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libs/langchain_v1/examples/mcp_stateful_browser_example.py
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280
libs/langchain_v1/examples/mcp_stateful_browser_example.py
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"""Example demonstrating stateful MCP usage with Playwright browser automation.
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This example shows how to properly maintain browser sessions across multiple tool calls
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using the MCP (Model Context Protocol) client with stateful session management.
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The key difference between stateless and stateful MCP usage:
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- Stateless (default): Each tool call creates a new session, browser closes after each call
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- Stateful (recommended for browser automation): Single session maintained across all tool calls
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This solves the common issue where browser sessions terminate immediately after navigation,
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causing subsequent tool calls to fail with "Ref not found" errors.
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"""
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import asyncio
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from typing import Any
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from langchain.agents import create_agent
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from langchain_core.messages import HumanMessage
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from langgraph.checkpoint.memory import MemorySaver
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# Note: langchain-mcp-adapters must be installed separately
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# pip install langchain-mcp-adapters
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try:
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from langchain_mcp_adapters.client import MultiServerMCPClient
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from langchain_mcp_adapters.tools import load_mcp_tools
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except ImportError:
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raise ImportError(
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"Please install langchain-mcp-adapters: pip install langchain-mcp-adapters"
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)
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async def stateless_example_problematic():
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"""Problematic example showing stateless MCP usage that causes browser session termination.
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This demonstrates the INCORRECT way that leads to the reported issue.
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Each tool call creates a new browser session, causing state loss between calls.
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"""
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print("\n" + "=" * 80)
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print("PROBLEMATIC EXAMPLE: Stateless MCP Usage (Browser closes after each tool call)")
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print("=" * 80)
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# Initialize MCP client with Playwright server
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client = MultiServerMCPClient(
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{
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"playwright": {
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"command": "npx",
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"args": [
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"@playwright/mcp@latest",
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"--headless", # Run in headless mode
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"--isolated", # Allow multiple browser instances
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],
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"transport": "stdio",
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}
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}
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)
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# PROBLEM: Using get_tools() creates stateless tools
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# Each tool invocation will create a new session
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tools = await client.get_tools()
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print(f"✗ Loaded {len(tools)} stateless MCP tools")
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# Create model and agent
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from langchain_openai import ChatOpenAI
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model = ChatOpenAI(model="gpt-4", temperature=0)
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model_with_tools = model.bind_tools(tools)
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# Create agent with memory
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memory = MemorySaver()
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agent = create_agent(
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model_with_tools,
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tools,
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debug=True,
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system_prompt="You are a web testing engineer using Playwright tools.",
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checkpointer=memory,
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)
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# This will fail on the second tool call because the browser session is lost
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test_prompt = """
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1. Navigate to https://example.com
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2. Take a screenshot
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3. Click on the 'More information...' link
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"""
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try:
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config = {"configurable": {"thread_id": "test-thread"}}
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response = await agent.ainvoke(
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{"messages": [HumanMessage(content=test_prompt)]},
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config=config,
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)
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print("Response:", response["messages"][-1].content)
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except Exception as e:
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print(f"❌ Error (expected): {e}")
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print("This error occurs because the browser session was terminated after navigation")
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async def stateful_example_correct():
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"""Correct example showing stateful MCP usage that maintains browser sessions.
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This demonstrates the CORRECT way to use MCP tools for browser automation.
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A single browser session is maintained across all tool calls.
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"""
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print("\n" + "=" * 80)
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print("CORRECT EXAMPLE: Stateful MCP Usage (Browser session persists)")
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print("=" * 80)
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# Initialize MCP client with Playwright server
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client = MultiServerMCPClient(
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{
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"playwright": {
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"command": "npx",
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"args": [
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"@playwright/mcp@latest",
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"--headless", # Run in headless mode
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"--isolated", # Allow multiple browser instances
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],
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"transport": "stdio",
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}
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}
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)
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# SOLUTION: Create a persistent session and load tools from it
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# This maintains the browser session across all tool calls
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async with client.session("playwright") as session:
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# Load tools with the persistent session
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tools = await load_mcp_tools(session)
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print(f"✓ Loaded {len(tools)} stateful MCP tools with persistent session")
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# Create model and agent
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from langchain_openai import ChatOpenAI
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model = ChatOpenAI(model="gpt-4", temperature=0)
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model_with_tools = model.bind_tools(tools)
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# Create agent with memory
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memory = MemorySaver()
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agent = create_agent(
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model_with_tools,
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tools,
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debug=True,
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system_prompt="You are a web testing engineer using Playwright tools.",
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checkpointer=memory,
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)
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# Complex multi-step browser interaction that requires session persistence
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test_prompt = """
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Please perform the following browser automation tasks:
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1. Navigate to https://example.com
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2. Take a screenshot of the page
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3. Click on the 'More information...' link
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4. Take another screenshot after navigation
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5. Go back to the previous page
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6. Verify you're back on example.com
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"""
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try:
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config = {"configurable": {"thread_id": "test-thread"}}
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response = await agent.ainvoke(
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{"messages": [HumanMessage(content=test_prompt)]},
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config=config,
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)
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print("✓ Success! All browser operations completed in the same session")
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print("Response:", response["messages"][-1].content)
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except Exception as e:
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print(f"❌ Unexpected error: {e}")
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# Session automatically cleaned up when exiting the context manager
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print("✓ Browser session properly closed after all operations")
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async def advanced_stateful_example():
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"""Advanced example showing complex browser automation with form filling and validation.
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This demonstrates a real-world scenario where session persistence is critical.
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"""
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print("\n" + "=" * 80)
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print("ADVANCED EXAMPLE: Complex Browser Automation with Session State")
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print("=" * 80)
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client = MultiServerMCPClient(
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{
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"playwright": {
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"command": "npx",
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"args": [
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"@playwright/mcp@latest",
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"--headless",
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],
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"transport": "stdio",
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}
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}
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)
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async with client.session("playwright") as session:
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tools = await load_mcp_tools(session)
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from langchain_openai import ChatOpenAI
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model = ChatOpenAI(model="gpt-4", temperature=0)
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model_with_tools = model.bind_tools(tools)
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memory = MemorySaver()
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agent = create_agent(
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model_with_tools,
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tools,
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debug=False, # Less verbose for this example
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system_prompt="""You are an expert web automation engineer.
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Use Playwright tools to interact with web pages and validate functionality.
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Always maintain the browser session state across operations.""",
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checkpointer=memory,
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)
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# Complex test scenario requiring persistent session
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test_scenario = """
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Perform a comprehensive test of a web form:
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1. Navigate to https://httpbin.org/forms/post
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2. Fill in the customer name field with "Test User"
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3. Fill in the telephone field with "555-1234"
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4. Fill in the email field with "test@example.com"
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5. Select "Large" for the pizza size
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6. Check the "Bacon" topping checkbox
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7. Fill in the delivery instructions with "Leave at door"
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8. Take a screenshot of the filled form
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9. Submit the form
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10. Verify the submission was successful by checking the response page
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"""
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try:
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config = {"configurable": {"thread_id": "form-test"}}
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response = await agent.ainvoke(
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{"messages": [HumanMessage(content=test_scenario)]},
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config=config,
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)
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print("✓ Complex form automation completed successfully!")
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print("All operations executed in a single browser session")
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except Exception as e:
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print(f"Error during form automation: {e}")
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async def main():
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"""Run all examples to demonstrate the difference between stateless and stateful MCP usage."""
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print("\n" + "🔍" * 40)
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print("MCP STATEFUL BROWSER AUTOMATION EXAMPLES")
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print("🔍" * 40)
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print("""
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This script demonstrates the critical difference between stateless and stateful
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MCP tool usage for browser automation.
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Key Learning:
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- Stateless (default): Browser closes after each tool call → State lost → Errors
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- Stateful (recommended): Browser persists across all tool calls → State maintained → Success
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The stateful approach is ESSENTIAL for:
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- Browser automation (Playwright, Puppeteer)
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- Database connections that need transactions
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- Any tools requiring persistent state between calls
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""")
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# Run the problematic stateless example
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try:
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await stateless_example_problematic()
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except Exception as e:
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print(f"Stateless example failed (expected): {e}")
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# Run the correct stateful example
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await stateful_example_correct()
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# Run the advanced example
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await advanced_stateful_example()
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print("\n" + "=" * 80)
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print("SUMMARY: Always use stateful sessions for browser automation!")
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print("Pattern: async with client.session('server_name') as session:")
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print(" tools = await load_mcp_tools(session)")
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print("=" * 80)
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if __name__ == "__main__":
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# Run the examples
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asyncio.run(main())
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