[BUG] MCP HTTP server connection fails in v2.1.84+ when server uses stateless transport (GET /mcp returns 405)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Description

Starting from v2.1.84, Claude Code fails to connect to MCP servers that use the stateless HTTP transport. The connection worked fine in v2.1.83.

Root Cause

Claude Code v2.1.84+ appears to send a GET request to the MCP endpoint (e.g. /mcp) to establish an SSE stream, as defined by the MCP spec 2025-11-25 resumability/streaming support.

MCP servers implemented using the stateless transport (e.g. Spring AI's WebMvcStatelessServerTransport) explicitly return HTTP 405 Method Not Allowed on GET requests — only POST is supported. This causes Claude Code to mark the server as failed instead of falling back to POST-only communication.

Expected Behavior

Claude Code should either:

• Option A: Gracefully fall back to POST-only mode when GET returns 405 (backwards compatibility with stateless MCP servers)
• Option B: Document that stateless/POST-only MCP servers are no longer supported and that servers must implement SSE via GET

Actual Behavior

Server is marked as failed. Error message in Claude Code:

Failed to connect to MCP server 'my-server': failed

Workaround

Switch the MCP server to the streamable transport (supports both GET for SSE and POST). This resolves the issue.

Environment

• Claude Code version: 2.1.84 (broken), 2.1.83 (working)
• MCP server: Spring AI spring-ai-starter-mcp-server-webmvc:1.1.3 with protocol: STATELESS
• MCP protocol version advertised by server: 2025-06-18
• Transport: HTTP (not stdio)

What Should Happen?

Claude Code should gracefully fall back to POST-only mode when GET returns 405, maintaining backwards compatibility with stateless MCP servers. Alternatively, if GET/SSE is now required, this breaking change should be documented in the release notes.

Error Messages/Logs

For the "Error Messages/Logs" field:


✗ Failed to connect to MCP server 'kara': failed


From Claude Code's internal logs (visible with `--debug`):


[Agent] Failed to connect to MCP server 'kara': failed


The server-side log shows the GET request hitting the endpoint and returning `HTTP 405 Method Not Allowed` — no error on the server side, it's intentional behavior of the stateless transport.

Steps to Reproduce

1. Set up an MCP HTTP server using the stateless transport (POST-only, no SSE)
2. Register it with Claude Code: claude mcp add --transport http my-server https://example.com/mcp
3. Run claude mcp list — server shows as ✗ Failed to connect in v2.1.84+
4. Downgrade to v2.1.83 — server shows as ✓ Connected

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

v2.1.83

Claude Code Version

v2.1.85

Platform

AWS Bedrock

Operating System

macOS

Terminal/Shell

Warp

Additional Information

_No response_