diff --git a/lib/content/conformance.sh b/lib/content/conformance.sh
index 1a7d54da..7158b62b 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 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 stats table data validate store snapshot crdt crdt-store sync md-import md-doc fed)
 
 OUT_JSON="lib/content/scoreboard.json"
 OUT_MD="lib/content/scoreboard.md"
@@ -48,6 +48,7 @@ run_suite() {
 (load "lib/content/compose.sx")
 (load "lib/content/stats.sx")
 (load "lib/content/table.sx")
+(load "lib/content/data.sx")
 (load "lib/content/page.sx")
 (load "lib/content/page-full.sx")
 (load "lib/content/markdown.sx")
diff --git a/lib/content/data.sx b/lib/content/data.sx
new file mode 100644
index 00000000..86009304
--- /dev/null
+++ b/lib/content/data.sx
@@ -0,0 +1,79 @@
+;; content-on-sx — portable data serialization.
+;;
+;; Converts documents to/from a plain SX data form, decoupling storage and
+;; transport from the Smalltalk instance shape. A document becomes
+;;   {:id :title :slug :tags :blocks (list block-data)}
+;; and a block becomes {:id :type :fields {...}} (section children recurse).
+;; content/from-data reconstructs real block objects.
+;;
+;; Requires (loaded by harness): block.sx, doc.sx, meta.sx, section.sx
+;; (mk-section), table.sx (mk-table).
+
+;; ── to-data ──
+(define
+  content/-fd-loop
+  (fn
+    (ks ivs acc)
+    (if
+      (= (len ks) 0)
+      acc
+      (let
+        ((k (first ks)))
+        (if
+          (= k "id")
+          (content/-fd-loop (rest ks) ivs acc)
+          (content/-fd-loop
+            (rest ks)
+            ivs
+            (assoc
+              acc
+              k
+              (if
+                (= k "children")
+                (map block->data (get ivs k))
+                (get ivs k)))))))))
+
+(define block->data (fn (b) {:fields (content/-fd-loop (keys (get b :ivars)) (get b :ivars) {}) :id (blk-id b) :type (blk-type b)}))
+
+(define content/to-data (fn (doc) {:blocks (map block->data (doc-blocks doc)) :slug (doc-slug doc) :id (doc-id doc) :title (doc-title doc) :tags (doc-tags doc)}))
+
+;; ── from-data ──
+(define
+  content/-field-pairs
+  (fn (fields) (map (fn (k) (list k (get fields k))) (keys fields))))
+
+(define
+  data->block
+  (fn
+    (d)
+    (let
+      ((type (get d :type)) (id (get d :id)) (fields (get d :fields)))
+      (cond
+        ((= type "section")
+          (mk-section id (map data->block (get fields "children"))))
+        ((= type "table")
+          (mk-table id (get fields "headers") (get fields "rows")))
+        (else (mk-block type id (content/-field-pairs fields)))))))
+
+(define
+  content/-meta-of
+  (fn
+    (data)
+    (let
+      ((m1 (if (= (get data :title) nil) {} (assoc {} :title (get data :title)))))
+      (let
+        ((m2 (if (= (get data :slug) nil) m1 (assoc m1 :slug (get data :slug)))))
+        (let
+          ((tags (get data :tags)))
+          (if
+            (or (= tags nil) (= (len tags) 0))
+            m2
+            (assoc m2 :tags tags)))))))
+
+(define
+  content/from-data
+  (fn
+    (data)
+    (doc-with-meta
+      (doc-new (get data :id) (map data->block (get data :blocks)))
+      (content/-meta-of data))))
diff --git a/lib/content/scoreboard.json b/lib/content/scoreboard.json
index 45b6981f..adee7c9d 100644
--- a/lib/content/scoreboard.json
+++ b/lib/content/scoreboard.json
@@ -13,6 +13,7 @@
     "compose": {"pass": 17, "fail": 0},
     "stats": {"pass": 17, "fail": 0},
     "table": {"pass": 15, "fail": 0},
+    "data": {"pass": 21, "fail": 0},
     "validate": {"pass": 23, "fail": 0},
     "store": {"pass": 29, "fail": 0},
     "snapshot": {"pass": 20, "fail": 0},
@@ -23,7 +24,7 @@
     "md-doc": {"pass": 12, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 502,
+  "total_pass": 523,
   "total_fail": 0,
-  "total": 502
+  "total": 523
 }
diff --git a/lib/content/scoreboard.md b/lib/content/scoreboard.md
index 1d2e416d..f144b600 100644
--- a/lib/content/scoreboard.md
+++ b/lib/content/scoreboard.md
@@ -17,6 +17,7 @@ _Generated by `lib/content/conformance.sh`_
 | compose | 17 | 0 | 17 |
 | stats | 17 | 0 | 17 |
 | table | 15 | 0 | 15 |
+| data | 21 | 0 | 21 |
 | validate | 23 | 0 | 23 |
 | store | 29 | 0 | 29 |
 | snapshot | 20 | 0 | 20 |
@@ -26,4 +27,4 @@ _Generated by `lib/content/conformance.sh`_
 | md-import | 38 | 0 | 38 |
 | md-doc | 12 | 0 | 12 |
 | fed | 20 | 0 | 20 |
-| **Total** | **502** | **0** | **502** |
+| **Total** | **523** | **0** | **523** |
diff --git a/lib/content/tests/data.sx b/lib/content/tests/data.sx
new file mode 100644
index 00000000..33eca9e5
--- /dev/null
+++ b/lib/content/tests/data.sx
@@ -0,0 +1,93 @@
+;; Extension — portable data serialization (to-data / from-data round-trip).
+
+(st-bootstrap-classes!)
+(content/bootstrap!)
+(content-bootstrap-text!)
+(content-bootstrap-markdown!)
+(content-bootstrap-section!)
+(content-bootstrap-table!)
+
+;; ── block->data shape ──
+(define h (mk-heading "h" 2 "Hi"))
+(content-test "block->data id" (get (block->data h) :id) "h")
+(content-test "block->data type" (get (block->data h) :type) "heading")
+(content-test "block->data fields" (get (block->data h) :fields) {:text "Hi" :level 2})
+
+;; ── round-trip a mixed document with metadata ──
+(define
+  d
+  (doc-with-meta
+    (doc-append
+      (doc-append
+        (doc-append
+          (doc-append (doc-empty "post") (mk-heading "h" 1 "Title"))
+          (mk-text "p" "Body"))
+        (mk-image "img" "/c.png" "cat"))
+      (mk-list "l" true (list "a" "b")))
+    {:slug "s" :title "T" :tags (list "x" "y")}))
+
+(define rt (content/from-data (content/to-data d)))
+(content-test "rt id" (doc-id rt) "post")
+(content-test "rt title" (doc-title rt) "T")
+(content-test "rt slug" (doc-slug rt) "s")
+(content-test "rt tags" (doc-tags rt) (list "x" "y"))
+(content-test "rt ids" (doc-ids rt) (list "h" "p" "img" "l"))
+(content-test "rt render" (asHTML rt) (asHTML d))
+(content-test
+  "rt heading level"
+  (blk-send (doc-find rt "h") "level")
+  1)
+(content-test
+  "rt list items"
+  (blk-send (doc-find rt "l") "items")
+  (list "a" "b"))
+
+;; ── nested sections round-trip ──
+(define
+  ds
+  (doc-append
+    (doc-empty "d")
+    (mk-section
+      "s"
+      (list
+        (mk-heading "nh" 2 "N")
+        (mk-section "i" (list (mk-text "x" "deep")))))))
+(define rts (content/from-data (content/to-data ds)))
+(content-test "rt nested render" (asHTML rts) (asHTML ds))
+(content-test "rt nested tree-ids" (doc-tree-ids rts) (doc-tree-ids ds))
+(content-test
+  "rt nested deep-find"
+  (str (blk-send (doc-deep-find rts "x") "text"))
+  "deep")
+
+;; ── table round-trip ──
+(define
+  dtb
+  (doc-append
+    (doc-empty "d")
+    (mk-table "t" (list "A" "B") (list (list "1" "2")))))
+(define rtt (content/from-data (content/to-data dtb)))
+(content-test "rt table render" (asHTML rtt) (asHTML dtb))
+(content-test
+  "rt table headers"
+  (table-headers (doc-find rtt "t"))
+  (list "A" "B"))
+
+;; ── data is plain (no st-instance markers at top level) ──
+(define dat (content/to-data d))
+(content-test "data id field" (get dat :id) "post")
+(content-test "data block count" (len (get dat :blocks)) 4)
+(content-test
+  "data first block type"
+  (get (first (get dat :blocks)) :type)
+  "heading")
+
+;; ── empty doc round-trip ──
+(content-test
+  "rt empty ids"
+  (doc-ids (content/from-data (content/to-data (doc-empty "e"))))
+  (list))
+(content-test
+  "rt no-meta title nil"
+  (doc-title (content/from-data (content/to-data (doc-empty "e"))))
+  nil)
diff --git a/plans/content-on-sx.md b/plans/content-on-sx.md
index 2e512e88..49326b10 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` → **502/502** (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)
+`bash lib/content/conformance.sh` → **523/523** (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 serialization)
 
 ## Ground rules
 
@@ -92,9 +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] portable data serialization (`data.sx`: content/to-data + from-data, round-trips tree)
 
 ## Progress log
 
+- 2026-06-07 — Extension: portable data serialization (`data.sx`).
+  `content/to-data` converts a document to plain SX data
+  (`{:id :title :slug :tags :blocks [{:id :type :fields}]}`, sections recursing);
+  `content/from-data` reconstructs real block objects (section/table handled
+  specially, others generically via mk-block). Round-trips the full tree +
+  metadata (render-equal), decoupling storage/transport from the Smalltalk
+  instance shape. 21 tests; suite 523/523.
 - 2026-06-07 — Extension: document composition (`compose.sx`). `content/concat`
   / `content/prepend` / `content/concat-all` combine documents (keeping the
   first's id + metadata, concatenating blocks, immutable); `content/wrap-section`