Frappe Database Error Diagnosis & Resolution

Cross-ref: frappe-core-database (API syntax), frappe-errors-controllers (controller errors).

Error-to-Fix Mapping Table

Error / Exception	HTTP	Cause	Fix
`DuplicateEntryError`	409	Unique constraint violation on insert/rename	Check existence first OR catch and return existing
`DoesNotExistError`	404	`get_doc()` on missing record	Use `frappe.db.exists()` first OR catch exception
`LinkValidationError`	417	Link field points to non-existent record	Validate link target exists before save
`LinkExistsError`	N/A	Delete blocked by linked documents	Show linked docs to user; use `force=True` carefully
`MandatoryError`	417	Required field is empty on save	Set all mandatory fields before insert/save
`TimestampMismatchError`	N/A	Concurrent edit detected (`modified` changed)	Reload doc and retry, or inform user to refresh
`CharacterLengthExceededError`	417	String exceeds field maxlength / DB column size	Truncate input or increase field length
`DataTooLongException`	417	Value exceeds DB column storage capacity	Same as CharacterLengthExceededError
`InReadOnlyMode`	503	Write attempted during read-only mode	Check `frappe.flags.in_import` or site config