diff --git a/lib/content/clone.sx b/lib/content/clone.sx
new file mode 100644
index 00000000..3da5a293
--- /dev/null
+++ b/lib/content/clone.sx
@@ -0,0 +1,34 @@
+;; content-on-sx — block id remapping / clone.
+;;
+;; Deep-rewrite every block id in the tree (descending into sections) by applying
+;; a function. Enables collision-free composition: prefix one document's ids
+;; before concatenating it with another. Immutable; content is unchanged, only
+;; ids.
+;;
+;; Requires (loaded by harness): doc.sx, section.sx (section? /
+;; section-children / section-with-children).
+
+(define
+  block-remap-id
+  (fn
+    (b f)
+    (let
+      ((nb (blk-set b "id" (f (blk-id b)))))
+      (if
+        (section? nb)
+        (section-with-children
+          nb
+          (map (fn (c) (block-remap-id c f)) (section-children nb)))
+        nb))))
+
+(define
+  content/remap-ids
+  (fn
+    (doc f)
+    (doc-with-blocks
+      doc
+      (map (fn (b) (block-remap-id b f)) (doc-blocks doc)))))
+
+(define
+  content/prefix-ids
+  (fn (doc prefix) (content/remap-ids doc (fn (id) (str prefix id)))))
diff --git a/lib/content/conformance.sh b/lib/content/conformance.sh
index 11a0a257..2f233e49 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 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 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"
@@ -47,6 +47,7 @@ run_suite() {
 (load "lib/content/section.sx")
 (load "lib/content/compose.sx")
 (load "lib/content/tree-edit.sx")
+(load "lib/content/clone.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 1da17166..b714d62e 100644
--- a/lib/content/scoreboard.json
+++ b/lib/content/scoreboard.json
@@ -12,6 +12,7 @@
     "section": {"pass": 25, "fail": 0},
     "compose": {"pass": 17, "fail": 0},
     "tree-edit": {"pass": 17, "fail": 0},
+    "clone": {"pass": 10, "fail": 0},
     "stats": {"pass": 17, "fail": 0},
     "table": {"pass": 15, "fail": 0},
     "data": {"pass": 21, "fail": 0},
@@ -26,7 +27,7 @@
     "md-doc": {"pass": 12, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 551,
+  "total_pass": 561,
   "total_fail": 0,
-  "total": 551
+  "total": 561
 }
diff --git a/lib/content/scoreboard.md b/lib/content/scoreboard.md
index 1eefa8f5..4499a6ac 100644
--- a/lib/content/scoreboard.md
+++ b/lib/content/scoreboard.md
@@ -16,6 +16,7 @@ _Generated by `lib/content/conformance.sh`_
 | section | 25 | 0 | 25 |
 | compose | 17 | 0 | 17 |
 | tree-edit | 17 | 0 | 17 |
+| clone | 10 | 0 | 10 |
 | stats | 17 | 0 | 17 |
 | table | 15 | 0 | 15 |
 | data | 21 | 0 | 21 |
@@ -29,4 +30,4 @@ _Generated by `lib/content/conformance.sh`_
 | md-import | 38 | 0 | 38 |
 | md-doc | 12 | 0 | 12 |
 | fed | 20 | 0 | 20 |
-| **Total** | **551** | **0** | **551** |
+| **Total** | **561** | **0** | **561** |
diff --git a/lib/content/tests/clone.sx b/lib/content/tests/clone.sx
new file mode 100644
index 00000000..e5dcb292
--- /dev/null
+++ b/lib/content/tests/clone.sx
@@ -0,0 +1,55 @@
+;; Extension — block id remapping / clone.
+
+(st-bootstrap-classes!)
+(content/bootstrap!)
+(content-bootstrap-section!)
+
+(define
+  d
+  (doc-append
+    (doc-append (doc-empty "d") (mk-heading "h" 1 "Title"))
+    (mk-section "s" (list (mk-text "a" "A") (mk-text "b" "B")))))
+
+;; ── prefix-ids rewrites every id in the tree ──
+(define p (content/prefix-ids d "x-"))
+(content-test "prefix top-level ids" (doc-ids p) (list "x-h" "x-s"))
+(content-test
+  "prefix tree-ids"
+  (doc-tree-ids p)
+  (list "x-h" "x-s" "x-a" "x-b"))
+(content-test "prefix immutable" (doc-tree-ids d) (list "h" "s" "a" "b"))
+(content-test "prefix preserves content" (asHTML p) (asHTML d))
+(content-test
+  "prefix preserves nested content"
+  (str (blk-send (doc-deep-find p "x-a") "text"))
+  "A")
+
+;; ── custom remap fn ──
+(define u (content/remap-ids d (fn (id) (str id "!"))))
+(content-test "remap suffix" (doc-tree-ids u) (list "h!" "s!" "a!" "b!"))
+
+;; ── collision-free composition ──
+(define
+  d2
+  (doc-append (doc-empty "d2") (mk-heading "h" 2 "Other")))
+(define
+  combined
+  (content/concat
+    (content/prefix-ids d "left-")
+    (content/prefix-ids d2 "right-")))
+(content-test
+  "combined ids unique"
+  (doc-tree-ids combined)
+  (list "left-h" "left-s" "left-a" "left-b" "right-h"))
+(content-test "combined validates" (content/valid? combined) true)
+;; without prefixing, the shared id "h" collides
+(content-test
+  "unprefixed collides"
+  (content/valid? (content/concat d d2))
+  false)
+
+;; ── render of combined ──
+(content-test
+  "combined render"
+  (asHTML combined)
+  "<h1>Title</h1><section><p>A</p><p>B</p></section><h2>Other</h2>")
diff --git a/plans/content-on-sx.md b/plans/content-on-sx.md
index 4dbca23f..0ab3a5eb 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` → **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)
+`bash lib/content/conformance.sh` → **561/561** (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)
 
 ## Ground rules
 
@@ -93,11 +93,18 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 - [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] id remapping / clone (`clone.sx`: content/remap-ids + prefix-ids, collision-free compose)
 - [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: id remapping / clone (`clone.sx`).
+  `content/remap-ids` deep-rewrites every block id across the tree (sections
+  recurse) via a function; `content/prefix-ids` prefixes them. Enables
+  collision-free composition (prefix each doc before concat → validates clean,
+  where the unprefixed concat has duplicate ids). Content unchanged, only ids;
+  immutable. 10 tests; suite 561/561.
 - 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