Fix n8n HTTP MCP compatibility by filtering extra parameters before validation

[Copilot]

Description

n8n's remote HTTP MCP sends metadata fields (sessionId, action, chatInput, toolCallId) in tool arguments, causing Pydantic validation to fail with "Unexpected keyword argument" errors.

Changes

Core Fix

• Parameter filtering monkey-patch: Filters arguments against function signature before Pydantic validation in FunctionTool.run
• FastMCP upgrade: 2.10.5 → 2.13.1 for latest improvements

Testing

• n8n compatibility tests: 4 integration tests validating tool calls with extra parameters
• Test infrastructure fix: Cast Neo4j port to string in conftest.py for testcontainers compatibility

Example

# n8n sends this:
{
  "query": "MATCH (n) RETURN n",
  "sessionId": "abc-123",        # filtered out
  "action": "sendMessage",        # filtered out
  "chatInput": "show me data",    # filtered out
  "toolCallId": "tool-456"        # filtered out
}

# Tool receives only valid params:
{
  "query": "MATCH (n) RETURN n"
}

Type of Change

• Bug fix
• Project configuration change

Complexity

• MEDIUM

Complexity: Monkey-patches FastMCP's validation layer to filter parameters. Duplicates some core method logic to maintain functionality after filtering.

How Has This Been Tested?

• Integration tests (35 existing + 4 new n8n compatibility tests)
• Manual tests

Checklist

• Documentation has been updated
• Integration tests have been updated
• CHANGELOG.md updated if appropriate

Original prompt

i have an issue with using the cypher mcp server that has the following error when being queried by n8n using remote http mcp with the following error

System: You are a helpful assistant that is an expert at Cartography Neo4j Cypher queries. Assume the data requested is in the database \n\nFIRST Use your tools to research a proper query from the documents\n\nSECOND run the cypher for the results\nHuman: what cloudflare dns records are available\nAI: Calling Cartography Neo4j with input: {"sample_size":100,"tool":"get_neo4j_schema","id":"276a9908-a157-4eb3-bdf9-044c506220d4"}\nTool: [{"response":[{"type":"text","text":"4 validation errors for call[get_neo4j_schema]\nsessionId\n Unexpected keyword argument [type=unexpected_keyword_argument, input_value='197cdce49d30476c8aa3b74bb65aec4a', input_type=str]\n For further information visit https://errors.pydantic.dev/2.12/v/unexpected_keyword_argument\naction\n Unexpected keyword argument [type=unexpected_keyword_argument, input_value='sendMessage', input_type=str]\n For further information visit https://errors.pydantic.dev/2.12/v/unexpected_keyword_argument\nchatInput\n Unexpected keyword argument [type=unexpected_keyword_argument, input_value='what cloudflare dns records are available', input_type=str]\n For further information visit https://errors.pydantic.dev/2.12/v/unexpected_keyword_argument\ntoolCallId\n Unexpected keyword argument [type=unexpected_keyword_argument, input_value='276a9908-a157-4eb3-bdf9-044c506220d4', input_type=str]\n For further information visit https://errors.pydantic.dev/2.12/v/unexpected_keyword_argument"}]}]\nAI: Calling Cartography Neo4j with input: {"sample_size":100,"tool":"get_neo4j_schema","id":"e57d1db5-e6f6-401b-b94a-d6335fa12df8"}\nTool: [{"response":[{"type":"text","text":"4 validation errors for call[get_neo4j_schema]\nsessionId\n Unexpected keyword argument [type=unexpected_keyword_argument, input_value='197cdce49d30476c8aa3b74bb65aec4a', input_type=str]\n For further information visit https://errors.pydantic.dev/2.12/v/unexpected_keyword_argument\naction\n Unexpected keyword argument [type=unexpected_keyword_argument, input_value='sendMessage', input_type=str]\n For further information visit https://errors.pydantic.dev/2.12/v/unexpected_keyword_argument\nchatInput\n Unexpected keyword argument [type=unexpected_keyword_argument, input_value='what cloudflare dns records are available', input_type=str]\n For further information visit https://errors.pydantic.dev/2.12/v/unexpected_keyword_argument\ntoolCallId\n Unexpected keyword argument [type=unexpected_keyword_argument, input_value='e57d1db5-e6f6-401b-b94a-d6335fa12df8', input_type=str]\n For further information visit https://errors.pydantic.dev/2.12/v/unexpected_keyword_argument"}]}]

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.