diff --git a/mutations_create_post.go b/mutations_create_post.go
new file mode 100644
index 0000000..89c5c73
--- /dev/null
+++ b/mutations_create_post.go
@@ -0,0 +1,68 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+)
+
+// CreatePost — the canonical "make a new post" mutation. The bump_at is set
+// to the current time; the post is added to the postsByBumpedAt index; the
+// channels (if any) are added to the post_channels join map.
+type CreatePost struct {
+	AuthorID string
+	Title    string
+	Body     string
+	Channels []string
+
+	// Set by Validate, used by Apply and AffectedAuthors.
+	resolvedPostID string
+}
+
+func (m *CreatePost) Kind() string { return "create_post" }
+
+func (m *CreatePost) AffectedAuthors() []string {
+	return []string{m.AuthorID}
+}
+
+func (m *CreatePost) Validate(s *Store) error {
+	if strings.TrimSpace(m.AuthorID) == "" {
+		return &ValidationError{Field: "author_id", Message: "required"}
+	}
+	if _, ok := s.GetAuthor(m.AuthorID); !ok {
+		return &ValidationError{Field: "author_id", Message: fmt.Sprintf("unknown author %q", m.AuthorID)}
+	}
+	if strings.TrimSpace(m.Body) == "" {
+		return &ValidationError{Field: "body", Message: "required"}
+	}
+	for _, ch := range m.Channels {
+		if _, ok := s.GetChannel(ch); !ok {
+			return &ValidationError{Field: "channels", Message: fmt.Sprintf("unknown channel %q", ch)}
+		}
+	}
+	return nil
+}
+
+func (m *CreatePost) Apply(s *Store, lock LockRegion) error {
+	now := s.Now()
+	p := &Post{
+		ID:        s.NewID(),
+		AuthorID:  m.AuthorID,
+		Title:     m.Title,
+		Body:      m.Body,
+		BumpedAt:  now,
+		CreatedAt: now,
+		UpdatedAt: now,
+		Channels:  append([]string(nil), m.Channels...),
+	}
+	m.resolvedPostID = p.ID
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	s.posts[p.ID] = p
+	s.indexInsertPost(p)
+	if len(m.Channels) > 0 {
+		s.postChannels[p.ID] = append([]string(nil), m.Channels...)
+	}
+	return nil
+}
+
+func (m *CreatePost) ResolvedPostID() string { return m.resolvedPostID }