Multi Round-Trip Requests

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Overview

• Questions reach the user. The question goes to the MCP client connected to Bifrost, the user answers it there, and the answer goes back to the server on the retry.
• Servers only ask what the client can answer. Bifrost tells the server which kinds of questions the calling client supports.
• Any Bifrost node can take the retry. Bifrost keeps nothing between rounds, so in a cluster the retry can land on any node.
• Every round is logged. MCP Logs records each round, linked together, so you can follow the call from start to finish.

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How a question travels through Bifrost

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What gets passed on

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Requirements

• The MCP client connects to Bifrost’s gateway on 2026-07-28, at /mcp, a server’s own /mcp/<slug> endpoint or a Virtual MCP (under a server.base_path, /<prefix>/mcp), and declares the kinds of questions it can answer.
• The MCP server is connected on 2026-07-28. A server connected on an earlier version asks its questions in a way Bifrost can’t pass on, so its tool fails with an error instead. SSE servers can’t use 2026-07-28, so connect the server over HTTP or STDIO.

{
  "mcp": {
    "max_protocol_version": "auto",
    "client_configs": [
      {
        "name": "deploy",
        "connection_type": "http",
        "connection_string": "https://deploy.example.com/mcp",
        "tools_to_execute": ["*"],
        "max_protocol_version": "auto"
      }
    ]
  }
}

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Where questions can’t be passed on

• MCP clients on an earlier MCP version, or that don’t support the kind of question asked
• Chat completions and the Responses API, when Bifrost runs the tool in agent mode
• Tool execution through /v1/mcp/tool/execute
• Code mode
• Prompts and resource reads, through any path

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Logs

• A round that asked a question has the status Needs input, and shows what the server asked under Input requested.
• The retry shows the client’s answers under Answered, and the final result.
• A round that asked to retry later is marked Retry later.

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Troubleshooting

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The client gets an error instead of the question

• The client connects on an earlier MCP version. Check that it supports MCP 2026-07-28.
• The client doesn’t support that kind of question. A client that declares only form questions can’t answer a link question, and the other way around.
• The call didn’t come through the MCP gateway. Chat completions, the Responses API, agent mode and code mode can’t ask a user.

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The client supports questions, but the tool still fails

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The server keeps asking to retry

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Next Steps