diff --git a/api/error.go b/api/error.go
index af72dd1..2abcc01 100644
--- a/api/error.go
+++ b/api/error.go
@@ -13,6 +13,14 @@ type Error struct {
 	Arguments []interface{} `json:"arguments"`
 }
 
+// NewError returns a new error from a base error with errorarguments.
+func NewError(baseError Error, arguments ...interface{}) Error {
+	for _, arg := range arguments {
+		baseError.Put(arg)
+	}
+	return baseError
+}
+
 // Put adds an argument to the error
 // to be displayed back to API caller
 func (e *Error) Put(arg interface{}) {
diff --git a/api/response.go b/api/response.go
index a6ea101..2ea4e6f 100644
--- a/api/response.go
+++ b/api/response.go
@@ -16,14 +16,21 @@ type Response struct {
 	Err     Error
 }
 
-// NewResponse creates an empty response
-func NewResponse() *Response {
-	return &Response{
+// NewResponse creates an empty response. An optional error can be passed as its first argument.
+func NewResponse(errors ...Error) *Response {
+	res := &Response{
 		Status:  http.StatusOK,
 		Data:    make(ResponseData),
 		Err:     ErrorFailure(),
 		Headers: make(http.Header),
 	}
+
+	// optional error
+	if len(errors) == 1 {
+		res.Err = errors[0]
+	}
+
+	return res
 }
 
 // SetData adds/overrides a new response field
@@ -51,3 +58,17 @@ func (i *Response) MarshalJSON() ([]byte, error) {
 
 	return json.Marshal(fmt)
 }
+
+// Write writes to an HTTP response.
+func (i *Response) Write(w http.ResponseWriter) error {
+	w.WriteHeader(i.Status)
+	w.Header().Add("Content-Type", "application/json")
+
+	fmt, err := json.Marshal(i)
+	if err != nil {
+		return err
+	}
+	w.Write(fmt)
+
+	return nil
+}
diff --git a/internal/multipart/reader.go b/internal/multipart/reader.go
index 3db12cd..f62bc59 100644
--- a/internal/multipart/reader.go
+++ b/internal/multipart/reader.go
@@ -6,19 +6,18 @@ import (
 	"io"
 )
 
-// NewReader craetes a new reader
-func NewReader(_src io.Reader, _boundary string) (*Reader, error) {
-
+// NewReader creates a new reader from a reader and a boundary.
+func NewReader(r io.Reader, boundary string) (*Reader, error) {
 	reader := &Reader{
 		reader:   nil,
-		boundary: fmt.Sprintf("--%s", _boundary),
+		boundary: fmt.Sprintf("--%s", boundary),
 		Data:     make(map[string]*Component),
 	}
 
 	// 1. Create reader
-	dst, ok := _src.(*bufio.Reader)
+	dst, ok := r.(*bufio.Reader)
 	if !ok {
-		dst = bufio.NewReader(_src)
+		dst = bufio.NewReader(r)
 	}
 	reader.reader = dst
 
diff --git a/internal/multipart/types.go b/internal/multipart/types.go
index 78c291d..2419a28 100644
--- a/internal/multipart/types.go
+++ b/internal/multipart/types.go
@@ -2,17 +2,24 @@ package multipart
 
 import (
 	"bufio"
-	"errors"
 )
 
+// ConstError is a wrapper to set constant errors
+type ConstError string
+
+// Error implements error
+func (err ConstError) Error() string {
+	return string(err)
+}
+
 // ErrMissingDataName is set when a multipart variable/file has no name="..."
-var ErrMissingDataName = errors.New("data has no name")
+var ErrMissingDataName = ConstError("data has no name")
 
 // ErrDataNameConflict is set when a multipart variable/file name is already used
-var ErrDataNameConflict = errors.New("data name conflict")
+var ErrDataNameConflict = ConstError("data name conflict")
 
 // ErrNoHeader is set when a multipart variable/file has no (valid) header
-var ErrNoHeader = errors.New("data has no header")
+var ErrNoHeader = ConstError("data has no header")
 
 // Component represents a multipart variable/file
 type Component struct {
diff --git a/server.go b/server.go
index d9205d8..eadbb99 100644
--- a/server.go
+++ b/server.go
@@ -51,7 +51,6 @@ func New(configPath string) (*Server, error) {
 
 // ServeHTTP implements http.Handler and has to be called on each request
 func (s *Server) ServeHTTP(res http.ResponseWriter, req *http.Request) {
-
 	defer req.Body.Close()
 
 	// 1. build API request from HTTP request
@@ -73,7 +72,9 @@ func (s *Server) ServeHTTP(res http.ResponseWriter, req *http.Request) {
 	// 3. check if matching methodDef exists in config */
 	var methodDef = serviceDef.Method(req.Method)
 	if methodDef == nil {
-		httpError(res, api.ErrorUnknownMethod())
+		apiResponse := api.NewResponse(api.ErrorUnknownMethod())
+		apiResponse.Write(res)
+		logError(apiResponse)
 		return
 	}
 
@@ -86,7 +87,9 @@ func (s *Server) ServeHTTP(res http.ResponseWriter, req *http.Request) {
 
 	// Fail if argument check failed
 	if paramError.Code != api.ErrorSuccess().Code {
-		httpError(res, paramError)
+		apiResponse := api.NewResponse(paramError)
+		apiResponse.Write(res)
+		logError(apiResponse)
 		return
 	}
 
@@ -109,16 +112,17 @@ func (s *Server) ServeHTTP(res http.ResponseWriter, req *http.Request) {
 	// fail if found no handler
 	if serviceHandler == nil {
 		if serviceFound {
-			apiError := api.ErrorUncallableMethod()
-			apiError.Put(servicePath)
-			apiError.Put(req.Method)
-			httpError(res, apiError)
+			apiError := api.NewError(api.ErrorUncallableMethod(), servicePath, req.Method)
+			apiResponse := api.NewResponse(apiError)
+			apiResponse.Write(res)
+			logError(apiResponse)
 			return
 		}
 
-		apiError := api.ErrorUncallableService()
-		apiError.Put(servicePath)
-		httpError(res, apiError)
+		apiError := api.NewError(api.ErrorUncallableService(), servicePath)
+		apiResponse := api.NewResponse(apiError)
+		apiResponse.Write(res)
+		logError(apiResponse)
 		return
 	}
 
@@ -138,8 +142,8 @@ func (s *Server) ServeHTTP(res http.ResponseWriter, req *http.Request) {
 		}
 	}
 
-	// 3. build JSON apiResponse
-	httpPrint(res, apiResponse)
+	// 3. write to response
+	apiResponse.Write(res)
 	return
 
 }
diff --git a/util.go b/util.go
index 305304d..7a94034 100644
--- a/util.go
+++ b/util.go
@@ -1,9 +1,7 @@
 package aicra
 
 import (
-	"encoding/json"
 	"log"
-	"net/http"
 
 	"git.xdrm.io/go/aicra/api"
 	"git.xdrm.io/go/aicra/internal/config"
@@ -21,17 +19,17 @@ func (s *Server) extractParameters(store *reqdata.Store, methodParam map[string]
 	// for each param of the config
 	for name, param := range methodParam {
 
-		/* (1) Extract value */
+		// 1. extract value
 		p, isset := store.Set[name]
 
-		/* (2) Required & missing */
+		// 2. fail if required & missing
 		if !isset && !param.Optional {
 			apiError = api.ErrorMissingParam()
 			apiError.Put(name)
 			return nil, apiError
 		}
 
-		/* (3) Optional & missing: set default value */
+		// 3. optional & missing: set default value
 		if !isset {
 			p = &reqdata.Parameter{
 				Parsed: true,
@@ -47,12 +45,12 @@ func (s *Server) extractParameters(store *reqdata.Store, methodParam map[string]
 			continue
 		}
 
-		/* (4) Parse parameter if not file */
+		// 4. parse parameter if not file
 		if !p.File {
 			p.Parse()
 		}
 
-		/* (5) Fail on unexpected multipart file */
+		// 5. fail on unexpected multipart file
 		waitFile, gotFile := param.Type == "FILE", p.File
 		if gotFile && !waitFile || !gotFile && waitFile {
 			apiError = api.ErrorInvalidParam()
@@ -61,13 +59,13 @@ func (s *Server) extractParameters(store *reqdata.Store, methodParam map[string]
 			return nil, apiError
 		}
 
-		/* (6) Do not check if file */
+		// 6. do not check if file
 		if gotFile {
 			parameters[param.Rename] = p.Value
 			continue
 		}
 
-		/* (7) Check type */
+		// 7. check type
 		if s.Checkers.Run(param.Type, p.Value) != nil {
 
 			apiError = api.ErrorInvalidParam()
@@ -85,20 +83,7 @@ func (s *Server) extractParameters(store *reqdata.Store, methodParam map[string]
 	return parameters, apiError
 }
 
-// Prints an HTTP response
-func httpPrint(r http.ResponseWriter, res *api.Response) {
-	r.WriteHeader(res.Status)
-
-	// write this json
-	jsonResponse, _ := json.Marshal(res)
-	r.Header().Add("Content-Type", "application/json")
-	r.Write(jsonResponse)
-}
-
 // Prints an error as HTTP response
-func httpError(r http.ResponseWriter, e api.Error) {
-	JSON, _ := json.Marshal(e)
-	r.Header().Add("Content-Type", "application/json")
-	r.Write(JSON)
-	log.Printf("[http.fail] %s\n", e.Reason)
+func logError(res *api.Response) {
+	log.Printf("[http.fail] %v\n", res.Err)
 }