GetCaller() uintptr
-
-	Time(t time.Time) Error
-	GetTime() time.Time
-
-	Key(msg string, args ...any) Error
-	GetKey() string
-
-	Log(ctx context.Context) Error
-	Notify(ctx context.Context, opts ...zoptions.NotifyOption) Error
-}
diff --git a/zcore/zlog/zlog.go b/zcore/zlog/zlog.go
deleted file mode 100644
index d3e6811..0000000
--- a/zcore/zlog/zlog.go
+++ /dev/null
@@ -1 +0,0 @@
-package zlog