diff --git a/lib/content/conformance.sh b/lib/content/conformance.sh
index 4c4ab5b5..11a0a257 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 stats table 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 stats table data wire validate store snapshot crdt crdt-store sync md-import md-doc fed)
 
 OUT_JSON="lib/content/scoreboard.json"
 OUT_MD="lib/content/scoreboard.md"
@@ -46,6 +46,7 @@ run_suite() {
 (load "lib/content/text.sx")
 (load "lib/content/section.sx")
 (load "lib/content/compose.sx")
+(load "lib/content/tree-edit.sx")
 (load "lib/content/stats.sx")
 (load "lib/content/table.sx")
 (load "lib/content/data.sx")
diff --git a/lib/content/scoreboard.json b/lib/content/scoreboard.json
index 6a90f654..1da17166 100644
--- a/lib/content/scoreboard.json
+++ b/lib/content/scoreboard.json
@@ -11,6 +11,7 @@
     "text": {"pass": 20, "fail": 0},
     "section": {"pass": 25, "fail": 0},
     "compose": {"pass": 17, "fail": 0},
+    "tree-edit": {"pass": 17, "fail": 0},
     "stats": {"pass": 17, "fail": 0},
     "table": {"pass": 15, "fail": 0},
     "data": {"pass": 21, "fail": 0},
@@ -25,7 +26,7 @@
     "md-doc": {"pass": 12, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 534,
+  "total_pass": 551,
   "total_fail": 0,
-  "total": 534
+  "total": 551
 }
diff --git a/lib/content/scoreboard.md b/lib/content/scoreboard.md
index 9497efba..1eefa8f5 100644
--- a/lib/content/scoreboard.md
+++ b/lib/content/scoreboard.md
@@ -15,6 +15,7 @@ _Generated by `lib/content/conformance.sh`_
 | text | 20 | 0 | 20 |
 | section | 25 | 0 | 25 |
 | compose | 17 | 0 | 17 |
+| tree-edit | 17 | 0 | 17 |
 | stats | 17 | 0 | 17 |
 | table | 15 | 0 | 15 |
 | data | 21 | 0 | 21 |
@@ -28,4 +29,4 @@ _Generated by `lib/content/conformance.sh`_
 | md-import | 38 | 0 | 38 |
 | md-doc | 12 | 0 | 12 |
 | fed | 20 | 0 | 20 |
-| **Total** | **534** | **0** | **534** |
+| **Total** | **551** | **0** | **551** |
diff --git a/lib/content/tests/tree-edit.sx b/lib/content/tests/tree-edit.sx
new file mode 100644
index 00000000..43c41911
--- /dev/null
+++ b/lib/content/tests/tree-edit.sx
@@ -0,0 +1,91 @@
+;; Extension — deep tree editing (update/delete/insert into nested sections).
+
+(st-bootstrap-classes!)
+(content/bootstrap!)
+(content-bootstrap-section!)
+
+;; doc:  top  /  sec[ a, inner[ b ] ]
+(define
+  d
+  (doc-append
+    (doc-append (doc-empty "d") (mk-text "top" "T"))
+    (mk-section
+      "sec"
+      (list
+        (mk-text "a" "A")
+        (mk-section "inner" (list (mk-text "b" "B")))))))
+
+;; ── deep-update a nested block ──
+(define d1 (doc-deep-update d "b" "text" "Edited"))
+(content-test
+  "deep-update nested"
+  (str (blk-send (doc-deep-find d1 "b") "text"))
+  "Edited")
+(content-test
+  "deep-update immutable"
+  (str (blk-send (doc-deep-find d "b") "text"))
+  "B")
+(content-test
+  "deep-update top-level"
+  (str
+    (blk-send
+      (doc-deep-find (doc-deep-update d "top" "text" "X") "top")
+      "text"))
+  "X")
+(content-test
+  "deep-update mid-section"
+  (str
+    (blk-send (doc-deep-find (doc-deep-update d "a" "text" "AA") "a") "text"))
+  "AA")
+(content-test
+  "deep-update preserves tree"
+  (doc-tree-ids d1)
+  (doc-tree-ids d))
+
+;; ── deep-replace ──
+(define d2 (doc-deep-replace d "b" (mk-heading "b" 3 "H")))
+(content-test
+  "deep-replace type"
+  (blk-type (doc-deep-find d2 "b"))
+  "heading")
+(content-test
+  "deep-replace render"
+  (asHTML d2)
+  "<p>T</p><section><p>A</p><section><h3>H</h3></section></section>")
+
+;; ── deep-delete ──
+(define d3 (doc-deep-delete d "b"))
+(content-test "deep-delete removes nested" (doc-deep-find d3 "b") nil)
+(content-test
+  "deep-delete tree-ids"
+  (doc-tree-ids d3)
+  (list "top" "sec" "a" "inner"))
+(content-test "deep-delete immutable" (doc-tree-count d) 5)
+(content-test
+  "deep-delete mid-section"
+  (doc-tree-ids (doc-deep-delete d "a"))
+  (list "top" "sec" "inner" "b"))
+(content-test
+  "deep-delete top-level"
+  (doc-tree-ids (doc-deep-delete d "top"))
+  (list "sec" "a" "inner" "b"))
+
+;; ── deep-insert-into a nested section ──
+(define d4 (doc-deep-insert-into d "inner" (mk-text "c" "C")))
+(content-test
+  "insert-into nested"
+  (doc-tree-ids d4)
+  (list "top" "sec" "a" "inner" "b" "c"))
+(content-test
+  "insert-into found"
+  (str (blk-send (doc-deep-find d4 "c") "text"))
+  "C")
+(content-test
+  "insert-into outer section"
+  (doc-tree-ids (doc-deep-insert-into d "sec" (mk-divider "dv")))
+  (list "top" "sec" "a" "inner" "b" "dv"))
+(content-test "insert-into immutable" (doc-tree-count d) 5)
+(content-test
+  "insert-into render"
+  (asHTML d4)
+  "<p>T</p><section><p>A</p><section><p>B</p><p>C</p></section></section>")
diff --git a/lib/content/tree-edit.sx b/lib/content/tree-edit.sx
new file mode 100644
index 00000000..7f424862
--- /dev/null
+++ b/lib/content/tree-edit.sx
@@ -0,0 +1,96 @@
+;; content-on-sx — deep tree editing.
+;;
+;; Mutate blocks anywhere in the nested tree (descending into CtSection children),
+;; complementing the top-level doc ops and the deep-find read path. All return
+;; new documents (immutable).
+;;
+;; Requires (loaded by harness): doc.sx, section.sx (section? / section-children /
+;; section-with-children / section-append).
+
+;; map f over every block in the tree, replacing the one whose id matches.
+(define
+  block-tree-update
+  (fn
+    (blocks id f)
+    (map
+      (fn
+        (b)
+        (if
+          (= (blk-id b) id)
+          (f b)
+          (if
+            (section? b)
+            (section-with-children
+              b
+              (block-tree-update (section-children b) id f))
+            b)))
+      blocks)))
+
+;; remove the block with id from anywhere in the tree.
+(define
+  block-tree-delete
+  (fn
+    (blocks id)
+    (map
+      (fn
+        (b)
+        (if
+          (section? b)
+          (section-with-children
+            b
+            (block-tree-delete (section-children b) id))
+          b))
+      (filter (fn (b) (if (= (blk-id b) id) false true)) blocks))))
+
+;; append a block into the children of the section with section-id.
+(define
+  block-tree-insert-into
+  (fn
+    (blocks section-id block)
+    (map
+      (fn
+        (b)
+        (if
+          (section? b)
+          (if
+            (= (blk-id b) section-id)
+            (section-append b block)
+            (section-with-children
+              b
+              (block-tree-insert-into (section-children b) section-id block)))
+          b))
+      blocks)))
+
+;; ── document-level deep ops ──
+(define
+  doc-deep-update
+  (fn
+    (doc id field value)
+    (doc-with-blocks
+      doc
+      (block-tree-update
+        (doc-blocks doc)
+        id
+        (fn (b) (blk-set b field value))))))
+
+(define
+  doc-deep-replace
+  (fn
+    (doc id newblock)
+    (doc-with-blocks
+      doc
+      (block-tree-update (doc-blocks doc) id (fn (b) newblock)))))
+
+(define
+  doc-deep-delete
+  (fn
+    (doc id)
+    (doc-with-blocks doc (block-tree-delete (doc-blocks doc) id))))
+
+(define
+  doc-deep-insert-into
+  (fn
+    (doc section-id block)
+    (doc-with-blocks
+      doc
+      (block-tree-insert-into (doc-blocks doc) section-id block))))
diff --git a/plans/content-on-sx.md b/plans/content-on-sx.md
index b11f284b..4dbca23f 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` → **534/534** (Phases 1–4 COMPLETE + 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, doc stats, table block, HTML page wrapper + SEO page, doc composition, portable data + wire serialization)
+`bash lib/content/conformance.sh` → **551/551** (Phases 1–4 COMPLETE + 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 tree editing, doc stats, table block, HTML page wrapper + SEO page, doc composition, portable data + wire serialization)
 
 ## Ground rules
 
@@ -92,11 +92,17 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 - [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)
+- [x] deep tree editing (`tree-edit.sx`: doc-deep-update/replace/delete/insert-into)
 - [x] portable data serialization (`data.sx`: content/to-data + from-data, round-trips tree)
 - [x] wire serialization (`wire.sx`: content/to-wire + from-wire, SX-text on the wire)
 
 ## Progress log
 
+- 2026-06-07 — Extension: deep tree editing (`tree-edit.sx`). `doc-deep-update`
+  / `doc-deep-replace` / `doc-deep-delete` / `doc-deep-insert-into` mutate blocks
+  anywhere in the nested tree (descending into CtSection children), completing
+  tree mutation to match the deep-find read path; all immutable. 17 tests; suite
+  551/551.
 - 2026-06-07 — Extension: on-the-wire serialization (`wire.sx`).
   `content/to-wire` serialises a document to a transmittable SX-text string (data
   form + SX serializer); `content/from-wire` parses it back into a live document.