MCP server "flow" fails to reconnect - HTTP 400 at flow.team

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) 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?

**Bug Description**
The "flow" MCP server (https://flow.team/ai/mcp) authenticates
successfully but fails to reconnect every time with HTTP 400.

**Steps to Reproduce**
1. Run /mcp
2. Select "flow" server
3. OAuth completes ("Authentication successful")
4. Claude Code shows: "Got new credentials, but reconnecting to
flow failed: HTTP 400 at https://flow.team"

**Tried (all failed with same error)**
- Clear authentication → re-authenticate
- Full restart of Claude Code
- claude --debug (no additional detail in log)

**Expected Behavior**
Server should connect and expose tools after successful auth.

**Actual Behavior**
"Status: failed", "Issue: SSE error: Non-200 status code (400)"
Auth shows as ✓ authenticated, but connection still fails.

**Environment**
- Claude Code version: 2.1.263
- OS: Windows
- Server URL: https://flow.team/ai/mcp
- Config location: local (.claude.json, project-scoped)

### What Should Happen?

**Bug Description**
The "flow" MCP server (https://flow.team/ai/mcp) authenticates
successfully but fails to reconnect every time with HTTP 400.

**Steps to Reproduce**
1. Run /mcp
2. Select "flow" server
3. OAuth completes ("Authentication successful")
4. Claude Code shows: "Got new credentials, but reconnecting to
flow failed: HTTP 400 at https://flow.team"

**Tried (all failed with same error)**
- Clear authentication → re-authenticate
- Full restart of Claude Code
- claude --debug (no additional detail in log)

**Expected Behavior**
Server should connect and expose tools after successful auth.

**Actual Behavior**
"Status: failed", "Issue: SSE error: Non-200 status code (400)"
Auth shows as ✓ authenticated, but connection still fails.

**Environment**
- Claude Code version: 2.1.263
- OS: Windows
- Server URL: https://flow.team/ai/mcp
- Config location: local (.claude.json, project-scoped)

### Error Messages/Logs

```shell

```

### Steps to Reproduce

**Bug Description**
The "flow" MCP server (https://flow.team/ai/mcp) authenticates
successfully but fails to reconnect every time with HTTP 400.

**Steps to Reproduce**
1. Run /mcp
2. Select "flow" server
3. OAuth completes ("Authentication successful")
4. Claude Code shows: "Got new credentials, but reconnecting to
flow failed: HTTP 400 at https://flow.team"

**Tried (all failed with same error)**
- Clear authentication → re-authenticate
- Full restart of Claude Code
- claude --debug (no additional detail in log)

**Expected Behavior**
Server should connect and expose tools after successful auth.

**Actual Behavior**
"Status: failed", "Issue: SSE error: Non-200 status code (400)"
Auth shows as ✓ authenticated, but connection still fails.

**Environment**
- Claude Code version: 2.1.263
- OS: Windows
- Server URL: https://flow.team/ai/mcp
- Config location: local (.claude.json, project-scoped)

### Claude Model

None

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

_No response_

### Claude Code Version

version: 2.1.263

### Platform

Anthropic API

### Operating System

macOS

### Terminal/Shell

Terminal.app (macOS)

### Additional Information

_No response_

Start by reproducing the /mcp flow with https://flow.team/ai/mcp and capture the reconnect request around the OAuth success; claude --debug and the reported SSE HTTP 400 are the available entry points. Check the Windows/macOS environment discrepancy while isolating the failure. Done means the authenticated flow server reconnects successfully and exposes its tools.

Written by the indexing model from the issue text.