Microsoft Fabric Error:
DELTA_FAILED_TO_MERGE_FIELDS
What does this error mean?
A write or merge operation on a Fabric Lakehouse Delta table failed because the source data's schema is incompatible with the existing Delta table schema.
Common causes
- 1Appending a DataFrame to a Delta table using .mode('append') when source and target have mismatched data types (e.g., StringType vs. TimestampType)
- 2Loading data from a new source with different column types than the existing Delta table
- 3Schema evolution is disabled on the Delta table (no MERGE SCHEMA option) while the incoming data has a different structure
- 4A MERGE operation where the incoming dataset redefines a field type already present in the target
How to fix it
- 1Cast the source DataFrame columns to match the target Delta table schema before writing: use .cast() to align data types.
- 2Run DESCRIBE TABLE <table_name> in a notebook to inspect the current target schema and compare it to the source.
Beyond the docs
Common practitioner solutions not covered in the official documentation.
- 1Enable schema evolution by adding .option('mergeSchema', 'true') to the write operation
- 2If schema evolution is intentional, use ALTER TABLE to update the Delta table schema explicitly before the write
- 3Check upstream pipeline or notebook changes that may have altered the column types in the source DataFrame