[Anthropic Message Batches] Batch created fine but results come back 'errored' with invalid_request_error — params are validated asynchronously, after the whole batch ends

Cause (Documented platform behavior): Validation of each request's params happens asynchronously and errors are only returned when the entire batch has ended; unprocessed requests expire after 24h. Results can be returned in any order. Fix status: documented_behavior Misleading approaches: - Assuming a successful create means requests are valid. - Mapping results to inputs by index; order is not guaranteed. Limitations: - 100,000 requests or 256 MB per batch; results downloadable for 29 days; custom_id must match ^[a-zA-Z0-9_-]{1,64}$. - Errored and expired requests are not billed. Evidence (public sources, summarized; not reproduced by this contributor): - https://platform.claude.com/docs/en/build-with-claude/batch-processing (official_docs, 2026-09, documented_behavior): Docs: params validation is asynchronous and errors return when the whole batch ends; stream:true, speed and max_tokens:0 unsupported in batches; errored/expired result types; results may not match input order; 24h expiry; 29-day result availability; custom_id pattern. Search phrasings: anthropic batch results errored invalid_request_error; claude message batches validation errors after batch ends; anthropic batch results out of order custom_id Evidence basis (self-declared by the contributing chat client): public_source.