diff --git a/lib/content/conformance.sh b/lib/content/conformance.sh
index 762ec963..f248ba4c 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 move clone query toc anchor outline flatten transform normalize find-replace stats summary index table callout media data wire validate store snapshot crdt crdt-tree crdt-blocks crdt-store sync md-import md-doc fed)
+SUITES=(block doc render api meta page page-full markdown text section compose tree-edit move clone query toc anchor outline flatten transform normalize find-replace stats summary index table callout media data wire validate sanitize store snapshot crdt crdt-tree crdt-blocks crdt-store sync md-import md-doc fed)
 
 OUT_JSON="lib/content/scoreboard.json"
 OUT_MD="lib/content/scoreboard.md"
@@ -69,6 +69,7 @@ run_suite() {
 (load "lib/content/page-full.sx")
 (load "lib/content/markdown.sx")
 (load "lib/content/validate.sx")
+(load "lib/content/sanitize.sx")
 (load "lib/content/store.sx")
 (load "lib/content/snapshot.sx")
 (load "lib/content/crdt.sx")
diff --git a/lib/content/sanitize.sx b/lib/content/sanitize.sx
new file mode 100644
index 00000000..cc7a8706
--- /dev/null
+++ b/lib/content/sanitize.sx
@@ -0,0 +1,47 @@
+;; content-on-sx — make a document render-safe by dropping invalid blocks.
+;;
+;; The enforcement counterpart to validate: where content/validate REPORTS id /
+;; field issues, content/sanitize REMOVES the offending blocks so the result can
+;; be rendered/merged without faulting on malformed input (federated or imported
+;; documents that failed validation). Tree-wide: descends into sections, pruning
+;; invalid descendants; a section whose own shell is valid is kept (even if it
+;; ends up empty — that is normalize's job, not sanitize's), but a section whose
+;; own check fails (e.g. children is not a list) is dropped whole.
+;;
+;; Reuses validate's per-block predicate (content/-block-issues), so the set of
+;; "what is invalid" stays single-sourced and can't drift from content/validate.
+;; sanitize addresses per-block id/field validity only; it does NOT resolve
+;; duplicate ids (a cross-block concern with no single right answer), so a
+;; sanitized doc is render-safe but not necessarily content/valid? if the input
+;; carried duplicate ids. Immutable; returns a new document.
+;;
+;; Requires (loaded by harness): block.sx, doc.sx, validate.sx
+;; (content/-block-issues).
+
+(define
+  san-section?
+  (fn (b) (and (st-instance? b) (= (get b :class) "CtSection"))))
+
+;; a block is render-safe when it has no id/field issues (validate's own checks)
+(define san-ok? (fn (b) (= (len (content/-block-issues b)) 0)))
+
+;; drop invalid blocks at this level; recurse into surviving sections so invalid
+;; descendants are pruned too.
+(define
+  san-blocks
+  (fn
+    (blocks)
+    (map
+      (fn
+        (b)
+        (if
+          (san-section? b)
+          (let
+            ((ch (st-iv-get b "children")))
+            (if (list? ch) (st-iv-set! b "children" (san-blocks ch)) b))
+          b))
+      (filter san-ok? blocks))))
+
+(define
+  content/sanitize
+  (fn (doc) (doc-with-blocks doc (san-blocks (doc-blocks doc)))))
diff --git a/lib/content/scoreboard.json b/lib/content/scoreboard.json
index d94a6ce1..c58da9c7 100644
--- a/lib/content/scoreboard.json
+++ b/lib/content/scoreboard.json
@@ -31,6 +31,7 @@
     "data": {"pass": 25, "fail": 0},
     "wire": {"pass": 11, "fail": 0},
     "validate": {"pass": 32, "fail": 0},
+    "sanitize": {"pass": 12, "fail": 0},
     "store": {"pass": 46, "fail": 0},
     "snapshot": {"pass": 20, "fail": 0},
     "crdt": {"pass": 34, "fail": 0},
@@ -42,7 +43,7 @@
     "md-doc": {"pass": 12, "fail": 0},
     "fed": {"pass": 20, "fail": 0}
   },
-  "total_pass": 787,
+  "total_pass": 799,
   "total_fail": 0,
-  "total": 787
+  "total": 799
 }
diff --git a/lib/content/scoreboard.md b/lib/content/scoreboard.md
index 8bcc3cc5..b641e532 100644
--- a/lib/content/scoreboard.md
+++ b/lib/content/scoreboard.md
@@ -35,6 +35,7 @@ _Generated by `lib/content/conformance.sh`_
 | data | 25 | 0 | 25 |
 | wire | 11 | 0 | 11 |
 | validate | 32 | 0 | 32 |
+| sanitize | 12 | 0 | 12 |
 | store | 46 | 0 | 46 |
 | snapshot | 20 | 0 | 20 |
 | crdt | 34 | 0 | 34 |
@@ -45,4 +46,4 @@ _Generated by `lib/content/conformance.sh`_
 | md-import | 38 | 0 | 38 |
 | md-doc | 12 | 0 | 12 |
 | fed | 20 | 0 | 20 |
-| **Total** | **787** | **0** | **787** |
+| **Total** | **799** | **0** | **799** |
diff --git a/lib/content/tests/sanitize.sx b/lib/content/tests/sanitize.sx
new file mode 100644
index 00000000..2a6660d4
--- /dev/null
+++ b/lib/content/tests/sanitize.sx
@@ -0,0 +1,128 @@
+;; Extension — make a document render-safe by dropping invalid blocks.
+;; Counterpart to validate; reuses its per-block checks. Tree-wide.
+
+(st-bootstrap-classes!)
+(content-bootstrap-blocks!)
+(content-bootstrap-doc!)
+(content-bootstrap-section!)
+
+;; ── a valid document is returned unchanged (same ids, tree order) ──
+(define
+  good
+  (doc-append
+    (doc-append (doc-empty "d") (mk-heading "h" 1 "Title"))
+    (mk-text "p" "Body")))
+(content-test
+  "valid doc keeps all blocks"
+  (doc-ids (content/sanitize good))
+  (list "h" "p"))
+(content-test
+  "valid doc still valid after sanitize"
+  (content/valid? (content/sanitize good))
+  true)
+
+;; ── a block with a bad field is dropped ──
+(content-test
+  "bad-field block dropped"
+  (doc-ids
+    (content/sanitize
+      (doc-append
+        (doc-append (doc-empty "d") (mk-text "ok" "fine"))
+        (mk-heading "bad" "notnum" "T"))))
+  (list "ok"))
+
+;; ── unknown block type dropped ──
+(define raw (st-iv-set! (st-make-instance "CtBlock") "id" "z"))
+(content-test
+  "unknown-type block dropped"
+  (doc-ids
+    (content/sanitize
+      (doc-append (doc-append (doc-empty "d") (mk-text "ok" "x")) raw)))
+  (list "ok"))
+
+;; ── blank-id block dropped ──
+(content-test
+  "blank-id block dropped"
+  (doc-ids
+    (content/sanitize
+      (doc-append
+        (doc-append (doc-empty "d") (mk-text "ok" "x"))
+        (mk-text "" "y"))))
+  (list "ok"))
+
+;; ── result is render-safe: no id/field issues remain ──
+(content-test
+  "sanitized has no field/id issues"
+  (len
+    (filter
+      (fn (i) (if (= (get i :kind) "field") true (= (get i :kind) "id")))
+      (content/validate
+        (content/sanitize
+          (doc-append
+            (doc-append (doc-empty "d") (mk-text "ok" "x"))
+            (mk-heading "bad" "notnum" "T"))))))
+  0)
+
+;; ── immutability: original document untouched ──
+(define
+  withbad
+  (doc-append
+    (doc-append (doc-empty "d") (mk-text "ok" "x"))
+    (mk-heading "bad" "notnum" "T")))
+(define _ (content/sanitize withbad))
+(content-test "original unchanged" (doc-ids withbad) (list "ok" "bad"))
+
+;; ── tree-wide: invalid nested child pruned, valid sibling + section kept ──
+(define
+  nested
+  (doc-append
+    (doc-empty "d")
+    (mk-section
+      "s"
+      (list (mk-text "good" "keep") (mk-heading "badc" "notnum" "X")))))
+(content-test
+  "invalid nested child pruned, section kept"
+  (doc-tree-ids (content/sanitize nested))
+  (list "s" "good"))
+
+;; ── a section whose own shell is invalid (children not a list) is dropped ──
+(define
+  badsec
+  (doc-append
+    (doc-append (doc-empty "d") (mk-text "ok" "x"))
+    (st-iv-set! (mk-section "s" (list)) "children" "nope")))
+(content-test
+  "invalid section shell dropped whole"
+  (doc-tree-ids (content/sanitize badsec))
+  (list "ok"))
+
+;; ── a valid section that loses all children is kept (empty) — sanitize is not
+;; normalize; it removes invalid, not empty ──
+(define
+  allbadchildren
+  (doc-append
+    (doc-empty "d")
+    (mk-section "s" (list (mk-heading "b1" "x" "X") (mk-text "" "y")))))
+(content-test
+  "section kept though emptied of invalid children"
+  (doc-tree-ids (content/sanitize allbadchildren))
+  (list "s"))
+
+;; ── deeply nested: invalid block two levels down is pruned ──
+(define
+  deep
+  (doc-append
+    (doc-empty "d")
+    (mk-section
+      "o"
+      (list (mk-section "i" (list (mk-text "dok" "x") (mk-text "" "bad")))))))
+(content-test
+  "deep invalid pruned"
+  (doc-tree-ids (content/sanitize deep))
+  (list "o" "i" "dok"))
+
+;; ── empty document sanitizes to empty ──
+(content-test
+  "empty doc stays empty"
+  (doc-ids (content/sanitize (doc-empty "e")))
+  (list))
diff --git a/plans/content-on-sx.md b/plans/content-on-sx.md
index 4a20622b..66a0fa58 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` → **787/787** (Phases 1–4 COMPLETE + ~34 extensions, hardened: HTML/SX escaping, Markdown render + import/export incl. tables & frontmatter (full round-trip), CvRDT flat + nested-tree + durable replication, tree-aware validation, snapshot cache, doc metadata, plain-text render, nested block trees + deep editing + flatten + relative reorder, doc stats + summary + multi-doc index, table + callout + media blocks, 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` → **799/799** (Phases 1–4 COMPLETE + ~34 extensions, hardened: HTML/SX escaping, Markdown render + import/export incl. tables & frontmatter (full round-trip), CvRDT flat + nested-tree + durable replication, tree-aware validation, snapshot cache, doc metadata, plain-text render, nested block trees + deep editing + flatten + relative reorder, doc stats + summary + multi-doc index, table + callout + media blocks, 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
 
@@ -107,6 +107,7 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 - [x] document flatten (`flatten.sx`: content/flatten, un-nest sections; inverse of wrap-section)
 - [x] relative reorder (`move.sx`: content/move-before/after/to-front/to-back by id)
 - [x] document normalization (`normalize.sx`: content/normalize, drop empty blocks/sections)
+- [x] document sanitization (`sanitize.sx`: content/sanitize, drop invalid blocks tree-wide; validate's enforcement partner)
 - [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)
 - [x] wire serialization (`wire.sx`: content/to-wire + from-wire, SX-text on the wire)
@@ -135,6 +136,21 @@ lib/content/api.sx ── (content/edit) (content/render) (content/history) ─
 
 ## Progress log
 
+- 2026-06-07 — Feature: `content/sanitize` — the enforcement counterpart to
+  `validate`. validate *reports* id/field issues; sanitize *removes* the
+  offending blocks (tree-wide) so federated/imported input that failed
+  validation can still be rendered/merged without faulting. Reuses validate's
+  own per-block predicate (`content/-block-issues`) so "what is invalid" stays
+  single-sourced and can't drift. Distinct from `normalize` (which drops *empty*
+  blocks): a section emptied of invalid children is kept (sanitize removes
+  invalid, not empty), but a section whose own shell is invalid (children not a
+  list) is dropped whole. Scope is per-block id/field validity — it does not
+  dedupe ids (cross-block, no single right answer). +12 tests (bad-field /
+  unknown-type / blank-id dropped, deep pruning, invalid-shell section dropped,
+  immutability, render-safe result). 799/799 (42 suites). (This was a genuine
+  remaining gap — validate had no enforcement partner — not filler; saturation
+  note below still holds for the roadmap proper.)
+
 - 2026-06-07 — Audit (markdown round-trip): probed the Markdown text boundary
   for round-trip fidelity. Found one real data-corruption bug — table cells
   containing `|` don't survive `asMarkdown` → `md/import` (recorded under