diff --git a/lib/content/conformance.sh b/lib/content/conformance.sh
index 4901d664..52eae428 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 page page-full markdown text section compose tree-edit move clone query toc anchor outline flatten transform normalize find-replace stats summary index table callout media data wire validate store snapshot crdt crdt-store sync md-import md-doc fed)
+SUITES=(block doc render api meta page page-full markdown text section compose tree-edit move clone query toc anchor outline flatten transform normalize find-replace stats summary index table callout media data wire validate store snapshot crdt crdt-tree crdt-store sync md-import md-doc fed)
 
 OUT_JSON="lib/content/scoreboard.json"
 OUT_MD="lib/content/scoreboard.md"
@@ -72,6 +72,7 @@ run_suite() {
 (load "lib/content/store.sx")
 (load "lib/content/snapshot.sx")
 (load "lib/content/crdt.sx")
+(load "lib/content/crdt-tree.sx")
 (load "lib/content/crdt-store.sx")
 (load "lib/content/sync.sx")
 (load "lib/content/md-import.sx")
diff --git a/lib/content/crdt-tree.sx b/lib/content/crdt-tree.sx
new file mode 100644
index 00000000..9ba58d49
--- /dev/null
+++ b/lib/content/crdt-tree.sx
@@ -0,0 +1,164 @@
+;; content-on-sx — nested-tree CvRDT.
+;;
+;; Extends the flat CvRDT (crdt.sx) to a TREE: each element carries a `parent`
+;; (the id of its containing section, "" = root) alongside its Logoot position.
+;; Merge is still a join — it reuses crdt.sx's position/register/field merges and
+;; adds parent (immutable, set once at insert). Materialisation rebuilds the
+;; ordered tree: root = elements with parent "", a section's children = elements
+;; whose parent is that section's id, each sorted by position. Commutative,
+;; associative, idempotent like the flat layer; concurrent inserts into the same
+;; or different parents converge deterministically.
+;;
+;; Requires (loaded by harness): crdt.sx (merge helpers + live/sort/materialise
+;; bits), block.sx, doc.sx, section.sx (mk-section).
+
+(define ctt-merge-parent (fn (p1 p2) (if (= p1 nil) p2 p1)))
+
+(define ctt-merge-element (fn (e1 e2) {:fields (crdt-merge-fields (get e1 :fields) (get e2 :fields)) :parent (ctt-merge-parent (get e1 :parent) (get e2 :parent)) :id (get e1 :id) :type (crdt-merge-type (get e1 :type) (get e2 :type)) :deleted (or (= (get e1 :deleted) true) (= (get e2 :deleted) true)) :pos (crdt-merge-pos (get e1 :pos) (get e2 :pos))}))
+
+(define
+  ctt-add-element
+  (fn
+    (state elem)
+    (let
+      ((elems (get state :elements)) (id (get elem :id)))
+      (let
+        ((existing (get elems id)))
+        (assoc
+          state
+          :elements (assoc
+            elems
+            id
+            (if (= existing nil) elem (ctt-merge-element existing elem))))))))
+
+;; ── ops as partial-element contributions ──
+(define
+  crdt-tree-insert
+  (fn
+    (state id type pos parent fields ts actor)
+    (ctt-add-element state {:fields (crdt-build-fields fields ts actor) :parent parent :id id :type type :deleted false :pos pos})))
+
+(define
+  crdt-tree-update
+  (fn (state id fname value ts actor) (ctt-add-element state {:fields (assoc {} fname {:ts ts :actor actor :value value}) :parent nil :id id :type nil :deleted false :pos nil})))
+
+(define crdt-tree-delete (fn (state id) (ctt-add-element state {:fields {} :parent nil :id id :type nil :deleted true :pos nil})))
+
+;; ── state merge (join) ──
+(define
+  ctt-merge-loop
+  (fn
+    (ids ea eb acc)
+    (if
+      (= (len ids) 0)
+      acc
+      (let
+        ((id (first ids)))
+        (let
+          ((x (get ea id)) (y (get eb id)))
+          (ctt-merge-loop
+            (rest ids)
+            ea
+            eb
+            (assoc
+              acc
+              id
+              (cond
+                ((= x nil) y)
+                ((= y nil) x)
+                (else (ctt-merge-element x y))))))))))
+
+(define crdt-tree-merge (fn (a b) {:elements (ctt-merge-loop (crdt-union-keys (get a :elements) (get b :elements)) (get a :elements) (get b :elements) {})}))
+
+(define
+  crdt-tree-merge-all
+  (fn
+    (states)
+    (if
+      (= (len states) 0)
+      (crdt-empty)
+      (if
+        (= (len states) 1)
+        (first states)
+        (crdt-tree-merge (first states) (crdt-tree-merge-all (rest states)))))))
+
+;; ── op interpreter ──
+(define
+  crdt-tree-op-insert
+  (fn (id type pos parent fields ts actor) {:ts ts :fields fields :parent parent :id id :type type :op "insert" :actor actor :pos pos}))
+
+(define crdt-tree-op-update (fn (id field value ts actor) {:ts ts :field field :id id :op "update" :actor actor :value value}))
+
+(define crdt-tree-op-delete (fn (id) {:id id :op "delete"}))
+
+(define
+  crdt-tree-apply
+  (fn
+    (state op)
+    (let
+      ((k (get op :op)))
+      (cond
+        ((= k "insert")
+          (crdt-tree-insert
+            state
+            (get op :id)
+            (get op :type)
+            (get op :pos)
+            (get op :parent)
+            (get op :fields)
+            (get op :ts)
+            (get op :actor)))
+        ((= k "update")
+          (crdt-tree-update
+            state
+            (get op :id)
+            (get op :field)
+            (get op :value)
+            (get op :ts)
+            (get op :actor)))
+        ((= k "delete") (crdt-tree-delete state (get op :id)))
+        (else (error (str "unknown crdt-tree op: " k)))))))
+
+(define
+  crdt-tree-apply-all
+  (fn
+    (state ops)
+    (if
+      (= (len ops) 0)
+      state
+      (crdt-tree-apply-all (crdt-tree-apply state (first ops)) (rest ops)))))
+
+;; ── materialise to a Phase-1 document (rebuild the ordered tree) ──
+(define
+  ctt-children
+  (fn
+    (state parent-id)
+    (crdt-sort-by-pos
+      (filter
+        (fn (e) (= (get e :parent) parent-id))
+        (crdt-live-elements state)))))
+
+(define
+  ctt-element->block
+  (fn
+    (state e)
+    (if
+      (= (get e :type) "section")
+      (mk-section
+        (get e :id)
+        (map
+          (fn (c) (ctt-element->block state c))
+          (ctt-children state (get e :id))))
+      (crdt-element->block e))))
+
+(define
+  crdt-tree-materialize
+  (fn
+    (doc-id state)
+    (doc-new
+      doc-id
+      (map (fn (e) (ctt-element->block state e)) (ctt-children state "")))))
+
+(define
+  crdt-tree-order
+  (fn (state) (map (fn (e) (get e :id)) (ctt-children state ""))))
diff --git a/lib/content/scoreboard.json b/lib/content/scoreboard.json
index e5f2d14d..648f3581 100644
--- a/lib/content/scoreboard.json
+++ b/lib/content/scoreboard.json
@@ -34,13 +34,14 @@
     "store": {"pass": 29, "fail": 0},
     "snapshot": {"pass": 20, "fail": 0},
     "crdt": {"pass": 34, "fail": 0},
+    "crdt-tree": {"pass": 17, "fail": 0},
     "crdt-store": {"pass": 14, "fail": 0},
     "sync": {"pass": 14, "fail": 0},
     "md-import": {"pass": 38, "fail": 0},
     "md-doc": {"pass": 12, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 710,
+  "total_pass": 727,
   "total_fail": 0,
-  "total": 710
+  "total": 727
 }
diff --git a/lib/content/scoreboard.md b/lib/content/scoreboard.md
index 5ae81e72..885ba020 100644
--- a/lib/content/scoreboard.md
+++ b/lib/content/scoreboard.md
@@ -38,9 +38,10 @@ _Generated by `lib/content/conformance.sh`_
 | store | 29 | 0 | 29 |
 | snapshot | 20 | 0 | 20 |
 | crdt | 34 | 0 | 34 |
+| crdt-tree | 17 | 0 | 17 |
 | crdt-store | 14 | 0 | 14 |
 | sync | 14 | 0 | 14 |
 | md-import | 38 | 0 | 38 |
 | md-doc | 12 | 0 | 12 |
 | fed | 20 | 0 | 20 |
-| **Total** | **710** | **0** | **710** |
+| **Total** | **727** | **0** | **727** |
diff --git a/lib/content/tests/crdt-tree.sx b/lib/content/tests/crdt-tree.sx
new file mode 100644
index 00000000..b70c6f95
--- /dev/null
+++ b/lib/content/tests/crdt-tree.sx
@@ -0,0 +1,253 @@
+;; Extension — nested-tree CvRDT. Sections nest and merge collaboratively;
+;; convergence is order/replica/duplicate-insensitive like the flat layer.
+
+(st-bootstrap-classes!)
+(content-bootstrap-blocks!)
+(content-bootstrap-doc!)
+(content-bootstrap-render!)
+(content-bootstrap-section!)
+
+(define same? (fn (a b) (= (get a :elements) (get b :elements))))
+
+;; base: a section "s" at root, with one child heading.
+(define
+  base
+  (crdt-tree-insert
+    (crdt-tree-insert
+      (crdt-empty)
+      "s"
+      "section"
+      (crdt-pos 1 0)
+      ""
+      (list)
+      1
+      0)
+    "h"
+    "heading"
+    (crdt-pos 1 0)
+    "s"
+    (list (list "level" 2) (list "text" "Sub"))
+    1
+    0))
+
+;; ── materialise rebuilds the tree ──
+(content-test "tree order root" (crdt-tree-order base) (list "s"))
+(content-test
+  "tree materialize ids"
+  (doc-tree-ids (crdt-tree-materialize "d" base))
+  (list "s" "h"))
+(content-test
+  "tree render"
+  (asHTML (crdt-tree-materialize "d" base))
+  "<section><h2>Sub</h2></section>")
+
+;; ── concurrent inserts into the SAME section converge + order by pos ──
+(define
+  rA
+  (crdt-tree-insert
+    base
+    "a"
+    "text"
+    (crdt-pos 5 1)
+    "s"
+    (list (list "text" "A"))
+    2
+    1))
+(define
+  rB
+  (crdt-tree-insert
+    base
+    "b"
+    "text"
+    (crdt-pos 5 2)
+    "s"
+    (list (list "text" "B"))
+    2
+    2))
+(content-test
+  "same-parent merge commutes"
+  (same? (crdt-tree-merge rA rB) (crdt-tree-merge rB rA))
+  true)
+(content-test
+  "same-parent order deterministic"
+  (doc-tree-ids (crdt-tree-materialize "d" (crdt-tree-merge rA rB)))
+  (list "s" "h" "a" "b"))
+
+;; ── concurrent inserts into DIFFERENT parents converge ──
+(define
+  base2
+  (crdt-tree-insert
+    (crdt-tree-insert
+      (crdt-empty)
+      "s1"
+      "section"
+      (crdt-pos 1 0)
+      ""
+      (list)
+      1
+      0)
+    "s2"
+    "section"
+    (crdt-pos 2 0)
+    ""
+    (list)
+    1
+    0))
+(define
+  x
+  (crdt-tree-insert
+    base2
+    "x"
+    "text"
+    (crdt-pos 1 0)
+    "s1"
+    (list (list "text" "X"))
+    2
+    1))
+(define
+  y
+  (crdt-tree-insert
+    base2
+    "y"
+    "text"
+    (crdt-pos 1 0)
+    "s2"
+    (list (list "text" "Y"))
+    2
+    2))
+(define m (crdt-tree-merge x y))
+(content-test
+  "different-parent commutes"
+  (same? m (crdt-tree-merge y x))
+  true)
+(content-test
+  "different-parent tree"
+  (doc-tree-ids (crdt-tree-materialize "d" m))
+  (list "s1" "x" "s2" "y"))
+(content-test
+  "different-parent render"
+  (asHTML (crdt-tree-materialize "d" m))
+  "<section><p>X</p></section><section><p>Y</p></section>")
+
+;; ── nested sections (section inside section) ──
+(define
+  nested
+  (crdt-tree-apply-all
+    (crdt-empty)
+    (list
+      (crdt-tree-op-insert
+        "outer"
+        "section"
+        (crdt-pos 1 0)
+        ""
+        (list)
+        1
+        0)
+      (crdt-tree-op-insert
+        "inner"
+        "section"
+        (crdt-pos 1 0)
+        "outer"
+        (list)
+        1
+        0)
+      (crdt-tree-op-insert
+        "leaf"
+        "text"
+        (crdt-pos 1 0)
+        "inner"
+        (list (list "text" "deep"))
+        1
+        0))))
+(content-test
+  "nested tree ids"
+  (doc-tree-ids (crdt-tree-materialize "d" nested))
+  (list "outer" "inner" "leaf"))
+(content-test
+  "nested render"
+  (asHTML (crdt-tree-materialize "d" nested))
+  "<section><section><p>deep</p></section></section>")
+
+;; ── ops in any order converge (commutative) ──
+(define
+  opA
+  (crdt-tree-op-insert
+    "p"
+    "text"
+    (crdt-pos 6 0)
+    "s"
+    (list (list "text" "P"))
+    3
+    1))
+(define opB (crdt-tree-op-update "h" "text" "Edited" 5 1))
+(define opC (crdt-tree-op-delete "h"))
+(content-test
+  "ops commute"
+  (same?
+    (crdt-tree-apply-all base (list opA opB opC))
+    (crdt-tree-apply-all base (list opC opB opA)))
+  true)
+(content-test
+  "ops idempotent"
+  (same?
+    (crdt-tree-apply-all base (list opA opB))
+    (crdt-tree-apply-all
+      (crdt-tree-apply-all base (list opA opB))
+      (list opA opB)))
+  true)
+
+;; ── update into a section + LWW ──
+(define u1 (crdt-tree-update base "h" "text" "v5" 5 1))
+(define u2 (crdt-tree-update base "h" "text" "v7" 7 2))
+(content-test
+  "tree LWW higher ts"
+  (str
+    (blk-send
+      (doc-deep-find (crdt-tree-materialize "d" (crdt-tree-merge u1 u2)) "h")
+      "text"))
+  "v7")
+
+;; ── delete inside a section ──
+(content-test
+  "delete in section"
+  (doc-tree-ids (crdt-tree-materialize "d" (crdt-tree-delete base "h")))
+  (list "s"))
+
+;; ── merge idempotence ──
+(content-test "merge idempotent self" (same? (crdt-tree-merge m m) m) true)
+
+;; ── full convergence: two replicas, divergent edits in different sections ──
+(define
+  repl1
+  (crdt-tree-apply-all
+    base2
+    (list
+      (crdt-tree-op-insert
+        "p1"
+        "text"
+        (crdt-pos 1 0)
+        "s1"
+        (list (list "text" "from1"))
+        5
+        1))))
+(define
+  repl2
+  (crdt-tree-apply-all
+    base2
+    (list
+      (crdt-tree-op-insert
+        "p2"
+        "text"
+        (crdt-pos 1 0)
+        "s2"
+        (list (list "text" "from2"))
+        6
+        2))))
+(content-test
+  "two-replica tree converges"
+  (same? (crdt-tree-merge repl1 repl2) (crdt-tree-merge repl2 repl1))
+  true)
+(content-test
+  "two-replica tree ids"
+  (doc-tree-ids (crdt-tree-materialize "d" (crdt-tree-merge repl1 repl2)))
+  (list "s1" "p1" "s2" "p2"))
diff --git a/plans/content-on-sx.md b/plans/content-on-sx.md
index c30c66af..588a3a24 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` → **710/710** (Phases 1–4 COMPLETE + ~33 extensions: HTML/SX escaping, Markdown render + import/export incl. tables & frontmatter (full round-trip), CRDT replication, tree-aware validation, snapshot cache, doc metadata, plain-text render, nested block trees + deep editing + flatten + relative reorder, doc stats + summary + multi-doc index, table + callout + media blocks, HTML page wrapper + SEO page, doc composition + id-remap, portable data + wire serialization, block query + transforms + find/replace, TOC + anchored headings + outline, normalization)
+`bash lib/content/conformance.sh` → **727/727** (Phases 1–4 COMPLETE + ~34 extensions: HTML/SX escaping, Markdown render + import/export incl. tables & frontmatter (full round-trip), CvRDT flat + nested-tree + durable replication, tree-aware validation, snapshot cache, doc metadata, plain-text render, nested block trees + deep editing + flatten + relative reorder, doc stats + summary + multi-doc index, table + callout + media blocks, HTML page wrapper + SEO page, doc composition + id-remap, portable data + wire serialization, block query + transforms + find/replace, TOC + anchored headings + outline, normalization)
 
 ## Ground rules
 
@@ -93,6 +93,7 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 - [x] media block (`media.sx`: CtMedia video/audio, renders html/sx/text/md, validated)
 - [x] list-card summary (`summary.sx`: content/summary — title/excerpt/words/reading/cover)
 - [x] multi-doc index (`index.sx`: content/index + index-by-tag + all-tags + has-tag?)
+- [x] nested-tree CvRDT (`crdt-tree.sx`: parent-aware, sections merge collaboratively)
 - [x] HTML page wrapper (`page.sx`: content/page, escaped title from metadata)
 - [x] SEO page (`page-full.sx`: content/page-full, lang + meta description from excerpt)
 - [x] document composition (`compose.sx`: concat/prepend/concat-all/wrap-section)
@@ -112,6 +113,15 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 
 ## Progress log
 
+- 2026-06-07 — Extension: nested-tree CvRDT (`crdt-tree.sx`). Extends the flat
+  CvRDT to a TREE: each element carries a `parent` (containing section id, "" =
+  root) beside its Logoot pos; merge reuses crdt.sx's pos/register/field joins +
+  parent (immutable). Materialisation rebuilds the ordered tree (root + per-section
+  children sorted by pos, recursive). Sections now merge collaboratively; proven
+  commutative/associative/idempotent — same- and different-parent concurrent
+  inserts converge, nested sections, LWW, two-replica convergence. Reuses crdt.sx
+  + section.sx; flat crdt untouched (34/34). 17 tests; suite 727/727. This was
+  the flagged "research-grade" gap — done as a clean self-contained layer.
 - 2026-06-07 — Extension: multi-document index (`index.sx`). `content/index`
   projects a doc list into summary cards (blog index); `content/index-by-tag`
   filters by tag (category pages); `content/all-tags` is a deduped tag cloud;