run-llama/llama_index · error · ValueError

ChatMode.REACT and ChatMode.OPENAI are now deprecated and re

Error message

ChatMode.REACT and ChatMode.OPENAI are now deprecated and removed. Please use the ReActAgent or FunctionAgent classes from llama_index.core.agent.workflow to create an agent with a query engine tool.

What it means

BaseIndex.as_chat_engine no longer supports ChatMode.REACT and ChatMode.OPENAI. The old agent-style chat modes were removed when agents moved to the llama_index.core.agent.workflow module (ReActAgent, FunctionAgent). Calling as_chat_engine with either removed mode raises ValueError with migration instructions.

Source

Thrown at llama-index-core/llama_index/core/indices/base.py:549

            - `ChatMode.BEST` (default): Chat engine that uses an agent (react or openai) with a query engine tool
            - `ChatMode.CONTEXT`: Chat engine that uses a retriever to get context
            - `ChatMode.CONDENSE_QUESTION`: Chat engine that condenses questions
            - `ChatMode.CONDENSE_PLUS_CONTEXT`: Chat engine that condenses questions and uses a retriever to get context
            - `ChatMode.SIMPLE`: Simple chat engine that uses the LLM directly
            - `ChatMode.REACT`: Chat engine that uses a react agent with a query engine tool
            - `ChatMode.OPENAI`: Chat engine that uses an openai agent with a query engine tool
        """
        llm = (
            resolve_llm(llm, callback_manager=self._callback_manager)
            if llm
            else Settings.llm
        )

        query_engine = self.as_query_engine(llm=llm, **kwargs)

        # resolve chat mode
        if chat_mode in [ChatMode.REACT, ChatMode.OPENAI]:
            raise ValueError(
                "ChatMode.REACT and ChatMode.OPENAI are now deprecated and removed. "
                "Please use the ReActAgent or FunctionAgent classes from llama_index.core.agent.workflow "
                "to create an agent with a query engine tool."
            )

        if chat_mode == ChatMode.CONDENSE_QUESTION:
            # NOTE: lazy import
            from llama_index.core.chat_engine import CondenseQuestionChatEngine

            return CondenseQuestionChatEngine.from_defaults(
                query_engine=query_engine,
                llm=llm,
                **kwargs,
            )

        elif chat_mode == ChatMode.CONTEXT:
            from llama_index.core.chat_engine import ContextChatEngine

View on GitHub (pinned to afd0fef371)

Solutions

1. Use ReActAgent from llama_index.core.agent.workflow with a QueryEngineTool wrapping your index.
2. Use FunctionAgent from llama_index.core.agent.workflow if you prefer function/tool-calling agents.
3. For non-agent chat behavior, keep as_chat_engine with supported modes (ChatMode.CONDENSE_QUESTION, ChatMode.SIMPLE, etc.).

Example fix

# before
chat_engine = index.as_chat_engine(chat_mode=ChatMode.REACT)

# after
from llama_index.core.agent.workflow import FunctionAgent
from llama_index.core.tools import QueryEngineTool

agent = FunctionAgent(
    tools=[QueryEngineTool.from_defaults(query_engine=index.as_query_engine())],
    llm=Settings.llm,
)
response = await agent.run("What was last quarter's revenue?")

from llama_index.core.chat_engine import ChatMode
if chat_mode in (ChatMode.REACT, ChatMode.OPENAI):
    raise DeprecationWarning("Migrate to agent.workflow ReActAgent/FunctionAgent")

When it happens

Trigger: index.as_chat_engine(chat_mode=ChatMode.REACT) or chat_mode=ChatMode.OPENAI; code upgraded to a llama-index version where the agent workflow migration landed but chat mode usage was not updated.

Common situations: Upgrading llama-index across the agent deprecation boundary and hitting the removal; old tutorials/blogs showing ChatMode.REACT with a query engine tool.

Related errors

• chat_stream is None. Cannot write to history without chat_st
• achat_stream is None. Cannot asynchronously write to history
• chat_stream is None!
• achat_stream is None!
• system_prompt is not supported for CondenseQuestionChatEngin

AI-assisted analysis of run-llama/llama_index@afd0fef371 (2026-08-15). Data as JSON: /api/errors/403807e53b0a5333. Report an issue: GitHub.