Tool calls fail on Claude 4.6 models

[jcbjoe]

pipecat version

0.0.105

Python version

3.11.5

Operating System

MacOS 26.2

Issue description

The new Anthropic Sonnet 4.6 does not support assistant prefilling. This causes tool calls to fail with the following error:

PipelineTask#1: Something went wrong: ErrorFrame#0(error: Unknown error occurred: Error code: 400 - {'type': 'error', 'error': {'type': 'invalid_request_error', 'message': 'This model does not support assistant message prefill. The conversation must end with a user message.'}, 'request_id': ''}, fatal: False)

A workaround for this is to append a blank message to the llm request before it is sent and is the solution the LiveKit team opted for: livekit/agents#4973

Reproduction steps

Use a tool call with a claud 4.6 model (Opus or Sonnet)

Expected behavior

No errors and the flow work as expected

Actual behavior

Pipeline crashes

Logs

PipelineTask#1: Something went wrong: ErrorFrame#0(error: Unknown error occurred: Error code: 400 - {'type': 'error', 'error': {'type': 'invalid_request_error', 'message': 'This model does not support assistant message prefill. The conversation must end with a user message.'}, 'request_id': 'req_011CYzCWrrmTBzStvAz2XWKE'}, fatal: False)

[markbackman]

Thanks for reporting this.

Can you say more about how you're hitting this issue? Are you manually adding an assistant message at the end of the context or is there a scenario where you end up with an assistant message as the last message?

AFAIK, Pipecat should always end up with a user message as the last message, but perhaps there's a case we're not considering.

[jcbjoe]

Hey @markbackman This only seems to happen when you try and execute a tool call. Essentially what I witnessed was the following flow of messages:

• User: Whats the weather?
• (Execute tool call)
• Assistant: One moment while I check that.
• (Tool call ends and and assistant is the last message)