diff --git a/lib/host/blog.sx b/lib/host/blog.sx
index 8929e9fd..3a5882a8 100644
--- a/lib/host/blog.sx
+++ b/lib/host/blog.sx
@@ -22,6 +22,41 @@
 (define host/blog-use-store! (fn (b) (set! host/blog-store b)))
 (define host/blog--key (fn (slug) (str "blog:" slug)))
 
+;; ── content-addressing: a universal CID over the canonical form ─────
+;; Every object (content/type/relation/constraint post) carries a stable :cid =
+;; hash of its CONTENT. The runtime has no hash primitive, so the canon serializer
+;; and a tail-recursive double-hash are built here. Canon SORTS keys, so the CID is
+;; identical across processes regardless of dict insertion / hash-seed order. The
+;; :slug (a mutable name) and any prior :cid are excluded — the CID hashes content
+;; only. git-shaped: slug = mutable name -> CID = immutable content identity.
+(define host/blog--canon
+  (fn (v)
+    (let ((t (type-of v)))
+      (cond
+        ((= t "dict")
+          (str "{" (join "|"
+            (map (fn (k) (str k "=" (host/blog--canon (get v k))))
+              (filter (fn (k) (and (not (= k "slug")) (not (= k "cid"))))
+                (sort (keys v))))) "}"))
+        ((= t "list") (str "[" (join "|" (map host/blog--canon v)) "]"))
+        ((= t "nil") "~")
+        (else (str v))))))
+(define host/blog--hash-go
+  (fn (s i n h1 h2)
+    (if (>= i n)
+      (str h1 "-" h2)
+      (let ((c (char-code (substr s i 1))))
+        (host/blog--hash-go s (+ i 1) n
+          (mod (+ (* h1 131) c) 1000000007)
+          (mod (+ (* h2 137) c) 998244353))))))
+(define host/blog--cid-of
+  (fn (rec) (let ((s (host/blog--canon rec))) (str "z" (host/blog--hash-go s 0 (len s) 7 11)))))
+;; the single choke point for every record write: stamps the content CID, then puts.
+(define host/blog--write!
+  (fn (slug rec)
+    (persist/backend-kv-put host/blog-store (host/blog--key slug)
+      (merge rec {:cid (host/blog--cid-of rec)}))))
+
 ;; slug from a title: lowercase, words joined by '-'. (Punctuation kept simple.)
 (define host/blog-slugify
   (fn (title)
@@ -38,7 +73,7 @@
 (define host/blog-put!
   (fn (slug title sx-content status)
     (let ((prev (host/blog-get slug)))
-      (persist/backend-kv-put host/blog-store (host/blog--key slug)
+      (host/blog--write! slug
         (merge (if prev prev {})
           {:slug slug :title title :sx-content sx-content :status status})))))
 (define host/blog-delete!
@@ -63,6 +98,15 @@
           {:slug slug :title (get r :title) :status (get r :status)}))
       (host/blog-slugs))))
 
+;; a post's content CID — its global, location-independent identity (nil if unknown).
+(define host/blog-cid (fn (slug) (get (host/blog-get slug) :cid)))
+;; reverse lookup: a slug whose record has this CID (nil if none). Scan; not for renders.
+(define host/blog-by-cid
+  (fn (cid)
+    (reduce
+      (fn (acc slug) (if acc acc (if (= (host/blog-cid slug) cid) slug acc)))
+      nil (host/blog-slugs))))
+
 ;; ── render ──────────────────────────────────────────────────────────
 ;; A post's sx_content is SX element markup -> HTML via render-page (which supplies
 ;; the server env so components resolve + keyword attrs are kept).
@@ -358,8 +402,7 @@
   (fn (slug schema)
     (let ((r (host/blog-get slug)))
       (when r
-        (persist/backend-kv-put host/blog-store (host/blog--key slug)
-          (merge r {:schema schema}))))))
+        (host/blog--write! slug (merge r {:schema schema}))))))
 
 ;; every element tag in a parsed content tree, recursively (the heads of nested
 ;; lists) — so "requires h1" matches an h1 even inside an article/section wrapper.
@@ -397,7 +440,7 @@
   (fn (slug title symmetric label inverse-label)
     (begin
       (when (not (host/blog-exists? slug))
-        (persist/backend-kv-put host/blog-store (host/blog--key slug)
+        (host/blog--write! slug
           {:slug slug :title title
            :sx-content (str "(article (h1 \"" title "\") (p \"A relation — posts link to each other through it. Its symmetry and labels live on this post.\"))")
            :status "published"
diff --git a/lib/host/tests/blog.sx b/lib/host/tests/blog.sx
index 708ab308..6e130abe 100644
--- a/lib/host/tests/blog.sx
+++ b/lib/host/tests/blog.sx
@@ -659,6 +659,30 @@
   (contains? (dream-resp-body (host-bl-oapp (host-bl-req "/open-post/"))) "<p>o</p>")
   true)
 
+;; ── content-addressing: every object carries a stable CID ───────────
+;; A CID is the hash of the canonical (key-sorted) content; the slug (a name) and
+;; any prior :cid are excluded. Same content -> same CID, across slugs and processes.
+(host/blog-use-store! (persist/open))
+(host/blog-put! "cid-a" "Same Body" "(p \"same\")" "published")
+(host/blog-put! "cid-b" "Same Body" "(p \"same\")" "published")
+(host-bl-test "put! stamps a non-nil CID"
+  (and (not (nil? (host/blog-cid "cid-a"))) (> (len (host/blog-cid "cid-a")) 1)) true)
+(host-bl-test "content-addressed: identical content -> identical CID (slug excluded)"
+  (= (host/blog-cid "cid-a") (host/blog-cid "cid-b")) true)
+(host-bl-test "CID changes when content changes"
+  (let ((before (host/blog-cid "cid-a")))
+    (host/blog-put! "cid-a" "Same Body" "(p \"different now\")" "published")
+    (not (= before (host/blog-cid "cid-a"))))
+  true)
+(host-bl-test "canon excludes :slug and :cid"
+  (= (host/blog--canon {:slug "x" :cid "old" :title "T"})
+     (host/blog--canon {:title "T"}))
+  true)
+(host-bl-test "by-cid reverse lookup finds a slug with that CID"
+  (not (nil? (host/blog-by-cid (host/blog-cid "cid-b")))) true)
+(host-bl-test "by-cid of an unknown CID is nil"
+  (host/blog-by-cid "znope-nope") nil)
+
 (define
   host-bl-tests-run!
   (fn ()
diff --git a/plans/relations-as-posts.md b/plans/relations-as-posts.md
index 8299cdbc..e1f390c6 100644
--- a/plans/relations-as-posts.md
+++ b/plans/relations-as-posts.md
@@ -11,6 +11,38 @@ itself in its own graph.
 
 Supersedes the hardcoded `:candidates "types"/"tags"/"all"` field of `host/blog-rel-kinds`.
 
+## Content-addressability is universal (foundational)
+
+**Every object carries a content-address (CID) — content-posts, type-posts, relation-posts,
+constraint-posts, all of them.** A CID is the hash of the object's *canonical* form: a recursive,
+**key-sorted** serialization (so insertion order, and any process-seed-dependent dict ordering, is
+irrelevant — identical content always yields an identical CID). The runtime has no hash primitive,
+so the canon serializer + a tail-recursive double-hash are built in SX (`host/blog--canon`,
+`host/blog--cid-of`); the slug is excluded from the hash (it's a *name*, not content).
+
+The model is **git-shaped**: the **slug is a mutable name → CID** (a branch pointing at a commit);
+the **CID is the immutable content identity** (the commit). Editing a post mints a new CID; the slug
+follows. Type evolution is the same — a type *version* is content-addressed, instances reference the
+version they were created against. Two objects with identical content *are* the same object (same
+CID) — correct content-addressing semantics.
+
+**Why it's foundational (federation).** A CID is a **global, location-independent identity**, so:
+
+- **Types flow across `fed-sx`.** The same type *definition* on any node has the same CID → a
+  **shared, content-addressed vocabulary**. Federated *instances* reference type CIDs, so a receiving
+  node can *interpret* them. This is linked-data/RDF realised on the post graph, and it generalises
+  ActivityPub itself: AP has a *fixed* type vocabulary (Note/Article/Person, Create/Follow/Like) —
+  the metamodel makes that vocabulary **extensible and user-defined**.
+- **Structure / behaviour trust-split** (the federation boundary): type **structure** (schema,
+  relations, signatures) is declarative and federates *freely* — sharing a definition is sharing a
+  hash. **Behaviour** (Slice 9 lifecycles/effects) does **not** federate naively: you never run a
+  remote node's lifecycle with *your* effect primitives (their "ship" could `charge-card`).
+  Behaviour federates only under high trust, with the effects **re-bound** to local, audited
+  primitives (their orchestration, your effects). `fed-sx` is already trust-gated — that's the lever.
+
+Build order: stamp a stable CID on every object first (additive — slug-addressing stays the working
+key), then a `cid → slug` index, then migrate references / type versioning, then federation.
+
 ## North star — the metamodel as a system-construction kit
 
 The destination this is all heading toward: the host stops being "a blog" and becomes a