could not attach to MCP server home assistant

[billyconte]

Hi-

I just got two error messages from Claude desktop. Has been working flawlessly for weeks now, until now. How do I go about troubleshooting this?

[github-actions]

ℹ️ Information Needed

To provide a helpful analysis, I need some additional information:

• Installation method - Are you using uvx, pip, docker, or the Home Assistant add-on?
• Logs - Please provide the MCP logs from Claude Desktop to see the specific error. You can find these at ~/Library/Logs/Claude/mcp.log (macOS) or %APPDATA%\Claude\logs\mcp.log (Windows).
• ha-mcp version - Run uvx ha-mcp --version (or your platform's equivalent) to identify the version you are running.
• Claude Desktop configuration - Share the relevant section of your claude_desktop_config.json (please redact your HASS_TOKEN and URL).

---

💡 Please reply to this issue with the requested information.

🤖 Automated triage by Issue Bot

[billyconte]

Also, I can confirm the token is still active in HA.

1. Using uvx (Mac)
2. see below
3. ha-mcp 6.7.0

{
"mcpServers": {
"Home Assistant": {
"command": "/Users/billyconte/.local/bin/uvx",
"args": [
"--refresh",
"ha-mcp@latest"
],
"env": {
"HOMEASSISTANT_URL": "http://192.168.x.x:8123",
"HOMEASSISTANT_TOKEN": "redacted"
}
}
},
"preferences": {
"coworkScheduledTasksEnabled": false,
"sidebarMode": "chat"
}
}

2026-02-19T18:25:50.770Z [info] [Home Assistant] Server started and connected successfully
2026-02-19T18:25:50.779Z [info] [Home Assistant] Message from client: {"method":"initialize","params":{"protocolVersion":"2025-11-25","capabilities":{"extensions":{"io.modelcontextprotocol/ui":{"mimeTypes":["text/html;profile=mcp-app"]}}},"clientInfo":{"name":"claude-ai","version":"0.1.0"}},"jsonrpc":"2.0","id":0}
2026-02-19T18:25:59.265Z [info] [Home Assistant] Server transport closed
2026-02-19T18:25:59.265Z [info] [Home Assistant] Client transport closed
2026-02-19T18:25:59.265Z [info] [Home Assistant] Server transport closed unexpectedly, this is likely due to the process exiting early. If you are developing this MCP server you can add output to stderr (i.e. console.error('...') in JavaScript, print('...', file=sys.stderr) in python) and it will appear in this log.
2026-02-19T18:25:59.265Z [error] [Home Assistant] Server disconnected. For troubleshooting guidance, please visit our debugging documentation
2026-02-19T18:25:59.265Z [info] [Home Assistant] Client transport closed

[github-actions]

This issue is caused by a breaking change in the FastMCP library (version 3.0.0), which was released on February 18, 2026. Because ha-mcp v6.7.0 did not have a strict upper version limit on this dependency, new installations or refreshes are automatically pulling the incompatible version, leading to the silent crash immediately after the server is initialized by Claude Desktop.

A fix has already been committed to the master branch to pin the dependency to <3.0.0, but until a new version (e.g., v6.7.1) is published to PyPI, users will continue to encounter this crash when using the @latest tag.

🔍 Analysis

The root cause of this "could not attach" error is an overnight breaking change in the underlying fastmcp library. FastMCP 3.0.0 introduced several API changes that cause ha-mcp to crash silently shortly after the initialize request is received from the client. This explains why your logs show the server starting but then closing the transport exactly 8.5–9 seconds after initialization.

This is a known issue that affected all users who performed a fresh install or used the --refresh flag in uvx after February 18. The maintainers have already applied a fix in the repository, but it is likely not yet available in the stable v6.7.0 release you are currently using.

🛠️ Suggested Steps

1. Wait for v6.7.1: A new release should be available shortly that includes the fix. Once it's out, your existing command will work again.
2. Apply Temporary Workaround: You can force uvx to use a compatible version of the library by updating your Claude Desktop configuration. Change your args to include the specific dependency pin:

   "args": [
     "--refresh",
     "--with",
     "fastmcp<3.0.0",
     "ha-mcp@latest"
   ]

3. Restart Claude Desktop: After updating the configuration, fully quit and restart Claude Desktop to force uvx to recreate the environment with the correct library version.

📊 Technical Analysis (for maintainers)

Root Cause

FastMCP 3.0.0 (released 2026-02-18) introduced breaking changes that cause the server to exit silently after the initialize message is received. The ha-mcp 6.7.0 release has an unbounded dependency (fastmcp>=2.11.0), causing uvx and pip to pull the broken v3.0.0 version.

Affected Code

• pyproject.toml - The dependency was not pinned to <3.0.0 in the 6.7.0 release.
• src/ha_mcp/server.py - The initialization logic is incompatible with FastMCP 3.x's new internal state handling and constructor requirements.

Proposed Fix

The fix has already been applied to the master branch in commit ad00b183. It pins fastmcp to <3.0.0.

--- a/pyproject.toml
+++ b/pyproject.toml
@@ -25,7 +25,7 @@
 ]
 
 dependencies = [
-    "fastmcp>=2.11.0",
+    "fastmcp>=2.11.0,<3.0.0",  # Pin: v3.0.0 has breaking changes (see #644, #645, #648)
     "httpx[socks]>=0.27.0,<1.0",

Related Issues/PRs

• fix: pin fastmcp<3.0.0 to prevent silent server crashes #650 - fix: pin fastmcp<3.0.0 to prevent silent server crashes
• ha-mcp main() crashes silently on stdio launch (Claude Desktop & Claude Code, all versions tested) #644, Installation fails silently on macOS due to fastmcp 3.0.0 incompatibility — fix + workaround #645, [BUG] Server disconnects immediately after initialize handshake with Claude Desktop #648 - Other issues tracking FastMCP 3.0 compatibility

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🤖 Automated analysis by Issue Bot

[kingpanther13]

Should be fixed with 6.7.1 can you confirm after updating/reinstalling?

[billyconte]

Yes it just started working again today, thank you!

[alanaraujoeu]

Hi! I ahve this issue too. I´m sure that I am using the correct secret, basic and id :(

[kingpanther13]

@alanaraujoeu you're commenting on a very old issue, and you're not providing very much information. I Would need to know your install method, connection method, and etc to be able to troubleshoot this. If you're using Claude Desktop please check the pinned issue first, if that doesn't fix it then create a new issue and we'll go from there.