Parallel calls to MCP Server tools showing as failure, but executions happen.

[GreenOakMick]

Bug Description

I have built an MCP Server against the Helpscout API to perform conversation drafting, reply, add notes, and conversation retrieval. Using a simple chat message AI Agent with the MCP Server, all tools are working when called individually in between AI Model calls. However, if the AI Model (I am currently using OpenAI 4.1 mini) calls multiple MCP Server tools at the same time, only one of the tool calls will come back successful and the others will show an error with the following message:

Failed to execute tool "Create_Note"
Error: this.resolveFunctions[extra.sessionId] is not a function

'Create_Note' is the name of the tool. I assume the 'this.resolveFunctions' is referring to internal code handling MCP tool calls.

This issue only happens when multiple tool calls happen at the same time. If each tool call is separated by an AI Model call, they all succeed. Additionally, even though the tool call fails, the actual workflow run happens and is successful. It seems the only failure is the exchange between the AI Agent and the MCP tool call. And again, only when running in parallel with other tool calls. Otherwise, they always work.

To Reproduce

1. Create an MCP Server with one or more sub-workflow tools
2. Create a workflow that runs 'on chat message' and add a single AI Agent node with the MCP Server as a tool. I used OpenAI 4.1 mini.
3. Send a command to the AI Agent that would require it to call the tool multiple times at once. For example, I asked the agent to create 3 separate notes.

When you see multiple MCP tool calls in a row, you can confirm they all have the same start time and you should see this error.

Expected behavior

Tool calls are either handled in parallel correctly or run serially where the outcome of each tool call is properly accounted for.

Operating System

Ubuntu Linux 22.04

n8n Version

1.91.3

Node.js Version

Current Node version in the Docker Image

Database

PostgreSQL

Execution mode

queue

[Joffcom]

Hey @GreenOakMick,

We have created an internal ticket to look into this which we will be tracking as "GHC-2177"