diff --git a/client.go b/client.go
index b9827e4..1443557 100644
--- a/client.go
+++ b/client.go
@@ -50,12 +50,12 @@ func buildClient(s net.Conn, ctl ControllerSet, serverCh serverChannelSet) (*cli
 	// 4. Write into socket
 	_, err := res.Send(s)
 	if err != nil {
-		return nil, fmt.Errorf("Upgrade write error: %s", err)
+		return nil, fmt.Errorf("upgrade write: %w", err)
 	}
 
 	if res.GetStatusCode() != 101 {
 		s.Close()
-		return nil, fmt.Errorf("Upgrade error (HTTP %d)", res.GetStatusCode())
+		return nil, fmt.Errorf("upgrade failed (HTTP %d)", res.GetStatusCode())
 	}
 
 	/* (2) Initialise client
diff --git a/internal/http/upgrade/header.go b/internal/http/upgrade/header.go
index 1e5f06b..9503c0c 100644
--- a/internal/http/upgrade/header.go
+++ b/internal/http/upgrade/header.go
@@ -38,7 +38,7 @@ func ReadHeader(b []byte) (*Header, error) {
 	parts := bytes.Split(b, []byte(": "))
 
 	if len(parts) != 2 {
-		return nil, fmt.Errorf("Invalid HTTP header format '%s'", b)
+		return nil, fmt.Errorf("invalid HTTP header format '%s'", b)
 	}
 
 	// 2. Create instance
diff --git a/internal/http/upgrade/line.go b/internal/http/upgrade/line.go
index a463220..842b4a1 100644
--- a/internal/http/upgrade/line.go
+++ b/internal/http/upgrade/line.go
@@ -67,7 +67,7 @@ func (r *Line) extractHttpMethod(b []byte) error {
 	// case "DELETE":  r.method = DELETE
 
 	default:
-		return fmt.Errorf("Invalid HTTP method '%s', expected 'GET'", b)
+		return fmt.Errorf("invalid HTTP method '%s', expected 'GET'", b)
 	}
 
 	return nil
@@ -98,7 +98,7 @@ func (r *Line) extractHttpVersion(b []byte) error {
 	extractor := regexp.MustCompile(`^HTTP/([1-9])(?:\.([0-9]))?$`)
 
 	if !extractor.Match(b) {
-		return fmt.Errorf("HTTP version, expected INT or INT.INT, got '%s'", b)
+		return fmt.Errorf("invalid HTTP version, expected INT or INT.INT, got '%s'", b)
 	}
 
 	// 2. Extract version number
diff --git a/internal/http/upgrade/request.go b/internal/http/upgrade/request.go
index 1a7c0f8..45c1a5f 100644
--- a/internal/http/upgrade/request.go
+++ b/internal/http/upgrade/request.go
@@ -49,7 +49,7 @@ func Parse(r io.Reader) (request *Request, err error) {
 	// 1. Get chunk reader
 	cr := reader.NewReader(r)
 	if err != nil {
-		return req, fmt.Errorf("Error while creating chunk reader: %s", err)
+		return req, fmt.Errorf("create chunk reader: %w", err)
 	}
 
 	// 2. Parse header line by line
@@ -139,7 +139,7 @@ func (r *Request) parseHeader(b []byte) error {
 	head, err := ReadHeader(b)
 	if err != nil {
 		r.code = BadRequest
-		return fmt.Errorf("Error parsing header: %s", err)
+		return fmt.Errorf("parse header: %w", err)
 	}
 
 	// 2. Manage header
diff --git a/internal/http/upgrade/request_test.go b/internal/http/upgrade/request_test.go
index 3d12b30..7af15fa 100644
--- a/internal/http/upgrade/request_test.go
+++ b/internal/http/upgrade/request_test.go
@@ -6,23 +6,6 @@ import (
 	"testing"
 )
 
-// // 1. Parse request
-// req, _ := request.Parse(s)
-
-// // 3. Build response
-// res := req.BuildResponse()
-
-// // 4. Write into socket
-// _, err := res.Send(s)
-// if err != nil {
-// 	return nil, fmt.Errorf("Upgrade write error: %s", err)
-// }
-
-// if res.GetStatusCode() != 101 {
-// 	s.Close()
-// 	return nil, fmt.Errorf("Upgrade error (HTTP %d)\n", res.GetStatusCode())
-// }
-
 func TestEOFSocket(t *testing.T) {
 
 	socket := &bytes.Buffer{}
diff --git a/internal/uri/parser.go b/internal/uri/parser.go
index e35a4ee..e581c23 100644
--- a/internal/uri/parser.go
+++ b/internal/uri/parser.go
@@ -44,7 +44,7 @@ func FromString(s string) (*Scheme, error) {
 
 	// 1. Manage '/' at the start
 	if len(s) < 1 || s[0] != '/' {
-		return nil, fmt.Errorf("URI must begin with '/'")
+		return nil, fmt.Errorf("invalid URI; must start with '/'")
 	}
 
 	// 2. Split by '/'
@@ -102,7 +102,7 @@ func (s Scheme) GetMatch(n uint8) ([]string, error) {
 
 	// 1. Index out of range
 	if n > uint8(len(s)) {
-		return nil, fmt.Errorf("Index out of range")
+		return nil, fmt.Errorf("index out of range")
 	}
 
 	// 2. Iterate to find index (exclude strings)
@@ -125,7 +125,7 @@ func (s Scheme) GetMatch(n uint8) ([]string, error) {
 	}
 
 	// 3. If nothing found -> return empty set
-	return nil, fmt.Errorf("Index out of range (max: %d)", ni)
+	return nil, fmt.Errorf("index out of range (max: %d)", ni)
 
 }
 
diff --git a/message.go b/message.go
index 9ddc43e..f00f757 100644
--- a/message.go
+++ b/message.go
@@ -13,25 +13,25 @@ func (c constErr) Error() string { return string(c) }
 
 const (
 	// ErrUnmaskedFrame error
-	ErrUnmaskedFrame = constErr("Received unmasked frame")
+	ErrUnmaskedFrame constErr = "Received unmasked frame"
 	// ErrTooLongControlFrame error
-	ErrTooLongControlFrame = constErr("Received a control frame that is fragmented or too long")
+	ErrTooLongControlFrame constErr = "Received a control frame that is fragmented or too long"
 	// ErrInvalidFragment error
-	ErrInvalidFragment = constErr("Received invalid fragmentation")
+	ErrInvalidFragment constErr = "Received invalid fragmentation"
 	// ErrUnexpectedContinuation error
-	ErrUnexpectedContinuation = constErr("Received unexpected continuation frame")
+	ErrUnexpectedContinuation constErr = "Received unexpected continuation frame"
 	// ErrInvalidSize error
-	ErrInvalidSize = constErr("Received invalid payload size")
+	ErrInvalidSize constErr = "Received invalid payload size"
 	// ErrInvalidPayload error
-	ErrInvalidPayload = constErr("Received invalid utf8 payload")
+	ErrInvalidPayload constErr = "Received invalid utf8 payload"
 	// ErrInvalidCloseStatus error
-	ErrInvalidCloseStatus = constErr("Received invalid close status")
+	ErrInvalidCloseStatus constErr = "Received invalid close status"
 	// ErrInvalidOpCode error
-	ErrInvalidOpCode = constErr("Received invalid OpCode")
+	ErrInvalidOpCode constErr = "Received invalid OpCode"
 	// ErrReservedBits error
-	ErrReservedBits = constErr("Received reserved bits")
+	ErrReservedBits constErr = "Received reserved bits"
 	// ErrCloseFrame error
-	ErrCloseFrame = constErr("Received close Frame")
+	ErrCloseFrame constErr = "Received close Frame"
 )
 
 // Maximum Header Size  = Final/OpCode + isMask/Length + Length + Mask
diff --git a/server.go b/server.go
index 9102b25..f9a43a6 100644
--- a/server.go
+++ b/server.go
@@ -68,7 +68,7 @@ func (s *Server) Bind(uriStr string, f ControllerFunc) error {
 	// 1. Build URI parser
 	uriScheme, err := uri.FromString(uriStr)
 	if err != nil {
-		return fmt.Errorf("Cannot build URI: %s", err)
+		return fmt.Errorf("cannot build URI: %w", err)
 	}
 
 	// 2. Create controller
@@ -94,7 +94,7 @@ func (s *Server) Launch() error {
 	// 2. Bind socket to listen
 	s.sock, err = net.Listen("tcp", url)
 	if err != nil {
-		return fmt.Errorf("Listen socket: %s", err)
+		return fmt.Errorf("listen: %w", err)
 	}
 
 	defer s.sock.Close()