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