Error contract

Row errors are a single list aligned to the request array, using {} for rows without errors. A row's non-field error is reported under DRF's NON_FIELD_ERRORS_KEY (non_field_errors by default). Field errors include malformed keys (Invalid value for 'ticker': expected a string, got list., raised before any collection runs) and lookup misses (Ticker 'XYZ' not found.: the field's error= template or Object '{value}' not found., with {value} formatted from the coerced key).

Status	Code	When	Body
400	invalid	malformed rows, validation errors	index-aligned list
403	batch_permission_denied	a target fails the object permissions	index-aligned list
404	batch_target_not_found	a target pk is missing or out of scope	index-aligned list
409	batch_conflict	a uniqueness rule is violated	index-aligned list
409	batch_integrity_conflict	the database rejected the write	{"detail": ...}
413	batch_size_limit_exceeded	more than batch_max_size rows	{"detail": ...}
504	batch_collection_timeout	async collection timed out	{"detail": ...}

The checks run in this order, and the first failing one answers: malformed rows (400) → not found (404) → permission (403) → validation (400) → conflicts (409). Any error writes nothing. This format is stable and independent of DRF's own LIST_SERIALIZER_ERRORS_AS_DICT transition.