Anthropic API 400 invalid_request_error: what it means and how to fix it

What it means

There was an issue with the format or content of your request. This error type may also be used for other 4XX status codes not listed in the docs.

Causes

• Malformed or invalid request format or content
• Sending a prefilled last assistant message to Claude 4.6 and later models ("This model does not support assistant message prefill. The conversation must end with a user message.")
• Editing, reordering, filtering or reconstructing thinking or redacted_thinking blocks in the latest assistant message before sending them back
• Sending thinking: {"type": "enabled"} to Claude 4.7 and later models, or thinking: {"type": "adaptive"} to Claude 4.5 and earlier models
• Sending thinking: {"type": "disabled"} to models where thinking is always on
• Sending tool_choice type "tool" or "any" to models that do not support forced tool use
• Declaring an Anthropic-defined tool type the requested model does not support
• Sending thinking.block_binding without the thinking-binding-controls-2026-08-01 beta header

How to fix it

1. 1
   Fix 1.

   Check the request format and parameters against the API reference and the Common validation errors section of the errors page.

2. 2
   Fix 2.

   On models that do not support prefill, use structured outputs, system prompt instructions, or output_config.format instead.

3. 3
   Fix 3.

   Pass thinking and redacted_thinking blocks back exactly as received, including blocks whose thinking field is empty.

4. 4
   Fix 4.

   Use adaptive thinking on models that removed extended thinking, and thinking: {"type": "enabled", "budget_tokens": N} on models that only support extended thinking.

5. 5
   Fix 5.

   Use tool_choice auto (the default) with strict tool use, or structured outputs, on models that reject forced tool use.