diff --git a/lib/content/scoreboard.json b/lib/content/scoreboard.json
index d59732a3..12236831 100644
--- a/lib/content/scoreboard.json
+++ b/lib/content/scoreboard.json
@@ -8,7 +8,7 @@
     "markdown": {"pass": 20, "fail": 0},
     "text": {"pass": 20, "fail": 0},
     "section": {"pass": 25, "fail": 0},
-    "validate": {"pass": 17, "fail": 0},
+    "validate": {"pass": 23, "fail": 0},
     "store": {"pass": 29, "fail": 0},
     "snapshot": {"pass": 20, "fail": 0},
     "crdt": {"pass": 34, "fail": 0},
@@ -17,7 +17,7 @@
     "md-import": {"pass": 24, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 410,
+  "total_pass": 416,
   "total_fail": 0,
-  "total": 410
+  "total": 416
 }
diff --git a/lib/content/scoreboard.md b/lib/content/scoreboard.md
index 9b570f2b..d311d047 100644
--- a/lib/content/scoreboard.md
+++ b/lib/content/scoreboard.md
@@ -12,7 +12,7 @@ _Generated by `lib/content/conformance.sh`_
 | markdown | 20 | 0 | 20 |
 | text | 20 | 0 | 20 |
 | section | 25 | 0 | 25 |
-| validate | 17 | 0 | 17 |
+| validate | 23 | 0 | 23 |
 | store | 29 | 0 | 29 |
 | snapshot | 20 | 0 | 20 |
 | crdt | 34 | 0 | 34 |
@@ -20,4 +20,4 @@ _Generated by `lib/content/conformance.sh`_
 | sync | 14 | 0 | 14 |
 | md-import | 24 | 0 | 24 |
 | fed | 20 | 0 | 20 |
-| **Total** | **410** | **0** | **410** |
+| **Total** | **416** | **0** | **416** |
diff --git a/lib/content/tests/validate.sx b/lib/content/tests/validate.sx
index 0cd113be..d01f935c 100644
--- a/lib/content/tests/validate.sx
+++ b/lib/content/tests/validate.sx
@@ -1,8 +1,10 @@
-;; Extension — document integrity validation.
+;; Extension — document integrity validation (tree-aware: descends into sections).
+;; (Conformance loads section.sx before this suite.)
 
 (st-bootstrap-classes!)
 (content-bootstrap-blocks!)
 (content-bootstrap-doc!)
+(content-bootstrap-section!)
 
 ;; ── a fully valid document ──
 (define
@@ -114,3 +116,51 @@
       (mk-heading "hh" 2 "H"))
     (mk-text "tt" "T")))
 (content-test "all well-formed types valid" (content/valid? allgood) true)
+
+;; ── tree-aware: descends into sections ──
+(define
+  nested
+  (doc-append
+    (doc-empty "d")
+    (mk-section
+      "s"
+      (list (mk-heading "nh" 1 "H") (mk-text "np" "ok")))))
+(content-test "valid nested section" (content/valid? nested) true)
+
+(define
+  nested-bad
+  (doc-append
+    (doc-empty "d")
+    (mk-section "s" (list (mk-heading "nh" "notnum" "H")))))
+(content-test
+  "nested bad field detected"
+  (content/issue-kinds nested-bad)
+  (list "field"))
+
+;; valid section block itself
+(content-test
+  "section valid"
+  (content/valid? (doc-append (doc-empty "d") (mk-section "s" (list))))
+  true)
+(content-test
+  "section bad children"
+  (content/issue-kinds
+    (doc-append
+      (doc-empty "d")
+      (st-iv-set! (mk-section "s" (list)) "children" "nope")))
+  (list "field"))
+
+;; duplicate id across a section boundary (top-level id == nested id)
+(define
+  dup-tree
+  (doc-append
+    (doc-append (doc-empty "d") (mk-text "x" "top"))
+    (mk-section "s" (list (mk-text "x" "nested")))))
+(content-test
+  "tree-wide duplicate detected"
+  (len
+    (filter
+      (fn (i) (= (get i :kind) "duplicate"))
+      (content/validate dup-tree)))
+  1)
+(content-test "tree dup not valid" (content/valid? dup-tree) false)
diff --git a/lib/content/validate.sx b/lib/content/validate.sx
index 3a37cefc..537053b6 100644
--- a/lib/content/validate.sx
+++ b/lib/content/validate.sx
@@ -1,9 +1,10 @@
 ;; content-on-sx — document integrity validation.
 ;;
-;; Guards imports, edits and federated input: checks each block's id and the
-;; required fields/types for its kind, plus document-level duplicate ids. Returns
-;; a list of issue dicts {:id :kind :detail}; an empty list means valid. Dispatch
-;; on block type is a validation-boundary concern, not core behaviour.
+;; Guards imports, edits and federated input: walks the whole block TREE (into
+;; nested sections) checking each block's id and required fields/types, plus
+;; tree-wide duplicate ids. Returns issue dicts {:id :kind :detail}; empty = ok.
+;; Tree detection is inline (class + st-iv-get) so this file needs no section.sx.
+;; Dispatch on block type is a validation-boundary concern, not core behaviour.
 ;;
 ;; Requires (loaded by harness): block.sx, doc.sx.
 
@@ -35,6 +36,30 @@
 
 (define ct-uniq (fn (xs) (ct-uniq-loop xs (list))))
 
+;; ── tree flatten (descends into CtSection children; guards malformed children) ──
+(define
+  ct-section-block?
+  (fn (b) (and (st-instance? b) (= (get b :class) "CtSection"))))
+(define
+  ct-tree-blocks
+  (fn
+    (blocks)
+    (if
+      (= (len blocks) 0)
+      (list)
+      (let
+        ((b (first blocks)))
+        (append
+          (cons
+            b
+            (if
+              (ct-section-block? b)
+              (let
+                ((ch (st-iv-get b "children")))
+                (if (list? ch) (ct-tree-blocks ch) (list)))
+              (list)))
+          (ct-tree-blocks (rest blocks)))))))
+
 ;; ── id checks ──
 (define
   content/-id-issues
@@ -120,32 +145,36 @@
               id
               (list? (blk-get b "items"))
               "list items must be a list")))
+        ((= t "section")
+          (ct-field-issue
+            id
+            (list? (blk-get b "children"))
+            "section children must be a list"))
         (else (list (ct-issue id "type" (str "unknown block type: " t))))))))
 
 (define
   content/-block-issues
   (fn (b) (append (content/-id-issues b) (content/-field-issues b))))
 
-;; ── document-level: duplicate ids ──
+;; ── duplicate ids across the whole tree ──
 (define
   content/-dup-issues
   (fn
-    (doc)
-    (let
-      ((ids (doc-ids doc)))
-      (map
-        (fn (id) (ct-issue id "duplicate" (str "duplicate block id: " id)))
-        (ct-uniq
-          (filter (fn (id) (> (ct-count-in id ids) 1)) ids))))))
+    (ids)
+    (map
+      (fn (id) (ct-issue id "duplicate" (str "duplicate block id: " id)))
+      (ct-uniq (filter (fn (id) (> (ct-count-in id ids) 1)) ids)))))
 
 ;; ── public ──
 (define
   content/validate
   (fn
     (doc)
-    (append
-      (content/-dup-issues doc)
-      (ct-flatmap content/-block-issues (doc-blocks doc)))))
+    (let
+      ((all (ct-tree-blocks (doc-blocks doc))))
+      (append
+        (content/-dup-issues (map (fn (b) (blk-id b)) all))
+        (ct-flatmap content/-block-issues all)))))
 
 (define
   content/valid?
diff --git a/plans/content-on-sx.md b/plans/content-on-sx.md
index d25236d7..4e89ba25 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` → **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)
+`bash lib/content/conformance.sh` → **416/416** (Phases 1–4 COMPLETE + 10 extensions: HTML/SX escaping, Markdown render+import, CRDT replication, tree-aware validation, snapshot cache, doc metadata, plain-text render, nested block trees)
 
 ## Ground rules
 
@@ -80,7 +80,7 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 - [x] asSx wire string-escaping (`String>>sxEscaped`: \ and " in SX literals)
 - [x] Markdown render mode (`asMarkdown:` / `content/render doc "md"`)
 - [x] durable CRDT replication (`crdt-store.sx`: ops on persist, replay + converge)
-- [x] document validation (`validate.sx`: ids, per-type fields, duplicate ids)
+- [x] document validation (`validate.sx`: ids, per-type fields, duplicate ids; tree-aware — descends into sections, tree-wide dup ids, section field check)
 - [x] Markdown import adapter (`md-import.sx`: text → blocks, round-trips export)
 - [x] snapshot cache over replay (`snapshot.sx`: cache-not-primary, transparent)
 - [x] document metadata (`meta.sx`: title/slug/tags + Ghost title plumbing)
@@ -89,6 +89,12 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 
 ## Progress log
 
+- 2026-06-07 — Refinement: tree-aware validation. `validate.sx` now flattens the
+  whole block tree (descending into `CtSection` children, guarding malformed
+  non-list children) so field checks and duplicate-id detection cover nested
+  blocks and span section boundaries; added a `section` field-type case. Inline
+  tree detection (class + st-iv-get) keeps it free of a section.sx dependency.
+  +6 tests; suite 416/416.
 - 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