6 changed files with 349 additions and 11 deletions
--- a/README.md
+++ b/README.md
@@ -18,6 +18,13 @@ Released v1.0.0 2024-06-14.  Works as intended.  No known bugs.
 -   if output is a tty, outputs pretty color logs
 -   if output is not a tty, outputs json
 -   supports delivering each log message via a webhook
+-   emits every `slog` attribute: those passed to a log call, those
+    accumulated with `WithAttrs`, and those qualified by `WithGroup`.
+    `slog.Group` values nest, and `slog.LogValuer` values are resolved. In
+    json output attributes are object fields (groups become nested objects);
+    in console output they are appended as `key=value` pairs, with grouped
+    keys written as `group.key=value`. See
+    [Attribute output](#attribute-output) for the details worth knowing

 ## Planned Features

@@ -60,6 +67,43 @@ func main() {
 }
 ```

+## Attribute output
+
+Attributes reach every handler: the ones passed to the log call, the ones
+accumulated with `WithAttrs`, and the ones qualified by the groups open at
+the time they were attached. `slog.Group` values nest, and
+`slog.LogValuer` values are resolved to the value they stand for. A few
+behaviours are worth knowing before you rely on them.
+
+**Your values are never modified.** Whatever you log is read and rendered,
+never written to. A map or a slice you pass to `slog.Any` comes back from
+the logger exactly as you handed it over, even when a group later uses the
+same key.
+
+**Durations are nanoseconds in json, and readable on the console.** The
+json and webhook payloads emit a `slog.Duration` as a number of
+nanoseconds, matching `slog.NewJSONHandler`, so a consumer can compare and
+aggregate the field without parsing it. The console line emits the same
+duration as `3s`, matching `slog.NewTextHandler`, because a person reads
+it.
+
+**The json payload is an object, with the consequences an object has.**
+The record's own fields are named `Time`, `Level`, `Message` and `PC`, and
+they own those names: an attribute keyed after one of them is dropped from
+the json and webhook output. A key logged more than once keeps its last
+value there for the same reason, with one exception: when both are
+`slog.Group` values sharing a key, the two groups are merged into one
+object holding the members of both, rather than the second replacing the
+first. Neither the collision nor the merge applies to the console output,
+which is a line of text: every pair appears, in order. If you need a field
+called `message`, pick a key that does not collide - the collision is
+silent.
+
+**Empty things follow the `slog.Handler` contract.** An empty `Attr` is
+ignored, an empty group is elided along with its key, a group with an
+empty key is inlined into its parent, and `WithGroup("")` returns the
+handler unchanged.
+
 ## License

 [WTFPL](./LICENSE)
--- a/TODO.md
+++ b/TODO.md
@@ -24,6 +24,10 @@ files it depends on: .golangci.yml, REPO_POLICIES.md, .editorconfig,

 # Completed Steps

+* 2026-08-10: fixed every handler discarding slog attributes: console,
+    JSON and webhook handlers now emit record attributes, accumulate
+    WithAttrs without mutating the receiver, and honour WithGroup;
+    slog.Group values nest and LogValuer values are resolved
 * 2026-02-08: fixed JSONHandler deadlock from recursive log.Println,
    with regression test; tagged 1.0.1
 * 2024-06-14: 1.0 prep: lint and fmt enforced in Docker build, call
@@ -46,6 +50,8 @@ files it depends on: .golangci.yml, REPO_POLICIES.md, .editorconfig,
    the working tree
 * Pick one tag scheme before the next release (v1.0.0 vs 1.0.1 are
    inconsistent)
+* Tag v1.0.2, with the leading v, once the attribute fix lands, so
+    consuming repos can move off pseudo-version pins in one step
 * Fix RELP output to cache (from old TODO)
 * Re-add RELP delivery over TCP to remote rsyslog imrelp; removed
    2024-06-14 because it did not build (README planned feature)
--- a/attrs.go
+++ b/attrs.go
@@ -0,0 +1,255 @@
+package simplelog
+
+import (
+	"encoding/json"
+	"log/slog"
+	"strconv"
+	"strings"
+	"unicode"
+)
+
+// handlerAttrs is the attribute state every handler carries: the attributes
+// accumulated by WithAttrs, plus the groups opened by WithGroup. Its methods
+// never mutate the receiver, so handlers derived from a common parent stay
+// independent of each other.
+type handlerAttrs struct {
+	attrs  []slog.Attr
+	groups []string
+}
+
+// withAttrs returns a copy carrying attrs in addition to those already held.
+// The attributes are qualified by the groups that are open at the time they
+// are attached, as the slog.Handler contract requires.
+func (h handlerAttrs) withAttrs(attrs []slog.Attr) handlerAttrs {
+	qualified := qualifyAttrs(h.groups, attrs)
+	combined := make([]slog.Attr, 0, len(h.attrs)+len(qualified))
+	combined = append(combined, h.attrs...)
+	combined = append(combined, qualified...)
+	return handlerAttrs{attrs: combined, groups: h.groups}
+}
+
+// withGroup returns a copy with a further group open. An empty name is a no-op,
+// per the slog.Handler contract.
+func (h handlerAttrs) withGroup(name string) handlerAttrs {
+	if name == "" {
+		return h
+	}
+	groups := make([]string, 0, len(h.groups)+1)
+	groups = append(groups, h.groups...)
+	groups = append(groups, name)
+	return handlerAttrs{attrs: h.attrs, groups: groups}
+}
+
+// forRecord returns the accumulated attributes followed by the record's own,
+// the latter qualified by any open groups.
+func (h handlerAttrs) forRecord(record slog.Record) []slog.Attr {
+	own := qualifyAttrs(h.groups, recordAttrs(record))
+	all := make([]slog.Attr, 0, len(h.attrs)+len(own))
+	all = append(all, h.attrs...)
+	all = append(all, own...)
+	return all
+}
+
+// qualifyAttrs nests attrs inside the given open groups, innermost last.
+func qualifyAttrs(groups []string, attrs []slog.Attr) []slog.Attr {
+	for i := len(groups) - 1; i >= 0; i-- {
+		attrs = []slog.Attr{{
+			Key:   groups[i],
+			Value: slog.GroupValue(attrs...),
+		}}
+	}
+	return attrs
+}
+
+// recordAttrs collects the attributes a record carries. They live in
+// unexported fields, so they are only reachable through Record.Attrs - which
+// is why marshaling a slog.Record directly loses every one of them.
+func recordAttrs(record slog.Record) []slog.Attr {
+	attrs := make([]slog.Attr, 0, record.NumAttrs())
+	record.Attrs(func(attr slog.Attr) bool {
+		attrs = append(attrs, attr)
+		return true
+	})
+	return attrs
+}
+
+// groupMap is a JSON object this package built for itself: the payload of a
+// record, or one of the nested objects a slog.Group becomes.
+//
+// The distinct type is what keeps the handler from writing into data the
+// caller still owns. A value handed to slog.Any goes into the payload by
+// reference - copying every logged map and slice would be a real cost for no
+// gain, since rendering only reads. The one place the handler writes into a
+// value already in the payload is the group merge below, and a type assertion
+// to groupMap can only succeed on a map this package allocated: a caller's
+// map[string]any is a different type and never matches, however it was keyed.
+// So the invariant holds by construction - nothing reachable from the caller
+// is ever written to, only read.
+type groupMap map[string]any
+
+// recordToMap renders a record, with its handler's attributes, as the JSON
+// object the JSON and webhook handlers emit. The record's own fields keep the
+// names they have always had, and win a collision with an attribute key.
+func recordToMap(record slog.Record, attrs handlerAttrs) groupMap {
+	fields := attrsToMap(attrs.forRecord(record))
+	fields["Time"] = record.Time
+	fields["Level"] = record.Level
+	fields["Message"] = record.Message
+	fields["PC"] = record.PC
+	return fields
+}
+
+// attrsToMap renders attributes as a JSON object in which groups are nested
+// objects. A group named more than once is merged rather than duplicated;
+// any other repeated key keeps the last value, as a JSON object must.
+func attrsToMap(attrs []slog.Attr) groupMap {
+	fields := make(groupMap, len(attrs))
+	for _, attr := range attrs {
+		addAttrToMap(fields, attr)
+	}
+	return fields
+}
+
+func addAttrToMap(fields groupMap, attr slog.Attr) {
+	value := attr.Value.Resolve()
+	if attr.Key == "" && value.Any() == nil {
+		// An empty Attr is ignored, per the slog.Handler contract.
+		return
+	}
+
+	if value.Kind() == slog.KindGroup {
+		group := value.Group()
+		if len(group) == 0 {
+			// An empty group is elided, as is its key.
+			return
+		}
+		// A group with an empty key is inlined into its parent.
+		target := fields
+		if attr.Key != "" {
+			// Merge only into a group this package built. Anything else
+			// at this key - including a map the caller logged - is
+			// replaced rather than written into, so the caller's own
+			// data structure is never touched.
+			nested, ok := fields[attr.Key].(groupMap)
+			if !ok {
+				nested = make(groupMap, len(group))
+				fields[attr.Key] = nested
+			}
+			target = nested
+		}
+		for _, member := range group {
+			addAttrToMap(target, member)
+		}
+		return
+	}
+
+	fields[attr.Key] = jsonValue(value)
+}
+
+// jsonValue converts a resolved slog.Value into something encoding/json can
+// render usefully. Values it cannot marshal - and errors, which marshal to an
+// empty object - fall back to their slog string form, so a value is never
+// rendered as an empty object.
+//
+// Values are returned as they were given, not copied: nothing here or in its
+// callers writes to a value the caller supplied.
+func jsonValue(value slog.Value) any {
+	switch value.Kind() {
+	case slog.KindString:
+		return value.String()
+	case slog.KindInt64:
+		return value.Int64()
+	case slog.KindUint64:
+		return value.Uint64()
+	case slog.KindFloat64:
+		return value.Float64()
+	case slog.KindBool:
+		return value.Bool()
+	case slog.KindDuration:
+		// Nanoseconds as a number, which is what slog.NewJSONHandler
+		// emits. A JSON consumer can then compare and aggregate the
+		// field; the "3s" form would have to be parsed first, and no
+		// common log pipeline knows how.
+		return value.Duration().Nanoseconds()
+	case slog.KindTime:
+		return value.Time()
+	default:
+		// KindAny, and anything a future Go release adds.
+		return jsonAnyValue(value)
+	}
+}
+
+func jsonAnyValue(value slog.Value) any {
+	held := value.Any()
+	if _, ok := held.(json.Marshaler); !ok {
+		if err, ok := held.(error); ok {
+			return err.Error()
+		}
+	}
+	if _, err := json.Marshal(held); err != nil {
+		return value.String()
+	}
+	return held
+}
+
+// attrsToText renders attributes as the space separated key=value pairs the
+// console handler appends to a log line. Groups become dotted key prefixes.
+//
+// Values take their slog string form, so a duration reads as "3s" here where
+// the json output carries nanoseconds as a number. That is the same split the
+// stdlib makes between slog.NewTextHandler and slog.NewJSONHandler: the
+// console line is read by a person, the json line by a program.
+func attrsToText(attrs []slog.Attr) string {
+	var out strings.Builder
+	for _, attr := range attrs {
+		appendAttrText(&out, "", attr)
+	}
+	return out.String()
+}
+
+func appendAttrText(out *strings.Builder, prefix string, attr slog.Attr) {
+	value := attr.Value.Resolve()
+	if attr.Key == "" && value.Any() == nil {
+		return
+	}
+
+	if value.Kind() == slog.KindGroup {
+		group := value.Group()
+		if len(group) == 0 {
+			return
+		}
+		nested := prefix
+		if attr.Key != "" {
+			nested = prefix + attr.Key + "."
+		}
+		for _, member := range group {
+			appendAttrText(out, nested, member)
+		}
+		return
+	}
+
+	out.WriteString(" ")
+	// The key is quoted on the same terms as the value, and as a whole
+	// including its group prefix, because that is the token a reader has to
+	// find the "=" in. slog.NewTextHandler quotes "prefix+key" the same way,
+	// so a key like "a=b" reads as "a=b"=v2 rather than the ambiguous
+	// a=b=v2, which parses as the key "a" with the value "b=v2".
+	out.WriteString(quoteIfNeeded(prefix + attr.Key))
+	out.WriteString("=")
+	out.WriteString(quoteIfNeeded(value.String()))
+}
+
+// quoteIfNeeded quotes a key or a value only when leaving it bare would make
+// the key=value pairs ambiguous, matching how the stdlib text handler reads.
+func quoteIfNeeded(text string) string {
+	if text == "" {
+		return `""`
+	}
+	for _, r := range text {
+		if unicode.IsSpace(r) || !unicode.IsPrint(r) ||
+			r == '"' || r == '=' {
+			return strconv.Quote(text)
+		}
+	}
+	return text
+}
--- a/console_handler.go
+++ b/console_handler.go
@@ -10,7 +10,9 @@ import (
 	"github.com/fatih/color"
 )

-type ConsoleHandler struct{}
+type ConsoleHandler struct {
+	attrs handlerAttrs
+}

 func NewConsoleHandler() *ConsoleHandler {
 	return &ConsoleHandler{}
@@ -42,12 +44,13 @@ func (c *ConsoleHandler) Handle(
 	}
 	fmt.Println(
 		colorFunc(
-			"%s [%s] %s:%d: %s",
+			"%s [%s] %s:%d: %s%s",
 			timestamp,
 			record.Level,
 			file,
 			line,
 			record.Message,
+			attrsToText(c.attrs.forRecord(record)),
 		),
 	)
 	return nil
@@ -61,9 +64,15 @@ func (c *ConsoleHandler) Enabled(
 }

 func (c *ConsoleHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
-	return c
+	if len(attrs) == 0 {
+		return c
+	}
+	return &ConsoleHandler{attrs: c.attrs.withAttrs(attrs)}
 }

 func (c *ConsoleHandler) WithGroup(name string) slog.Handler {
-	return c
+	if name == "" {
+		return c
+	}
+	return &ConsoleHandler{attrs: c.attrs.withGroup(name)}
 }
--- a/json_handler.go
+++ b/json_handler.go
@@ -8,14 +8,19 @@ import (
 	"os"
 )

-type JSONHandler struct{}
+type JSONHandler struct {
+	attrs handlerAttrs
+}

 func NewJSONHandler() *JSONHandler {
 	return &JSONHandler{}
 }

 func (j *JSONHandler) Handle(ctx context.Context, record slog.Record) error {
-	jsonData, _ := json.Marshal(record)
+	jsonData, err := json.Marshal(recordToMap(record, j.attrs))
+	if err != nil {
+		return fmt.Errorf("error marshaling log record: %w", err)
+	}
 	fmt.Fprintln(os.Stdout, string(jsonData))
 	return nil
 }
@@ -25,9 +30,15 @@ func (j *JSONHandler) Enabled(ctx context.Context, level slog.Level) bool {
 }

 func (j *JSONHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
-	return j
+	if len(attrs) == 0 {
+		return j
+	}
+	return &JSONHandler{attrs: j.attrs.withAttrs(attrs)}
 }

 func (j *JSONHandler) WithGroup(name string) slog.Handler {
-	return j
+	if name == "" {
+		return j
+	}
+	return &JSONHandler{attrs: j.attrs.withGroup(name)}
 }
--- a/webhook_handler.go
+++ b/webhook_handler.go
@@ -12,6 +12,7 @@ import (

 type WebhookHandler struct {
 	webhookURL string
+	attrs      handlerAttrs
 }

 func (w *WebhookHandler) Enabled(ctx context.Context, level slog.Level) bool {
@@ -19,11 +20,23 @@ func (w *WebhookHandler) Enabled(ctx context.Context, level slog.Level) bool {
 }

 func (w *WebhookHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
-	return w
+	if len(attrs) == 0 {
+		return w
+	}
+	return &WebhookHandler{
+		webhookURL: w.webhookURL,
+		attrs:      w.attrs.withAttrs(attrs),
+	}
 }

 func (w *WebhookHandler) WithGroup(name string) slog.Handler {
-	return w
+	if name == "" {
+		return w
+	}
+	return &WebhookHandler{
+		webhookURL: w.webhookURL,
+		attrs:      w.attrs.withGroup(name),
+	}
 }

 func NewWebhookHandler(webhookURL string) (*WebhookHandler, error) {
@@ -34,7 +47,7 @@ func NewWebhookHandler(webhookURL string) (*WebhookHandler, error) {
 }

 func (w *WebhookHandler) Handle(ctx context.Context, record slog.Record) error {
-	jsonData, err := json.Marshal(record)
+	jsonData, err := json.Marshal(recordToMap(record, w.attrs))
 	if err != nil {
 		return fmt.Errorf("error marshaling event: %v", err)
 	}