diff --git a/ai/transcript.ml b/ai/transcript.ml
index cc8b34f7..6fab4a4f 100644
--- a/ai/transcript.ml
+++ b/ai/transcript.ml
@@ -235,6 +235,11 @@ let rendered_body cfg fallback item_s =
   | Some view -> view
   | None -> fallback item_s
 
+let content_row cfg body =
+  row ~main:`center ~grow:1.0 ~min_width:0
+    [ column ~grow:1.0 ~max_width:cfg.content_width ~min_width:0
+        ~cross:`stretch ~padding_horizontal:8 ~padding_vertical:4 [ body ] ]
+
 let grouped_item cfg (item_s : item Signal.signal) : t =
   scoped (fun _context ->
     let item = sample item_s in
@@ -279,7 +284,7 @@ let tool_group cfg (row_s : row Signal.signal) : t =
       in
       Signal.set open_ (Some (not current))
     in
-    accordion
+    content_row cfg (accordion
       ~text:(reactive
                (fun (items : item list) (active : bool) ->
                   let n = List.length items in
@@ -292,7 +297,7 @@ let tool_group cfg (row_s : row Signal.signal) : t =
       ~accessibility_identifier:"lui-ai-tool-group"
       [ column ~cross:`stretch ~gap:6
           [ keyed ~source:items_s ~key:(fun (i : item) -> i.id)
-              ~cmp:String.compare ~mount:(grouped_item cfg) ] ])
+              ~cmp:String.compare ~mount:(grouped_item cfg) ] ]))
 
 let builtin_body cfg context (item_s : item Signal.signal) : t =
   ignore context;
@@ -315,9 +320,7 @@ let item_row_of cfg context (item_s : item Signal.signal) : t =
     ?style_class:
       (match cfg.item_class item with "" -> None | s -> Some s)
     ~data_attrs:(reactive cfg.item_data_attrs item_s)
-    [ row ~main:`center ~grow:1.0
-        [ column ~grow:1.0 ~max_width:cfg.content_width ~cross:`stretch
-            ~padding_horizontal:8 ~padding_vertical:4 [ body ] ] ]
+    [ content_row cfg body ]
 
 let row_of cfg (row_s : row Signal.signal) : t =
   scoped (fun context ->
diff --git a/docs/agent-guide/proposed/bugfix/2026-10-19-bounded-tool-group-layout.md b/docs/agent-guide/proposed/bugfix/2026-10-19-bounded-tool-group-layout.md
new file mode 100644
index 00000000..e7258c51
--- /dev/null
+++ b/docs/agent-guide/proposed/bugfix/2026-10-19-bounded-tool-group-layout.md
@@ -0,0 +1,57 @@
+# Bounded Tool Group Layout
+
+## Problem
+
+Mapre's actual browser consumer shows a completed tool-group summary spanning
+1160px while ordinary message content is bounded to 768px. The transcript's
+Tool_group path bypasses the centered content column used by Single rows.
+
+## Proposal
+
+Give grouped tools the same centered bounded content surface as messages while
+keeping the outer scroll viewport full width. Share the static layout helper
+between Single and Tool_group, preserve host renderers, keyed child identity,
+reactive metadata and disclosure state. Do not add app-specific CSS, vendor a
+copy, alter Dune files or add another scroll owner.
+
+## Alternatives considered
+
+### Constrain the scroll viewport
+
+Rejected because it recreates ignored wheel input in blank conversation gutters.
+
+### Override the consumer with layout CSS
+
+Rejected because the generic transcript already owns content_width and must
+apply it consistently across row kinds and platforms.
+
+## Acceptance criteria
+
+- Native mounted grouped-only transcripts respect an explicit non-default
+  content_width and include centered content layout; the case fails first.
+- Existing host-rendering, streaming, keyed lifetime, disclosure and transcript
+  tests pass. The outer viewport still grows across the conversation pane.
+- The actual Mapre browser consumer has bounded centered activity, accessible
+  disclosure and stable host details on desktop and mobile.
+- Submit the upstream PR and consume an installed reviewed package; no vendoring.
+
+## Risks
+
+- Group summaries gain the same horizontal/vertical padding as message content;
+  disclosure height changes must retain the host's scroll anchoring behavior.
+
+## Questions
+
+None. This completes the existing transcript layout contract and the user's
+explicit request to fix shared LUI issues upstream before consumer integration.
+
+## Verification
+
+The new grouped-only mounted native case fails before the fix because no
+centered/bounded content node exists. It now passes with a non-default 560px
+content_width; all 25 AI cases and all 148 native test_lui cases pass. Melange
+and install targets compile. The helper is shared with ordinary message rows;
+no scroll viewport, event handler, public signature or dependency changes.
+The consumer's desktop browser first reports an unbounded 1160px summary.
+Final installed-package consumer acceptance remains pending; this decision
+stays proposed until that actual browser proof is complete.
diff --git a/test/test_lui_ai.ml b/test/test_lui_ai.ml
index 51dba2fb..b7130ed5 100644
--- a/test/test_lui_ai.ml
+++ b/test/test_lui_ai.ml
@@ -607,6 +607,20 @@ let test_bounded_layout () =
   ignore bounded;
   drive_dispose s
 
+let test_group_bounded_layout () =
+  let view _ctx source _send =
+    Lui_ai.Transcript.transcript ~content_width:560 ~messages:source () in
+  let s = drive_mount
+      ~initial:[ tool_item "t1" `completed;
+                 tool_item "t2" `completed ] ~view in
+  ignore (find_node s (fun n ->
+      n.Drive.Model.kind = "row"
+      && node_prop n "main" = Some (StringValue "center")));
+  ignore (find_node s (fun n ->
+      n.Drive.Model.kind = "column"
+      && node_prop n "max-width" = Some (IntValue 560)));
+  drive_dispose s
+
 let test_container_data_attrs_signal () =
   let attrs_ref = ref None in
   let view _ctx model_s _send =
@@ -653,6 +667,7 @@ let tests =
       [ Alcotest.test_case "grouped tail key" `Quick test_grouped_tail_key;
         Alcotest.test_case "grouped pin" `Quick test_grouped_pin;
         Alcotest.test_case "bounded layout" `Quick test_bounded_layout;
+        Alcotest.test_case "group bounded layout" `Quick test_group_bounded_layout;
         Alcotest.test_case "container attrs signal" `Quick
           test_container_data_attrs_signal;
         Alcotest.test_case "streaming updates same row" `Quick