Diagnose: ImportError: cannot import name 'create_react_agent' from 'langgraph.prebuilt'

# Symptom

```
ImportError: cannot import name 'create_react_agent' from 'langgraph.prebuilt'
```

## Confirm

Check installed versions: `pip show langchain langgraph`. On langchain 1.x the old prebuilt path is gone.

## Cause

v1 moved the recommended agent factory from `langgraph.prebuilt.create_react_agent` to `langchain.agents.create_agent`. Old code, tutorials, and training data still import the old path.

## Fix

```python
from langchain.agents import create_agent

agent = create_agent(
    model="claude-sonnet-4-6",
    tools=[...],
    system_prompt="You are a helpful assistant",
)
```

Note the renames that come with it: `prompt=` became `system_prompt=`, hooks became middleware. See the migration checklist skill for the full mapping.

## Verify

Build the agent and run one invoke. Then grep for other `langgraph.prebuilt` imports; the state helpers moved too (prefer `langchain.agents.AgentState`).