diff --git a/lib/content/conformance.sh b/lib/content/conformance.sh
index 762a39fa..092223c4 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 toc anchor outline transform normalize find-replace 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 toc anchor outline flatten transform normalize find-replace 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"
@@ -52,6 +52,7 @@ run_suite() {
 (load "lib/content/toc.sx")
 (load "lib/content/anchor.sx")
 (load "lib/content/outline.sx")
+(load "lib/content/flatten.sx")
 (load "lib/content/transform.sx")
 (load "lib/content/normalize.sx")
 (load "lib/content/find-replace.sx")
diff --git a/lib/content/flatten.sx b/lib/content/flatten.sx
new file mode 100644
index 00000000..d8e81e7c
--- /dev/null
+++ b/lib/content/flatten.sx
@@ -0,0 +1,34 @@
+;; content-on-sx — document flatten.
+;;
+;; Un-nests a sectioned document into a flat block sequence: each section is
+;; replaced inline by its (recursively flattened) children, dropping the section
+;; wrapper. The inverse of content/wrap-section, for flat export targets.
+;; Immutable; inline tree handling (no section.sx dep).
+;;
+;; Requires (loaded by harness): block.sx, doc.sx.
+
+(define
+  flat-section?
+  (fn (b) (and (st-instance? b) (= (get b :class) "CtSection"))))
+
+(define
+  flat-blocks
+  (fn
+    (blocks)
+    (if
+      (= (len blocks) 0)
+      (list)
+      (let
+        ((b (first blocks)))
+        (append
+          (if
+            (flat-section? b)
+            (let
+              ((ch (st-iv-get b "children")))
+              (if (list? ch) (flat-blocks ch) (list)))
+            (list b))
+          (flat-blocks (rest blocks)))))))
+
+(define
+  content/flatten
+  (fn (doc) (doc-with-blocks doc (flat-blocks (doc-blocks doc)))))
diff --git a/lib/content/scoreboard.json b/lib/content/scoreboard.json
index 8a800c42..2a803553 100644
--- a/lib/content/scoreboard.json
+++ b/lib/content/scoreboard.json
@@ -17,6 +17,7 @@
     "toc": {"pass": 8, "fail": 0},
     "anchor": {"pass": 6, "fail": 0},
     "outline": {"pass": 14, "fail": 0},
+    "flatten": {"pass": 10, "fail": 0},
     "transform": {"pass": 12, "fail": 0},
     "normalize": {"pass": 11, "fail": 0},
     "find-replace": {"pass": 10, "fail": 0},
@@ -34,7 +35,7 @@
     "md-doc": {"pass": 12, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 635,
+  "total_pass": 645,
   "total_fail": 0,
-  "total": 635
+  "total": 645
 }
diff --git a/lib/content/scoreboard.md b/lib/content/scoreboard.md
index 429e0235..9eb912ec 100644
--- a/lib/content/scoreboard.md
+++ b/lib/content/scoreboard.md
@@ -21,6 +21,7 @@ _Generated by `lib/content/conformance.sh`_
 | toc | 8 | 0 | 8 |
 | anchor | 6 | 0 | 6 |
 | outline | 14 | 0 | 14 |
+| flatten | 10 | 0 | 10 |
 | transform | 12 | 0 | 12 |
 | normalize | 11 | 0 | 11 |
 | find-replace | 10 | 0 | 10 |
@@ -37,4 +38,4 @@ _Generated by `lib/content/conformance.sh`_
 | md-import | 38 | 0 | 38 |
 | md-doc | 12 | 0 | 12 |
 | fed | 20 | 0 | 20 |
-| **Total** | **635** | **0** | **635** |
+| **Total** | **645** | **0** | **645** |
diff --git a/lib/content/tests/flatten.sx b/lib/content/tests/flatten.sx
new file mode 100644
index 00000000..b9cd8286
--- /dev/null
+++ b/lib/content/tests/flatten.sx
@@ -0,0 +1,72 @@
+;; Extension — document flatten (un-nest sections).
+
+(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-text "b" "B")))))
+
+;; ── one level un-nested ──
+(define f (content/flatten d))
+(content-test "flatten ids" (doc-ids f) (list "h" "a" "b"))
+(content-test
+  "flatten no sections"
+  (content/types f)
+  (list "heading" "text" "text"))
+(content-test "flatten immutable" (doc-ids d) (list "h" "s"))
+(content-test "flatten render" (asHTML f) "<h1>Top</h1><p>A</p><p>B</p>")
+
+;; ── deep nesting fully flattened ──
+(define
+  deep
+  (doc-append
+    (doc-empty "d")
+    (mk-section
+      "o"
+      (list
+        (mk-text "x" "X")
+        (mk-section
+          "i"
+          (list (mk-text "y" "Y") (mk-heading "z" 2 "Z")))))))
+(content-test
+  "deep flatten ids"
+  (doc-ids (content/flatten deep))
+  (list "x" "y" "z"))
+
+;; ── inverse of wrap-section ──
+(define
+  plain
+  (doc-append
+    (doc-append (doc-empty "p") (mk-text "a" "A"))
+    (mk-text "b" "B")))
+(content-test
+  "flatten . wrap == identity ids"
+  (doc-ids (content/flatten (content/wrap-section plain "sec")))
+  (doc-ids plain))
+(content-test
+  "flatten . wrap == identity render"
+  (asHTML (content/flatten (content/wrap-section plain "sec")))
+  (asHTML plain))
+
+;; ── already-flat doc unchanged ──
+(content-test
+  "flat unchanged"
+  (asHTML (content/flatten plain))
+  (asHTML plain))
+
+;; ── empty section disappears ──
+(content-test
+  "empty section flattens away"
+  (doc-ids
+    (content/flatten (doc-append (doc-empty "d") (mk-section "s" (list)))))
+  (list))
+
+;; ── empty doc ──
+(content-test
+  "flatten empty"
+  (doc-ids (content/flatten (doc-empty "e")))
+  (list))
diff --git a/plans/content-on-sx.md b/plans/content-on-sx.md
index 865481b6..92a4b5eb 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` → **635/635** (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 + transforms + find/replace, TOC + anchored headings + outline, normalization)
+`bash lib/content/conformance.sh` → **645/645** (Phases 1–4 COMPLETE + ~28 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 editing + flatten, doc stats, table block, HTML page wrapper + SEO page, doc composition + id-remap, portable data + wire serialization, block query + transforms + find/replace, TOC + anchored headings + outline, normalization)
 
 ## Ground rules
 
@@ -99,6 +99,7 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 - [x] TOC rendering (`toc.sx`: content/toc-markdown + toc-html from headings)
 - [x] anchored-heading render (`anchor.sx`: content/html-anchored, functional TOC links)
 - [x] document outline (`outline.sx`: content/outline, nested heading tree)
+- [x] document flatten (`flatten.sx`: content/flatten, un-nest sections; inverse of wrap-section)
 - [x] document normalization (`normalize.sx`: content/normalize, drop empty blocks/sections)
 - [x] global find/replace (`find-replace.sx`: content/find-replace across text-bearing blocks)
 - [x] portable data serialization (`data.sx`: content/to-data + from-data, round-trips tree)
@@ -106,6 +107,11 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 
 ## Progress log
 
+- 2026-06-07 — Extension: document flatten (`flatten.sx`). `content/flatten`
+  un-nests a sectioned doc into a flat block sequence (each section replaced
+  inline by its recursively-flattened children, wrapper dropped) — the inverse of
+  content/wrap-section, for flat export targets. Inline tree handling; immutable.
+  10 tests; suite 645/645.
 - 2026-06-07 — Extension: nested document outline (`outline.sx`).
   `content/outline` builds a hierarchical heading tree from content/headings —
   each node `{:id :text :level :children}`, headings nesting under the nearest