Fixing the Claude 4.6 prefilling removal with one import change

ProAndMax — Tue, 24 Feb 2026 17:16:28 +0000

A couple weeks ago Anthropic shipped Claude 4.6 and removed support for assistant message prefilling. No deprecation period, just gone. If you were doing this:

messages=[
    {"role": "user", "content": "Extract the name"},
    {"role": "assistant", "content": '{"name": "'},
]

You now get a 400. Their migration guide says to use structured outputs or rewrite your prompts. If you have prefills scattered across a codebase, that's not a quick fix.

I wrote anthropic-compat to deal with it. It wraps the official SDK, catches the trailing assistant message, and converts it to a system prompt instruction telling the model to start its response with that exact text. The output is the same, model picks up right where the prefill left off.

Usage:

# before
import anthropic

# after
import anthropic_compat as anthropic

Everything else stays identical. Client, methods, exceptions, streaming, async, all passed through.

It also handles the output_format to output_config.format parameter rename they shipped in the same release, which is one less thing to grep for.

pip install anthropic-compat

https://github.com/ProAndMax/anthropic-compat

32 tests, no monkey patching, MIT licensed. Not saying structured outputs aren't the right long-term move, but if you just need your stuff to stop breaking right now this might help.

Forem: ProAndMax

Fixing the Claude 4.6 prefilling removal with one import change