# Source schema

Reads the data source's own schema, as last introspected from the warehouse or uploaded as DDL. This is the layer beneath the ontology: every table the source has, including ones the ontology does not model at all.

Use it during triage when a fix needs data the ontology does not carry yet, for instance to inspect the columns of the unmapped tables a `reference_tables` fix proposal names. See [Issues](/reference/mcp/issues/).

## Parameters

| Name | Type | Notes |
| --- | --- | --- |
| `project_id` | UUID, required | The project whose source to read |
| `tables` | list of strings | Fully qualified `schema.table` names to detail, at most 20 per call. Omit to list every table without columns |

## Response

| Field | Notes |
| --- | --- |
| `schema_version` | The source-schema snapshot number and when it was taken |
| `tables` | `[{name, status, column_count}]` |
| `detailed` | Only when `tables` was passed: `[{name, status, columns: [{name, data_type, nullable?, description?}]}]` |
| `error` | Set instead of the keys above when the request fails. A project with no source schema yet returns an error telling you to sync it or upload a DDL first |

Status has three values, and the difference matters when deciding whether a table needs modeling:

| Status | Meaning |
| --- | --- |
| `modeled` | Placed or curated in the ontology |
| `tracked` | Mirrored by the ontology’s physical layer, but not modeled: no domain, no description |
| `untracked` | The ontology has no record of it |

**This is an interactive lookup, capped at 20 detailed tables per call.** For bulk work across a whole schema, [`cassis schema pull`](/reference/cli/schema/) writes the same schema to a gitignored local snapshot an agent can grep in one pass. Paging through this tool during a modeling sweep is the slow way to do it.

The schema here is observed state, not the ontology. Adding a table to the ontology means writing its file in the repository or creating it in the app, not calling a tool: see the [capability matrix](/reference/capabilities/).