Anthropic 400 invalid_request_error: read it as config, and check spend limits before you retry

TL;DR: A 400 from the Anthropic API means the request itself is wrong, or your own spend limit stopped it. Retrying the same bytes never helps. Read the error message. Fix that field and resend.

The fix

1. Read the error message. It names the field or parameter that is wrong. Fix that field and resend.
2. If the message begins "You have reached your specified API usage limits" (or "specified workspace API usage limits"), your own spend limit is the cause, not your request. Raise or remove the limit in Console under Settings, then Billing. The message says when access resumes.
3. Catch-all rule: this error type is also used for other 4xx codes not listed in the docs. If the message does not name a field, check the request shape against the endpoint reference anyway.
4. Retry policy: never blind-retry a 400. Same request, same 400, forever. Fix first.
5. Distinguish "your request is malformed" from "your spend limit stopped you" by reading the message first.
6. Own spend limit hit: 400. Tier monthly spend cap hit: 429 with error code enforcedspendlimit_reached. Know which one you are looking at.

When to use this

• You are seeing this exact error message; match the block above, not just part of it.
• The failing call matches the scenario in the title: Anthropic 400 invalidrequesterror.
• You want the fastest verified fix before digging through logs.

When not to use this

• Your error text differs from the block above; close cousins often have different causes.
• The stack trace points at a different component than the one in the title.
• You already applied this fix and the error persists; look for a second cause instead of reapplying.

Compatibility

• Not pinned to a specific version; follows current Anthropic behavior.

Anthropic's error reference says invalidrequesterror covers request format/content issues, is also used for other unlisted 4xx codes, and that a 400 is returned when usage reaches an org or workspace spend limit you set. Verified live at https://platform.claude.com/docs/en/api/errors. The spend-limit message wording is documented on the rate limits page.