diff --git a/apps/web/src/chat/threads-api.ts b/apps/web/src/chat/threads-api.ts
index d83e6044b..a1801a46c 100644
--- a/apps/web/src/chat/threads-api.ts
+++ b/apps/web/src/chat/threads-api.ts
@@ -482,8 +482,10 @@ export type RoomMessage = {
   readonly authorName: string;
   readonly body: string;
   readonly at: string;
-  /** Native in-reply-to children: the room's sub-threads. */
-  readonly replies: readonly RoomMessage[];
+  /** In-reply-to parent, when this turn named one and it matched a known
+   * message — metadata for the sub-thread panel, never used to hide a turn
+   * from the main timeline. */
+  readonly parentMessageId: string | undefined;
 };
 
 function authorName(address: string): string {
@@ -517,8 +519,10 @@ async function readRoomFolder(tenantId: string, folder: "INBOX" | "Sent"): Promi
   }));
 }
 
-/** The room timeline: every root turn oldest first, its in-reply-to chain
- * nested beneath it as the room's sub-threads. */
+/** The room timeline: every turn oldest first, flat — no turn is ever
+ * dropped from the main list. A turn whose in-reply-to names a known
+ * message keeps that as `parentMessageId`, metadata for the sub-thread
+ * panel to walk the ancestor chain of whichever turn the person opened. */
 export async function readRoom(tenantId: string): Promise<readonly RoomMessage[]> {
   const [inbox, sent] = await Promise.all([
     readRoomFolder(tenantId, "INBOX"),
@@ -526,26 +530,35 @@ export async function readRoom(tenantId: string): Promise<readonly RoomMessage[]
   ]);
   const turns = [...inbox, ...sent].sort((a, b) => Date.parse(a.at) - Date.parse(b.at));
   const known = new Set(turns.map((turn) => turn.messageId));
-  const children = new Map<string, RoomTurn[]>();
-  const roots: RoomTurn[] = [];
-  for (const turn of turns) {
-    const parentId = turn.parentId;
-    if (parentId === undefined || !known.has(parentId)) {
-      roots.push(turn);
-      continue;
-    }
-    children.set(parentId, [...(children.get(parentId) ?? []), turn]);
-  }
-  const build = (turn: RoomTurn): RoomMessage => ({
+  return turns.map((turn) => ({
     id: turn.id,
     messageId: turn.messageId,
     author: turn.author,
     authorName: turn.authorName,
     body: turn.body,
     at: turn.at,
-    replies: (children.get(turn.messageId) ?? []).map(build),
-  });
-  return roots.map(build);
+    parentMessageId:
+      turn.parentId !== undefined && known.has(turn.parentId) ? turn.parentId : undefined,
+  }));
+}
+
+/** The ancestor chain of a turn, oldest first, ending with the turn itself
+ * — what the sub-thread panel shows for the turn the person opened. */
+export function ancestorChain(
+  messages: readonly RoomMessage[],
+  messageId: string,
+): readonly RoomMessage[] {
+  const byMessageId = new Map(messages.map((message) => [message.messageId, message]));
+  const chain: RoomMessage[] = [];
+  let current = byMessageId.get(messageId);
+  const seen = new Set<string>();
+  while (current !== undefined && !seen.has(current.messageId)) {
+    seen.add(current.messageId);
+    chain.unshift(current);
+    current =
+      current.parentMessageId === undefined ? undefined : byMessageId.get(current.parentMessageId);
+  }
+  return chain;
 }
 
 /** The one send seam for a room: a single mailbox send addressed to every
diff --git a/apps/web/src/pages/workbench-room-page.tsx b/apps/web/src/pages/workbench-room-page.tsx
index d437675d0..afd169569 100644
--- a/apps/web/src/pages/workbench-room-page.tsx
+++ b/apps/web/src/pages/workbench-room-page.tsx
@@ -22,6 +22,7 @@ import { useEffect, useState } from "react";
 import { Markdown } from "@/chat/markdown";
 import {
   agentInitials,
+  ancestorChain,
   listRoomParticipants,
   readRoom,
   sendToRoom,
@@ -82,10 +83,11 @@ function RoomComposer({
 
 function RoomMessageRow({
   message,
-  onOpenReplies,
+  onReply,
 }: {
   readonly message: RoomMessage;
-  readonly onOpenReplies: (message: RoomMessage) => void;
+  /** Undefined in the sub-thread panel, where a row is read-only context. */
+  readonly onReply?: (message: RoomMessage) => void;
 }) {
   return (
     <div className="chat-thread-message" data-author={message.author}>
@@ -96,13 +98,9 @@ function RoomMessageRow({
       </span>
       <div className="chat-thread-body">
         <Markdown text={message.body} />
-        {message.replies.length === 0 ? null : (
-          <button
-            type="button"
-            className="room-replies-link"
-            onClick={() => onOpenReplies(message)}
-          >
-            {message.replies.length === 1 ? "1 reply" : `${String(message.replies.length)} replies`}
+        {onReply === undefined ? null : (
+          <button type="button" className="room-replies-link" onClick={() => onReply(message)}>
+            Reply
           </button>
         )}
       </div>
@@ -320,7 +318,8 @@ function Room({ roomTenantId }: { readonly roomTenantId: string }) {
 
   const messages = timeline.data ?? [];
   const latestMessage = [...messages].sort((a, b) => Date.parse(b.at) - Date.parse(a.at))[0];
-  const opened = messages.find((message) => message.id === openThread);
+  const openedChain = openThread === null ? [] : ancestorChain(messages, openThread);
+  const opened = openedChain.at(-1);
   const failure: unknown = timeline.error ?? participants.error;
 
   if (failure !== null && failure !== undefined && timeline.data === undefined) {
@@ -352,7 +351,7 @@ function Room({ roomTenantId }: { readonly roomTenantId: string }) {
                   <RoomMessageRow
                     key={message.id}
                     message={message}
-                    onOpenReplies={(target) => setOpenThread(target.id)}
+                    onReply={(target) => setOpenThread(target.messageId)}
                   />
                 ))}
               </div>
@@ -380,12 +379,8 @@ function Room({ roomTenantId }: { readonly roomTenantId: string }) {
               </Button>
             </div>
             <div className="chat-thread-messages">
-              {[opened, ...opened.replies].map((message) => (
-                <RoomMessageRow
-                  key={message.id}
-                  message={message}
-                  onOpenReplies={() => undefined}
-                />
+              {openedChain.map((message) => (
+                <RoomMessageRow key={message.id} message={message} />
               ))}
             </div>
             <RoomComposer