diff --git a/mutations_add_comment.go b/mutations_add_comment.go
new file mode 100644
index 0000000..66a02f6
--- /dev/null
+++ b/mutations_add_comment.go
@@ -0,0 +1,116 @@
+package main
+
+import (
+	"fmt"
+	"strings"
+)
+
+// AddComment — appends a comment to a post. Bumps the post. Computes the
+// comment path from the parent comment's path. Pre-validated; the lock is
+// held during apply so the path computation + index update is atomic.
+type AddComment struct {
+	PostID   string
+	ParentID string // empty for top-level
+	AuthorID string
+	Body     string
+
+	// Set by Apply.
+	resolvedCommentID string
+	resolvedPath      string
+	resolvedDepth     int
+}
+
+func (m *AddComment) Kind() string { return "add_comment" }
+
+func (m *AddComment) AffectedAuthors() []string {
+	return []string{m.AuthorID}
+}
+
+func (m *AddComment) 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.PostID) == "" {
+		return &ValidationError{Field: "post_id", Message: "required"}
+	}
+	post, ok := s.GetPost(m.PostID)
+	if !ok {
+		return &ValidationError{Field: "post_id", Message: fmt.Sprintf("unknown post %q", m.PostID)}
+	}
+	if post.IsDeleted {
+		return &ValidationError{Field: "post_id", Message: "post is deleted"}
+	}
+	if m.ParentID != "" {
+		comments := s.ListComments(m.PostID)
+		found := false
+		for _, c := range comments {
+			if c.ID == m.ParentID && !c.IsDeleted {
+				found = true
+				break
+			}
+		}
+		if !found {
+			return &ValidationError{Field: "parent_id", Message: fmt.Sprintf("unknown parent %q on post %q", m.ParentID, m.PostID)}
+		}
+	}
+	if strings.TrimSpace(m.Body) == "" {
+		return &ValidationError{Field: "body", Message: "required"}
+	}
+	return nil
+}
+
+func (m *AddComment) Apply(s *Store, lock LockRegion) error {
+	now := s.Now()
+	id := s.NewID()
+
+	// Compute path under the lock. Look up parent's path if any.
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	var path string
+	var depth int
+	if m.ParentID != "" {
+		parent, ok := s.comments[m.ParentID]
+		if !ok {
+			return fmt.Errorf("parent comment disappeared mid-apply: %s", m.ParentID)
+		}
+		path = parent.Path + "." + id
+		depth = parent.Depth + 1
+	} else {
+		path = id
+		depth = 0
+	}
+
+	c := &Comment{
+		ID:        id,
+		PostID:    m.PostID,
+		ParentID:  m.ParentID,
+		Path:      path,
+		Depth:     depth,
+		AuthorID:  m.AuthorID,
+		Body:      m.Body,
+		CreatedAt: now,
+		UpdatedAt: now,
+	}
+	m.resolvedCommentID = id
+	m.resolvedPath = path
+	m.resolvedDepth = depth
+	s.comments[id] = c
+	s.indexInsertComment(c)
+
+	// Bump the post.
+	post, ok := s.posts[m.PostID]
+	if !ok {
+		return fmt.Errorf("post disappeared mid-apply: %s", m.PostID)
+	}
+	post.BumpedAt = now
+	post.UpdatedAt = now
+	s.indexUpdatePost(post)
+	return nil
+}
+
+func (m *AddComment) ResolvedCommentID() string { return m.resolvedCommentID }
+func (m *AddComment) ResolvedPath() string      { return m.resolvedPath }
+func (m *AddComment) ResolvedDepth() int        { return m.resolvedDepth }