[BUG] MCP Proxy: SSE stream repeatedly drops with 409 Conflict, reconnection fails

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?

The claude.ai MCP proxy (mcp-proxy.anthropic.com) consistently drops SSE
connections to a custom MCP server, and reconnection attempts fail with HTTP
409 "Conflict".

## Evidence (from debug logs)

Over the past 10 days of usage:

• 63 successful connections — initial connect always works (~3s)
• 227 connection drops — SSE stream breaks after varying uptimes
• 31 total failures — max reconnection attempts (2) exceeded, server

becomes unreachable

• Drop rate: 360% (drops per connection)

### Uptime before drop (distribution):
| Range | Drops |
|-------|-------|
| < 30s | 11 |
| 30–60s | 3 |
| 1–2 min | 5 |
| 2–5 min | 25 |
| > 5 min | 68 |

### Error pattern (always the same):
MCP server "claude.ai Walter PO": CLAUDEAI-PROXY connection dropped after Xs
uptime
MCP server "claude.ai Walter PO": Connection error: Streamable HTTP error:
Failed to open SSE stream: Conflict
MCP server "claude.ai Walter PO": Connection error: Failed to reconnect SSE
stream: Streamable HTTP error: Failed to open SSE stream: Conflict
MCP server "claude.ai Walter PO": Connection error: Maximum reconnection
attempts (2) exceeded.

## Root cause

The Anthropic MCP proxy returns HTTP 409 Conflict on reconnection,
suggesting it still considers the old session active when the client tries to
re-establish the SSE stream. The upstream MCP server itself (v3.0.0) connects
correctly every time — the issue is in the proxy layer.

## Impact

• MCP tools become unavailable mid-conversation with no way to recover except

restarting Claude Code or using /mcp

• Affects productivity significantly when relying on MCP integrations

## Environment

• Claude Code CLI (macOS)
• MCP server connected via claude.ai proxy transport
• Logs from ~/.claude/debug/*.txt

## Suggested improvements

1. Increase max reconnection attempts beyond 2
2. Fix 409 Conflict on reconnect — invalidate old session before

re-establishing

1. Add automatic reconnection on tool call if connection was lost

What Should Happen?

1. SSE stream should be stable — Once connected, the SSE stream should remain

open as long as the Claude Code session is active, not drop every 2–5 minutes

1. Reconnection should succeed — When a drop occurs, the proxy should accept

the reconnection instead of returning 409 Conflict. The old session should be
invalidated server-side so a new SSE stream can be established

1. More than 2 retry attempts — 2 retries is too aggressive a cutoff. The

client should retry with exponential backoff (e.g. 5–10 attempts over ~60s)

1. Graceful recovery — If the connection was lost mid-conversation, the next

tool call should trigger an automatic reconnect transparently, rather than
requiring manual /mcp or a full restart

Error Messages/Logs

Steps to Reproduce

1. Set up a custom MCP server connected via claude.ai proxy transport

(configured in Claude Code MCP settings)

1. Start a Claude Code session and use any tool from the MCP server — initial

connection succeeds

1. Continue working for 2–5 minutes without calling MCP tools
2. Call an MCP tool again — the SSE stream has silently dropped, and

reconnection fails with 409 Conflict

1. Observe the error chain in ~/.claude/debug/*.txt:

connection dropped after Xs uptime
→ Failed to open SSE stream: Conflict
→ Failed to reconnect SSE stream: [...] Conflict
→ Maximum reconnection attempts (2) exceeded

1. MCP server is now unreachable for the rest of the session — only

recoverable via /mcp or restarting Claude Code

Reproduction rate

• ~100% reproducible — happens in virtually every session that lasts more than

a few minutes

• 227 drops across 63 connections over 10 days confirms this is systemic, not

intermittent

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.71

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_