diff --git a/nanobot/channels/websocket.py b/nanobot/channels/websocket.py
index 74c8077f4..9527c0dd7 100644
--- a/nanobot/channels/websocket.py
+++ b/nanobot/channels/websocket.py
@@ -34,7 +34,7 @@ from nanobot.utils.media_decode import (
     save_base64_data_url,
 )
 from nanobot.webui.cli_apps_api import normalize_cli_app_mentions
-from nanobot.webui.forking import create_webui_chat_fork
+from nanobot.webui.forking import handle_webui_fork_chat
 from nanobot.webui.gateway_services import GatewayServices
 from nanobot.webui.http_utils import (
     normalize_config_path as _normalize_config_path,
@@ -670,49 +670,7 @@ class WebSocketChannel(BaseChannel):
             await self._hydrate_after_subscribe(new_id)
             return
         if t == "fork_chat":
-            source_chat_id = envelope.get("source_chat_id")
-            raw_index = envelope.get("before_user_index")
-            if not _is_valid_chat_id(source_chat_id):
-                await self._send_event(connection, "error", detail="invalid source_chat_id")
-                return
-            if (
-                isinstance(raw_index, bool)
-                or not isinstance(raw_index, int)
-                or raw_index < 0
-            ):
-                await self._send_event(connection, "error", detail="invalid before_user_index")
-                return
-            if self.gateway.session_manager is None:
-                await self._send_event(connection, "error", detail="session_manager_unavailable")
-                return
-
-            try:
-                forked = create_webui_chat_fork(
-                    self.gateway.session_manager,
-                    source_chat_id=source_chat_id,
-                    before_user_index=raw_index,
-                    title=envelope.get("title") if isinstance(envelope.get("title"), str) else None,
-                )
-                if forked is None:
-                    await self._send_event(connection, "error", detail="invalid fork source or index")
-                    return
-                fork_id, fork_key = forked
-            except Exception as exc:
-                self.logger.warning("fork_chat failed: {}", exc)
-                await self._send_event(connection, "error", detail="fork_chat_failed")
-                return
-
-            scope = self._workspaces.scope_for_session_key(fork_key)
-            self._attach(connection, fork_id)
-            await self._send_event(connection, "attached", chat_id=fork_id)
-            await self._send_event(
-                connection,
-                "session_updated",
-                chat_id=fork_id,
-                scope="metadata",
-                workspace_scope=scope.payload(),
-            )
-            await self._hydrate_after_subscribe(fork_id)
+            await handle_webui_fork_chat(self, connection, envelope)
             return
         if t == "attach":
             cid = envelope.get("chat_id")
diff --git a/nanobot/webui/forking.py b/nanobot/webui/forking.py
index c867ffc66..247cb8e6f 100644
--- a/nanobot/webui/forking.py
+++ b/nanobot/webui/forking.py
@@ -2,7 +2,10 @@
 
 from __future__ import annotations
 
+import re
 import uuid
+from collections.abc import Mapping
+from typing import Any
 
 from nanobot.session.manager import SessionManager
 from nanobot.session.webui_turns import WEBUI_TITLE_METADATA_KEY, clean_generated_title
@@ -13,6 +16,12 @@ from nanobot.webui.transcript import (
     write_session_messages_as_transcript,
 )
 
+_WEBUI_CHAT_ID_RE = re.compile(r"^[A-Za-z0-9_:-]{1,64}$")
+
+
+def _valid_webui_chat_id(value: Any) -> bool:
+    return isinstance(value, str) and _WEBUI_CHAT_ID_RE.match(value) is not None
+
 
 def create_webui_chat_fork(
     session_manager: SessionManager,
@@ -52,3 +61,53 @@ def create_webui_chat_fork(
         session_manager.delete_session(target_key)
         raise
     return new_id, target_key
+
+
+async def handle_webui_fork_chat(channel: Any, connection: Any, envelope: Mapping[str, Any]) -> None:
+    """Handle the WebUI/desktop ``fork_chat`` websocket command.
+
+    ``websocket.py`` owns the transport. This module owns WebUI fork semantics:
+    validate the request, clone session/transcript state, attach the new chat,
+    and hydrate the client.
+    """
+    source_chat_id = envelope.get("source_chat_id")
+    raw_index = envelope.get("before_user_index")
+    if not _valid_webui_chat_id(source_chat_id):
+        await channel._send_event(connection, "error", detail="invalid source_chat_id")
+        return
+    if isinstance(raw_index, bool) or not isinstance(raw_index, int) or raw_index < 0:
+        await channel._send_event(connection, "error", detail="invalid before_user_index")
+        return
+
+    session_manager = channel.gateway.session_manager
+    if session_manager is None:
+        await channel._send_event(connection, "error", detail="session_manager_unavailable")
+        return
+
+    try:
+        forked = create_webui_chat_fork(
+            session_manager,
+            source_chat_id=source_chat_id,
+            before_user_index=raw_index,
+            title=envelope.get("title") if isinstance(envelope.get("title"), str) else None,
+        )
+        if forked is None:
+            await channel._send_event(connection, "error", detail="invalid fork source or index")
+            return
+        fork_id, fork_key = forked
+    except Exception as exc:
+        channel.logger.warning("fork_chat failed: {}", exc)
+        await channel._send_event(connection, "error", detail="fork_chat_failed")
+        return
+
+    scope = channel._workspaces.scope_for_session_key(fork_key)
+    channel._attach(connection, fork_id)
+    await channel._send_event(connection, "attached", chat_id=fork_id)
+    await channel._send_event(
+        connection,
+        "session_updated",
+        chat_id=fork_id,
+        scope="metadata",
+        workspace_scope=scope.payload(),
+    )
+    await channel._hydrate_after_subscribe(fork_id)