diff --git a/mutations_upsert_channel.go b/mutations_upsert_channel.go
new file mode 100644
index 0000000..fa6dee3
--- /dev/null
+++ b/mutations_upsert_channel.go
@@ -0,0 +1,53 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+)
+
+// UpsertChannel — create or update a channel. Channels are lightweight tags.
+type UpsertChannel struct {
+	Slug        string
+	Name        string
+	Description string
+}
+
+func (m *UpsertChannel) Kind() string { return "upsert_channel" }
+
+func (m *UpsertChannel) AffectedAuthors() []string {
+	return nil // channels don't attribute to an author
+}
+
+func (m *UpsertChannel) Validate(s *Store) error {
+	if strings.TrimSpace(m.Slug) == "" {
+		return &ValidationError{Field: "slug", Message: "required"}
+	}
+	for _, r := range m.Slug {
+		if r == ' ' || r == '\t' || r == '\n' || r == '/' {
+			return &ValidationError{Field: "slug", Message: "must not contain whitespace or slashes"}
+		}
+	}
+	if strings.TrimSpace(m.Name) == "" {
+		return &ValidationError{Field: "name", Message: "required"}
+	}
+	return nil
+}
+
+func (m *UpsertChannel) Apply(s *Store, lock LockRegion) error {
+	now := s.Now()
+	c := &Channel{
+		Slug:        m.Slug,
+		Name:        m.Name,
+		Description: m.Description,
+		CreatedAt:   now,
+	}
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if existing, ok := s.channels[m.Slug]; ok {
+		c.CreatedAt = existing.CreatedAt
+	}
+	s.channels[m.Slug] = c
+	return nil
+}
+
+var _ = fmt.Sprintf // keep fmt for future error formatting