diff --git a/attrs_test.go b/attrs_test.go
new file mode 100644
index 0000000..086a0d8
--- /dev/null
+++ b/attrs_test.go
@@ -0,0 +1,410 @@
+package simplelog
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"errors"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"strings"
+	"testing"
+	"time"
+)
+
+// These tests assert on the bytes the handlers actually emit, because that is
+// where the defect lives: the handlers accept attributes and then throw them
+// away, so nothing short of reading the output proves they survived.
+
+// captureStdout redirects os.Stdout for the duration of fn and returns what was
+// written to it. Both the console and the JSON handler write to os.Stdout, so
+// this is the only way to see their real output.
+func captureStdout(t *testing.T, fn func()) string {
+	t.Helper()
+
+	r, w, err := os.Pipe()
+	if err != nil {
+		t.Fatalf("os.Pipe: %v", err)
+	}
+
+	original := os.Stdout
+	os.Stdout = w
+
+	collected := make(chan string, 1)
+	go func() {
+		var buf bytes.Buffer
+		_, _ = io.Copy(&buf, r)
+		collected <- buf.String()
+	}()
+
+	defer func() {
+		os.Stdout = original
+		_ = r.Close()
+	}()
+
+	fn()
+
+	os.Stdout = original
+	if err := w.Close(); err != nil {
+		t.Fatalf("close pipe writer: %v", err)
+	}
+
+	return <-collected
+}
+
+// testRecord builds an INFO record carrying the given attributes.
+func testRecord(message string, attrs ...slog.Attr) slog.Record {
+	record := slog.NewRecord(time.Now(), slog.LevelInfo, message, 0)
+	record.AddAttrs(attrs...)
+	return record
+}
+
+// decodeLine parses a single line of JSON handler output.
+func decodeLine(t *testing.T, output string) map[string]any {
+	t.Helper()
+
+	line := strings.TrimSpace(output)
+	if line == "" {
+		t.Fatal("handler emitted no output")
+	}
+
+	var decoded map[string]any
+	if err := json.Unmarshal([]byte(line), &decoded); err != nil {
+		t.Fatalf("output is not valid JSON: %v\noutput: %s", err, line)
+	}
+	return decoded
+}
+
+// wantField asserts that a decoded JSON object has key with the given value.
+func wantField(t *testing.T, decoded map[string]any, key string, want any) {
+	t.Helper()
+
+	got, ok := decoded[key]
+	if !ok {
+		t.Fatalf("field %q missing from output: %v", key, decoded)
+	}
+	if got != want {
+		t.Fatalf("field %q = %v, want %v", key, got, want)
+	}
+}
+
+// wantGroup asserts that a decoded JSON object has key holding a nested object.
+func wantGroup(t *testing.T, decoded map[string]any, key string) map[string]any {
+	t.Helper()
+
+	got, ok := decoded[key]
+	if !ok {
+		t.Fatalf("group %q missing from output: %v", key, decoded)
+	}
+	group, ok := got.(map[string]any)
+	if !ok {
+		t.Fatalf("field %q = %v, want a nested object", key, got)
+	}
+	return group
+}
+
+// wantContains asserts that console output contains a fragment.
+func wantContains(t *testing.T, output, fragment string) {
+	t.Helper()
+
+	if !strings.Contains(output, fragment) {
+		t.Fatalf("output does not contain %q\noutput: %s", fragment, output)
+	}
+}
+
+// wantNotContains asserts that console output does not contain a fragment.
+func wantNotContains(t *testing.T, output, fragment string) {
+	t.Helper()
+
+	if strings.Contains(output, fragment) {
+		t.Fatalf("output unexpectedly contains %q\noutput: %s", fragment, output)
+	}
+}
+
+// castTarget is a slog.LogValuer: the handler must resolve it rather than
+// serialising the struct itself.
+type castTarget struct {
+	id string
+}
+
+func (c castTarget) LogValue() slog.Value {
+	return slog.StringValue(c.id)
+}
+
+var _ slog.LogValuer = castTarget{}
+
+func TestJSONHandlerEmitsRecordAttrs(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewJSONHandler()
+		record := testRecord(
+			"casting",
+			slog.String("device", "livingroom"),
+			slog.String("file", "movie.mp4"),
+			slog.Int("attempt", 3),
+		)
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	decoded := decodeLine(t, output)
+	wantField(t, decoded, "Message", "casting")
+	wantField(t, decoded, "device", "livingroom")
+	wantField(t, decoded, "file", "movie.mp4")
+	wantField(t, decoded, "attempt", float64(3))
+}
+
+func TestJSONHandlerWithAttrsAccumulates(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewJSONHandler().
+			WithAttrs([]slog.Attr{slog.String("service", "cattbox")}).
+			WithAttrs([]slog.Attr{slog.String("component", "caster")})
+		record := testRecord("casting", slog.String("device", "livingroom"))
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	decoded := decodeLine(t, output)
+	wantField(t, decoded, "service", "cattbox")
+	wantField(t, decoded, "component", "caster")
+	wantField(t, decoded, "device", "livingroom")
+}
+
+func TestJSONHandlerWithAttrsDoesNotMutateReceiver(t *testing.T) {
+	parent := NewJSONHandler()
+	first := parent.WithAttrs([]slog.Attr{slog.String("worker", "first")})
+	second := parent.WithAttrs([]slog.Attr{slog.String("worker", "second")})
+
+	firstOutput := captureStdout(t, func() {
+		if err := first.Handle(context.Background(), testRecord("work")); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+	secondOutput := captureStdout(t, func() {
+		if err := second.Handle(context.Background(), testRecord("work")); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+	parentOutput := captureStdout(t, func() {
+		if err := parent.Handle(context.Background(), testRecord("work")); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	wantField(t, decodeLine(t, firstOutput), "worker", "first")
+	wantField(t, decodeLine(t, secondOutput), "worker", "second")
+
+	if _, present := decodeLine(t, parentOutput)["worker"]; present {
+		t.Fatalf("parent handler leaked an attribute from a derived handler: %s", parentOutput)
+	}
+}
+
+func TestJSONHandlerWithGroupNestsAttrs(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewJSONHandler().
+			WithGroup("cast").
+			WithAttrs([]slog.Attr{slog.String("device", "livingroom")})
+		record := testRecord("casting", slog.String("file", "movie.mp4"))
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	decoded := decodeLine(t, output)
+	wantField(t, decoded, "Message", "casting")
+
+	group := wantGroup(t, decoded, "cast")
+	wantField(t, group, "device", "livingroom")
+	wantField(t, group, "file", "movie.mp4")
+}
+
+func TestJSONHandlerResolvesValues(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewJSONHandler()
+		record := testRecord(
+			"cast failed",
+			slog.Group("request", slog.Int("status", 502), slog.String("method", "POST")),
+			slog.Any("target", castTarget{id: "chromecast-7"}),
+			slog.Any("error", errors.New("connection refused")),
+		)
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	decoded := decodeLine(t, output)
+	wantField(t, decoded, "target", "chromecast-7")
+	wantField(t, decoded, "error", "connection refused")
+
+	group := wantGroup(t, decoded, "request")
+	wantField(t, group, "status", float64(502))
+	wantField(t, group, "method", "POST")
+}
+
+func TestConsoleHandlerEmitsRecordAttrs(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewConsoleHandler()
+		record := testRecord(
+			"casting",
+			slog.String("device", "livingroom"),
+			slog.String("file", "movie.mp4"),
+			slog.Int("attempt", 3),
+		)
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	wantContains(t, output, "casting")
+	wantContains(t, output, "device=livingroom")
+	wantContains(t, output, "file=movie.mp4")
+	wantContains(t, output, "attempt=3")
+}
+
+func TestConsoleHandlerQuotesValuesNeedingIt(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewConsoleHandler()
+		record := testRecord("casting", slog.String("file", "The Movie.mp4"))
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	wantContains(t, output, `file="The Movie.mp4"`)
+}
+
+func TestConsoleHandlerWithAttrsAccumulates(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewConsoleHandler().
+			WithAttrs([]slog.Attr{slog.String("service", "cattbox")}).
+			WithAttrs([]slog.Attr{slog.String("component", "caster")})
+		record := testRecord("casting", slog.String("device", "livingroom"))
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	wantContains(t, output, "service=cattbox")
+	wantContains(t, output, "component=caster")
+	wantContains(t, output, "device=livingroom")
+}
+
+func TestConsoleHandlerWithAttrsDoesNotMutateReceiver(t *testing.T) {
+	parent := NewConsoleHandler()
+	first := parent.WithAttrs([]slog.Attr{slog.String("worker", "first")})
+	second := parent.WithAttrs([]slog.Attr{slog.String("worker", "second")})
+
+	firstOutput := captureStdout(t, func() {
+		if err := first.Handle(context.Background(), testRecord("work")); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+	secondOutput := captureStdout(t, func() {
+		if err := second.Handle(context.Background(), testRecord("work")); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+	parentOutput := captureStdout(t, func() {
+		if err := parent.Handle(context.Background(), testRecord("work")); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	wantContains(t, firstOutput, "worker=first")
+	wantNotContains(t, firstOutput, "worker=second")
+	wantContains(t, secondOutput, "worker=second")
+	wantNotContains(t, secondOutput, "worker=first")
+	wantNotContains(t, parentOutput, "worker=")
+}
+
+func TestConsoleHandlerWithGroupQualifiesAttrs(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewConsoleHandler().
+			WithGroup("cast").
+			WithAttrs([]slog.Attr{slog.String("device", "livingroom")})
+		record := testRecord("casting", slog.String("file", "movie.mp4"))
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	wantContains(t, output, "cast.device=livingroom")
+	wantContains(t, output, "cast.file=movie.mp4")
+}
+
+func TestConsoleHandlerResolvesValues(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := NewConsoleHandler()
+		record := testRecord(
+			"cast failed",
+			slog.Group("request", slog.Int("status", 502)),
+			slog.Any("target", castTarget{id: "chromecast-7"}),
+			slog.Any("error", errors.New("connection refused")),
+		)
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	wantContains(t, output, "request.status=502")
+	wantContains(t, output, "target=chromecast-7")
+	wantContains(t, output, `error="connection refused"`)
+}
+
+func TestWebhookHandlerEmitsAttrs(t *testing.T) {
+	bodies := make(chan []byte, 1)
+	server := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			body, err := io.ReadAll(r.Body)
+			if err != nil {
+				t.Errorf("read webhook body: %v", err)
+			}
+			bodies <- body
+			w.WriteHeader(http.StatusOK)
+		},
+	))
+	defer server.Close()
+
+	handler, err := NewWebhookHandler(server.URL)
+	if err != nil {
+		t.Fatalf("NewWebhookHandler: %v", err)
+	}
+
+	withAttrs := handler.WithAttrs([]slog.Attr{slog.String("service", "cattbox")})
+	record := testRecord("casting", slog.String("device", "livingroom"))
+	if err := withAttrs.Handle(context.Background(), record); err != nil {
+		t.Fatalf("Handle: %v", err)
+	}
+
+	var body []byte
+	select {
+	case body = <-bodies:
+	case <-time.After(5 * time.Second):
+		t.Fatal("webhook handler posted nothing")
+	}
+
+	decoded := decodeLine(t, string(body))
+	wantField(t, decoded, "service", "cattbox")
+	wantField(t, decoded, "device", "livingroom")
+}
+
+// TestMultiplexHandlerPassesAttrsThrough guards the composite handler that
+// package init installs: attributes must survive the multiplex too.
+func TestMultiplexHandlerPassesAttrsThrough(t *testing.T) {
+	output := captureStdout(t, func() {
+		handler := (&MultiplexHandler{handlers: []ExtendedHandler{NewJSONHandler()}}).
+			WithAttrs([]slog.Attr{slog.String("service", "cattbox")})
+		record := testRecord("casting", slog.String("device", "livingroom"))
+		if err := handler.Handle(context.Background(), record); err != nil {
+			t.Fatalf("Handle: %v", err)
+		}
+	})
+
+	decoded := decodeLine(t, output)
+	wantField(t, decoded, "service", "cattbox")
+	wantField(t, decoded, "device", "livingroom")
+}