Top-level output_format returns a 400 on Claude Sonnet 5.5
Returns 400
Applies to claude-sonnet-5-5 on the Anthropic Messages API.
Anthropic's docs describe this error but don't quote the exact message, so we don't either.
How to fix it
Move the schema to output_config.format.
Before
{
"max_tokens": 1024,
"messages": [
{
"content": "Summarize this ticket.",
"role": "user"
}
],
"model": "claude-sonnet-4-5-20250929",
"output_format": {
"schema": {
"type": "object"
},
"type": "json_schema"
}
}After (Claude Sonnet 5.5)
{
"max_tokens": 1024,
"messages": [
{
"content": "Summarize this ticket.",
"role": "user"
}
],
"model": "claude-sonnet-5-5",
"output_config": {
"format": {
"schema": {
"type": "object"
},
"type": "json_schema"
}
}
}Official source
Theoutput_formatparameter is deprecated. To use it anyway, add thestructured-outputs-2025-11-13beta header. Without it, the API returns a 400 error. Useoutput_config.formatinstead.
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
- Assistant message prefill returns a 400 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
- 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
How do I fix this when moving to claude-sonnet-5-5?
Move the schema to output_config.format.
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.