from_openapi server raises tons of Pydantic errors trying to parse JSON returned from the upstream api

[dbsanfte]

I've been trying to debug an issue in fastmcp over the past few days related to JSON deserialisation.

I've stood up a FastMCP instance using the from_openapi method, against an OpenAPI json generated here:

https://converter.swagger.io/api/convert?url=https%3A%2F%2Fdevelopers.topdesk.com%2Fswagger%2Fincident_specification_3.8.5.yaml

This is the TopDesk Incident API:

https://developers.topdesk.com/explorer/?page=incident

When my local LLM calls the tool on the MCP server, the MCP server correctly and successfully calls the correct endpoint on the Topdesk API upstream, and it gets a list of incidents back. These appear to be well-structured JSON. But the MCP server fails to parse it with a bunch of Pydantic errors.

See below:

70 validation errors for CallToolResult
content.0.TextContent.type
  Field required [type=missing, input_value={'id': '34e2e1c4-f929-402... 'partialIncidents': []}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/missing\ncontent.0.TextContent.text
  Field required [type=missing, input_value={'id': '34e2e1c4-f929-402... 'partialIncidents': []}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/missing\ncontent.0.ImageContent.type
  Field required [type=missing, input_value={'id': '34e2e1c4-f929-402... 'partialIncidents': []}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/missing\ncontent.0.ImageContent.data
  Field required [type=missing, input_value={'id': '34e2e1c4-f929-402... 'partialIncidents': []}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/missing\ncontent.0.ImageContent.mimeType
  Field required [type=missing, input_value={'id': '34e2e1c4-f929-402... 'partialIncidents': []}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/missing\ncontent.0.EmbeddedResource.type
  Field required [type=missing, input_value={'id': '34e2e1c4-f929-402... 'partialIncidents': []}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/missing\ncontent.0.EmbeddedResource.resource
  Field required [type=missing, input_value={'id': '34e2e1c4-f929-402... 'partialIncidents': []}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/missing\ncontent.1.TextContent.type
  Field required [type=missing, input_value={'id': '72f59a2c-7a15-435... 'partialIncidents': []}, input_type=dict]
    For further information visit https://errors.pydantic.dev/2.11/v/missing\ncontent.1.TextContent.text
  Field required [type=missing, input_value={'id': '72f59a2c-7a15-435... 'partialIncidents': []}, input_type=dict]
...

The tool call itself made by the LLM is:

{
      "role": "assistant",
      "content": null,
      "tool_calls": [
        {
          "index": 0,
          "id": "919000339",
          "function": {
            "arguments": "{\"input\": \"{\\\"status\\\": \\\"firstLine\\\"}\"}",
            "name": "get_incidents"
          },
          "type": "function"
        }
      ]
    }

And that gives me:

2025-04-16 17:13:39,235 - httpx - INFO - HTTP Request: GET https://myinstance.topdesk.net/tas/api/incidents "HTTP/1.1 206 Partial Content"

So the error is definitely in the server somewhere.

I'm not quite sure where to go from here but I'm happy to help debug more if necessary.

[jlowin]

I believe this is the same issue as #186 and may already be fixed on main, which will be released as 2.2 shortly. If that is not the case, please re-open and we will try to solve ASAP

[jlowin]

Update: 2.2 is out now if you'd like to confirm

[dbsanfte]

@jlowin sadly I can still reproduce this using the code from the latest main (91fba14)

[jlowin]

I'm sorry to hear that! Can you share any code so I can replicate against that API?

[dbsanfte]

Let me see what I can throw together tomorrow, will ping

[dbsanfte]

https://github.com/dbsanfte/topdesk-mcp-repro-bug

That should do the job

[jlowin]

@dbsanfte thanks so much for this complete repro! I'm going through and I think I'm seeing the same behavior (getting some odd 404 stuff too, not sure why). I'm on the road through Tuesday but will try to find time to continue working on this.

[liukidar]

Any updates on this? I'm also experiencing the same behaviour with other openapi schemas.

EDIT: I realised the openapi schema had a 'webhook' entry which apparently wasn't standard and rather than being ignored, it was causing the issue. Removing it fixed the problem for me.

[jlowin]

@liukidar is it possible to share an MRE? I just create a schema with extra fields and it didn't have an issue, it looks like the OpenAPI-Pydantic models all have extra=True.