diff --git a/lib/content/conformance.sh b/lib/content/conformance.sh
index a25d508b..411b9394 100755
--- a/lib/content/conformance.sh
+++ b/lib/content/conformance.sh
@@ -15,7 +15,7 @@ if [ ! -x "$SX_SERVER" ]; then
   fi
 fi
 
-SUITES=(block doc render api meta markdown text validate store snapshot crdt crdt-store sync md-import fed)
+SUITES=(block doc render api meta markdown text section validate store snapshot crdt crdt-store sync md-import fed)
 
 OUT_JSON="lib/content/scoreboard.json"
 OUT_MD="lib/content/scoreboard.md"
@@ -44,6 +44,7 @@ run_suite() {
 (load "lib/content/api.sx")
 (load "lib/content/meta.sx")
 (load "lib/content/text.sx")
+(load "lib/content/section.sx")
 (load "lib/content/markdown.sx")
 (load "lib/content/validate.sx")
 (load "lib/content/store.sx")
diff --git a/lib/content/scoreboard.json b/lib/content/scoreboard.json
index 167c48e6..d59732a3 100644
--- a/lib/content/scoreboard.json
+++ b/lib/content/scoreboard.json
@@ -7,6 +7,7 @@
     "meta": {"pass": 27, "fail": 0},
     "markdown": {"pass": 20, "fail": 0},
     "text": {"pass": 20, "fail": 0},
+    "section": {"pass": 25, "fail": 0},
     "validate": {"pass": 17, "fail": 0},
     "store": {"pass": 29, "fail": 0},
     "snapshot": {"pass": 20, "fail": 0},
@@ -16,7 +17,7 @@
     "md-import": {"pass": 24, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 385,
+  "total_pass": 410,
   "total_fail": 0,
-  "total": 385
+  "total": 410
 }
diff --git a/lib/content/scoreboard.md b/lib/content/scoreboard.md
index 2cfaa99d..9b570f2b 100644
--- a/lib/content/scoreboard.md
+++ b/lib/content/scoreboard.md
@@ -11,6 +11,7 @@ _Generated by `lib/content/conformance.sh`_
 | meta | 27 | 0 | 27 |
 | markdown | 20 | 0 | 20 |
 | text | 20 | 0 | 20 |
+| section | 25 | 0 | 25 |
 | validate | 17 | 0 | 17 |
 | store | 29 | 0 | 29 |
 | snapshot | 20 | 0 | 20 |
@@ -19,4 +20,4 @@ _Generated by `lib/content/conformance.sh`_
 | sync | 14 | 0 | 14 |
 | md-import | 24 | 0 | 24 |
 | fed | 20 | 0 | 20 |
-| **Total** | **385** | **0** | **385** |
+| **Total** | **410** | **0** | **410** |
diff --git a/lib/content/section.sx b/lib/content/section.sx
new file mode 100644
index 00000000..2c20f8f7
--- /dev/null
+++ b/lib/content/section.sx
@@ -0,0 +1,103 @@
+;; content-on-sx — nested block trees (section container).
+;;
+;; CtSection is a block whose ivar `children` is an ordered list of blocks (any
+;; type, including nested sections → arbitrary depth). This turns the document
+;; from a flat sequence into the ordered TREE of the architecture sketch.
+;;
+;; Self-contained: CtSection answers asHTML/asSx/asText/asMarkdown: by folding
+;; its children's renderings — pure polymorphic recursion, so it composes with
+;; the existing render boundary with no changes to block.sx or render.sx. (The
+;; relevant per-block render bootstrap must be loaded for the children.)
+;;
+;; Requires (loaded by harness): block.sx, doc.sx, render.sx (asHTML/asSx);
+;; markdown.sx / text.sx for those formats on children.
+
+(define
+  content-bootstrap-section!
+  (fn
+    ()
+    (begin
+      (st-class-define! "CtSection" "CtBlock" (list "children"))
+      (ct-def-method! "CtSection" "children" "children ^ children")
+      (ct-def-method! "CtSection" "type" "type ^ #section")
+      (ct-def-method!
+        "CtSection"
+        "asHTML"
+        "asHTML ^ '<section>' , (children inject: '' into: [:a :b | a , (b asHTML)]) , '</section>'")
+      (ct-def-method!
+        "CtSection"
+        "asSx"
+        "asSx ^ '(section ' , (children inject: '' into: [:a :b | a , (b asSx)]) , ')'")
+      (ct-def-method!
+        "CtSection"
+        "asText"
+        "asText ^ (children inject: '' into: [:a :b | (b asText = '') ifTrue: [a] ifFalse: [(a = '' ifTrue: [b asText] ifFalse: [a , ' ' , b asText])]])")
+      (ct-def-method!
+        "CtSection"
+        "asMarkdown:"
+        "asMarkdown: nl ^ (children inject: '' into: [:a :b | a , (a = '' ifTrue: [''] ifFalse: [nl , nl]) , (b asMarkdown: nl)])")
+      true)))
+
+(define
+  mk-section
+  (fn
+    (id children)
+    (st-iv-set!
+      (st-iv-set! (st-make-instance "CtSection") "id" id)
+      "children"
+      children)))
+
+(define
+  section?
+  (fn (b) (and (st-instance? b) (= (get b :class) "CtSection"))))
+
+(define section-children (fn (sec) (st-send sec "children" (list))))
+
+;; copy-on-write child edits (return a new section)
+(define
+  section-with-children
+  (fn (sec children) (st-iv-set! sec "children" children)))
+(define
+  section-append
+  (fn
+    (sec block)
+    (section-with-children sec (append (section-children sec) (list block)))))
+
+;; ── tree traversal (descends into nested sections) ──
+(define
+  block-deep-find
+  (fn
+    (blocks id)
+    (if
+      (= (len blocks) 0)
+      nil
+      (let
+        ((b (first blocks)))
+        (if
+          (= (blk-id b) id)
+          b
+          (let
+            ((nested (if (section? b) (block-deep-find (section-children b) id) nil)))
+            (if (= nested nil) (block-deep-find (rest blocks) id) nested)))))))
+
+(define doc-deep-find (fn (doc id) (block-deep-find (doc-blocks doc) id)))
+
+(define
+  block-tree-ids
+  (fn
+    (blocks)
+    (if
+      (= (len blocks) 0)
+      (list)
+      (let
+        ((b (first blocks)))
+        (append
+          (cons
+            (blk-id b)
+            (if (section? b) (block-tree-ids (section-children b)) (list)))
+          (block-tree-ids (rest blocks)))))))
+
+(define doc-tree-ids (fn (doc) (block-tree-ids (doc-blocks doc))))
+
+(define block-tree-count (fn (blocks) (len (block-tree-ids blocks))))
+(define doc-tree-count (fn (doc) (len (doc-tree-ids doc))))
diff --git a/lib/content/tests/section.sx b/lib/content/tests/section.sx
new file mode 100644
index 00000000..36c1c649
--- /dev/null
+++ b/lib/content/tests/section.sx
@@ -0,0 +1,99 @@
+;; Extension — nested block trees (CtSection container).
+
+(st-bootstrap-classes!)
+(content/bootstrap!)
+(content-bootstrap-markdown!)
+(content-bootstrap-text!)
+(content-bootstrap-section!)
+
+(define nl (str "\n"))
+
+;; ── a section is a block ──
+(define
+  sec
+  (mk-section
+    "s"
+    (list (mk-heading "h" 2 "Hi") (mk-text "p" "Body"))))
+(content-test "section is block" (block? sec) true)
+(content-test "section? yes" (section? sec) true)
+(content-test "section? no on text" (section? (mk-text "x" "y")) false)
+(content-test "section type" (blk-type sec) "section")
+(content-test "section id" (blk-id sec) "s")
+(content-test
+  "section children count"
+  (len (section-children sec))
+  2)
+
+;; ── recursive render ──
+(content-test
+  "section html"
+  (asHTML sec)
+  "<section><h2>Hi</h2><p>Body</p></section>")
+(content-test "section sx" (asSx sec) "(section (h2 \"Hi\")(p \"Body\"))")
+(content-test "section text" (asText sec) "Hi Body")
+(content-test
+  "empty section html"
+  (asHTML (mk-section "e" (list)))
+  "<section></section>")
+
+;; ── nested in a document ──
+(define
+  d
+  (doc-append
+    (doc-append (doc-empty "d") (mk-heading "top" 1 "Top"))
+    sec))
+(content-test
+  "doc with section html"
+  (asHTML d)
+  "<h1>Top</h1><section><h2>Hi</h2><p>Body</p></section>")
+(content-test "doc top-level ids" (doc-ids d) (list "top" "s"))
+
+;; ── arbitrary depth ──
+(define
+  deep
+  (mk-section
+    "outer"
+    (list
+      (mk-text "a" "A")
+      (mk-section
+        "inner"
+        (list (mk-text "b" "B") (mk-heading "c" 3 "C"))))))
+(content-test
+  "deep html"
+  (asHTML deep)
+  "<section><p>A</p><section><p>B</p><h3>C</h3></section></section>")
+(content-test "deep text" (asText deep) "A B C")
+
+;; ── tree traversal descends into sections ──
+(define dd (doc-append (doc-empty "d") deep))
+(content-test "deep-find nested" (blk-id (doc-deep-find dd "b")) "b")
+(content-test
+  "deep-find deeper"
+  (str (blk-send (doc-deep-find dd "c") "text"))
+  "C")
+(content-test "deep-find missing" (doc-deep-find dd "zzz") nil)
+(content-test
+  "deep-find top-level"
+  (blk-id (doc-deep-find dd "outer"))
+  "outer")
+(content-test
+  "tree-ids flattened"
+  (doc-tree-ids dd)
+  (list "outer" "a" "inner" "b" "c"))
+(content-test "tree-count" (doc-tree-count dd) 5)
+(content-test "top-level ids still flat" (doc-ids dd) (list "outer"))
+
+;; ── copy-on-write child edits ──
+(define sec2 (section-append sec (mk-divider "dv")))
+(content-test "section-append" (len (section-children sec2)) 3)
+(content-test
+  "section-append immutable"
+  (len (section-children sec))
+  2)
+(content-test
+  "section-append renders"
+  (asHTML sec2)
+  "<section><h2>Hi</h2><p>Body</p><hr></section>")
+
+;; ── markdown of a section (children joined by blank line) ──
+(content-test "section markdown" (asMarkdown sec) (str "## Hi" nl nl "Body"))
diff --git a/plans/content-on-sx.md b/plans/content-on-sx.md
index b72c083b..d25236d7 100644
--- a/plans/content-on-sx.md
+++ b/plans/content-on-sx.md
@@ -19,7 +19,7 @@ injected adapter, not core.
 
 ## Status (rolling)
 
-`bash lib/content/conformance.sh` → **385/385** (Phases 1–4 COMPLETE + 9 extensions: HTML/SX escaping, Markdown render+import, CRDT replication, validation, snapshot cache, doc metadata, plain-text render)
+`bash lib/content/conformance.sh` → **410/410** (Phases 1–4 COMPLETE + 10 extensions: HTML/SX escaping, Markdown render+import, CRDT replication, validation, snapshot cache, doc metadata, plain-text render, nested block trees)
 
 ## Ground rules
 
@@ -85,9 +85,18 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 - [x] snapshot cache over replay (`snapshot.sx`: cache-not-primary, transparent)
 - [x] document metadata (`meta.sx`: title/slug/tags + Ghost title plumbing)
 - [x] plain-text render + excerpt (`text.sx`: asText, content/excerpt)
+- [x] nested block trees (`section.sx`: CtSection container, recursive render, deep-find)
 
 ## Progress log
 
+- 2026-06-07 — Extension: nested block trees (`section.sx`). `CtSection` is a
+  block whose `children` ivar is a list of blocks (incl. nested sections →
+  arbitrary depth), turning the flat document into the ordered TREE from the
+  architecture sketch. Self-contained: it answers asHTML/asSx/asText/asMarkdown:
+  by folding children's renderings (pure polymorphic recursion — no changes to
+  block.sx/render.sx). `mk-section`, `section-children`, `section-append` (cow),
+  and tree traversal `doc-deep-find` / `doc-tree-ids` / `doc-tree-count` that
+  descend into sections. 25 tests; suite 410/410.
 - 2026-06-07 — Extension: plain-text render + excerpts (`text.sx`). Fourth
   boundary format via polymorphic `asText` (heading/text/code/quote→text,
   image→alt, embed/divider→"", list→", "-joined); the document joins non-empty