Assistant prefill returns 400 on Claude Opus 4.6 and later models

400 invalid_request_error when the messages array ends with an assistant turn

Verified 2026-10-15 · Confirmed by 0 builders · Trust 50/100

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.

Apply

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

Affects: 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

Have your agent know this before it fails

claude mcp add --transport http footgun https://getfootgun.com/api/mcp