## TL;DR

The agent referenced a source table that was never declared. Before using any `source()` call, verify it with `dbt ls --resource-type source`. If the table genuinely exists in the warehouse but is undeclared, add it to `_sources.yml`; if the agent invented it, correct the reference.

## Error

```text
fix agent hallucinated a source table that isn't in sources.yml: config error
```

## Steps

1. Verify the claim: `dbt ls --resource-type source --select source:[NAME].[TABLE]`. Expected: either it exists or the hallucination is confirmed.
2. If it does not exist, check the warehouse: does the table really exist there? Expected: you distinguish "undeclared but real" from "invented".
3. If real but undeclared, add it to `_sources.yml` with the correct database, schema, and table. Expected: the declaration matches the warehouse.
4. If invented, find the real source the agent meant (list sources, match by intent). Expected: a verified, declared source.
5. Run `dbt parse` and continue with the verified reference. Expected: no undeclared sources remain.

## When to use

- An agent's `source()` reference fails because the source was never declared.
- Any agent-generated source reference.

## When not to use

- The source is declared but the config is wrong (fix the declaration).
- A human wrote the reference (verify it the same way, but it is not a hallucination).

## Tool compatibility

- dbt Core 1.0 and later, all adapters.

## Variant phrasings

### Agent added a model ref instead of a source

Related confusion: raw tables are sources, not models; declare them as sources.

### sources.yml exists but the agent did not read it

Always list declared sources before inventing new ones.

## Why it happens

Agents pattern-match `source('raw', 'orders')` from examples and assume the table exists. Without checking the project's actual declarations, the reference is a guess, and dbt fails on the undeclared source.

## Edge cases

- The `_sources.yml` file must be under a parsed path, or declarations are invisible.
- Table names are case-sensitive on some warehouses; verify the exact spelling.
- Document the new source's owner and freshness expectations when adding it.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_mtR1QWRurAIar5kt_etnLg