"at least one message is required" with Sonnet 4.5

[rfernandesit]

What happened?

When using Anthropic’s Sonnet 4.5 model in LibreChat, API requests intermittently fail with a 400 error stating "messages: at least one message is required".
This issue does not occur with other Anthropic models (e.g. Sonnet or Opus 4.0).
Request payloads appear valid and include at least one user message.

Version Information

v0.8.0

Steps to Reproduce

1. Start a new chat in LibreChat using Sonnet 4.5.

2. Select some MCP servers and send a normal message as a user.

3. Observe the 400 error response: "messages: at least one message is required" after some tools calling

What browsers are you seeing the problem on?

Chrome

Relevant log output

Something went wrong. Here's the specific error message we encountered: An error occurred while processing the request: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages: at least one message is required"},"request_id":"req_011CUrjK4ugKvWiRNYRDgMFL"}

Screenshots

No response

Code of Conduct

• I agree to follow this project's Code of Conduct

[danny-avila]

Select some MCP servers and send a normal message as a user.

You are likely overloading the context window with many MCP servers selected.

You should also update, instructions (docker):
https://www.librechat.ai/docs/local/docker#update-librechat

[rfernandesit]

Not really!
Even with one mcp server the error occurs. It seems more related to multiple tool calls then the number of selected mcp servers.

Got another error now.

[danny-avila]

No issue here:

I can also see you're using an older version and you should update:

Updating Instructions (docker):
https://www.librechat.ai/docs/local/docker#update-librechat

[rfernandesit]

We were using claude-sonnet-4-5 as model. When we changed to claude-sonnet-4-5-20250929 it started working. I reverted to the older one, and I was able to reproduce the error again.

[isaacraja]

I've seen this error happening when the mcp server returns a big response. limiting
the response length from the mcp server fixed the issue.