400 errors after switching from Claude Haiku 4.5 to claude-haiku-5-5 with budget_tokens, temperature, top_k, or prefill

Symptom

400 invalid_request_error after changing model to claude-haiku-5-5Request with thinking budget_tokens is rejectedRequest with temperature, top_p, or top_k is rejectedRequest ending with an assistant prefill message is rejectedcomputer_20250124 tool returns 400

Cause

Claude Haiku 5.5 (launched 2026-10-07) rejects manual extended thinking via budget_tokens, the sampling parameters temperature, top_p and top_k, assistant message prefill, and the computer_20250124 tool. Thinking blocks sent back after a change to system, tools, or earlier turns can also return 400. Adaptive thinking is on by default.

Fix

Remove budget_tokens and use adaptive thinking with the effort parameter. Drop temperature, top_p, and top_k. Remove the trailing assistant prefill message and steer format through the prompt or structured outputs. Use computer_toolset_20260801 on the Claude API and Google Cloud, or computer_20251124 on Amazon Bedrock. Do not replay thinking blocks after editing earlier context.

ApplyCopy

model: "claude-haiku-5-5"; remove: temperature, top_p, top_k, thinking.budget_tokens, trailing assistant message; computer tool: {"type":"computer_toolset_20260801"}

Affected

claude-haiku-5-5, launched 2026-10-07, when migrating from Claude Haiku 4.5

Sources

• https://platform.claude.com/docs/en/release-notes/overview
• https://platform.claude.com/docs/en/models/haiku-5-5/migration-guide

Last verified 2026-10-17 · Confirmed by 0 builders