Occasional Flaky Hanging of MCP Server Calls in Open-WebUI (using mcpo wrapper)

[gwpl]

Hi everyone!

I thought you might still be trying Open-WebUI and may have run into the issue where MCP servers sometimes hang and the execute icon just spins. Any tips on how to address that?

If you’d like to try my setup—or just see how an MCP server wrapped in an OpenAPI wrapper can be used as a tool in Open-WebUI—here’s outline based on the official modelcontextprotocol rust-sdk. It works about 95% of the time… but in ~5% of cases, it just hangs 🤷

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Background / My observations

• I’ve been brushing up on MCP and experimenting with examples from both claude-code and Open-WebUI.
• I’m starting to suspect that Open-WebUI is simply not fully baked yet (I’ve hit issues multiple times)... but maybe someone here knows a trick to make it rock-solid? :)

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How I do it

1. Open-WebUI requires an OpenAPI server endpoint.

2. So you need to wrap your MCP server in an OpenAPI wrapper.

3. I use the official mcpo from Open-WebUI.

   (side note: maybe this is where I’m going wrong and should try a different wrapper?)

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The problem: flaky hanging

• It happens SOMETIMES, and the issue is flaky: calls in Open-WebUI can hang indefinitely—the “execute” icon just spins and spins…

• For example, I might ask for 6 tool calls and 2 out of 6 will fail; other times I use it repeatedly and it works every single time.

• Still, it should never spin forever. Even after a docker restart it continues spinning, there’s no built-in timeout…

  (Maybe it’s an Open-WebUI bug, not mcpo, but since it’s a tooling-usage issue, the mcpo repo maintainers/community seem like the best audience.)

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Step-by-step setup

1. Run the example server from the official rust-sdk (e.g. servers_std_io):

   git clone https://github.com/modelcontextprotocol/rust-sdk/
   cd rust-sdk
   cargo run -p mcp-server-examples --example servers_std_io

   Or build the binary and use it:

   cargo build -p mcp-server-examples --example servers_std_io
   rust_sdk_stdio_cli="$(readlink -f ./target/release/examples/servers_std_io)"
   echo "$rust_sdk_stdio_cli"

2. Launch that server via mcpo (using uvx):

   git clone https://github.com/open-webui/mcpo
   cd mcpo
   uvx mcpo --name rust-sdk-example-stdio --port 4371 -- /home/$USER/github/modelcontextprotocol/rust-sdk/target/debug/examples/servers_std_io

3. Verify it works at:
   http://127.0.0.1:4371/docs

4. Add it as a Tool in Open-WebUI:
   http://127.0.0.1:4371

5. Test by opening a chat with GPT-4o and OpenAI o4-mini.

6. IMPORTANT: In the chat settings, switch Function/Tool Calling from “Default” to “Native”—only then do OpenAI models actually invoke the tool.

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Results

• Single calls—especially to the example “weather” and “time” servers from the [open-webui/openapi-servers repo]—generally work fine.

• But when I ask the model to perform multiple calls (e.g. the “call all provided tools” demo):

  • Some calls return valid results,
  • Others return an empty JSON {} and hang with the spinning icon indefinitely.