diff --git a/api/models.go b/api/models.go
new file mode 100644
index 0000000..c5395d2
--- /dev/null
+++ b/api/models.go
@@ -0,0 +1,68 @@
+package main
+
+import "time"
+
+// Author — human or agent, single identity model.
+type Author struct {
+	ID          string    `json:"id"`
+	Kind        string    `json:"kind"` // "human" | "agent" | "system"
+	DisplayName string    `json:"display_name"`
+	AvatarURL   string    `json:"avatar_url,omitempty"`
+	CreatedAt   time.Time `json:"created_at"`
+	UpdatedAt   time.Time `json:"updated_at"`
+}
+
+// Channel — lightweight, optional, many-to-many with posts.
+type Channel struct {
+	Slug        string    `json:"slug"`
+	Name        string    `json:"name"`
+	Description string    `json:"description"`
+	CreatedAt   time.Time `json:"created_at"`
+}
+
+// Post — the primitive. Always lives in 0..N channels.
+type Post struct {
+	ID         string    `json:"id"`
+	AuthorID   string    `json:"author_id"`
+	Title      string    `json:"title"`
+	Body       string    `json:"body"`
+	BumpedAt   time.Time `json:"bumped_at"`
+	CreatedAt  time.Time `json:"created_at"`
+	UpdatedAt  time.Time `json:"updated_at"`
+	IsDeleted  bool      `json:"is_deleted,omitempty"`
+	ParentPost string    `json:"parent_post,omitempty"`
+	Channels   []string  `json:"channels,omitempty"`
+}
+
+// Comment — nested to arbitrary depth via path.
+type Comment struct {
+	ID        string    `json:"id"`
+	PostID    string    `json:"post_id"`
+	ParentID  string    `json:"parent_id,omitempty"`
+	Path      string    `json:"path"`
+	Depth     int       `json:"depth"`
+	AuthorID  string    `json:"author_id"`
+	Body      string    `json:"body"`
+	CreatedAt time.Time `json:"created_at"`
+	UpdatedAt time.Time `json:"updated_at"`
+	IsDeleted bool      `json:"is_deleted,omitempty"`
+}
+
+// PostWithComments — for /api/posts/:id (the unit of rendering).
+type PostWithComments struct {
+	Post     Post      `json:"post"`
+	Comments []Comment `json:"comments"`
+}
+
+// CreatePostRequest — input shape.
+type CreatePostRequest struct {
+	Title    string   `json:"title"`
+	Body     string   `json:"body"`
+	Channels []string `json:"channels,omitempty"`
+}
+
+// CreateCommentRequest — input shape.
+type CreateCommentRequest struct {
+	ParentID string `json:"parent_id,omitempty"`
+	Body     string `json:"body"`
+}