diff --git a/README.md b/README.md
index 44df488..1873269 100644
--- a/README.md
+++ b/README.md
@@ -48,6 +48,10 @@ registered on the default guild — instant, one server — or on the applicatio
 with `WithGlobalCommands`, which reaches every server the bot is in and takes up
 to an hour to propagate.
 
+`Interaction.UserID()` is who invoked it, from `member.user` in a guild and from
+`user` in a DM — read either field alone and permission checks see an empty id in
+the other context.
+
 Path segments are percent-escaped and queries built with `url.Values`.
 `Webhook.Token` and `Interaction.Token` are `Secret` — `[REDACTED]` in logs and
 JSON, `.Reveal()` to read.
diff --git a/interactions.go b/interactions.go
index 321c43e..c434986 100644
--- a/interactions.go
+++ b/interactions.go
@@ -25,6 +25,7 @@ type Interaction struct {
 	GuildID   string          `json:"guild_id"`
 	ChannelID string          `json:"channel_id"`
 	Member    Member          `json:"member"`
+	User      Author          `json:"user"`
 	Data      InteractionData `json:"data"`
 }
 
@@ -33,6 +34,17 @@ type Member struct {
 	User Author `json:"user"`
 }
 
+// UserID is who invoked the interaction, wherever Discord put them: a guild
+// interaction carries the caller under `member.user`, a DM under `user`. Reading
+// only one of the two makes every permission check in the other context see an
+// empty id.
+func (i Interaction) UserID() string {
+	if id := i.Member.User.ID; id != "" {
+		return id
+	}
+	return i.User.ID
+}
+
 // InteractionData is the invoked command + its options. For a component
 // interaction (type 3) the command fields are empty and CustomID/Values carry
 // the clicked component's id and the selected value(s) instead.
diff --git a/interactions_autocomplete_test.go b/interactions_autocomplete_test.go
index 8ddd5da..36b859d 100644
--- a/interactions_autocomplete_test.go
+++ b/interactions_autocomplete_test.go
@@ -34,6 +34,22 @@ func TestFocusedNoneWhenAbsent(t *testing.T) {
 	}
 }
 
+// Discord puts the caller under `member.user` in a guild and under `user` in a
+// DM. A permission check that reads only one sees an empty id in the other.
+func TestInteractionUserIDCoversGuildAndDM(t *testing.T) {
+	guild := Interaction{Member: Member{User: Author{ID: "u1"}}}
+	if got := guild.UserID(); got != "u1" {
+		t.Fatalf("guild UserID() = %q, want u1", got)
+	}
+	dm := Interaction{User: Author{ID: "u2"}}
+	if got := dm.UserID(); got != "u2" {
+		t.Fatalf("dm UserID() = %q, want u2", got)
+	}
+	if got := (Interaction{}).UserID(); got != "" {
+		t.Fatalf("empty UserID() = %q, want empty", got)
+	}
+}
+
 func TestInteractionsRespondNoAllowedMentions(t *testing.T) {
 	s := transport.NewStub()
 	in := &Interactions{rt: s, def: &defaults{guilds: &Guilds{rt: s}}}