Agent engine + MCP is broken on 1.25.0

[rrondir]

Describe the Bug:
When deploying an agent with a remote mcp using McpToolset the following error happens: Error creating remote agent: cannot pickle '_thread.lock' object

I reverted my local adk to 1.24.1 and deployed with 1.25.0 as a dependency and I got the error:

AttributeError: 'MCPSessionManager' object has no attribute '_lock_map_lock'

Steps to Reproduce:
Please provide a numbered list of steps to reproduce the behavior:

Just deploy an agent using MCPToolset with StreamableHttpParams. If needed I can reach via a gcp support case with more info

Expected Behavior:
Deploy to agent engine and it connects to a remote mcp

Observed Behavior:
ADK 1.25.0 -> Pickle error can't deploy

Environment Details:

• ADK Library Version (pip show google-adk): ADK 1.25.0
• Desktop OS:** [e.g., macOS, Linux, Windows] Linux
• Python Version (python -V): 3.11

Model Information:

• Are you using LiteLLM: No
• Which model is being used: gemini-3.0-pro-preview

Regression:
Did this work in a previous version of ADK? If so, which one? it worked in 1.24.1 for deploying and running, albeit working half the time due to the bug that was attempted to get fixed in 1.25.0

How often has this issue occurred?:

• Always (100%)