diff --git a/fs/rc/rcserver/rcserver.go b/fs/rc/rcserver/rcserver.go
index 2067a59ce..3c6945ebc 100644
--- a/fs/rc/rcserver/rcserver.go
+++ b/fs/rc/rcserver/rcserver.go
@@ -186,7 +186,7 @@ func writeError(path string, in rc.Params, w http.ResponseWriter, err error, sta
 	})
 	if err != nil {
 		// can't return the error at this point
-		fs.Errorf(nil, "rc: failed to write JSON output: %v", err)
+		fs.Errorf(nil, "rc: writeError: failed to write JSON output from %#v: %v", in, err)
 	}
 }
 
@@ -270,6 +270,9 @@ func (s *Server) handlePost(w http.ResponseWriter, r *http.Request, path string)
 		writeError(path, in, w, errors.Errorf("authentication must be set up on the rc server to use %q or the --rc-no-auth flag must be in use", path), http.StatusForbidden)
 		return
 	}
+
+	inOrig := in.Copy()
+
 	if call.NeedsRequest {
 		// Add the request to RC
 		in["_request"] = r
@@ -282,7 +285,7 @@ func (s *Server) handlePost(w http.ResponseWriter, r *http.Request, path string)
 	// Check to see if it is async or not
 	isAsync, err := in.GetBool("_async")
 	if rc.NotErrParamNotFound(err) {
-		writeError(path, in, w, err, http.StatusBadRequest)
+		writeError(path, inOrig, w, err, http.StatusBadRequest)
 		return
 	}
 	delete(in, "_async") // remove the async parameter after parsing so vfs operations don't get confused
@@ -297,7 +300,7 @@ func (s *Server) handlePost(w http.ResponseWriter, r *http.Request, path string)
 		w.Header().Add("x-rclone-jobid", fmt.Sprintf("%d", jobID))
 	}
 	if err != nil {
-		writeError(path, in, w, err, http.StatusInternalServerError)
+		writeError(path, inOrig, w, err, http.StatusInternalServerError)
 		return
 	}
 	if out == nil {
@@ -308,8 +311,8 @@ func (s *Server) handlePost(w http.ResponseWriter, r *http.Request, path string)
 	err = rc.WriteJSON(w, out)
 	if err != nil {
 		// can't return the error at this point - but have a go anyway
-		writeError(path, in, w, err, http.StatusInternalServerError)
-		fs.Errorf(nil, "rc: failed to write JSON output: %v", err)
+		writeError(path, inOrig, w, err, http.StatusInternalServerError)
+		fs.Errorf(nil, "rc: handlePost: failed to write JSON output: %v", err)
 	}
 }
 
diff --git a/fs/rc/rcserver/rcserver_test.go b/fs/rc/rcserver/rcserver_test.go
index 678157b5d..9ada37a92 100644
--- a/fs/rc/rcserver/rcserver_test.go
+++ b/fs/rc/rcserver/rcserver_test.go
@@ -8,6 +8,7 @@ import (
 	"io/ioutil"
 	"net/http"
 	"net/http/httptest"
+	"os"
 	"regexp"
 	"strings"
 	"testing"
@@ -17,6 +18,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	_ "github.com/rclone/rclone/backend/local"
+	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/accounting"
 	"github.com/rclone/rclone/fs/rc"
 )
@@ -28,6 +30,21 @@ const (
 	remoteURL       = "[" + testFs + "]/" // initial URL path to fetch from that remote
 )
 
+func TestMain(m *testing.M) {
+	// Pretend to be rclone version if we have a version string parameter
+	if os.Args[len(os.Args)-1] == "version" {
+		fmt.Printf("rclone %s\n", fs.Version)
+		os.Exit(0)
+	}
+	// Pretend to error if we have an unknown command
+	if os.Args[len(os.Args)-1] == "unknown_command" {
+		fmt.Printf("rclone %s\n", fs.Version)
+		fmt.Fprintf(os.Stderr, "Unknown command\n")
+		os.Exit(1)
+	}
+	os.Exit(m.Run())
+}
+
 // Test the RC server runs and we can do HTTP fetches from it.
 // We'll do the majority of the testing with the httptest framework
 func TestRcServer(t *testing.T) {
@@ -456,6 +473,73 @@ func TestRC(t *testing.T) {
 	testServer(t, tests, &opt)
 }
 
+func TestRCWithAuth(t *testing.T) {
+	tests := []testRun{{
+		Name:        "core-command",
+		URL:         "core/command",
+		Method:      "POST",
+		Body:        `command=version`,
+		ContentType: "application/x-www-form-urlencoded",
+		Status:      http.StatusOK,
+		Expected: fmt.Sprintf(`{
+	"error": false,
+	"result": "rclone %s\n"
+}
+`, fs.Version),
+	}, {
+		Name:        "core-command-bad-returnType",
+		URL:         "core/command",
+		Method:      "POST",
+		Body:        `command=version&returnType=POTATO`,
+		ContentType: "application/x-www-form-urlencoded",
+		Status:      http.StatusInternalServerError,
+		Expected: `{
+	"error": "Unknown returnType \"POTATO\"",
+	"input": {
+		"command": "version",
+		"returnType": "POTATO"
+	},
+	"path": "core/command",
+	"status": 500
+}
+`,
+	}, {
+		Name:        "core-command-stream",
+		URL:         "core/command",
+		Method:      "POST",
+		Body:        `command=version&returnType=STREAM`,
+		ContentType: "application/x-www-form-urlencoded",
+		Status:      http.StatusOK,
+		Expected: fmt.Sprintf(`rclone %s
+{}
+`, fs.Version),
+	}, {
+		Name:        "core-command-stream-error",
+		URL:         "core/command",
+		Method:      "POST",
+		Body:        `command=unknown_command&returnType=STREAM`,
+		ContentType: "application/x-www-form-urlencoded",
+		Status:      http.StatusOK,
+		Expected: fmt.Sprintf(`rclone %s
+Unknown command
+{
+	"error": "exit status 1",
+	"input": {
+		"command": "unknown_command",
+		"returnType": "STREAM"
+	},
+	"path": "core/command",
+	"status": 500
+}
+`, fs.Version),
+	}}
+	opt := newTestOpt()
+	opt.Serve = true
+	opt.Files = testFs
+	opt.NoAuth = true
+	testServer(t, tests, &opt)
+}
+
 func TestMethods(t *testing.T) {
 	tests := []testRun{{
 		Name:     "options",