[BUG] Regression in 2.1.76: HTTP MCP server that worked previously now fails with "failed to parse JSON"

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?

An HTTP MCP server (Graylog 7.0.4) that was working last week now fails with Error: failed to parse JSON in Claude Code. The Graylog server has not
changed — this is a Claude Code regression introduced in a recent update.

MCP server config (in ~/.claude.json):
{
"type": "http",
"url": "http://<host>:9000/api/mcp"
}

Verified the endpoint works correctly with curl:
curl -X POST -H "Content-Type: application/json" \
-H "Authorization: Basic <token>" \
-d "{\"jsonrpc\":\"2.0\",\"method\":\"initialize\",\"params\":{\"protocolVersion\":\"2024-11-05\",\"capabilities\":{}},\"id\":1}" \
http://<host>:9000/api/mcp

Server response (HTTP 200):
{
"jsonrpc": "2.0",
"id": 1,
"result": {
"protocolVersion": "2025-06-18",
"capabilities": {
"prompts": { "listChanged": false },
"resources": { "subscribe": false, "listChanged": false },
"tools": { "listChanged": false }
},
"serverInfo": { "name": "Graylog", "version": "7.0.4" }
}
}

Observed behavior: Claude Code shows Error: failed to parse JSON and the server shows as failed in /mcp.

Expected behavior: Claude Code should connect successfully. It was doing so last week with this exact server.

Suspected cause: Claude Code may not be handling the server's protocolVersion: "2025-06-18" response correctly, or the HTTP MCP transport handshake
changed in a recent update and broke compatibility.

Claude Code last updated: March 13, 2026 (~8:37 PM local time) — the MCP connection broke after this update.

Claude Code version: 2.1.76
OS: Windows 11 Enterprise
Graylog version: 7.0.4 (unchanged)

What Should Happen?

Claude Code should connect successfully. It was doing so last week with this exact server.

Error Messages/Logs

Steps to Reproduce

1. Configure an HTTP MCP server in ~/.claude.json under a project's mcpServers:

"graylog": {
"type": "http",
"url": "http://<host>:9000/api/mcp",
"headers": {
"Authorization": "Basic <token>"
}
}

1. Start Claude Code in that project directory
2. Run /mcp to check server status
3. Observe Error: failed to parse JSON on the graylog server entry

Additional context for reproduction:

• The MCP server must respond with protocolVersion: "2025-06-18" in its initialize response
• A GET to /api/mcp returns HTTP 405 (server only accepts POST)
• A POST with a valid JSON-RPC initialize request returns HTTP 200 with valid JSON

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

2.1.76

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

_No response_