Assistant message prefill returns a 400 on Claude Sonnet 5.5
Returns 400
Applies to claude-sonnet-5-5 on the Anthropic Messages API.
Error message, quoted from Anthropic's docs:
This model does not support assistant message prefill. The conversation must end with a user message.How to fix it
End the conversation with a user message. Use structured outputs for output format, the system prompt for preambles, and the user turn for continuations.
{
"max_tokens": 1024,
"messages": [
{
"content": "Summarize this ticket.",
"role": "user"
},
{
"content": "{",
"role": "assistant"
}
],
"model": "claude-sonnet-4-5-20250929"
}{
"max_tokens": 1024,
"messages": [
{
"content": "Summarize this ticket.",
"role": "user"
}
],
"model": "claude-sonnet-5-5"
}Official source
Claude Sonnet 5.5 rejects a prefilled last assistant turn with a 400 error
https://platform.claude.com/docs/en/models/sonnet-5-5/migration-guide#migrating-from-sonnet-45 (last checked 2026-11-02)
Other changes for the same models
- temperature, top_p and top_k return an error on Claude Sonnet 5.5
- tool_choice any or tool returns a 400 on Claude Sonnet 5.5
- thinking type enabled with budget_tokens returns a 400 on Claude Sonnet 5.5
- thinking type disabled returns a 400 on Claude Sonnet 5.5
- Top-level output_format returns a 400 on Claude Sonnet 5.5
- computer_20250124 isn’t accepted by Claude Sonnet 5.5 or Haiku 5.5
- fine-grained-tool-streaming-2025-05-14 with a toolset returns a 400
- Beta headers to remove when moving to Claude Sonnet 5.5
- Claude Sonnet 5.5 thinks by default when thinking is omitted
FAQ
What does "This model does not support assistant message prefill. The conversation must end with a user message." mean?
Assistant message prefill returns a 400 on Claude Sonnet 5.5 The fix: End the conversation with a user message. Use structured outputs for output format, the system prompt for preambles, and the user turn for continuations.
How do I fix this when moving to claude-sonnet-5-5?
End the conversation with a user message. Use structured outputs for output format, the system prompt for preambles, and the user turn for continuations.
Where is this documented?
In Anthropic’s official docs: https://platform.claude.com/docs/en/models/sonnet-5-5/migration-guide#migrating-from-sonnet-45, last checked 2026-11-02.