Assistant prefill returns 400 on Claude Opus 4.6 and later models

Symptom

400 invalid_request_error when the messages array ends with an assistant turnA prefilled '{' that used to force JSON output stops working after a model upgrade

Cause

Ending messages with a partial assistant turn for the model to continue is rejected on Opus 4.6 and later Opus models and on Haiku 5.5. Code that used prefill to force JSON or skip a preamble fails as soon as the model id changes.

Fix

End messages with a user turn. Use structured outputs through output_config.format for JSON, or put the format rule in the system field. On Amazon Bedrock, where structured outputs are not offered for Opus 5.5, use instructions and validate the reply.

ApplyCopy

output_config: { format: { type: "json_schema", schema: { type: "object", properties: { answer: { type: "string" } }, required: ["answer"], additionalProperties: false } } }

Affected

Claude Opus 4.6, 4.7, 4.8, 5, 5.5 and Haiku 5.5 (Haiku 4.5 still accepts prefill), as of 2026-10

Sources

• https://platform.claude.com/docs/en/models/opus-5-5/migration-guide
• https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide
• https://platform.claude.com/docs/en/build-with-claude/structured-outputs

Last verified 2026-10-15 · Confirmed by 0 builders