## TL;DR
Confirm the source table actually holds rows for that partition date, then rerun the backfill for just that partition, because the staging query produced no output either because the upstream partition landed late or because the partition filter in the conf points at a date with no source rows. Start with the Chronon group_by backfill log and the staging table for the failing partition, work on one affected call, record, or run first, and prove the fix there before running the full set. Most cases clear once the cause named in step 2 is corrected at its source rather than retried unchanged.

```text
Chronon group by backfill failed error "staging query produced no output for partition"
```

## Use this when
- You see this exact message or symptom: Chronon group by backfill failed error "staging query produced no output for partition"
- One run, record, call, or object is affected and you can name it
- A retry without changing anything reproduces the same result

## Not for this skill when
- The system fails with a different message and none of the signals here appear
- Nothing has failed yet and you are doing first time setup or general planning
- Every service from this vendor is down for all users, which is a vendor outage to wait out first

## Steps
1. Reproduce the failure once using the Chronon group_by backfill log and the staging table for the failing partition, and record the time, the workspace or account, and the one item it names. Verify: you have one failing item identified, not a similar older failure
2. Inspect the conf partition column and format, the ds value the job ran with, and a row count on the source table for that exact partition. Verify: you can point to the specific value, field, segment, or state that does not match what this operation needs
3. Apply the fix at that source: Confirm the source table actually holds rows for that partition date, then rerun the backfill for just that partition, because the staging query produced no output either because the upstream partition landed late or because the partition filter in the conf points at a date with no source rows. Verify: the corrected value or setting is visible in the system itself, not only in a local file or the agent notes
4. Re-run the original operation on the single affected item first. Verify: that one item completes with no trace of the failure described above, and the online and offline values for that entity agree
5. Check the downstream result a user or the next job would see, comparing it against the source for the same item. Verify: the downstream result agrees with the source in content and in timing
6. Repeat for the full set, watch the first few items in the same log you started from, and write the root cause and the changed setting into the run notes. Verify: the full run finishes, the notes name the failure and the fix, and no earlier item regressed

## Variant phrasings
### Chronon group by backfill failed error "staging query produced no output for partition"
The subject-first phrasing of the same failure. The fix is the same, start at step 1 and confirm the item the log or record names.
### Tool-first phrasing of this failure
The phrasing searchers use once they know which system raised it. It is the same underlying problem, not a separate bug.
### Why does this keep happening after a retry
The repeat-occurrence phrasing. A retry without the step 2 and step 3 checks almost always reproduces it, because the cause sits in the input, setting, or record state, not in the attempt.

Compatibility: Chronon on Spark, group_by confs with daily partitions, Hive or Iceberg source tables.

## Why it happens
Chronon treats an empty staging result as a hard failure so a silent empty partition cannot be mistaken for real zeros downstream. The empty result almost always traces to an upstream partition that had not landed yet or a partition format mismatch such as yyyy-MM-dd against yyyyMMdd.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_aVjzu95r2FWn5fTBWAPkyA