run-llama/llama_index · error · ValueError

Streaming is not enabled. Please use chat() instead.

Error message

Streaming is not enabled. Please use chat() instead.

What it means

Raised by CondenseQuestionChatEngine.stream_chat() when the underlying query_engine returns a non-streaming response. The code checks whether the condensed query produced a response with response_gen; if streaming was not requested from the query engine, there is no token generator to expose and the engine raises ValueError telling you to use chat() instead.

Source

Thrown at llama-index-core/llama_index/core/chat_engine/condense_question.py:273

        # Record response
        if (
            isinstance(query_response, StreamingResponse)
            and query_response.response_gen is not None
        ):
            # override the generator to include writing to chat history
            self._memory.put(ChatMessage(role=MessageRole.USER, content=message))
            response = StreamingAgentChatResponse(
                chat_stream=response_gen_from_query_engine(query_response.response_gen),
                sources=[tool_output],
            )
            thread = Thread(
                target=response.write_response_to_history,
                args=(self._memory,),
            )
            response.write_response_to_history_thread = thread
            thread.start()
        else:
            raise ValueError("Streaming is not enabled. Please use chat() instead.")
        return response

    @trace_method("chat")
    async def achat(
        self, message: str, chat_history: Optional[List[ChatMessage]] = None
    ) -> AgentChatResponse:
        chat_history = chat_history or await self._memory.aget(input=message)

        # Generate standalone question from conversation context and last message
        condensed_question = await self._acondense_question(chat_history, message)

        log_str = f"Querying with: {condensed_question}"
        logger.info(log_str)
        if self._verbose:
            print(log_str)

        # TODO: right now, query engine uses class attribute to configure streaming,
        #       we are moving towards separate streaming and non-streaming methods.

View on GitHub (pinned to afd0fef371)

Solutions

1. Build the query engine with streaming: engine = index.as_query_engine(streaming=True), then pass it to CondenseQuestionChatEngine.from_defaults
2. Or call chat() instead of stream_chat() if streaming is not required
3. For retriever-based engines, verify streaming support of the LLM (some custom LLMs lack astream_complete)

Example fix

# before
qe = index.as_query_engine()  # streaming=False by default
chat = CondenseQuestionChatEngine.from_defaults(query_engine=qe)
chat.stream_chat('hello')  # ValueError: Streaming is not enabled

# after
qe = index.as_query_engine(streaming=True)
chat = CondenseQuestionChatEngine.from_defaults(query_engine=qe)
resp = chat.stream_chat('hello')
for token in resp.response_gen:
    print(token, end='')

qe = index.as_query_engine(streaming=True)  # set at construction, not later
assert getattr(qe, 'streaming', True) is not False, 'Query engine must be built with streaming=True to use stream_chat()'

try:
    resp = engine.stream_chat(msg)
except ValueError as e:
    if 'Streaming is not enabled' in str(e):
        resp = engine.chat(msg)  # graceful fallback to non-streaming
    else:
        raise

When it happens

Trigger: engine = CondenseQuestionChatEngine.from_defaults(query_engine=RetrieverQueryEngine.from_defaults(retriever, streaming=False)); engine.stream_chat('hi'). Any query engine constructed without streaming=True (the default) whose query() path is exercised by stream_chat.

Common situations: Calling stream_chat on an engine built from a default query engine; building the query engine from an index (index.as_query_engine()) which defaults to streaming=False; swapping engines in a streaming UI without updating construction flags.

Related errors

• Streaming is not enabled. Please use achat() instead.
• response_gen is only available for streaming responses. Set
• Expected Response object, got {type(answer_obj)} instead.
• code_execute_fn must be provided for CodeActAgent
• system_prompt is not supported for CondenseQuestionChatEngin

AI-assisted analysis of run-llama/llama_index@afd0fef371 (2026-08-15). Data as JSON: /api/errors/58e14a86a85215e3. Report an issue: GitHub.