diff --git a/webui/src/components/ChatList.tsx b/webui/src/components/ChatList.tsx
index f0d14c2d6..1c7cf3980 100644
--- a/webui/src/components/ChatList.tsx
+++ b/webui/src/components/ChatList.tsx
@@ -40,6 +40,7 @@ import {
   visibleSessionsForGroup,
   type ChatGroupLabels,
 } from "@/lib/chat-groups";
+import { writeDraggedSession } from "@/lib/session-drag";
 import { cn } from "@/lib/utils";
 import type { ChatSummary, SidebarDensity, SidebarSortMode } from "@/lib/types";
 
@@ -276,10 +277,19 @@ export const ChatList = memo(function ChatList({
                           <button
                             type="button"
                             onClick={() => onSelect(s.key)}
+                            draggable={!active}
+                            onDragStart={(event) => {
+                              if (active) {
+                                event.preventDefault();
+                                return;
+                              }
+                              writeDraggedSession(event.dataTransfer, s.key);
+                            }}
                             aria-current={active ? "page" : undefined}
                             title={tooltipTitle}
                             className={cn(
                               "min-w-0 flex-1 overflow-hidden text-left",
+                              !active && "cursor-grab active:cursor-grabbing",
                               compact ? "py-1" : "py-1.5",
                               projectMode && "pl-7",
                             )}
diff --git a/webui/src/components/thread/ThreadComposer.tsx b/webui/src/components/thread/ThreadComposer.tsx
index 66de104e6..c11de8373 100644
--- a/webui/src/components/thread/ThreadComposer.tsx
+++ b/webui/src/components/thread/ThreadComposer.tsx
@@ -105,6 +105,10 @@ import {
   isSideChannelLifecycle,
   slashCommandLifecycle,
 } from "@/lib/slash-command";
+import {
+  hasDraggedSession,
+  readDraggedSession,
+} from "@/lib/session-drag";
 import { formatQuotedUserMessage } from "@/lib/user-message-quote";
 import { cn } from "@/lib/utils";
 
@@ -1606,10 +1610,13 @@ export function ThreadComposer({
     [isStreaming, onStop, recentSlashCommands, resizeTextarea, skillQuery, value],
   );
 
-  const chooseMentionCandidate = useCallback(
-    (candidate: MentionCandidate) => {
-      if (!cliAppMention) return;
+  const insertMentionCandidate = useCallback(
+    (candidate: MentionCandidate, start: number, end: number) => {
       if (candidate.kind === "session") {
+        const alreadySelected = activeSessionMentions.some(
+          (mention) => mention.session_key === candidate.mention.session_key,
+        );
+        if (!alreadySelected && activeSessionMentions.length >= SESSION_MENTIONS_LIMIT) return;
         const name = candidate.name.toLowerCase();
         setSelectedSessionMentions([
           ...activeSessionMentions.filter((mention) => (
@@ -1619,10 +1626,13 @@ export function ThreadComposer({
           candidate.mention,
         ]);
       }
-      const suffix = value.slice(cliAppMention.end);
-      const mention = `@${candidate.name}${suffix.startsWith(" ") ? "" : " "}`;
-      const next = `${value.slice(0, cliAppMention.start)}${mention}${suffix}`;
-      const nextCursor = cliAppMention.start + mention.length;
+      const prefix = value.slice(0, start);
+      const suffix = value.slice(end);
+      const leadingSpace = prefix && !/\s$/.test(prefix) ? " " : "";
+      const trailingSpace = /^\s/.test(suffix) ? "" : " ";
+      const mention = `${leadingSpace}@${candidate.name}${trailingSpace}`;
+      const next = `${prefix}${mention}${suffix}`;
+      const nextCursor = prefix.length + mention.length;
       setValue(next);
       setCursorPosition(nextCursor);
       setCliAppMenuDismissed(true);
@@ -1636,9 +1646,40 @@ export function ThreadComposer({
         el.setSelectionRange(nextCursor, nextCursor);
       });
     },
-    [activeSessionMentions, cliAppMention, resizeTextarea, value],
+    [activeSessionMentions, resizeTextarea, value],
   );
 
+  const chooseMentionCandidate = useCallback(
+    (candidate: MentionCandidate) => {
+      if (!cliAppMention) return;
+      insertMentionCandidate(candidate, cliAppMention.start, cliAppMention.end);
+    },
+    [cliAppMention, insertMentionCandidate],
+  );
+
+  const handleSessionDrop = useCallback((event: React.DragEvent) => {
+    if (!hasDraggedSession(event.dataTransfer)) return false;
+    event.preventDefault();
+    if (disabled) return true;
+    const sessionKey = readDraggedSession(event.dataTransfer);
+    const mention = availableSessionMentions.find(
+      (candidate) => candidate.session_key === sessionKey,
+    );
+    if (!mention) return true;
+    const caret = textareaRef.current?.selectionStart ?? value.length;
+    insertMentionCandidate(
+      {
+        kind: "session",
+        name: mention.name,
+        displayName: mention.title || mention.name,
+        mention,
+      },
+      caret,
+      textareaRef.current?.selectionEnd ?? caret,
+    );
+    return true;
+  }, [availableSessionMentions, disabled, insertMentionCandidate, value.length]);
+
   const clearComposerText = useCallback((restoreFocus = true) => {
     setValue("");
     setSelectedSessionMentions([]);
@@ -2068,9 +2109,18 @@ export function ThreadComposer({
         submit();
       }}
       onDragEnter={onDragEnter}
-      onDragOver={onDragOver}
+      onDragOver={(event) => {
+        if (hasDraggedSession(event.dataTransfer)) {
+          event.preventDefault();
+          event.dataTransfer.dropEffect = "copy";
+        } else {
+          onDragOver(event);
+        }
+      }}
       onDragLeave={onDragLeave}
-      onDrop={onDrop}
+      onDrop={(event) => {
+        if (!handleSessionDrop(event)) onDrop(event);
+      }}
       className={cn("relative w-full", isHero ? "px-0" : "px-1 pb-1.5 pt-1 sm:px-0")}
     >
       {showSlashMenu ? (
diff --git a/webui/src/lib/session-drag.ts b/webui/src/lib/session-drag.ts
new file mode 100644
index 000000000..ee7aecf61
--- /dev/null
+++ b/webui/src/lib/session-drag.ts
@@ -0,0 +1,18 @@
+export const SESSION_DRAG_TYPE = "application/x-nanobot-session-key";
+
+export function hasDraggedSession(dataTransfer: DataTransfer): boolean {
+  return Array.from(dataTransfer.types).includes(SESSION_DRAG_TYPE);
+}
+
+export function readDraggedSession(dataTransfer: DataTransfer): string | null {
+  const sessionKey = dataTransfer.getData(SESSION_DRAG_TYPE).trim();
+  return sessionKey || null;
+}
+
+export function writeDraggedSession(
+  dataTransfer: DataTransfer,
+  sessionKey: string,
+): void {
+  dataTransfer.effectAllowed = "copy";
+  dataTransfer.setData(SESSION_DRAG_TYPE, sessionKey);
+}
diff --git a/webui/src/tests/chat-list.test.tsx b/webui/src/tests/chat-list.test.tsx
index e2b0e3b3b..b9a3a7937 100644
--- a/webui/src/tests/chat-list.test.tsx
+++ b/webui/src/tests/chat-list.test.tsx
@@ -2,6 +2,7 @@ import { fireEvent, render, screen, within } from "@testing-library/react";
 import { afterEach, describe, expect, it, vi } from "vitest";
 
 import { ChatList } from "@/components/ChatList";
+import { SESSION_DRAG_TYPE } from "@/lib/session-drag";
 import type { ChatSummary } from "@/lib/types";
 
 function session(overrides: Partial<ChatSummary>): ChatSummary {
@@ -47,6 +48,39 @@ describe("ChatList", () => {
     vi.unstubAllGlobals();
   });
 
+  it("exposes inactive chats as session mention drag sources", () => {
+    const dataTransfer = {
+      effectAllowed: "",
+      setData: vi.fn(),
+    };
+    render(
+      <ChatList
+        sessions={[
+          session({ chatId: "active", title: "Active chat" }),
+          session({ chatId: "reference", title: "Reference chat" }),
+        ]}
+        activeKey="websocket:active"
+        onSelect={vi.fn()}
+        onRequestDelete={vi.fn()}
+        onTogglePin={vi.fn()}
+        onRequestRename={vi.fn()}
+        onToggleArchive={vi.fn()}
+      />,
+    );
+
+    expect(screen.getByRole("button", { name: "Active chat" }))
+      .toHaveAttribute("draggable", "false");
+    const reference = screen.getByRole("button", { name: "Reference chat" });
+    expect(reference).toHaveAttribute("draggable", "true");
+
+    fireEvent.dragStart(reference, { dataTransfer });
+
+    expect(dataTransfer.setData).toHaveBeenCalledWith(
+      SESSION_DRAG_TYPE,
+      "websocket:reference",
+    );
+  });
+
   it("orders chats by latest session activity by default", () => {
     const sessions = [
       session({
diff --git a/webui/src/tests/thread-composer.test.tsx b/webui/src/tests/thread-composer.test.tsx
index e3e3f91a9..5976770df 100644
--- a/webui/src/tests/thread-composer.test.tsx
+++ b/webui/src/tests/thread-composer.test.tsx
@@ -3,6 +3,7 @@ import userEvent from "@testing-library/user-event";
 import { afterEach, describe, expect, it, vi } from "vitest";
 
 import { ThreadComposer } from "@/components/thread/ThreadComposer";
+import { SESSION_DRAG_TYPE } from "@/lib/session-drag";
 import type { ChatSummary, CliAppInfo, McpPresetInfo, SlashCommand } from "@/lib/types";
 
 vi.mock("@/lib/imageEncode", () => ({
@@ -1593,6 +1594,47 @@ describe("ThreadComposer", () => {
     });
   });
 
+  it("turns a dropped sidebar session into the shared structured mention", () => {
+    const onSend = vi.fn();
+    render(
+      <ThreadComposer
+        onSend={onSend}
+        placeholder="Type your message..."
+        sessions={[session("pricing", "收费设计", "讨论云存储")]}
+      />,
+    );
+
+    const input = screen.getByLabelText("Message input") as HTMLTextAreaElement;
+    fireEvent.change(input, { target: { value: "Compare notes" } });
+    input.setSelectionRange(7, 7);
+    const dataTransfer = {
+      types: [SESSION_DRAG_TYPE],
+      effectAllowed: "copy",
+      dropEffect: "none",
+      files: [],
+      getData: (type: string) => (
+        type === SESSION_DRAG_TYPE ? "websocket:pricing" : ""
+      ),
+    };
+
+    fireEvent.dragEnter(input, { dataTransfer });
+    fireEvent.dragOver(input, { dataTransfer });
+    fireEvent.drop(input, { dataTransfer });
+
+    expect(input).toHaveValue("Compare @收费设计 notes");
+    expect(screen.getByTestId("composer-session-mention-收费设计"))
+      .toHaveTextContent("@收费设计");
+
+    fireEvent.click(screen.getByRole("button", { name: "Send message" }));
+    expect(onSend).toHaveBeenCalledWith("Compare @收费设计 notes", undefined, {
+      sessionMentions: [{
+        name: "收费设计",
+        session_key: "websocket:pricing",
+        title: "收费设计",
+      }],
+    });
+  });
+
   it("disambiguates duplicate and capability-colliding session names", () => {
     render(
       <ThreadComposer