fix(cli,session): remove nest_asyncio, make chat-profile config async-safe

.prettierignore

@@ -1 +1,2 @@
 pnpm-lock.yaml
+.pytest_cache

backend/chainlit/cli/__init__.py

@@ -4,13 +4,17 @@
 import sys
 
 import click
-import nest_asyncio
 import uvicorn
 
-# Not sure if it is necessary to call nest_asyncio.apply() before the other imports
-nest_asyncio.apply()
-
-# ruff: noqa: E402
+# nest_asyncio was intentionally removed (see https://github.com/Chainlit/chainlit/issues/2767).
+# nest_asyncio ≤ 1.6.0 patches loop.run_until_complete() via
+# asyncio.ensure_future(future, loop=self), where the ``loop=`` keyword was
+# deprecated in Python 3.8 and **removed in Python 3.14** (bpo-39529).
+# Applying it at import time silently corrupted asyncio task registration:
+# asyncio.current_task() returned None inside running coroutines, which
+# caused anyio.NoEventLoopError on every static-asset request (HTTP 500 /
+# white page) on Python 3.14.  The entry point asyncio.run(start()) is a
+# top-level call and has never needed re-entrant loop support.
 from chainlit.auth import ensure_jwt_secret
 from chainlit.cache import init_lc_cache
 from chainlit.config import (

backend/chainlit/session.py

@@ -325,43 +325,61 @@ def __init__(
         )
         self.language = match.group(1) if match else "en-US"
 
-        self.config: ChainlitConfig = self.get_config()
+        # Start with global config; chat-profile overrides are applied
+        # asynchronously via resolve_config() after construction.
+        from chainlit.config import config as global_config
+
+        self.config: ChainlitConfig = global_config
 
         ws_sessions_id[self.id] = self
         ws_sessions_sid[socket_id] = self
 
     def get_config(self) -> "ChainlitConfig":
         """
-        Return the config for this session: overridden if chat profile exists and has overrides, else global config.
+        Return the config for this session.
+
+        If ``resolve_config()`` has already been awaited the returned
+        object includes any chat-profile overrides; otherwise it falls
+        back to the global config.
+        """
+        return self.config
+
+    async def resolve_config(self) -> "ChainlitConfig":
+        """
+        Resolve chat-profile config overrides asynchronously.
+
+        Must be awaited from an async context (e.g. the SocketIO
+        ``connect`` handler) *after* the session has been constructed.
+        Once overrides are successfully applied the result is cached
+        on ``self.config`` and subsequent calls return immediately.
         """
         from chainlit.config import config as global_config
 
-        # If no chat profile, always fallback to global config
+        # Nothing to resolve when there is no chat profile.
         if not self.chat_profile:
-            return global_config
-        # If already computed, use self.config
-        if hasattr(self, "config") and self.config:
             return self.config
-        # Try to compute overrides
-        cfg = global_config
-        if global_config.code.set_chat_profiles:
-            import asyncio
 
+        # Already resolved — return cached value.
+        if self.config is not global_config:
+            return self.config
+
+        if global_config.code.set_chat_profiles:
             try:
-                profiles = asyncio.get_event_loop().run_until_complete(
-                    global_config.code.set_chat_profiles(self.user, self.language)
+                profiles = await global_config.code.set_chat_profiles(
+                    self.user, self.language
                 )
                 current_profile = next(
                     (p for p in profiles if p.name == self.chat_profile), None
                 )
                 if current_profile and getattr(
                     current_profile, "config_overrides", None
                 ):
-                    cfg = global_config.with_overrides(current_profile.config_overrides)
+                    self.config = global_config.with_overrides(
+                        current_profile.config_overrides
+                    )
             except Exception:
                 pass
-        self.config = cfg
-        return cfg
+        return self.config
 
     def restore(self, new_socket_id: str):
         """Associate a new socket id to the session."""

backend/chainlit/socket.py

@@ -188,7 +188,7 @@ def emit_call_fn(event: Literal["ask", "call_fn"], data, timeout):
         unquote(url_encoded_chat_profile) if url_encoded_chat_profile else None
     )
 
-    WebsocketSession(
+    session = WebsocketSession(
         id=session_id,
         socket_id=sid,
         emit=emit_fn,
@@ -202,6 +202,11 @@ def emit_call_fn(event: Literal["ask", "call_fn"], data, timeout):
         environ=environ,
     )
 
+    # Resolve chat-profile config overrides asynchronously.
+    # This must happen after construction so that the session is
+    # registered and the event loop is available.
+    await session.resolve_config()
+
     return True
 
 

backend/pyproject.toml

@@ -43,7 +43,6 @@ dependencies = [
     "syncer>=2.0.3,<3.0.0",
     "asyncer>=0.0.8,<0.1.0",
     "mcp>=1.11.0,<2.0.0",
-    "nest-asyncio>=1.6.0,<2.0.0",
     "click>=8.1.3,<9.0.0",
     "tomli>=2.0.1,<3.0.0",
     "pydantic>=2.7.2,<3",
@@ -95,6 +94,7 @@ tests = [
 ]
 mypy = [
     "mypy>=1.13,<2.0.0",
+    "hatchling",
     "types-requests>=2.31.0.2,<3.0.0",
     "types-aiofiles>=23.1.0.5,<26.0.0",
     "mypy-boto3-dynamodb>=1.34.113,<2.0.0",

backend/tests/test_cli.py

@@ -0,0 +1,33 @@
+"""Regression tests for chainlit.cli import behaviour.
+
+Ensures nest_asyncio is not imported by chainlit.cli.
+
+Background
+----------
+nest_asyncio ≤ 1.6.0 patches ``asyncio.BaseEventLoop.run_until_complete`` via
+``asyncio.ensure_future(future, loop=self)``.  The ``loop=`` keyword was
+deprecated in Python 3.8 and **removed in Python 3.14** (bpo-39529).  When
+``nest_asyncio.apply()`` ran at module-import time it silently corrupted asyncio
+task registration: ``asyncio.current_task()`` returned ``None`` inside running
+coroutines, causing ``anyio.NoEventLoopError`` on every static-asset request
+and a white screen for users on Python 3.14.
+
+See https://github.com/Chainlit/chainlit/issues/2767
+"""
+
+import chainlit.cli
+
+
+def test_nest_asyncio_not_in_cli_namespace():
+    """chainlit.cli must not expose nest_asyncio in its module namespace.
+
+    If ``import nest_asyncio`` is ever re-added to cli/__init__.py this test
+    will fail immediately, preventing the Python 3.14 regression from
+    being reintroduced.
+    """
+    assert not hasattr(chainlit.cli, "nest_asyncio"), (
+        "chainlit.cli exposes 'nest_asyncio' in its namespace. "
+        "Remove 'import nest_asyncio' and 'nest_asyncio.apply()' from "
+        "backend/chainlit/cli/__init__.py — nest_asyncio breaks Python 3.14 "
+        "via the removed loop= kwarg in asyncio.ensure_future (bpo-39529)."
+    )