In-place replacement fails with Anthropic error when translating selected text

[fniessen]

Please update gptel first -- errors are often fixed by the time they're reported.

• I have updated gptel to the latest commit and tested that the issue still exists

Bug Description

I want to translate a script comment written in French into English.

1. I add my translation instruction, and select the relevant text (including the instruction itself),

2. I choose the "in-place" replacement option, and then run the command.

Despite this, I receive the error message...

Anthropic error: ((HTTP/2 400) invalid_request_error) This model does not support assistant message prefill. The conversation must end with a user message.

... shown below.

Am I using the feature correctly? If so, any idea why is this error occurring?

Backend

None

Steps to Reproduce

1. I add my translation instruction, and select the relevant text (including the instruction itself),
2. I choose the "in-place" replacement option, and then run the command.

Additional Context

Not relevant, I think. But Emacs 31.1 on WSL.

Backtrace

Log Information