MCP Server passes all JSON-RPC tests but Agent Builder fails with 424 (“Failed Dependency”)

API
1

Hi! I’m experiencing a persistent 424 “Failed Dependency” error when connecting my custom MCP server to Agent Builder.

All JSON-RPC tests (initialize, notifications/initialized, tools/list, tools/call) pass successfully via curl, and the server is fully compliant with protocol version 2025-06-18.
However, inside Agent Builder, the UI never reaches my endpoint — the network log shows repeated calls to
https://api.openai.com/v1/dashboard/responses/mcp/list_actions → 424 (Failed Dependency).

You can verify the server here:
:link: https://invoicereader-production-fod.up.railway.app/debug/last

Based on community threads, this seems to match the known “424 TaskGroup” bug in Agent Builder where the dashboard’s internal list_actions phase fails even though the MCP handshake succeeded.

The problem is not reproducible via API / curl, only within the Agent Builder UICould you please confirm if this is a known dashboard-side issue or a change in how Agent Builder expects to retrieve the tool list (list_actions vs tools/list)?

Thank you!

3 Likes
2

Hi @FRobledoRuiz Welcome to the forum!

I think this is related to a known issue, please see this github post with a solution

1 Like
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I think it’s an issue within the GUI itself, I will make sure it’s raised again though.

1 Like
4

Thanks a lot! I will see the link and the solution.

Best regards

5

I’ve conducted some additional tests, and it appears that the problem indeed lies within the GUI. It would be helpful if this case could be reviewed or reopened for further investigation.

1 Like
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Hello! We're looking into this, but in the meantime, we've found this is tied to streamable HTTP session lifecycle, and that setting stateless_http=True is the current workaround. Let me know if that works for you.

2 Likes
8

We’re using stateless_http=True and still seeing this problem (occasionally - maybe 1 in 25 tool calls) with the agent-builder workflow we’re trying to use from our chatkit-based app.

server has mcp==1.16.0, fastmcp==2.12.4 and is behind an openresty LB