Object of type datetime is not JSON serializable

[paulhugel]

MCP-Dataverse Bug Report

Context

• Environment: Apple Silicon M4

Mac mini, Jan.app, Visual Studio Code with Codex, GitHub Copilot, GitHub Copilot extensions enabled.

• Goal: Connect to Dataverse MCP server (https://mcp.dataverse.org/sse) via Jan and VS Code.

Configuration

Jan.app (working config)

{
  "command": "npx",
  "args": ["-y", "mcp-remote", "https://mcp.dataverse.org/sse", "--transport", "sse-only"],
  "env": {},
  "active": true
}

VS Code (settings.json)

{
  "mcp": {
    "servers": {
      "mcp-dataverse": {
        "type": "sse",
        "url": "https://mcp.dataverse.org/sse"
      }
    }
  }
}

Problem

When invoking get_croissant_record with a DOI (e.g., 10.7910/DVN/RI9JFU), the server throws:

Object of type datetime is not JSON serializable

Root Cause

• The mcp-dataverse server code returns raw Python datetime objects in responses.
• JSON has no native datetime type, so serialization fails.

Fix (Server-Side)

Patch the Python code in mcp-dataverse to serialize datetime objects:

import datetime, json

def _json_default(o):
    if isinstance(o, (datetime.datetime, datetime.date)):
        return o.isoformat()
    return str(o)

# Example usage when sending tool results
json.dumps(payload, default=_json_default, ensure_ascii=False)

Notes

• This bug is inside the MCP server, not in Jan.app or VS Code configuration.
• Hosted endpoint (https://mcp.dataverse.org/sse) cannot be patched by end users. Only maintainers can fix it.
• Workaround: Run a local clone of mcp-dataverse, apply the patch, and register that server in Jan or VS Code.

Status

• Jan.app: Successfully connects via mcp-remote with --transport sse-only.
• Jan.app: Successfully lists 8 tools available
• VS Code: Can register MCP server, but same datetime serialization bug will appear until server is patched.

New Evidence (SSE + curl Reproduction)

The issue was reproduced end-to-end using plain curl over SSE. This confirms the problems are entirely server-side.

How to Reproduce

1. Open SSE stream (Terminal A):

curl -N -H "Accept: text/event-stream" https://mcp.dataverse.org/sse

This returns an endpoint with a session id, e.g.:

event: endpoint
data: /messages/?session_id=0e4fd22090504f7999724c944acb7e6b

2. In a second terminal (Terminal B), send requests to that exact endpoint:

curl -sS -X POST 'https://mcp.dataverse.org/messages/?session_id=0e4fd22090504f7999724c944acb7e6b' \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":"1","method":"initialize","params":{"protocolVersion":"1.0","capabilities":{},"clientInfo":{"name":"curl","version":"1.0"}}}'

curl -sS -X POST 'https://mcp.dataverse.org/messages/?session_id=0e4fd22090504f7999724c944acb7e6b' \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":"2","method":"tools/list","params":{}}'

Responses to POSTs are 202 Accepted; actual results appear in the SSE stream (Terminal A).

Observed SSE Results

• tools/list returned the following tools (abridged):

{"jsonrpc":"2.0","id":"2","result":{"tools":[
  {"name":"fetch","description":"Fetches a website...","endpoint":"/fetch"},
  {"name":"onboarding","description":"Get the onboarding instruction...","endpoint":"/onboarding"},
  {"name":"get_croissant_record","description":"Convert a dataset to Croissant ML format...","endpoint":"/get_croissant_record"},
  {"name":"datatool","description":"Process a file in a dataset...","endpoint":"/tools/datatool"},
  {"name":"overview","endpoint":"/overview"},
  {"name":"overview_datasets","endpoint":"/overview/datasets"},
  {"name":"overview_files","endpoint":"/overview/files"},
  {"name":"search_datasets","endpoint":"/search/datasets"}
]}}

tools/call Behavior

• Calling get_croissant_record produced the known serialization error:

{"jsonrpc":"2.0","id":"6","result":{"content":[{"type":"text","text":"Object of type datetime is not JSON serializable"}],"isError":true}}

• Calling other listed tools by their names returned "Unknown tool":

onboarding      -> Unknown tool
overview        -> Unknown tool
fetch           -> Unknown tool
datatool        -> Unknown tool
overview_datasets -> Unknown tool
search_datasets -> Unknown tool

Conclusions

• Bug 1 (serialization): get_croissant_record returns Python datetime objects that are not JSON-serializable. Needs ISO 8601 conversion (see Fix section above).
• Bug 2 (dispatch mismatch): tools/list advertises multiple tools, but tools/call cannot invoke most of them by name. This suggests a mismatch between the registered tool names and the dispatcher’s routing logic. The server should ensure the same keys used in tools/list are accepted by tools/call.

Suggested Server Fixes

• Serialization: Apply a default handler to json.dumps (or pre-convert datetimes) so all responses are JSON-serializable.
• Tool dispatch: Align the tool registry and call handler so that names returned by tools/list are invokable by tools/call without leading slashes or alternate identifiers. Add tests asserting that every listed tool name is callable.

Impact

• Jan.app, VS Code, and any MCP client using SSE will experience both issues: inability to call most advertised tools and failure on get_croissant_record due to datetime serialization.

VS Code Notes

• Expected behavior mirrors the curl results above since VS Code speaks the same MCP methods. If you have specific VS Code error output (e.g., "Unknown tool: …" or the datetime error surfaced in the tool result panel), include it here to strengthen the report.

[4tikhonov]

Thanks for the report, seems like something is missing in the metadata record itself. But we'll investigate properly.