diff --git a/lib/content/conformance.sh b/lib/content/conformance.sh
index 44845292..4697c991 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 clone query 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 clone query transform 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"
@@ -49,6 +49,7 @@ run_suite() {
 (load "lib/content/tree-edit.sx")
 (load "lib/content/clone.sx")
 (load "lib/content/query.sx")
+(load "lib/content/transform.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 f21aeccc..9f9b976b 100644
--- a/lib/content/scoreboard.json
+++ b/lib/content/scoreboard.json
@@ -14,6 +14,7 @@
     "tree-edit": {"pass": 17, "fail": 0},
     "clone": {"pass": 10, "fail": 0},
     "query": {"pass": 13, "fail": 0},
+    "transform": {"pass": 12, "fail": 0},
     "stats": {"pass": 17, "fail": 0},
     "table": {"pass": 15, "fail": 0},
     "data": {"pass": 21, "fail": 0},
@@ -28,7 +29,7 @@
     "md-doc": {"pass": 12, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 574,
+  "total_pass": 586,
   "total_fail": 0,
-  "total": 574
+  "total": 586
 }
diff --git a/lib/content/scoreboard.md b/lib/content/scoreboard.md
index 1f2d0463..e1d94b7e 100644
--- a/lib/content/scoreboard.md
+++ b/lib/content/scoreboard.md
@@ -18,6 +18,7 @@ _Generated by `lib/content/conformance.sh`_
 | tree-edit | 17 | 0 | 17 |
 | clone | 10 | 0 | 10 |
 | query | 13 | 0 | 13 |
+| transform | 12 | 0 | 12 |
 | stats | 17 | 0 | 17 |
 | table | 15 | 0 | 15 |
 | data | 21 | 0 | 21 |
@@ -31,4 +32,4 @@ _Generated by `lib/content/conformance.sh`_
 | md-import | 38 | 0 | 38 |
 | md-doc | 12 | 0 | 12 |
 | fed | 20 | 0 | 20 |
-| **Total** | **574** | **0** | **574** |
+| **Total** | **586** | **0** | **586** |
diff --git a/lib/content/tests/transform.sx b/lib/content/tests/transform.sx
new file mode 100644
index 00000000..bf1a47b1
--- /dev/null
+++ b/lib/content/tests/transform.sx
@@ -0,0 +1,90 @@
+;; Extension — tree-wide block transforms.
+
+(st-bootstrap-classes!)
+(content/bootstrap!)
+(content-bootstrap-section!)
+
+(define
+  d
+  (doc-append
+    (doc-append (doc-empty "d") (mk-heading "h" 1 "Top"))
+    (mk-section
+      "s"
+      (list (mk-text "a" "A") (mk-heading "h2" 2 "Sub")))))
+
+;; ── map-type bumps heading levels everywhere ──
+(define
+  d1
+  (content/map-type
+    d
+    "heading"
+    (fn (b) (blk-set b "level" (+ (blk-get b "level") 1)))))
+(content-test
+  "map-type top heading"
+  (blk-send (doc-deep-find d1 "h") "level")
+  2)
+(content-test
+  "map-type nested heading"
+  (blk-send (doc-deep-find d1 "h2") "level")
+  3)
+(content-test
+  "map-type leaves text"
+  (str (blk-send (doc-deep-find d1 "a") "text"))
+  "A")
+(content-test
+  "map-type immutable"
+  (blk-send (doc-deep-find d "h") "level")
+  1)
+(content-test "map-type preserves tree" (doc-tree-ids d1) (doc-tree-ids d))
+
+;; ── set-field-on rewrites all text blocks ──
+(define d2 (content/set-field-on d "text" "text" "REDACTED"))
+(content-test
+  "set-field nested text"
+  (str (blk-send (doc-deep-find d2 "a") "text"))
+  "REDACTED")
+(content-test
+  "set-field count"
+  (len
+    (filter
+      (fn (b) (= (str (blk-get b "text")) "REDACTED"))
+      (list (doc-deep-find d2 "a"))))
+  1)
+
+;; ── map-blocks with custom predicate ──
+(define
+  d3
+  (content/map-blocks
+    d
+    (fn (b) (= (blk-id b) "h2"))
+    (fn (b) (blk-set b "text" "Changed"))))
+(content-test
+  "map-blocks predicate hit"
+  (str (blk-send (doc-deep-find d3 "h2") "text"))
+  "Changed")
+(content-test
+  "map-blocks predicate miss"
+  (str (blk-send (doc-deep-find d3 "h") "text"))
+  "Top")
+
+;; ── image src rewrite (cdn migration) ──
+(define di (doc-append (doc-empty "d") (mk-image "img" "/old.png" "x")))
+(content-test
+  "image src rewrite"
+  (str
+    (blk-send
+      (doc-find (content/set-field-on di "image" "src" "/cdn/new.png") "img")
+      "src"))
+  "/cdn/new.png")
+
+;; ── no matching blocks → unchanged ──
+(content-test
+  "no match unchanged"
+  (asHTML (content/map-type d "embed" (fn (b) b)))
+  (asHTML d))
+
+;; ── render after transform ──
+(content-test
+  "render after map-type"
+  (asHTML d1)
+  "<h2>Top</h2><section><p>A</p><h3>Sub</h3></section>")
diff --git a/lib/content/transform.sx b/lib/content/transform.sx
new file mode 100644
index 00000000..98ce5111
--- /dev/null
+++ b/lib/content/transform.sx
@@ -0,0 +1,52 @@
+;; content-on-sx — tree-wide block transforms.
+;;
+;; The write counterpart to query: apply a function to every matching block
+;; across the tree (descending into sections), returning a new document. For
+;; bulk edits — rewrite image srcs, bump heading levels, sanitise text. Tree
+;; detection/rebuild is inline (class + st-iv-get/set!) so this needs no
+;; section.sx. Immutable.
+;;
+;; Requires (loaded by harness): block.sx, doc.sx.
+
+(define
+  xf-section?
+  (fn (b) (and (st-instance? b) (= (get b :class) "CtSection"))))
+
+(define
+  block-tree-transform
+  (fn
+    (blocks pred f)
+    (map
+      (fn
+        (b)
+        (let
+          ((nb (if (pred b) (f b) b)))
+          (if
+            (xf-section? nb)
+            (let
+              ((ch (st-iv-get nb "children")))
+              (if
+                (list? ch)
+                (st-iv-set! nb "children" (block-tree-transform ch pred f))
+                nb))
+            nb)))
+      blocks)))
+
+(define
+  content/map-blocks
+  (fn
+    (doc pred f)
+    (doc-with-blocks doc (block-tree-transform (doc-blocks doc) pred f))))
+
+(define
+  content/map-type
+  (fn
+    (doc type f)
+    (content/map-blocks doc (fn (b) (= (blk-type b) type)) f)))
+
+;; convenience: set a field on every block of a type.
+(define
+  content/set-field-on
+  (fn
+    (doc type field value)
+    (content/map-type doc type (fn (b) (blk-set b field value)))))
diff --git a/plans/content-on-sx.md b/plans/content-on-sx.md
index 048bffbe..3fe45e26 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` → **574/574** (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 + id-remap, portable data + wire serialization, block query + TOC)
+`bash lib/content/conformance.sh` → **586/586** (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 + id-remap, portable data + wire serialization, block query + TOC + transforms)
 
 ## Ground rules
 
@@ -95,11 +95,18 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 - [x] deep tree editing (`tree-edit.sx`: doc-deep-update/replace/delete/insert-into)
 - [x] id remapping / clone (`clone.sx`: content/remap-ids + prefix-ids, collision-free compose)
 - [x] block query + TOC (`query.sx`: content/select/select-type/count-type/headings)
+- [x] block transforms (`transform.sx`: content/map-blocks/map-type/set-field-on)
 - [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: tree-wide block transforms (`transform.sx`). The write
+  counterpart to query: `content/map-blocks` (predicate) / `content/map-type` /
+  `content/set-field-on` apply a function to every matching block across the tree
+  (sections rebuilt), for bulk edits (cdn src rewrites, heading-level bumps, text
+  sanitisation). Inline tree rebuild (no section.sx dep); immutable. 12 tests;
+  suite 586/586.
 - 2026-06-07 — Extension: block query + TOC (`query.sx`). `content/select`
   (predicate) / `content/select-type` / `content/count-type` / `content/select-ids`
   collect blocks across the whole tree (sections recurse); `content/headings`