diff --git a/internal/transport/error.go b/internal/transport/error.go
index b4d3d79..c2a7b00 100644
--- a/internal/transport/error.go
+++ b/internal/transport/error.go
@@ -98,6 +98,10 @@ func (e *transportError) WithCauses(causes ...zeerr.Error) zeerr.Error {
 	return e
 }
 
+func (e *transportError) WithInternalErr(_ error) zeerr.Error {
+	return e
+}
+
 func (e transportError) Error() string {
 	return e.internalMsg
 }
diff --git a/zeerr/error.go b/zeerr/error.go
index 3e064f7..83e348c 100644
--- a/zeerr/error.go
+++ b/zeerr/error.go
@@ -46,6 +46,10 @@ type Error interface {
 	Causes() []Error
 	// WithCauses is used to attach causes to the error.
 	WithCauses(causes ...Error) Error
+	// WithInternalErr is used to attach internal error to the error.
+	// Internal errors are not exposed to the client and are used for
+	// debugging purposes.
+	WithInternalErr(internalErr error) Error
 	// Error implements the error interface.
 	Error() string
 }
diff --git a/zeerr/standard_error.go b/zeerr/standard_error.go
index 3b2c6ed..cab0e14 100644
--- a/zeerr/standard_error.go
+++ b/zeerr/standard_error.go
@@ -24,6 +24,8 @@ type standardError struct {
 	publicMsg   string
 
 	causes []Error
+
+	internalErr error
 }
 
 // NewError creates a new error.
@@ -118,6 +120,11 @@ func (e standardError) Causes() []Error {
 }
 
 // WithCauses is used to attach causes to the error.
+// Causes should be of type Error and it's used to create
+// complex nested error structures.
+//
+// Causes will not be unwrapped by the standard error implementation.
+// To include internal error use WithInternalErr instead.
 func (e *standardError) WithCauses(causes ...Error) Error {
 	for _, c := range causes {
 		if c != nil {
@@ -128,6 +135,20 @@ func (e *standardError) WithCauses(causes ...Error) Error {
 	return e
 }
 
+// WithInternalErr is used to attach an internal error to the error.
+// This error will be unwrapped by the standard error implementation.
+// Internal error will not be included in the encoded transport error.
+func (e *standardError) WithInternalErr(internalErr error) Error {
+	e.internalErr = internalErr
+
+	return e
+}
+
+// Unwrap returns an internal error.
+func (e *standardError) Unwrap() error {
+	return e.internalErr
+}
+
 // Error implements the error interface.
 func (e *standardError) Error() string {
 	return e.formattedErr()
@@ -142,5 +163,11 @@ func (e *standardError) formattedErr() string {
 		buf.WriteString(cause.Error())
 	}
 
+	if e.internalErr != nil {
+		buf.WriteString("\n\t")
+		buf.WriteString("internal error: ")
+		buf.WriteString(e.internalErr.Error())
+	}
+
 	return buf.String()
 }