diff --git a/ebox-flex.el b/ebox-flex.el
index e342090..a425c35 100644
--- a/ebox-flex.el
+++ b/ebox-flex.el
@@ -40,6 +40,8 @@
                   (cache-key signature rendered &optional side-effects))
 (declare-function ebox--render-with-cache
                   "ebox-incremental" (node &optional force cache-probe))
+(declare-function ebox-tree--register-legacy-flex-item-adapter
+                  "ebox-tree" (adapter token source content))
 (declare-function ebox-create "ebox" (&rest plist))
 (declare-function ebox-get "ebox" (box property))
 (declare-function ebox--ensure-node-id "ebox" (node))
@@ -80,6 +82,14 @@
   '(:order :flex :flex-grow :flex-shrink :flex-basis :align-self)
   "Properties that belong to flex item metadata.")
 
+(defconst ebox--flex-item-direct-adapter-token
+  (make-symbol "ebox-flex-item-direct-adapter")
+  "Opaque provenance token for a legacy Flex item without a Box wrapper.")
+
+(defconst ebox--flex-item-wrapper-adapter-token
+  (make-symbol "ebox-flex-item-wrapper-adapter")
+  "Opaque provenance token for a legacy Flex item with one Box wrapper.")
+
 (defconst ebox--flex-container-layout-prop-keys
   '(:flex-direction :flex-wrap :flex-flow
     :justify-content :align-items :align-content
@@ -519,11 +529,15 @@ programmatic port and are therefore rejected here."
        (not (eq (plist-get source :ebox-type) 'box))))
 
 (defun ebox--flex-bind-source-viewport (source viewport thunk)
-  "Call THUNK with VIEWPORT bound for composite flex item SOURCE."
-  (if (and viewport (ebox--flex-composite-source-p source))
-      (let ((ebox-viewport-width viewport))
-        (funcall thunk))
-    (funcall thunk)))
+  "Call THUNK in SOURCE's detached Flex-item render context.
+VIEWPORT is bound only for composite SOURCE nodes; the Flex parent context is
+always bound because SOURCE remains a direct Flex participant while rendered
+as a detached geometry root."
+  (let ((ebox--render-root-parent-kind 'flex))
+    (if (and viewport (ebox--flex-composite-source-p source))
+        (let ((ebox-viewport-width viewport))
+          (funcall thunk))
+      (funcall thunk))))
 
 (defvar ebox--flex-sized-render-observation-table
   (make-hash-table :test 'eq :weakness 'key)
@@ -575,13 +589,18 @@ this avoids signature and retained-cost work for one-off responsive sizes."
 Direct box flex items are measured intrinsically.  Composite items receive
 VIEWPORT for descendants so auto and max-width layouts can resolve against
 their containing block, while stack/column containers do not stretch their own
-outer measurement width to VIEWPORT."
+  outer measurement width to VIEWPORT."
   (if (ebox--flex-composite-source-p source)
-      (let ((ebox-viewport-width viewport)
-            (ebox--intrinsic-layout-measurement t))
-        (ebox--render-with-cache source))
-    (let ((ebox-viewport-width nil))
-      (ebox--render-with-cache source))))
+      (ebox--flex-bind-source-viewport
+       source viewport
+       (lambda ()
+         (let ((ebox--intrinsic-layout-measurement t))
+           (ebox--render-with-cache source))))
+    (ebox--flex-bind-source-viewport
+     source nil
+     (lambda ()
+       (let ((ebox-viewport-width nil))
+         (ebox--render-with-cache source))))))
 
 (defun ebox--flex-measured-render-context-compatible-p
     (source measured-viewport final-viewport)
@@ -1115,12 +1134,14 @@ Return a sized render entry containing the rendered string and dimensions."
                 (ebox--flex-sized-entry-rendered fragment-entry))
                ((and fragment-key
                      (fboundp 'ebox--render-cache-render-with-scroll-actions))
-                (let ((ebox-viewport-width item-viewport))
-                  (pcase-let ((`(,captured-rendered ,captured-side-effects)
-                               (ebox--render-cache-render-with-scroll-actions
-                                (or node source))))
-                    (setq fragment-side-effects captured-side-effects)
-                    captured-rendered)))
+                (ebox--flex-bind-source-viewport
+                 source item-viewport
+                 (lambda ()
+                   (pcase-let ((`(,captured-rendered ,captured-side-effects)
+                                (ebox--render-cache-render-with-scroll-actions
+                                 (or node source))))
+                     (setq fragment-side-effects captured-side-effects)
+                     captured-rendered))))
                (t
                 (ebox--flex-bind-source-viewport
                  source item-viewport
@@ -1629,12 +1650,18 @@ metadata and rendering decisions."
            (source (copy-sequence source))
            (declarations
             (append (plist-get source :ebox-style-declarations)
-                    (ebox-style-compile-declarations item-props))))
+                    (ebox-style-compile-declarations item-props)))
+           (adapter (list :ebox-type 'flex-item
+                          :display '(block flow)
+                          :node source
+                          :props item-props))
+           (token (if box-props
+                      ebox--flex-item-wrapper-adapter-token
+                    ebox--flex-item-direct-adapter-token)))
       (plist-put source :ebox-style-declarations declarations)
-      (list :ebox-type 'flex-item
-            :display '(block flow)
-            :node source
-            :props item-props))))
+      (ebox-tree--register-legacy-flex-item-adapter
+       adapter token source
+       (and box-props (plist-get source :ebox-content-node))))))
 
 ;;;###autoload
 (defun ebox-flex (&rest items)
diff --git a/ebox-incremental.el b/ebox-incremental.el
index ec8b1f2..42b2edf 100644
--- a/ebox-incremental.el
+++ b/ebox-incremental.el
@@ -26,7 +26,11 @@
 (declare-function ebox--replay-scroll-cache-actions
                   "ebox" (actions &optional portable))
 (declare-function ebox-tree-validate-declarative-root
-                  "ebox-tree" (root))
+                  "ebox-tree"
+                  (root &optional validate-participation-p root-parent-kind))
+(declare-function ebox-tree-validate-indexed-participation
+                  "ebox-tree"
+                  (node-table parent-table node-ids &optional source))
 (declare-function ebox-tree-clear-runtime-identities
                   "ebox-tree" (root))
 (declare-function ebox-tree-copy-node-structure
@@ -5815,9 +5819,32 @@ complete preorder first-owner and postorder last-box semantics."
           :dirty-set
           (ebox-incremental--candidate-dirty-set-from-touched
            old-state candidate-root touched)
+          :participation-node-ids
+          (ebox-incremental--candidate-participation-node-ids touched)
           :touched-count (length touched)
           :removed-count (length removed))))
 
+(defun ebox-incremental--candidate-participation-node-ids (touched)
+  "Return final nodes whose participation context changed in TOUCHED."
+  (let ((seen (make-hash-table :test 'equal)) result)
+    (cl-labels ((record
+                 (node)
+                 (when-let* ((node-id (and node (plist-get node :node-id))))
+                   (unless (gethash node-id seen)
+                     (puthash node-id t seen)
+                     (push node-id result)))))
+      (dolist (entry touched)
+        (pcase-let ((`(,old-node ,new-node ,_parent-id ,_children-changed)
+                     entry))
+          (record new-node)
+          ;; Added/replaced children already occur in TOUCHED.  Revisit every
+          ;; direct child only when the parent's layout role itself changed.
+          (when (or (null old-node)
+                    (not (eq (ebox-tree--child-layout-kind old-node 'parent)
+                             (ebox-tree--child-layout-kind new-node 'parent))))
+            (ebox-tree-for-each-direct-child new-node #'record))))
+      (nreverse result))))
+
 (defun ebox-incremental--candidate-local-index-delta
     (old-state candidate-root &optional path-copy-trace range-index-deltas)
   "Return local indexes and dirty entries for CANDIDATE-ROOT.
@@ -6138,6 +6165,8 @@ replace those O(n) table copies without changing this delta contract."
                         :native-node-postorder native-node-postorder)))
             (list :index index
                   :dirty-set dirty-set
+                  :participation-node-ids
+                  (ebox-incremental--candidate-participation-node-ids touched)
                   :touched-node-ids
                   (mapcar
                    (lambda (entry)
@@ -6538,7 +6567,7 @@ must be recomputed in the next publication."
   (unless (and (listp next-root) (not (stringp next-root)))
     (error "Ebox declarative root must be an Ebox node"))
   ;; Validate before copying so shared references and cycles remain visible.
-  (ebox-tree-validate-declarative-root next-root)
+  (ebox-tree-validate-declarative-root next-root t)
   (let* ((candidate-root
           (ebox-tree-clear-runtime-identities
            (ebox-tree-copy-node-structure next-root)))
@@ -6572,6 +6601,12 @@ must be recomputed in the next publication."
     ;; published, and path copying introduces no new sibling or ref
     ;; relations outside the replaced anchors.  Re-walking the whole
     ;; tree here made every commit pay one full-page validation.
+    (let ((index (plist-get local-preparation :index)))
+      (ebox-tree-validate-indexed-participation
+       (plist-get index :node-table)
+       (plist-get index :parent-table)
+       (plist-get local-preparation :participation-node-ids)
+       'author))
     (setq local-preparation
           (plist-put local-preparation
                      :detached-identity-history detached-history))
diff --git a/ebox-layout.el b/ebox-layout.el
index 920995b..65f12b4 100644
--- a/ebox-layout.el
+++ b/ebox-layout.el
@@ -1624,6 +1624,9 @@ Internal implementation of `ebox-render' for box nodes."
 (defvar ebox--surface-materialization-active nil
   "Non-nil while an Ebox TP producer is running raw layout.")
 
+(defvar ebox--render-root-parent-kind nil
+  "Formatting context of a detached root rendered for its owning parent.")
+
 (defun ebox--render-layout (node)
   "Render layout NODE directly to a multi-line propertized string.
 NODE can be:
diff --git a/ebox-surface.el b/ebox-surface.el
index 646daa3..8d93ce1 100644
--- a/ebox-surface.el
+++ b/ebox-surface.el
@@ -33,6 +33,7 @@
 (defvar ebox--viewport-dependent-subtree-cache)
 (defvar ebox--viewport-height-dependent-subtree-cache)
 (defvar ebox--render-runtime-revision)
+(defvar ebox--render-root-parent-kind)
 (defvar ebox--render-cache-table)
 (defvar ebox--render-cache-signature-cache)
 (defvar ebox--layout-fragments-table)
@@ -960,7 +961,8 @@ PREVIOUS-STATE's node-object table."
          (and subjects (plist-get subjects :node-subject-table))
          bindings-by-subject states-by-subject style-state-table
          stylesheet-signature
-         selector-tree-token stylesheet-subject-local-p))))
+         selector-tree-token stylesheet-subject-local-p
+         ebox--render-root-parent-kind))))
     (list :surface-root surface-root
           :node-root node-root
           :objects-by-node objects-by-node
@@ -1056,7 +1058,10 @@ PREVIOUS-STATE's node-object table."
             (puthash node object objects-by-node)
             (puthash node-id object objects-by-id))))
       (unless (gethash (plist-get root :node-id) objects-by-id)
-        (error "Ebox native projection lost its root object")))
+        (error "Ebox native projection lost its root object"))
+      (when subject-table
+        (ebox-tree-validate-indexed-participation
+         nodes parents touched-ids 'computed)))
     objects-by-node))
 
 (defun ebox-surface--complete-projection
@@ -1089,7 +1094,8 @@ the projection roots a second time."
      (and subjects (plist-get subjects :node-subject-table))
      bindings-by-subject states-by-subject style-state-table
      stylesheet-signature
-     selector-tree-token stylesheet-subject-local-p)
+     selector-tree-token stylesheet-subject-local-p
+     ebox--render-root-parent-kind)
     projection))
 
 (defun ebox-surface--projection-result
@@ -1369,7 +1375,8 @@ When PRESERVE-IDENTITIES-P is non-nil, retain existing Ebox node and region
 identities while always discarding TP handles and render-cache attachments."
   (unless (and (listp source) (not (stringp source)))
     (error "Ebox surface source must be an Ebox node"))
-  (ebox-tree-validate-declarative-root source)
+  (ebox-tree-validate-declarative-root
+   source t ebox--render-root-parent-kind)
   (let ((candidate (ebox-tree-copy-node-structure source)))
     (if preserve-identities-p
         (ebox-surface--clear-runtime-attachments candidate)
@@ -1697,7 +1704,8 @@ dependencies, or any malformed index conservatively preserves root planning."
 (defun ebox-surface--ensure-node-tree
     (context parent node table subject-table bindings-by-subject
              states-by-subject style-state-table stylesheet-signature
-             selector-tree-token stylesheet-subject-local-p)
+             selector-tree-token stylesheet-subject-local-p
+             &optional parent-kind parent-node grandparent-node)
   "Ensure styled NODE descendants below PARENT in CONTEXT and fill TABLE."
   (let ((object
          (tp-object-ensure context parent
@@ -1712,13 +1720,19 @@ dependencies, or any malformed index conservatively preserves root planning."
        style-state-table
        stylesheet-signature selector-tree-token
        stylesheet-subject-local-p))
-    (dolist (child (ebox-tree--children-raw node))
-      (ebox-surface--ensure-node-tree
-       context object child table subject-table bindings-by-subject
-       states-by-subject style-state-table stylesheet-signature
-       selector-tree-token stylesheet-subject-local-p))
+    (let ((child-kind (ebox-tree--child-layout-kind node parent-kind)))
+      (ebox-tree-for-each-direct-child
+       node
+       (lambda (child)
+         (ebox-surface--ensure-node-tree
+          context object child table subject-table bindings-by-subject
+          states-by-subject style-state-table stylesheet-signature
+          selector-tree-token stylesheet-subject-local-p
+          child-kind node parent-node))))
     (when subject-table
-      (ebox-style-sync-flex-item node))
+      (ebox-style-sync-flex-item node)
+      (ebox-tree--validate-node-parent-participation
+       node parent-kind parent-node grandparent-node 'computed))
     object))
 
 (defun ebox-surface--node-object-table (objects-by-node)
@@ -1951,6 +1965,9 @@ valid because their declarative layout and viewport did not change."
                   (binding (plist-get style-state :binding)))
         (ebox-style-apply-computed node (tp-binding-read binding))
         (ebox-style-sync-flex-item node)))
+    (when owner-ids
+      (ebox-tree-validate-indexed-participation
+       nodes (plist-get state :parent-table) owner-ids 'computed))
     state))
 
 (defun ebox-surface--rendered-role-topology-signature
diff --git a/ebox-tree.el b/ebox-tree.el
index c9d8f87..275a0b4 100644
--- a/ebox-tree.el
+++ b/ebox-tree.el
@@ -2,7 +2,7 @@
 
 ;;; Commentary:
 ;; Owns runtime identity, display accessors, child traversal, parent paths, and
-;; flex participation metadata.  It does not own layout measurement or buffer
+;; parent participation metadata.  It does not own layout measurement or buffer
 ;; editing.
 
 ;;; Code:
@@ -13,6 +13,7 @@
 (require 'ecss-selector)
 (require 'ebox-style)
 (require 'ebox-child-range)
+(require 'ebox-layout-config)
 
 (declare-function ebox-get "ebox" (box property))
 (declare-function ebox--ensure-node-id "ebox" (node))
@@ -28,6 +29,11 @@
   "Dynamic snapshot-detail-local node -> region ids cache.
 This is an optimization for snapshot capture, not buffer runtime state.")
 (defvar ebox--flex-item-prop-keys)
+(defvar ebox--flex-item-direct-adapter-token)
+(defvar ebox--flex-item-wrapper-adapter-token)
+(defvar ebox-tree--legacy-flex-item-adapter-proofs
+  (make-hash-table :test #'eq :weakness 'key)
+  "Weak adapter -> construction-shape proofs for legacy Flex items.")
 
 (defun ebox-tree-computed-display (node)
   "Return NODE's canonical CSS-like display pair."
@@ -164,6 +170,44 @@ The returned nodes remain owned by their tree.  Callers may inspect them but
 must not modify a published runtime tree through this accessor."
   (ebox-tree--children-raw node))
 
+(defun ebox-tree-for-each-direct-child (node function)
+  "Call FUNCTION for each direct child of NODE without materializing Ranges."
+  (let ((type (and (listp node) (plist-get node :ebox-type)))
+        (sequence (and (listp node)
+                       (plist-get node :ebox-child-sequence))))
+    (when (memq type '(flex grid))
+      (when-let* ((box (plist-get node :box)))
+        (funcall function box)))
+    (if sequence
+        (ebox-child-range--fold
+         sequence
+         (lambda (ignored child)
+           (funcall function child)
+           ignored)
+         nil)
+      (pcase type
+        ('box
+         (if (plist-member node :children)
+             (dolist (child (plist-get node :children))
+               (funcall function child))
+           (when-let* ((child (plist-get node :ebox-content-node)))
+             (funcall function child))))
+        ((or 'concat 'stack 'flex 'grid)
+         (dolist (child
+                  (or (plist-get node :children)
+                      (pcase type
+                        ('concat
+                         (delq nil (list (plist-get node :left)
+                                         (plist-get node :right))))
+                        ('stack
+                         (delq nil (list (plist-get node :top)
+                                         (plist-get node :bottom)))))))
+           (funcall function child)))
+        ('flex-item
+         (when-let* ((child (plist-get node :node)))
+           (funcall function child))))))
+  node)
+
 (defun ebox-tree-layout-children (node)
   "Return layout container children for NODE in render order.
 New row/column containers store flat `:children'.  Legacy `ebox-concat' and
@@ -360,10 +404,17 @@ original object identity."
                      (setq copy (without (without copy :top) :bottom))))
                   (plist-put copy :ebox-child-sequence
                              (ebox-child-range--build (nreverse segments))))
-              (ebox-tree-copy-with-direct-child-replacements
-               node
-               (mapcar (lambda (child) (cons child (copy-node child)))
-                       (ebox-tree--children-raw node))))))))
+              (let ((copy
+                     (ebox-tree-copy-with-direct-child-replacements
+                      node
+                      (mapcar (lambda (child)
+                                (cons child (copy-node child)))
+                              (ebox-tree--children-raw node)))))
+                ;; Only this exact deep-copy traversal has proven every copied
+                ;; child derives from the authentic source tree.  Generic
+                ;; path-copy replacements must not launder adapter provenance.
+                (ebox-tree--remint-legacy-flex-item-adapter-proof
+                 node copy)))))))
     (copy-node root)))
 
 (defun ebox-tree--runtime-identity-node-shell
@@ -488,7 +539,225 @@ Host references are root-global opaque metadata and are compared with `equal'."
       (visit root))
     root))
 
-(defun ebox-tree-validate-declarative-root (root)
+(defconst ebox-tree--common-participation-keys
+  '(:order :align-self)
+  "Participation properties shared by Flex and Grid children.")
+
+(defconst ebox-tree--flex-participation-keys
+  '(:flex :flex-grow :flex-shrink :flex-basis)
+  "Properties accepted only on a direct Flex child Box.")
+
+(defconst ebox-tree--grid-participation-keys
+  '(:grid-column :grid-row :grid-column-span :grid-row-span :justify-self)
+  "Properties accepted only on a direct Grid child Box.")
+
+(defconst ebox-tree--participation-keys
+  (append ebox-tree--common-participation-keys
+          ebox-tree--flex-participation-keys
+          ebox-tree--grid-participation-keys)
+  "Complete canonical parent-participation property domain.")
+
+(defun ebox-tree--style-specifies-participation-p (style key)
+  "Return non-nil when computed STYLE has a declaration winner for KEY."
+  (and style
+       (ebox-style--specified-property-p style (ebox-style-schema-id key))))
+
+(defun ebox-tree--declarations-specify-participation-p (node key)
+  "Return non-nil when NODE author declarations specify participation KEY."
+  (and (listp node)
+       (plist-member (ebox-style-node-declarations node)
+                     (ebox-style-schema-id key))))
+
+(defun ebox-tree--author-specifies-participation-p (node key)
+  "Return non-nil when NODE's author source specifies participation KEY."
+  (if (eq (plist-get node :ebox-type) 'flex-item)
+      (let* ((adapter-source (plist-get node :node))
+             (content (and (listp adapter-source)
+                           (plist-get adapter-source :ebox-content-node)))
+             (projected-p
+              (or (and (listp adapter-source)
+                       (plist-get adapter-source :ebox-computed-style))
+                  (and (listp content)
+                       (plist-get content :ebox-computed-style)))))
+        (or (ebox-tree--declarations-specify-participation-p
+             adapter-source key)
+            (ebox-tree--declarations-specify-participation-p content key)
+            (and (not projected-p)
+                 (plist-member (plist-get node :props) key))))
+    (or (ebox-tree--declarations-specify-participation-p node key)
+        (and (null (plist-get node :ebox-computed-style))
+             (plist-member node key)))))
+
+(defun ebox-tree--node-participation-keys (node source)
+  "Return NODE participation keys from AUTHOR or COMPUTED SOURCE."
+  (let ((flex-item-p (eq (plist-get node :ebox-type) 'flex-item))
+        (style (plist-get node :ebox-computed-style))
+        keys)
+    (dolist (key ebox-tree--participation-keys)
+      (when
+          (pcase source
+            ('author
+             (ebox-tree--author-specifies-participation-p node key))
+            ('computed
+             (if flex-item-p
+                 (let* ((adapter-source (plist-get node :node))
+                        (adapter-style
+                         (and (listp adapter-source)
+                              (plist-get adapter-source
+                                         :ebox-computed-style)))
+                        (content (and (listp adapter-source)
+                                      (plist-get adapter-source
+                                                 :ebox-content-node)))
+                        (content-style
+                         (and (listp content)
+                              (plist-get content :ebox-computed-style))))
+                   (and (plist-member (plist-get node :props) key)
+                        (or (ebox-tree--style-specifies-participation-p
+                             adapter-style key)
+                            (ebox-tree--style-specifies-participation-p
+                             content-style key))))
+               (and (plist-member node key)
+                    (ebox-tree--style-specifies-participation-p style key))))
+            (_ (error "Unknown Ebox participation source: %S" source)))
+        (push key keys)))
+    (nreverse keys)))
+
+(defun ebox-tree--child-layout-kind (node parent-kind)
+  "Return layout kind governing NODE children, preserving transparent adapters."
+  (pcase (plist-get node :ebox-type)
+    ('flex-item parent-kind)
+    ('flex 'flex)
+    ('grid 'grid)
+    (_ (pcase (if (and (eq (plist-get node :ebox-kind) 'box)
+                       (plist-get node :ebox-layout-config))
+                  (ebox-layout-config-kind
+                   (plist-get node :ebox-layout-config))
+                (ebox-tree-display-inner node))
+         ('flow 'normal)
+         (kind kind)))))
+
+(defun ebox-tree--legacy-flex-item-adapter-p (node token)
+  "Return non-nil when NODE has the exact legacy Flex adapter TOKEN shape."
+  (let ((proof (gethash node ebox-tree--legacy-flex-item-adapter-proofs))
+        (source (plist-get node :node)))
+    (and token
+         (eq (plist-get node :ebox-type) 'flex-item)
+         (vectorp proof)
+         (= (length proof) 3)
+         (eq (aref proof 0) token)
+         (eq (aref proof 1) source)
+         (listp source)
+         (not (stringp source))
+         (let ((children (ebox-tree--children-raw node)))
+           (and (eq (car children) source)
+                (null (cdr children))))
+         (if (eq token ebox--flex-item-wrapper-adapter-token)
+             (let ((content (plist-get source :ebox-content-node)))
+               (and (eq (plist-get source :ebox-type) 'box)
+                    (listp content)
+                    (not (stringp content))
+                    (eq (aref proof 2) content)))
+           (null (aref proof 2))))))
+
+(defun ebox-tree--register-legacy-flex-item-adapter
+    (adapter token source content)
+  "Register ADAPTER's exact TOKEN, SOURCE, and optional CONTENT identities."
+  (puthash adapter (vector token source content)
+           ebox-tree--legacy-flex-item-adapter-proofs)
+  adapter)
+
+(defun ebox-tree--remint-legacy-flex-item-adapter-proof (old new)
+  "Bind NEW adapter provenance to copied children when OLD is authentic."
+  (cond
+   ((ebox-tree--legacy-flex-item-adapter-p
+     old ebox--flex-item-direct-adapter-token)
+    (ebox-tree--register-legacy-flex-item-adapter
+     new ebox--flex-item-direct-adapter-token (plist-get new :node) nil))
+   ((ebox-tree--legacy-flex-item-adapter-p
+     old ebox--flex-item-wrapper-adapter-token)
+    (let ((wrapper (plist-get new :node)))
+      (ebox-tree--register-legacy-flex-item-adapter
+       new ebox--flex-item-wrapper-adapter-token
+       wrapper (plist-get wrapper :ebox-content-node))))
+   (t
+    (remhash new ebox-tree--legacy-flex-item-adapter-proofs)
+    new)))
+
+(defun ebox-tree--legacy-flex-item-duplicate-p (node parent grandparent)
+  "Return non-nil when NODE is an exact duplicate inside a legacy adapter."
+  (or
+   (and (ebox-tree--legacy-flex-item-adapter-p
+         parent ebox--flex-item-direct-adapter-token)
+        (eq node (plist-get parent :node)))
+   (and (ebox-tree--legacy-flex-item-adapter-p
+         parent ebox--flex-item-wrapper-adapter-token)
+        (let ((wrapper (plist-get parent :node)))
+          (and (eq node wrapper)
+               (eq (plist-get wrapper :ebox-type) 'box)
+               (listp (plist-get wrapper :ebox-content-node)))))
+   (and (ebox-tree--legacy-flex-item-adapter-p
+         grandparent ebox--flex-item-wrapper-adapter-token)
+        (let ((wrapper (plist-get grandparent :node)))
+          (and (eq parent wrapper)
+               (eq (plist-get wrapper :ebox-type) 'box)
+               (eq node (plist-get wrapper :ebox-content-node)))))))
+
+(defun ebox-tree--validate-node-parent-participation
+    (node parent-kind parent grandparent source)
+  "Validate NODE participation under PARENT-KIND from AUTHOR or COMPUTED SOURCE."
+  (unless (ebox-tree--legacy-flex-item-duplicate-p node parent grandparent)
+    (let ((keys (ebox-tree--node-participation-keys node source)))
+      (when keys
+        (when (eq (plist-get node :ebox-kind) 'text)
+          (error "Ebox Text cannot carry participation properties: %S" keys))
+        (pcase parent-kind
+          ('flex
+           (when (cl-intersection
+                  keys ebox-tree--grid-participation-keys :test #'eq)
+             (error "Grid participation requires a direct Grid parent: %S"
+                    keys)))
+          ('grid
+           (when (cl-intersection
+                  keys ebox-tree--flex-participation-keys :test #'eq)
+             (error "Flex participation requires a direct Flex parent: %S"
+                    keys)))
+          (_ (error "Ebox participation requires a Flex or Grid parent: %S"
+                    keys))))))
+  node)
+
+(defun ebox-tree--indexed-parent-context
+    (node-id node-table parent-table)
+  "Return (PARENT-KIND PARENT GRANDPARENT) for NODE-ID in indexed tree."
+  (let* ((parent-id (gethash node-id parent-table))
+         (parent (and parent-id (gethash parent-id node-table)))
+         (grandparent-id (and parent-id (gethash parent-id parent-table)))
+         (grandparent (and grandparent-id
+                           (gethash grandparent-id node-table)))
+         (layout-owner parent))
+    (while (and layout-owner
+                (eq (plist-get layout-owner :ebox-type) 'flex-item))
+      (let ((owner-id (plist-get layout-owner :node-id)))
+        (setq owner-id (and owner-id (gethash owner-id parent-table))
+              layout-owner (and owner-id (gethash owner-id node-table)))))
+    (list (and layout-owner
+               (ebox-tree--child-layout-kind layout-owner nil))
+          parent grandparent)))
+
+(defun ebox-tree-validate-indexed-participation
+    (node-table parent-table node-ids &optional source)
+  "Validate NODE-IDS in indexed final tree using AUTHOR or COMPUTED SOURCE."
+  (setq source (or source 'author))
+  (dolist (node-id (delete-dups (copy-sequence node-ids)))
+    (when-let* ((node (gethash node-id node-table)))
+      (pcase-let ((`(,parent-kind ,parent ,grandparent)
+                   (ebox-tree--indexed-parent-context
+                    node-id node-table parent-table)))
+        (ebox-tree--validate-node-parent-participation
+         node parent-kind parent grandparent source))))
+  node-table)
+
+(defun ebox-tree-validate-declarative-root
+    (root &optional validate-participation-p root-parent-kind)
   "Validate ROOT for one declarative runtime commit.
 
 The runtime tree must be a proper tree: a node object cannot appear in two
@@ -502,7 +771,7 @@ and all explicit identities are compared with `equal'.  Return ROOT on success."
         (active (make-hash-table :test 'eq))
         (range-refs (make-hash-table :test 'equal)))
     (cl-labels
-        ((visit (node)
+        ((visit (node parent-kind parent grandparent)
            (when (and (listp node) (not (stringp node)))
              (when (gethash node active)
                (error "Ebox declarative tree contains a cycle"))
@@ -510,6 +779,9 @@ and all explicit identities are compared with `equal'.  Return ROOT on success."
                (error "Ebox declarative tree reuses one node object"))
              (puthash node t seen)
              (puthash node t active)
+             (when validate-participation-p
+               (ebox-tree--validate-node-parent-participation
+                node parent-kind parent grandparent 'author))
              (let* ((keys (make-hash-table :test 'equal))
                     (type (plist-get node :ebox-type))
                     (raw (and (memq type '(concat stack flex grid))
@@ -547,9 +819,11 @@ and all explicit identities are compared with `equal'.  Return ROOT on success."
                        (error "Ebox declarative siblings use duplicate key %S"
                               key))
                      (puthash key t keys))
-                 (visit child)))
+                 (visit child
+                        (ebox-tree--child-layout-kind node parent-kind)
+                        node parent)))
              (remhash node active))))
-      (visit root))
+      (visit root root-parent-kind nil nil))
     root))
 
 (defun ebox-tree-clear-runtime-identities (root)
diff --git a/tests/ebox-commit-tests.el b/tests/ebox-commit-tests.el
index 466fc40..f857895 100644
--- a/tests/ebox-commit-tests.el
+++ b/tests/ebox-commit-tests.el
@@ -1260,6 +1260,91 @@ remain retained identities."
       (when (buffer-live-p buffer)
         (kill-buffer buffer)))))
 
+(ert-deftest ebox-candidate-rejects-invalid-final-parent-participation ()
+  "A detached replacement must be revalidated after its final graft."
+  (let* ((buffer
+          (ebox-render-to-buffer
+           (generate-new-buffer-name " *ebox-participation-rollback*")
+           (ebox-column
+            (ebox-create :key 'target :host-ref 'target
+                         :content "Stable" :width '(80)))))
+         (surface (with-current-buffer buffer ebox-surface--buffer-surface))
+         (revision (tp-surface-revision surface))
+         (before (ebox-commit-test--buffer-string buffer))
+         (candidate (ebox-candidate-begin buffer)))
+    (unwind-protect
+        (progn
+          ;; Detached subtrees do not know their parent yet, so recording the
+          ;; replacement is legal.  The final Column graft is authoritative.
+          (ebox-candidate-replace-host-ref
+           candidate 'target
+           (ebox-create :key 'target :host-ref 'target
+                        :content "Invalid" :width '(80) :flex-grow 1))
+          (should-error (ebox-commit buffer candidate) :type 'error)
+          (should (= (tp-surface-revision surface) revision))
+          (should (equal (ebox-commit-test--buffer-string buffer) before)))
+      (when (buffer-live-p buffer)
+        (kill-buffer buffer)))))
+
+(ert-deftest ebox-candidate-cannot-launder-legacy-adapter-provenance ()
+  "A public path copy must not authenticate a replacement as adapter output."
+  (let* ((adapter
+          (ebox-flex-item
+           (ebox-create :host-ref 'target :content "Stable" :width '(80))))
+         (buffer
+          (ebox-render-to-buffer
+           (generate-new-buffer-name " *ebox-adapter-proof-rollback*")
+           (ebox-column adapter)))
+         (surface (with-current-buffer buffer ebox-surface--buffer-surface))
+         (revision (tp-surface-revision surface))
+         (before (ebox-commit-test--buffer-string buffer))
+         (candidate (ebox-candidate-begin buffer))
+         (replacement
+          (ebox-create :host-ref 'target :content "Invalid" :width '(80))))
+    (plist-put replacement :flex-grow 1)
+    (unwind-protect
+        (progn
+          (ebox-candidate-replace-host-ref candidate 'target replacement)
+          (should-error (ebox-commit buffer candidate) :type 'error)
+          (should (= (tp-surface-revision surface) revision))
+          (should (equal (ebox-commit-test--buffer-string buffer) before)))
+      (when (buffer-live-p buffer)
+        (kill-buffer buffer)))))
+
+(ert-deftest ebox-candidate-participation-validation-stays-changed-local ()
+  "One replacement must not participation-validate every sibling."
+  (let* ((children
+          (cl-loop for index below 200
+                   collect
+                   (if (= index 99)
+                       (ebox-create :key index :host-ref 'target
+                                    :content (number-to-string index))
+                     (ebox-create :key index
+                                  :content (number-to-string index)))))
+         (buffer
+          (ebox-render-to-buffer
+           (generate-new-buffer-name " *ebox-participation-local*")
+           (apply #'ebox-column children)))
+         (candidate (ebox-candidate-begin buffer))
+         (original
+          (symbol-function 'ebox-tree-validate-indexed-participation))
+         author-node-counts)
+    (unwind-protect
+        (progn
+          (ebox-candidate-replace-host-ref
+           candidate 'target
+           (ebox-create :key 99 :host-ref 'target :content "changed"))
+          (cl-letf
+              (((symbol-function 'ebox-tree-validate-indexed-participation)
+                (lambda (node-table parent-table node-ids &optional source)
+                  (when (eq source 'author)
+                    (push (length node-ids) author-node-counts))
+                  (funcall original node-table parent-table node-ids source))))
+            (ebox-commit buffer candidate))
+          (should (equal author-node-counts '(2))))
+      (when (buffer-live-p buffer)
+        (kill-buffer buffer)))))
+
 (ert-deftest ebox-candidate-root-replacement-reuses-candidate-validity-contract ()
   "Root replacement rejects invalid, other-buffer, stale, and sealed use."
   (let ((first (generate-new-buffer " *ebox-root-valid-first*"))
diff --git a/tests/ebox-dsl-tests.el b/tests/ebox-dsl-tests.el
index a6d595a..bbb6cd2 100644
--- a/tests/ebox-dsl-tests.el
+++ b/tests/ebox-dsl-tests.el
@@ -421,6 +421,106 @@
     (should (= (ebox-dsl-test--tree-count box (lambda (_node) t)) 5))
     (should (= (ebox-dsl-test--tree-count box #'ebox-node-kind) 5))))
 
+(ert-deftest ebox-canonical-participation-validates-the-final-parent-context ()
+  "Participation should be legal only under its matching direct parent layout."
+  (cl-labels
+      ((child (&rest props)
+         (apply #'ebox-box-create
+                :layout (ebox-normal-layout-create)
+                :children (list (ebox-text-create :value "A"))
+                props))
+       (parent (layout child)
+         (ebox-box-create :layout layout :children (list child))))
+    (should (stringp
+             (ebox-render
+              (parent (ebox-flex-layout-create) (child :flex-grow 1)))))
+    (should (stringp
+             (ebox-render
+              (parent (ebox-grid-layout-create) (child :grid-row 1)))))
+    (should-error
+     (ebox-render (child :flex-grow 1)) :type 'error)
+    (should-error
+     (ebox-render
+      (parent (ebox-normal-layout-create) (child :flex-grow 1)))
+     :type 'error)
+    (should-error
+     (ebox-render
+      (parent (ebox-flex-layout-create) (child :grid-row 1)))
+     :type 'error)
+    (should-error
+     (ebox-render
+      (parent (ebox-grid-layout-create) (child :flex-grow 1)))
+     :type 'error)
+    (let ((text (ebox-text-create :value "A")))
+      (plist-put text :flex-grow 1)
+      (should-error
+       (ebox-render (parent (ebox-flex-layout-create) text)) :type 'error))
+    ;; The final candidate is authoritative after ECSS projection too.
+    (let ((ebox-style-stylesheet (ecss-stylesheet-create)))
+      (ebox-style-add-rule "box" (list :flex-grow 1))
+      (should-error
+       (ebox-render
+        (parent (ebox-normal-layout-create) (child)))
+       :type 'error))
+    ;; Detached Flex geometry retains its final parent during computed style.
+    (let ((ebox-style-stylesheet (ecss-stylesheet-create))
+          (detached (child)))
+      (plist-put detached :class "detached-flex-item")
+      (ebox-style-add-rule ".detached-flex-item" (list :flex-grow 1))
+      (let ((ebox--render-root-parent-kind 'flex))
+        (should (stringp (ebox-render detached))))
+      ;; The dynamic context is exact and cannot authorize a later root.
+      (should-error (ebox-render detached) :type 'error))
+    (let ((ebox-style-stylesheet (ecss-stylesheet-create))
+          (detached (child)))
+      (plist-put detached :class "detached-flex-item")
+      (ebox-style-add-rule ".detached-flex-item" (list :grid-row 1))
+      (let ((ebox--render-root-parent-kind 'flex))
+        (should-error (ebox-render detached) :type 'error)))
+    ;; A real legacy adapter exempts only its two exact duplicate nodes.
+    (let* ((illegal-grandchild (child :flex-grow 1))
+           (nested (parent (ebox-normal-layout-create) illegal-grandchild))
+           (adapter (ebox-flex-item nested :flex-grow 1 :padding 1)))
+      ;; The obsolete mutable depth field must have no authority.
+      (plist-put adapter :ebox-flex-item-adapter-depth 99)
+      (should-error
+       (ebox-render (ebox-flex :width '(100) adapter))
+       :type 'error))
+    ;; Provenance binds the exact constructor-time source identities.
+    (let* ((adapter (ebox-flex-item (ebox-create :content "original")))
+           (replacement (ebox-create :content "replacement")))
+      (plist-put replacement :flex-grow 1)
+      (plist-put adapter :node replacement)
+      (should-error
+       (ebox-render (ebox-column adapter)) :type 'error))
+    (let* ((adapter (ebox-flex-item (ebox-create :content "original")))
+           (source (plist-get adapter :node))
+           (replacement (ebox-create :content "replacement")))
+      (plist-put replacement :flex-grow 1)
+      (setq adapter
+            (ebox-tree-copy-with-direct-child-replacements
+             adapter (list (cons source replacement))))
+      (should-error
+       (ebox-render (ebox-column adapter)) :type 'error))
+    (let* ((adapter
+            (ebox-flex-item (ebox-create :content "original") :padding 1))
+           (wrapper (plist-get adapter :node))
+           (replacement (ebox-create :content "replacement")))
+      (plist-put replacement :flex-grow 1)
+      (plist-put wrapper :ebox-content-node replacement)
+      (should-error
+       (ebox-render (ebox-column adapter)) :type 'error))
+    ;; Internal layout and parent participation are orthogonal facts.
+    (should
+     (stringp
+      (ebox-render
+       (parent
+        (ebox-grid-layout-create)
+        (ebox-box-create
+         :layout (ebox-flex-layout-create)
+         :children (list (ebox-text-create :value "A"))
+         :grid-row 1)))))))
+
 (ert-deftest ebox-canonical-grid-revalidates-the-typed-config-boundary ()
   "GridConfig must reject frame properties and cross-kind property sets."
   (let ((extra-frame-prop (ebox-grid-layout-create))