diff --git a/.github/workflows/ci.yaml b/.github/workflows/ci.yaml
index 86ddede..6dbb1b5 100644
--- a/.github/workflows/ci.yaml
+++ b/.github/workflows/ci.yaml
@@ -15,22 +15,21 @@ jobs:
     steps:
       - uses: actions/setup-go@v3
         with:
-          go-version: 1.19
+          go-version: 1.22
       - uses: actions/checkout@v3
       - name: golangci-lint
-        uses: golangci/golangci-lint-action@v3
+        uses: golangci/golangci-lint-action@v6
         with:
-          # Optional: version of golangci-lint to use in form of v1.2 or v1.2.3 or `latest` to use the latest version
-          version: v1.50.1
+          version: v1.59.0
           args: --config .golangci.yaml
   unit-test:
     name: unit-test
     runs-on: ubuntu-latest
     steps:
-      - name: Set up Go 1.19
+      - name: Set up Go 1.22
         uses: actions/setup-go@v2
         with:
-          go-version: 1.19
+          go-version: 1.22
       - name: Check out code
         uses: actions/checkout@v2
       - name: unit-test
diff --git a/Makefile b/Makefile
index c07f284..2bd270d 100644
--- a/Makefile
+++ b/Makefile
@@ -97,3 +97,6 @@ spellcheck:
 		echo "ERROR: cspell not found, install it manually! Link: https://cspell.org/docs/getting-started"; \
 		exit 1; \
 	fi
+
+mod-tidy: ## run go mod tidy
+	go mod tidy
diff --git a/eris.go b/eris.go
index 512f938..cfbcf2a 100644
--- a/eris.go
+++ b/eris.go
@@ -2,6 +2,7 @@
 package eris
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"net/http"
@@ -95,6 +96,19 @@ func Errorf(format string, args ...any) statusError {
 	}
 }
 
+type joinError interface {
+	Unwrap() []error
+}
+
+// Join returns an error that wraps the given errors.
+func Join(errs ...error) error {
+	internal := errors.Join(errs...)
+	if internal == nil {
+		return nil
+	}
+	return wrap(internal, "join error", DEFAULT_ERROR_CODE_NEW)
+}
+
 // Wrap adds additional context to all error types while maintaining the type of the original error. Adds a default error code 'internal'
 //
 // This method behaves differently for each error type. For root errors, the stack trace is reset to the current
diff --git a/eris_test.go b/eris_test.go
index 6e7e254..3d050a4 100644
--- a/eris_test.go
+++ b/eris_test.go
@@ -9,6 +9,7 @@ import (
 	"testing"
 
 	"github.com/risingwavelabs/eris"
+	"github.com/stretchr/testify/assert"
 )
 
 var (
@@ -938,3 +939,32 @@ func TestWrapType(t *testing.T) {
 		t.Errorf("expected nil error if wrap nil error, but error was %v", erisErr)
 	}
 }
+
+func TestJoinError(t *testing.T) {
+	err := eris.Join(nil, nil)
+	if err != nil {
+		t.Errorf("join nil should be nil")
+	}
+	err = eris.Join(nil, fmt.Errorf("external error"))
+	if err == nil {
+		t.Errorf("join error should be error")
+	}
+	err = eris.Join(fmt.Errorf("err1"), nil, fmt.Errorf("err2"))
+	if err == nil {
+		t.Errorf("join errors should be error")
+	}
+	type joinError interface {
+		Unwrap() []error
+	}
+	if joinErr, ok := eris.Unwrap(err).(joinError); !ok {
+		if len(joinErr.Unwrap()) != 2 {
+			t.Errorf("join 2 errors should be 2 errors")
+		}
+	}
+
+	err1 := errors.New("err1")
+	err2 := errors.New("err2")
+	err = eris.Join(err1, err2)
+	assert.True(t, errors.Is(err, err1))
+	assert.True(t, errors.Is(err, err2))
+}
diff --git a/format.go b/format.go
index f723964..b0460f4 100644
--- a/format.go
+++ b/format.go
@@ -2,6 +2,7 @@ package eris
 
 import (
 	"fmt"
+	"strings"
 )
 
 // FormatOptions defines output options like omitting stack traces and inverting the error or stack order.
@@ -103,8 +104,11 @@ func ToCustomString(err error, format StringFormat) string {
 	if format.Options.InvertOutput {
 		errSep := false
 		if format.Options.WithExternal && upErr.ErrExternal != nil {
-			str += formatExternalStr(upErr.ErrExternal, format.Options.WithTrace)
-			if (format.Options.WithTrace && len(upErr.ErrRoot.Stack) > 0) || upErr.ErrRoot.Msg != "" {
+			externalStr := formatExternalStr(upErr.ErrExternal, format.Options.WithTrace)
+			str += externalStr
+			if strings.Contains(externalStr, "\n") {
+				str += "\n"
+			} else if (format.Options.WithTrace && len(upErr.ErrRoot.Stack) > 0) || upErr.ErrRoot.Msg != "" {
 				errSep = true
 				str += format.ErrorSep
 			}
@@ -124,10 +128,13 @@ func ToCustomString(err error, format StringFormat) string {
 		}
 		str += upErr.ErrRoot.formatStr(format)
 		if format.Options.WithExternal && upErr.ErrExternal != nil {
-			if (format.Options.WithTrace && len(upErr.ErrRoot.Stack) > 0) || upErr.ErrRoot.Msg != "" {
+			externalStr := formatExternalStr(upErr.ErrExternal, format.Options.WithTrace)
+			if strings.Contains(externalStr, "\n") {
+				str += "\n"
+			} else if (format.Options.WithTrace && len(upErr.ErrRoot.Stack) > 0) || upErr.ErrRoot.Msg != "" {
 				str += format.ErrorSep
 			}
-			str += formatExternalStr(upErr.ErrExternal, format.Options.WithTrace)
+			str += externalStr
 		}
 	}
 
@@ -250,7 +257,17 @@ func ToCustomJSON(err error, format JSONFormat) map[string]any {
 
 	jsonMap := make(map[string]any)
 	if format.Options.WithExternal && upErr.ErrExternal != nil {
-		jsonMap["external"] = formatExternalStr(upErr.ErrExternal, format.Options.WithTrace)
+
+		join, ok := upErr.ErrExternal.(joinError)
+		if !ok {
+			jsonMap["external"] = formatExternalStr(upErr.ErrExternal, format.Options.WithTrace)
+		} else {
+			var externals []map[string]any
+			for _, e := range join.Unwrap() {
+				externals = append(externals, ToCustomJSON(e, format))
+			}
+			jsonMap["externals"] = externals
+		}
 	}
 
 	if upErr.ErrRoot.Msg != "" || len(upErr.ErrRoot.Stack) > 0 {
@@ -311,10 +328,28 @@ type UnpackedError struct {
 
 // String formatter for external errors.
 func formatExternalStr(err error, withTrace bool) string {
+	type joinError interface {
+		Unwrap() []error
+	}
+
+	format := "%v"
 	if withTrace {
-		return fmt.Sprintf("%+v", err)
+		format = "%+v"
+	}
+	join, ok := err.(joinError)
+	if !ok {
+		return fmt.Sprintf(format, err)
+	}
+
+	var strs []string
+	for i, e := range join.Unwrap() {
+		lines := strings.Split(fmt.Sprintf(format, e), "\n")
+		for no, line := range lines {
+			lines[no] = fmt.Sprintf("\t%s", line)
+		}
+		strs = append(strs, fmt.Sprintf("%d>", i)+strings.Join(lines, "\n"))
 	}
-	return fmt.Sprint(err)
+	return strings.Join(strs, "\n")
 }
 
 // ErrRoot represents an error stack and the accompanying message.
diff --git a/format_test.go b/format_test.go
index 57f0772..0c796ae 100644
--- a/format_test.go
+++ b/format_test.go
@@ -3,7 +3,9 @@ package eris_test
 import (
 	"encoding/json"
 	"errors"
+	"fmt"
 	"reflect"
+	"regexp"
 	"testing"
 
 	"github.com/risingwavelabs/eris"
@@ -362,7 +364,7 @@ func TestFormatJSONWithStack(t *testing.T) {
 					t.Fatalf("%v: expected a 'message' field in the output but didn't find one { %v }", desc, errJSON)
 				}
 				if rootMap["message"] != tt.rootOutput["message"] {
-					t.Errorf("%v: expected { %v } got { %v }", desc, rootMap["message"], tt.rootOutput["message"])
+					t.Errorf("%v: expected { %v } got { %v }", desc, tt.rootOutput["message"], rootMap["message"])
 				}
 				if _, exists := rootMap["stack"]; !exists {
 					t.Fatalf("%v: expected a 'stack' field in the output but didn't find one { %v }", desc, errJSON)
@@ -381,7 +383,7 @@ func TestFormatJSONWithStack(t *testing.T) {
 						t.Fatalf("%v: expected a 'message' field in the output but didn't find one { %v }", desc, errJSON)
 					}
 					if wrapMap[i]["message"] != tt.wrapOutput[i]["message"] {
-						t.Errorf("%v: expected { %v } got { %v }", desc, wrapMap[i]["message"], tt.wrapOutput[i]["message"])
+						t.Errorf("%v: expected { %v } got { %v }", desc, tt.wrapOutput[i]["message"], wrapMap[i]["message"])
 					}
 					if _, exists := wrapMap[i]["stack"]; !exists {
 						t.Fatalf("%v: expected a 'stack' field in the output but didn't find one { %v }", desc, errJSON)
@@ -393,3 +395,190 @@ func TestFormatJSONWithStack(t *testing.T) {
 		})
 	}
 }
+
+func TestFormatJoinError(t *testing.T) {
+	tests := map[string]struct {
+		input           error
+		withTrace       bool
+		withExternal    bool
+		stringOutput    string
+		regexOutput     *regexp.Regexp
+		rootOutput      map[string]any
+		wrapOutput      []map[string]any
+		externalsOutput []map[string]any
+	}{
+		"without trace and external": {
+			input: eris.Wrap(eris.Join(
+				fmt.Errorf("fmt error"),
+				eris.Wrap(eris.Wrap(fmt.Errorf("external"), "wrap2"), "wrap1"),
+				eris.New("eris error"),
+			), "outer wrap"),
+			withTrace:    false,
+			withExternal: false,
+			stringOutput: "code(internal) outer wrap: code(unknown) join error",
+			rootOutput: map[string]any{
+				"message": "join error",
+			},
+			wrapOutput: []map[string]any{
+				{
+					"message": "outer wrap",
+				},
+			},
+		},
+		"without trace": {
+			input: eris.Wrap(eris.Join(
+				fmt.Errorf("fmt error"),
+				eris.Wrap(eris.Wrap(fmt.Errorf("external"), "wrap2"), "wrap1"),
+				eris.New("eris error"),
+			), "outer wrap"),
+			withTrace:    false,
+			withExternal: true,
+			stringOutput: `code(internal) outer wrap: code(unknown) join error
+0>	fmt error
+1>	code(internal) wrap1: code(internal) wrap2: external
+2>	code(unknown) eris error`,
+			rootOutput: map[string]any{
+				"message": "join error",
+			},
+			wrapOutput: []map[string]any{
+				{
+					"message": "outer wrap",
+				},
+			},
+			externalsOutput: []map[string]any{
+				{
+					"external": "fmt error",
+				}, {
+					"external": "external",
+					"root":     map[string]any{},
+					"wrap":     []map[string]any{},
+				}, {
+					"root": map[string]any{},
+				},
+			},
+		},
+		"with trace": {
+			input: eris.Wrap(eris.Join(
+				fmt.Errorf("fmt error"),
+				eris.Wrap(eris.Wrap(fmt.Errorf("external"), "wrap2"), "wrap1"),
+				eris.New("eris error"),
+			), "outer wrap"),
+			withTrace:    true,
+			withExternal: true,
+			regexOutput: regexp.MustCompile(`code\(internal\) outer wrap
+	eris_test\.TestFormatJoinError:\S+:\d+
+code\(unknown\) join error
+	eris_test\.TestFormatJoinError:\S+:\d+
+	eris_test\.TestFormatJoinError:\S+:\d+
+0>	fmt error
+1>	code\(internal\) wrap1
+		eris_test\.TestFormatJoinError:\S+:\d+
+	code\(internal\) wrap2
+		eris_test\.TestFormatJoinError:\S+:\d+
+		eris_test\.TestFormatJoinError:\S+:\d+
+	external
+2>	code\(unknown\) eris error
+		eris_test\.TestFormatJoinError:\S+:\d+`),
+			rootOutput: map[string]any{
+				"message": "join error",
+			},
+			wrapOutput: []map[string]any{
+				{
+					"message": "outer wrap",
+				},
+			},
+			externalsOutput: []map[string]any{
+				{
+					"external": "fmt error",
+				}, {
+					"external": "external",
+					"root":     map[string]any{},
+					"wrap":     []map[string]any{},
+				}, {
+					"root": map[string]any{},
+				},
+			},
+		},
+	}
+
+	for desc, tt := range tests {
+		t.Run(desc, func(t *testing.T) {
+			errStr := eris.ToCustomString(tt.input, eris.NewDefaultStringFormat(eris.FormatOptions{
+				WithTrace:    tt.withTrace,
+				WithExternal: tt.withExternal,
+			}))
+			if tt.stringOutput != "" && tt.stringOutput != errStr {
+				t.Errorf("%v: expected { %v } got { %v }", desc, tt.stringOutput, errStr)
+			}
+			if tt.regexOutput != nil && !tt.regexOutput.MatchString(errStr) {
+				t.Errorf("%v: expected match { %v } got { %v }", desc, tt.regexOutput.String(), errStr)
+			}
+
+			errJSON := eris.ToCustomJSON(tt.input, eris.NewDefaultJSONFormat(eris.FormatOptions{
+				WithTrace:    tt.withTrace,
+				WithExternal: tt.withExternal,
+			}))
+
+			// make sure messages are correct and stack elements exist (actual stack validation is in stack_test.go)
+			if rootMap, ok := errJSON["root"].(map[string]any); ok {
+				if _, exists := rootMap["message"]; !exists {
+					t.Fatalf("%v: expected a 'message' field in the output but didn't find one { %v }", desc, errJSON)
+				}
+				if rootMap["message"] != tt.rootOutput["message"] {
+					t.Errorf("%v: expected { %v } got { %v }", desc, tt.rootOutput["message"], rootMap["message"])
+				}
+				if _, exists := rootMap["stack"]; tt.withTrace && !exists {
+					t.Fatalf("%v: expected a 'stack' field in the output but didn't find one { %v }", desc, errJSON)
+				}
+			} else {
+				t.Errorf("%v: expected root error is malformed { %v }", desc, errJSON)
+			}
+
+			// make sure messages are correct and stack elements exist (actual stack validation is in stack_test.go)
+			if wrapMap, ok := errJSON["wrap"].([]map[string]any); ok {
+				if len(tt.wrapOutput) != len(wrapMap) {
+					t.Fatalf("%v: expected number of wrap layers { %v } doesn't match actual { %v }", desc, len(tt.wrapOutput), len(wrapMap))
+				}
+				for i := 0; i < len(wrapMap); i++ {
+					if _, exists := wrapMap[i]["message"]; !exists {
+						t.Fatalf("%v: expected a 'message' field in the output but didn't find one { %v }", desc, errJSON)
+					}
+					if wrapMap[i]["message"] != tt.wrapOutput[i]["message"] {
+						t.Errorf("%v: expected { %v } got { %v }", desc, tt.wrapOutput[i]["message"], wrapMap[i]["message"])
+					}
+					if _, exists := wrapMap[i]["stack"]; tt.withTrace && !exists {
+						t.Fatalf("%v: expected a 'stack' field in the output but didn't find one { %v }", desc, errJSON)
+					}
+				}
+			} else {
+				t.Errorf("%v: expected wrap error is malformed { %v }", desc, errJSON)
+			}
+
+			if externalsMap, ok := errJSON["externals"].([]map[string]any); ok {
+				if len(tt.externalsOutput) != len(externalsMap) {
+					t.Fatalf("%v: expected number of externals errors { %v } doesn't match actual { %v }", desc, len(tt.externalsOutput), len(externalsMap))
+				}
+				for i, externalsOutputItem := range tt.externalsOutput {
+					for key, val := range externalsOutputItem {
+						switch val.(type) {
+						case string:
+							if externalsMap[i][key] != val {
+								t.Errorf("%v: expected externals[%d][%s] { %v } got { %v }", desc, i, key, val, externalsMap[i][key])
+							}
+						case map[string]any:
+							if _, ok := externalsMap[i][key].(map[string]any); !ok {
+								t.Errorf("%v: expected externals[%d][%s] is object got { %v }", desc, i, key, externalsMap[i][key])
+							}
+						case []map[string]any:
+							if _, ok := externalsMap[i][key].([]map[string]any); !ok {
+								t.Errorf("%v: expected externals[%d][%s] is object array got { %v }", desc, i, key, externalsMap[i][key])
+							}
+						}
+					}
+				}
+			} else if tt.withExternal {
+				t.Errorf("%v: expected externals error is malformed { %v }", desc, errJSON)
+			}
+		})
+	}
+}
diff --git a/go.mod b/go.mod
index bb974c8..06a6631 100644
--- a/go.mod
+++ b/go.mod
@@ -2,4 +2,13 @@ module github.com/risingwavelabs/eris
 
 go 1.18
 
-require google.golang.org/grpc v1.53.0
+require (
+	github.com/stretchr/testify v1.9.0
+	google.golang.org/grpc v1.53.0
+)
+
+require (
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
+)
diff --git a/go.sum b/go.sum
index d8f5001..e610fca 100644
--- a/go.sum
+++ b/go.sum
@@ -1,4 +1,14 @@
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/testify v1.9.0 h1:HtqpIVDClZ4nwg75+f6Lvsy/wHu+3BoSGCbBAcpTsTg=
+github.com/stretchr/testify v1.9.0/go.mod h1:r2ic/lqez/lEtzL7wO/rwa5dbSLXVDPFyf8C91i36aY=
 google.golang.org/genproto v0.0.0-20230110181048-76db0878b65f h1:BWUVssLB0HVOSY78gIdvk1dTVYtT1y8SBWtPYuTJ/6w=
 google.golang.org/grpc v1.53.0 h1:LAv2ds7cmFV/XTS3XG1NneeENYrXGmorPxsBbptIjNc=
 google.golang.org/grpc v1.53.0/go.mod h1:OnIrk0ipVdj4N5d9IUoFUx72/VlD7+jUsHwZgwSMQpw=
 google.golang.org/protobuf v1.28.1 h1:d0NfwRgPtno5B1Wa6L2DAG+KivqkdutMf1UhdNx175w=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405 h1:yhCVgyC4o1eVCa2tZl7eS0r+SDo693bJlVdllGtEeKM=
+gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=