Re: streaming llm response

mark_ott · ‎10-01-2025

To implement streaming output for your agent in Databricks and resolve the error "This model does not support predict_stream method.", the key requirement is that your underlying MLflow model must support the predict_stream method. Most likely, your current registered MLflow model is not using a ChatModel implementation or LLM wrapper that supports streaming, so standard .predict() works but .predict_stream() does not.

Why This Error Occurs

Streaming interface: The MLflow model must implement the predict_stream method (using MLflow’s LLM/ChatModel interface).
Model registration: If you saved your model with MLflow but did not use an LLM/ChatModel wrapper that supports streaming, only standard prediction will work; streaming will fail.
Correct save: The model in MLflow must be saved using a method/class that exposes the streaming endpoint, not just the standard predict endpoint.

How to Resolve

1. Use a Supported ChatModel With Streaming

Ensure you are using an MLflow ChatModel implementation that supports streaming, e.g. OpenAI, Databricks MosaicML, or similar. When saving the model, use mlflow.langchain.save_model() or similar, specifying the appropriate class that includes the streaming method.

2. Implement Streaming in Your Model

Your ChatModel class (or whichever class is wrapped for MLflow model serving) should have a predict_stream method implemented.
In LangChain and LangGraph settings, ensure the LLM object supports streaming (set stream=True and use classes/interfaces that yield partial outputs).

3. Register and Deploy the Streaming Model

Save the model using the appropriate MLflow saving function that retains the streaming capabilities.
When registering/deploying, the model artifact must expose predict_stream.

4. Check Your Deployment Code

When deploying the agent, ensure your inference endpoint is properly configured to use the streaming schema per the latest MLflow documentation.

Example: MLflow Streaming ChatModel

python

import mlflow
from mlflow.langchain import save_model
from langchain.chat_models import ChatOpenAI

# Setup your LLM with streaming enabled
llm = ChatOpenAI(temperature=0.1, streaming=True)

# Save model using MLflow
save_model(
    llm,
    path="llm_model_streaming",
    mlflow_model_flavor="langchain"
)

Ensure the ChatModel (ChatOpenAI, MosaicML, etc.) supports streaming out of the box and is saved with that capability.

References to the Official Docs

The official [Databricks agent streaming guide], and MLflow ChatModel/Streaming documentation: confirm the streaming interface is present and properly implemented when you save and subsequently deploy the model.

Key Steps to Fix

Verify that your saving function in MLflow (e.g. save_model()) saves a streaming-capable ChatModel.
Re-register the model in MLflow after confirming that the underlying implementation is compatible with streaming.
Update deployment code or configs to use the streaming endpoint (predict_stream).

If the underlying LLM class or deployment does not support streaming, you must swap to a compatible class and redeploy.

Table: Error Cause and Resolution

Cause	Resolution
Model lacks `predict_stream` method	Save with streaming ChatModel
Wrong MLflow save function or model class	Use `mlflow.langchain.save_model`
LLM streaming not enabled in config	Set `stream=True` in LLM params

Implement these corrections, re-save and deploy your MLflow model, and the streaming output should work for your agent in Databricks.

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