n8n 1.119.1 breaks MCP tool calls with "toolCallId: Unexpected keyword argument" validation error

TL;DR

For "Error calling tool 'mytool': 1 validation error for call[mytool] toolCallId Unexpected keyword argument": i updated n8n to 1.119.1 and now my MCP tool calls fail.

Error calling tool 'my_tool': 1 validation error for call[my_tool] toolCallId Unexpected keyword argument

Steps

1. I updated n8n to 1.119.1 and now my MCP tool calls fail. The agent returns: "Error calling tool 'mytool': 1 validation error for call[mytool] toolCallId Unexpected keyword argument". My previous workflows ran fine before the update. In another workflow the agent just keeps running without ever calling the LLM or any tools. I am using an MCP server built with FastMCP. What changed and how do I fix this?
2. This was a regression in n8n 1.119.1: the update added strict validation of MCP tool-call arguments, so the toolCallId parameter n8n itself injects got rejected by tools whose schemas do not set additionalProperties: false (which is common with FastMCP-built servers). The error you are seeing is n8n's own validation tripping, not your MCP server. The fix is to update past it: the reversed check (only allow extra arguments if explicitly allowed) was released in n8n 2.3.5 and 1.123.14. Users on FastMCP servers confirmed the validation errors cleared after those releases. If you are stuck on an older version for now, a temporary workaround used by several people is to sanitize tool arguments on the server side, dropping keys like toolCallId and sessionId that are not in the function signature, but that is a hack: the real fix is the version with the corrected check.

When to use

You are seeing this: The agent returns: "Error calling tool 'mytool': 1 validation error for call[mytool] toolCallId Unexpected keyword argument". Use this skill when you run into "n8n 1.119.1 breaks MCP tool calls with "toolCallId: Unexpected keyword argument" validation error".

When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

Versions

Versions mentioned in the source: 1.119.1, n8n 2.3.5, 1.123.14. If you are on something much newer or older, the details may have shifted.

Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

Sources

• https://github.com/n8n-io/n8n/issues/21716

Someone asked: I updated n8n to 1.119.1 and now my MCP tool calls fail. The agent returns: "Error calling tool 'mytool': 1 validation error for call[mytool] toolCallId Unexpected keyword argument". My previous workflows ran fine before the update. In another workflow the agent just keeps running without ever calling the LLM or any tools. I am using an MCP server built with FastMCP. What changed and how do I fix this? The verified resolution: This was a regression in n8n 1.119.1: the update added strict validation of MCP tool-call arguments, so the toolCallId parameter n8n itself injects got rejected by tools whose schemas do not set additionalProperties: false (which is common with FastMCP-built servers). The error you are seeing is n8n's own validation tripping, not your MCP server. The fix is to update past it: the reversed check (only allow extra arguments if explicitly allowed) was released in n8n 2.3.5 and 1.123.14. Users on FastMCP servers confirmed the validation errors cleared after those releases. If you are stuck on an older version for now, a temporary workaround used by several people is to sanitize tool arguments on the server side, dropping keys like toolCallId and sessionId that are not in the function signature, but that is a hack: the real fix is the version with the corrected check. Thread: https://vectle.com/threads/thr_UVx9j6RM2nJ5bp27a25bZg