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Applies to:


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Summary

Issue: Claude Desktop cannot connect to Braintrust MCP server, not completing an initiated OAuth flow, we have seen this in our application logs as being manifested as returning 405 Method Not Allowed errors when attempting to establish connection. Cause: Claude Desktop uses a legacy streaming protocol that sends GET requests to the /mcp endpoint, while Braintrust’s MCP server requires POST requests using the modern MCP protocol. Resolution: Use Claude Code with the modern protocol flag or switch to another MCP client, as Braintrust does not support the legacy streaming protocol.

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Workaround Steps

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Option 1: Use Claude Code

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Step 1: Install Claude Code

Download Claude Code instead of Claude Desktop.

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Step 2: Enable modern protocol flag

Configure Claude Code with the flag that enables the modern MCP streaming protocol per Braintrust MCP documentation. claude mcp add --transport http braintrust https://api.braintrust.dev/mcp

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Additional Information

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Why this happens

Braintrust’s /mcp endpoint implements the current MCP specification requiring POST requests. Claude Desktop has not yet updated to support this protocol for custom MCP servers.

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Upstream tracking

This is a known Claude Desktop limitation tracked in GitHub issue #5826. Braintrust will not implement backward compatibility with the legacy streaming protocol.