Should I retry Anthropic 400 invalid_request_error?

No. Fix the cause first: Pass every thinking and redacted_thinking block from the last assistant turn back unchanged, including empty ones.

AI API errors / Anthropic (Claude)

Anthropic (Claude) · HTTP 400 · checked Oct 24, 2026

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

Do not retry: fix it first

400 invalid_request_error: `thinking` or `redacted_thinking` blocks in the latest assistant message cannot be modified. These blocks must remain as they were in the original response.

What it meansWhen you send an assistant turn back (usually in a tool-use loop), its thinking blocks must be exactly as the API returned them. Editing, reordering, filtering or rebuilding them breaks their signature.

How to fix it

Pass every thinking and redacted_thinking block from the last assistant turn back unchanged, including empty ones.
If your code filters content blocks by type before resending, keep both of those types.

Handle it in code

import anthropic

# The SDK retries 429, 5xx and connection errors with exponential backoff and honors retry-after.
client = anthropic.Anthropic(max_retries=6)

try:
    msg = client.messages.create(model="claude-sonnet-5-5", max_tokens=1024,
                                 messages=[{"role": "user", "content": "Hello"}])
except anthropic.RateLimitError as e:      # 429 after all retries
    print("still rate limited", e.response.headers.get("retry-after"))
except anthropic.APIStatusError as e:      # 529 overloaded, 500, 504...
    print(e.status_code, e.response.headers.get("request-id"))

The official SDKs already retry connection errors, 429 and 5xx twice with exponential backoff and honor retry-after; raise max_retries for batch jobs.

Same kind of error at other providers

Other Anthropic (Claude) errors

Source

Anthropic (Claude) error documentation: platform.claude.com/docs/en/api/errors (checked Oct 24, 2026)

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