Writes to managed Delta tables fail: Unknown feature 'icebergWriterCompatV1'

[randypitcherii]

Summary

Write operations (INSERT, UPDATE, DELETE) to managed Delta tables via the `uc_catalog` extension fail with a DeltaKernel error when the table has `icebergWriterCompatV1` in its writer features. This affects any managed Delta table in Databricks Unity Catalog that has UniForm (Iceberg compatibility) enabled.

This is a blocking issue for a legitimate, expected write path: `uc_catalog` uses Delta readers/writers, so writing to managed Delta tables over UC REST should work out of the box. The `icebergWriterCompatV1` feature is preventing that.

Error

```
DeltaKernel error: Unsupported: Unknown feature 'icebergWriterCompatV1'
```

Steps to Reproduce

```sql
INSTALL uc_catalog FROM core; LOAD uc_catalog;
INSTALL delta; LOAD delta;
CREATE SECRET (TYPE UC, TOKEN '', ENDPOINT '<workspace_url>', AWS_REGION 'us-east-1');
ATTACH '' AS uc (TYPE UC_CATALOG);

-- Any write operation fails:
INSERT INTO uc..<managed_delta_table> SELECT * FROM uc..<managed_delta_table> LIMIT 1;
```

The table was created via Databricks as a managed Delta table with UniForm enabled:

```sql
CREATE TABLE

USING DELTA
TBLPROPERTIES ('delta.universalFormat.enabledFormats' = 'iceberg', 'delta.enableIcebergCompatV2' = 'true')
```

Context

Databricks Unity Catalog adds `icebergWriterCompatV1` to the Delta protocol's `writerFeatures` when UniForm is enabled on a managed Delta table. DeltaKernel correctly rejects writes when it encounters an unknown writer feature per the Delta spec.

Since `uc_catalog` operates over the UC REST API using Delta readers/writers, writing to managed Delta tables is the expected and intended use case — not a workaround. Support for `icebergWriterCompatV1` needs to be added to unblock this path.

The upstream Delta project has a related feature request: delta-io/delta#4289.

Note: Reads work fine — this only affects write operations.

Expected Behavior

DeltaKernel should support the `icebergWriterCompatV1` writer feature so that writes to managed Delta tables with UniForm enabled work correctly via `uc_catalog`.

Environment

• DuckDB: 1.5.0
• Extensions: uc_catalog, delta (latest from core)
• Catalog: Databricks Unity Catalog

[randypitcherii]

Additional context: UC REST credential vending fails for Delta+UniForm tables

Setup

Using DuckDB's Delta extension via UC REST (uc_catalog extension + Delta reader/writer) against Databricks Unity Catalog.

Observed behavior

• All reads and writes to managed Delta tables with UniForm enabled fail with a Bad Request error from the temporary-table-credentials API (/api/2.0/unity-catalog/temporary-table-credentials).
• The same managed Delta tables are readable via the Iceberg REST path (using UniForm's Iceberg metadata), confirming the tables themselves are fine.
• Reads and writes to managed Iceberg tables via UC REST work for reads but not writes (separate issue).
• GRANT EXTERNAL USE SCHEMA ON SCHEMA ... TO ... was confirmed granted.

Root cause analysis

Databricks Unity Catalog creates managed Delta tables with UniForm enabled by default in many configurations. These tables carry the delta.enableIcebergCompatV2 / icebergWriterCompatV1 table property. The temporary-table-credentials API appears to reject credential requests for these Delta+UniForm tables, which blocks the uc_catalog extension from obtaining short-lived credentials for any read or write operation.

This suggests the issue is in the credential vending request rather than the Delta write path specifically.

Related

• consider table type when requesting secrets unity_catalog#68 — tracks the same Bad Request failure from the UC REST / uc_catalog side. The underlying cause is likely shared.

Reference links

• DuckDB Delta extension docs
• DuckDB UC Catalog extension
• Databricks temporary-table-credentials API
• Databricks UniForm documentation