zero parameter tools usage from langgraph agent

[gmann11]

We needed to override the tool args_schema for zero parameter tools use it with a langgraph agent:

    for tool in tools:
        if not 'properties' in tool.args_schema:
            tool.args_schema['properties'] = {}

if properties isn't defined as empty object in the tool, this error is returned:

openai.BadRequestError: Error code: 400 - {'error': {'message': "Invalid schema for function 'get-schema': In context=(), object schema missing properties.", 'type': 'invalid_request_error', 'param': 'tools[0].function.parameters', 'code': 'invalid_function_parameters'}},

[mjfwebb]

Hey @gmann11 thanks for the issue. Can you please provide a little more detail about how to reproduce this issue?

[gmann11]

Here's a short test program that shows the issue when using langgraph agent

test.py

.

[keremgocen]

I think langgraph is a bit more strict with the JSON schema compared to other clients (where they defaulted to something like "type": "object") so that's probably why we did not notice before.
Can you please try with this branch if possible https://github.com/neo4j/mcp/tree/bugfix/90-fix-input-schema-when-properties-are-empty? (you'd have to build the binary again locally)

I think this should fix the issue based on my local tests. We need to check other tools and probably do something similar and release a proper patch though.

[keremgocen]

Hey @gmann11, were you able to test with our potential fix? We are planning to create a patch for it, just wanted to confirm it works for your setup as well.

[gmann11]

Hi - I tested the fix. It looks like the get-schema is ok but list-gds-procedures still throws the error above.

[keremgocen]

Hi - I tested the fix. It looks like the get-schema is ok but list-gds-procedures still throws the error above.

Thanks for checking. Yes, that should be expected because I only added the patch for the get-schema tool at the time main...bugfix/90-fix-input-schema-when-properties-are-empty. And we noticed that's not the only tool with empty parameters. (list-gds-procedures is suffering from the same issue)

We will create a patch shortly to fix both. Thanks for your patience!

[keremgocen]

Hi again, checking the MCP spec again, we were able to confirm that properties is an optional field in the inputSchema https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/2025-06-18/schema.ts#L946.

So this does not seem to be a valid bug on the neo4j-mcp server, but an issue for more defensive clients where they are not allowing empty inputSchema parameters, and it should be raised as a bug with the langgraph agent I think.

However, one workaround is to provide an empty struct to the GetSchemaSpec and ListGDSProceduresSpec as mcp.WithInputSchema[struct{}]() and use a locally built version of the neo4j-mcp server until the issue with the client is fixed. Closing this as not a bug for now.