refactor(sl): split overlay columns from column_overrides and enforce TS/Python wire contract

Overlay sources now have two distinct collections: `columns:` for computed
columns (requiring `expr` + `type`) and `column_overrides:` for metadata
patches to inherited manifest columns. Composing or loading an overlay that
mixes the two — or references an unknown column — fails with a typed error.

Introduce `ResolvedSemanticLayerSource` / `resolvedSourceSchema` /
`toResolvedWire` as the strict shape sent to the Python engine, and add a
schema contract test that diffs Zod against the Pydantic JSON schema dumped
by `python -m semantic_layer dump-schema`. `SourceDefinition` is now
`extra="forbid"` on the Python side.

`loadAllSources` surfaces per-file load errors instead of swallowing them,
so validation/query paths can report manifest shard parse failures.

packages/cli/src/sl.ts

@@ -213,7 +213,11 @@ export async function runKtxSl(args: KtxSlArgs, io: KtxSlIo = process, deps: Ktx
       if (!source) {
         throw new Error(`Semantic-layer source "${args.connectionId}/${args.sourceName}" was not found`);
       }
-      const result = await validateLocalSlSource(source.yaml, { project, connectionId: args.connectionId });
+      const result = await validateLocalSlSource(source.yaml, {
+        project,
+        connectionId: args.connectionId,
+        sourceName: args.sourceName,
+      });
       if (!result.valid) {
         for (const error of result.errors) {
           io.stderr.write(`${error}\n`);

packages/context/package.json

@@ -153,6 +153,7 @@
     "@types/node": "^25.7.0",
     "@types/pg": "^8.20.0",
     "@vitest/coverage-v8": "^4.1.6",
+    "ajv": "8.20.0",
     "typescript": "^6.0.3",
     "vitest": "^4.1.6"
   },

packages/context/skills/dbt_ingest/SKILL.md

@@ -14,14 +14,14 @@ Use this skill for **uploaded** dbt projects (`dbt_project.yml` at stage root, `
 |-----|--------|--------|
 | `models:` entry with `columns:` | **Overlay** on the manifest table with the same name (after `discover_data` / `entity_details`) | One SL source per physical table; model name may differ from DB name — resolve with `read_raw_file` + warehouse context. |
 | `sources:` → `tables:` | Same as models; use `identifier` when present instead of logical `name`. | Schema + name must match how the connection sees tables. |
-| Column `description` | `descriptions.user` or merged `descriptions` map on the column | Do not overwrite `dbt` description keys from sync. |
+| Column `description` | `column_overrides[].descriptions.user` on the overlay | Do not overwrite `dbt` description keys from sync. |
 | `data_tests: not_null` / `unique` | Short hint in column `descriptions` or notes: “dbt: not null”, “dbt: unique” | Full structured metadata lands in manifest via **sync**; the skill keeps bundle-time SL text useful for the agent. |
 | `accepted_values` | Add a **brief** line in the column description: allowed values (truncate long lists) | Also mention enum-like use in `discover_data` / filters. |
 | `relationships` | Add or confirm `joins:` on the overlay **only** when `to` resolves to a real table via `read_raw_file` + `discover_data` / `entity_details` | If the ref cannot be resolved, capture the intent in a wiki page instead. |
 
 ## Physical schema grounding
 
-dbt YAML is documentation and test metadata; it is not permission to invent physical columns. Before writing any table-backed SL source, confirm the real warehouse shape with `discover_data`, `sl_discover`, or `entity_details` and use only confirmed column names in `columns:`, `grain:`, `joins:`, `segments:`, and `measures[].expr`.
+dbt YAML is documentation and test metadata; it is not permission to invent physical columns. Before writing any table-backed SL source, confirm the real warehouse shape with `discover_data`, `sl_discover`, or `entity_details` and use only confirmed column names in `column_overrides:`, computed-only `columns:`, `grain:`, `joins:`, `segments:`, and `measures[].expr`.
 
 For dbt context-source ingest, the dbt connection is usually not the warehouse connection. Call `sl_discover` without `connectionId` first, then write overlays to the connection that owns the matching manifest-backed source (for example `postgres-warehouse`), not to the dbt connection (for example `dbt-main`). If no matching manifest-backed source is visible on any warehouse connection, do not call `sl_write_source`; record `emit_unmapped_fallback` and keep the fact wiki-only.
 
@@ -61,7 +61,7 @@ SL source, `tables:` frontmatter, `sl_refs`, or `emit_unmapped_fallback`:
 
 ## 1.1 test hints (descriptions / meta)
 
-When YAML shows `accepted_values` or `not_null`, add **short** hints into `columns[].descriptions` (e.g. under `user`) or freeform column notes so chat and validation see intent before the next git sync refreshes `constraints` / `enum_values` in `_schema`. Keep hints under a few words when possible.
+When YAML shows `accepted_values` or `not_null`, add **short** hints into `column_overrides[].descriptions` (for example under `user`) or freeform column notes so chat and validation see intent before the next git sync refreshes `constraints` / `enum_values` in `_schema`. Keep hints under a few words when possible.
 
 ## Overlap with MetricFlow
 
@@ -71,6 +71,6 @@ If the same bundle also has MetricFlow `semantic_models:` / `metrics:`, the **`m
 
 - Do not run `dbt` CLI or assume `target/` / `manifest.json` exists in the upload.
 - Do not invent column names, grain keys, or measure expressions from dbt model names, descriptions, tests, or common naming patterns.
-- Do not write `columns:`, `grain:`, or `measures:` for a dbt model unless those exact column names are confirmed by dbt YAML columns or warehouse schema discovery.
+- Do not write computed `columns:`, `column_overrides:`, `grain:`, or `measures:` for a dbt model unless those exact column names are confirmed by dbt YAML columns or warehouse schema discovery.
 - Do not invent joins from `relationships` tests if the target model/table is not found in SL or the warehouse.
 - Do not read `peerFileIndex` paths — use `read_raw_file` only on `rawFiles` and `dependencyPaths` from the WorkUnit.

packages/context/skills/lookml_ingest/SKILL.md

@@ -12,7 +12,7 @@ LookML views map to SL sources, `measure:` to measures, `explore: { join: }` to
 
 | LookML | KTX form | Notes |
 |---|---|---|
-| `view: X { sql_table_name: …; measure:/dimension:/join: }` | **Overlay** at `<connId>/X.yaml` with `measures`, `columns` (computed), `joins`, `segments` | Manifest-backed; inherit grain/columns |
+| `view: X { sql_table_name: …; measure:/dimension:/join: }` | **Overlay** at `<connId>/X.yaml` with `measures`, computed-only `columns`, `column_overrides`, `joins`, `segments` | Manifest-backed; inherit grain/columns |
 | `view: X { derived_table: { sql: … } }` | **Standalone** with top-level `sql:`, explicit `grain:` + `columns:` | No manifest entry exists |
 | `view: X { sql_always_where: <p> }` | **Standalone** with `sql: SELECT * FROM <base> WHERE <p>` | Enforcement, not opt-in |
 | `explore: { join: Y { sql_on: …; relationship: … } }` | `joins:` entry `{ to: Y, on: "<local> = Y.<col>", relationship: … }` | On the overlay or standalone |
@@ -136,7 +136,8 @@ KTX overlay at `<connId>/fct_labs.yaml`:
 
 ```yaml
 name: fct_labs
-description: "Lab-order fact table. One row per lab order event."
+descriptions:
+  user: "Lab-order fact table. One row per lab order event."
 columns:
   - name: is_byol
     type: boolean

packages/context/skills/metabase_ingest/SKILL.md

@@ -79,7 +79,7 @@ SL source, `tables:` frontmatter, `sl_refs`, or `emit_unmapped_fallback`:
 
 For each card:
 1. Analyze `resolvedSql` + `resultMetadata`: identify base tables, aggregations, joins, filters, column types.
-2. **REQUIRED before any write**: call `sl_discover` for every candidate target source name. The response tells you whether the name is manifest-backed (`Type: table` or `Type: sql`). For manifest-backed names you MUST use the overlay shape (`name:` + `measures:`/`segments:`/`description:` only — no `sql:`, `table:`, `grain:`, or `columns:`); the tool will reject a standalone write and you'll have wasted the call. If `sl_discover` returns nothing for the name, you can write a standalone source. Also call `sl_read_source` on existing sources you intend to extend so you don't duplicate measures.
+2. **REQUIRED before any write**: call `sl_discover` for every candidate target source name. The response tells you whether the name is manifest-backed (`Type: table` or `Type: sql`). For manifest-backed names you MUST use the overlay shape (`name:` plus overlay fields such as `measures:`, `segments:`, `descriptions:`, `joins:`, `disable_joins:`, `column_overrides:`, and computed-only `columns:` entries with `expr` + `type`; no `sql:`, `table:`, `grain:`, or base-table `columns:`); the tool will reject a standalone write and you'll have wasted the call. If `sl_discover` returns nothing for the name, you can write a standalone source. Also call `sl_read_source` on existing sources you intend to extend so you don't duplicate measures.
 3. Include `rawPaths: ["cards/<id>.json"]` on every `sl_write_source`, `sl_edit_source`, and `wiki_write` call. If one artifact generalizes multiple near-duplicate cards, include each contributing card path and no unrelated cards.
 4. Decide:
    - Simple aggregation on a table that already has a source → `sl_edit_source` to add a measure.
@@ -98,7 +98,7 @@ measures:
     expr: "<expression>"
 ```
 
-Overlay shape: `name:` plus any of `measures:`, `segments:`, `descriptions:`, `joins:`, `disable_joins:`. Never include `sql:`, `table:`, `grain:`, or `columns:` on a manifest-backed name — those would shadow the manifest's schema and drop its joins. Overlay `joins:` are merged additively with the manifest's joins (deduped by `to` + `on`); use `disable_joins: ["<on-clause>"]` to suppress a specific manifest join. After the overlay exists, use `sl_edit_source` for further tweaks. See `sl_capture` skill for the canonical overlay rule.
+Overlay shape: `name:` plus any of `measures:`, `segments:`, `descriptions:`, `joins:`, `disable_joins:`, `exclude_columns:`, `column_overrides:`, or computed-only `columns:` entries with `expr` + `type`. Never include `sql:`, `table:`, `grain:`, or base-table `columns:` on a manifest-backed name — those would shadow the manifest's schema and drop its joins. Use `column_overrides:` for inherited column descriptions. Overlay `joins:` are merged additively with the manifest's joins (deduped by `to` + `on`); use `disable_joins: ["<on-clause>"]` to suppress a specific manifest join. After the overlay exists, use `sl_edit_source` for further tweaks. See `sl_capture` skill for the canonical overlay rule.
 
 **Join discovery:** When your card's SQL references warehouse tables (e.g. in `FROM` or `JOIN` clauses), call `sl_discover({ query: '<table>' })` before writing. The matching manifest entry's `name` is the value you use in `joins: [- to: <name>]` only when the card output exposes a local key that matches the target source grain (for example `account_id = mart_account_segments.account_id`). Do not declare a KTX join just because the card SQL joins that table internally. If the output only exposes display fields such as `account_name`, keep the SQL source self-contained or project the key before adding the join. Use `many_to_one` for FK-to-dimension joins, `one_to_many` for the reverse.
 

packages/context/skills/metricflow_ingest/SKILL.md

@@ -12,7 +12,7 @@ A MetricFlow `semantic_model` maps to an SL source; MetricFlow `measures` map to
 
 | MetricFlow | KTX form | Notes |
 |---|---|---|
-| `semantic_model: X { model: ref('t') }` with measures + dimensions | **Overlay** at `<connId>/X.yaml` with `measures`, `columns` (computed), `joins` | The `model:` ref resolves to a manifest table. |
+| `semantic_model: X { model: ref('t') }` with measures + dimensions | **Overlay** at `<connId>/X.yaml` with `measures`, computed-only `columns`, `column_overrides`, `joins` | The `model:` ref resolves to a manifest table. |
 | `semantic_model: X { model: source('s','t') }` | **Overlay** at `<connId>/X.yaml` over table `t`. | Same shape; `source()` still resolves to a physical table. |
 | `semantic_model: X { model: <literal> }` with no manifest entry | **Standalone** with explicit `sql:`, `grain:`, `columns:` | Happens when the dbt manifest isn't available. |
 | `semantic_model: Y { extends: X }` | **Merge** Y's measures/dimensions/entities into X's overlay, or write a single overlay named for the most-derived child (Y) containing both X's and Y's primitives | Do not emit a second overlay for X — flatten. |
@@ -84,7 +84,7 @@ If `sl_discover` errors because no such table exists, use `discover_data` and
 `entity_details` to find the warehouse target. If a SQL probe is still needed,
 call `sql_execution` with the same warehouse connection name, for example:
 `sql_execution({connectionName: "warehouse", sql: "SELECT 1 FROM analytics.orders LIMIT 0"})`.
-**Never invent column names** - every column in `columns:`, `grain:`, and
+**Never invent column names** - every column in computed `columns:`, `column_overrides:`, `grain:`, and
 `sql:` must be sourced from raw files, `entity_details`, or a successful SQL
 probe.
 

packages/context/skills/sl/SKILL.md

@@ -39,6 +39,10 @@ columns:                    # computed dimensions only
   - name: is_large_order
     type: boolean
     expr: "amount > 1000"
+column_overrides:           # metadata patches for inherited columns
+  - name: status
+    descriptions:
+      user: "Order lifecycle status."
 segments:
   - name: paid_non_refunded
     expr: "is_paid = true AND is_refunded = false"
@@ -51,6 +55,7 @@ joins:
 Rules:
 - Do **not** repeat base-table columns, grain, `table`, or `source_type` in an overlay — those are inherited.
 - Overlay columns MUST be computed (`expr` + `type`).
+- Use `column_overrides` to add descriptions or metadata to inherited manifest columns. Do not put `type` or `expr` in `column_overrides`.
 - `exclude_columns` hides specific manifest columns; `disable_joins` suppresses specific auto-detected joins.
 
 ### Standalone table sources
@@ -110,7 +115,7 @@ An SQL source is a one-shot answer: the aggregation is frozen, callers cannot re
 
 ### Columns
 
-Every standalone column requires `name` and `type`. Overlays have computed columns only.
+Every standalone column requires `name` and `type`. Overlays have computed columns in `columns:` and manifest column metadata patches in `column_overrides:`.
 
 - `type`: one of `string`, `number`, `boolean`, `time`. Map LookML `date`/`datetime`/`timestamp` → `time`. Map LookML `yesno` → `boolean`.
 - `role` (optional): `time` enables time-granularity queries (month, week, day). `default` is the implicit fallback.

packages/context/skills/sl_capture/SKILL.md

@@ -100,7 +100,33 @@ measures:
 
 **Extract repeated filter bundles into named segments.** If the same predicate appears on multiple measures of the same source, lift it to a `segments[]` entry and have each measure reference it. One edit updates every measure that depends on it.
 
-**Never write a standalone file on a manifest-backed name.** If `sl_discover({ query: "<table-or-source-name>" })` finds an existing schema for that name, you MUST write an overlay (`name:` + `measures:`/`segments:`/`descriptions:` only — no `sql:`, `table:`, `grain:`, `columns:`, `joins:`). A standalone with `sql:` or `table:` on a manifest-backed name clobbers the inherited columns and joins; `sl_write_source` and `sl_validate` both reject this shape with a clear fix hint. Always run `sl_discover` before your first write on any existing name.
+**Never write a standalone file on a manifest-backed name.** If `sl_discover({ query: "<table-or-source-name>" })` finds an existing schema for that name, you MUST write an overlay. A standalone with `sql:` or `table:` on a manifest-backed name clobbers the inherited columns and joins; `sl_write_source` and `sl_validate` both reject this shape with a clear fix hint. Always run `sl_discover` before your first write on any existing name.
+
+Overlay before/after examples:
+
+```yaml
+# Wrong: patches an inherited manifest column through columns:
+name: fct_orders
+columns:
+  - name: status
+    descriptions:
+      user: "Order lifecycle status."
+```
+
+```yaml
+# Right: patch inherited columns with column_overrides:
+name: fct_orders
+column_overrides:
+  - name: status
+    descriptions:
+      user: "Order lifecycle status."
+columns:
+  - name: is_large_order
+    type: boolean
+    expr: "amount > 1000"
+```
+
+Overlay YAML may include `measures:`, `segments:`, `descriptions:`, `joins:`, `disable_joins:`, `exclude_columns:`, `column_overrides:`, and computed-only `columns:` entries with `expr` and `type`. Do not include `sql:`, `table:`, `grain:`, or base-table `columns:`.
 
 **Prefer overlay decomposition over standalone SQL sources.** Before reaching for `source_type: sql`, check whether the metric decomposes into measures on existing overlays (including cross-source derived measures). Use `source_type: sql` only when:
 - The metric requires per-user/per-entity derivation that cannot be expressed as a single `expr` (e.g., `EXISTS` over a time-windowed subset), OR

packages/context/src/ingest/adapters/metricflow/import-semantic-models.test.ts

@@ -38,7 +38,7 @@ describe('importMetricflowSemanticModels', () => {
     const scoped = {
       getManifestEntry: vi.fn().mockResolvedValue(null),
       isManifestBacked: vi.fn().mockResolvedValue(false),
-      loadAllSources: vi.fn().mockResolvedValue([]),
+      loadAllSources: vi.fn().mockResolvedValue({ sources: [], loadErrors: [] }),
       loadSource: vi.fn().mockResolvedValue(null),
       writeSource: vi.fn().mockResolvedValue({ warnings: [] }),
     };
@@ -104,7 +104,7 @@ describe('importMetricflowSemanticModels', () => {
     const scoped = {
       getManifestEntry: vi.fn().mockResolvedValue(null),
       isManifestBacked: vi.fn().mockResolvedValue(false),
-      loadAllSources: vi.fn().mockResolvedValue([]),
+      loadAllSources: vi.fn().mockResolvedValue({ sources: [], loadErrors: [] }),
       loadSource: vi.fn().mockImplementation((connectionId: string, sourceName: string) =>
         Promise.resolve(sourceName === 'orders' ? { name: 'orders' } : null),
       ),
@@ -139,7 +139,7 @@ describe('importMetricflowSemanticModels', () => {
     const scoped = {
       getManifestEntry: vi.fn().mockResolvedValue(null),
       isManifestBacked: vi.fn().mockResolvedValue(false),
-      loadAllSources: vi.fn().mockResolvedValue([]),
+      loadAllSources: vi.fn().mockResolvedValue({ sources: [], loadErrors: [] }),
       loadSource: vi.fn().mockResolvedValue(null),
       writeSource: vi.fn().mockRejectedValueOnce(new Error('cannot write orders')).mockResolvedValue({ warnings: [] }),
     };
@@ -190,7 +190,7 @@ describe('importMetricflowSemanticModels', () => {
       isManifestBacked: vi.fn().mockImplementation(async (_connectionId: string, sourceName: string) => {
         return sourceName === 'orders';
       }),
-      loadAllSources: vi.fn().mockResolvedValue([]),
+      loadAllSources: vi.fn().mockResolvedValue({ sources: [], loadErrors: [] }),
       loadSource: vi.fn().mockResolvedValue(null),
       writeSource: vi.fn().mockImplementation(async (_connectionId: string, source: (typeof written)[number]) => {
         written.push(source);
@@ -268,7 +268,7 @@ describe('importMetricflowSemanticModels', () => {
       isManifestBacked: vi.fn().mockImplementation(async (_connectionId: string, sourceName: string) => {
         return sourceName === 'orders';
       }),
-      loadAllSources: vi.fn().mockResolvedValue([]),
+      loadAllSources: vi.fn().mockResolvedValue({ sources: [], loadErrors: [] }),
       loadSource: vi.fn().mockResolvedValue(null),
       writeSource: vi.fn().mockResolvedValue({ warnings: [] }),
     };
@@ -311,7 +311,7 @@ describe('importMetricflowSemanticModels', () => {
     const scoped = {
       getManifestEntry: vi.fn().mockResolvedValue(null),
       isManifestBacked: vi.fn().mockResolvedValue(false),
-      loadAllSources: vi.fn().mockResolvedValue([]),
+      loadAllSources: vi.fn().mockResolvedValue({ sources: [], loadErrors: [] }),
       loadSource: vi.fn().mockResolvedValue(null),
       writeSource: vi
         .fn()

packages/context/src/ingest/adapters/metricflow/import-semantic-models.ts

@@ -71,7 +71,7 @@ export async function importMetricflowSemanticModels(
   let crossModelSourcesCreated = 0;
 
   const preexistingSourceNames = new Set(
-    (await semanticLayerService.loadAllSources(input.connectionId)).map((source) => source.name),
+    (await semanticLayerService.loadAllSources(input.connectionId)).sources.map((source) => source.name),
   );
   const modelContexts: MetricflowSemanticModelImportContext[] = [];
   const sourceNameByModelRef = new Map<string, string>();

packages/context/src/ingest/ingest-bundle.runner.test.ts

@@ -187,7 +187,10 @@ const makeDeps = () => {
     loadAllSources: vi
       .fn()
       .mockImplementation((connectionId: string) =>
-        Promise.resolve(connectionId === 'warehouse-2' ? [{ name: 'looker__orders' }] : []),
+        Promise.resolve({
+          sources: connectionId === 'warehouse-2' ? [{ name: 'looker__orders' }] : [],
+          loadErrors: [],
+        }),
       ),
   };
   const slSearchService = {
@@ -1347,7 +1350,7 @@ describe('IngestBundleRunner — Stages 1 → 7', () => {
       frontmatter: { sl_refs: ['looker__b2b__sales_pipeline.arr'] },
     });
     deps.semanticLayerService.loadAllSources.mockImplementation((connectionId: string) =>
-      Promise.resolve([{ name: `${connectionId}_source` }]),
+      Promise.resolve({ sources: [{ name: `${connectionId}_source` }], loadErrors: [] }),
     );
     deps.agentRunner.runLoop.mockImplementation(async (params: any) => {
       if (params.telemetryTags.operationName === 'ingest-bundle-wu') {
@@ -1447,7 +1450,7 @@ describe('IngestBundleRunner — Stages 1 → 7', () => {
       parseArtifacts: { semanticModels: [{ name: 'orders' }] },
     });
     deps.semanticLayerService.loadAllSources.mockImplementation((connectionId: string) =>
-      Promise.resolve([{ name: `${connectionId}_source` }]),
+      Promise.resolve({ sources: [{ name: `${connectionId}_source` }], loadErrors: [] }),
     );
     const postProcessor = {
       run: vi.fn().mockResolvedValue({
@@ -1631,7 +1634,10 @@ describe('IngestBundleRunner — Stages 1 → 7', () => {
     const deps = makeDeps();
     deps.adapter.listTargetConnectionIds = vi.fn().mockResolvedValue(['postgres-warehouse']);
     deps.semanticLayerService.loadAllSources.mockImplementation((connectionId: string) =>
-      Promise.resolve(connectionId === 'postgres-warehouse' ? [{ name: 'stg_accounts' }] : []),
+      Promise.resolve({
+        sources: connectionId === 'postgres-warehouse' ? [{ name: 'stg_accounts' }] : [],
+        loadErrors: [],
+      }),
     );
 
     const runner = buildRunner(deps);
@@ -1659,7 +1665,10 @@ describe('IngestBundleRunner — Stages 1 → 7', () => {
 
   it('does not resolve qualified fallback table refs by source name alone', async () => {
     const deps = makeDeps();
-    deps.semanticLayerService.loadAllSources.mockResolvedValue([{ name: 'orders', table: 'sales.orders' }]);
+    deps.semanticLayerService.loadAllSources.mockResolvedValue({
+      sources: [{ name: 'orders', table: 'sales.orders' }],
+      loadErrors: [],
+    });
     const runner = buildRunner(deps);
 
     await expect(

packages/context/src/ingest/ingest-bundle.runner.ts

@@ -300,7 +300,7 @@ export class IngestBundleRunner {
     const blocks = await Promise.all(
       connectionIds.map(async (connectionId) => {
         try {
-          const sources = await this.deps.semanticLayerService.loadAllSources(connectionId);
+          const { sources } = await this.deps.semanticLayerService.loadAllSources(connectionId);
           const names = sources.map((source) => source.name).sort((left, right) => left.localeCompare(right));
           const body = names.length > 0 ? names.join('\n') : '(no sources yet)';
           return `## ${connectionId}\n${body}`;
@@ -329,7 +329,7 @@ export class IngestBundleRunner {
   ): Promise<boolean> {
     for (const connectionId of connectionIds) {
       try {
-        const sources = await semanticLayerService.loadAllSources(connectionId);
+        const { sources } = await semanticLayerService.loadAllSources(connectionId);
         if (sources.some((source) => semanticSourceMatchesTableRef(source, tableRef))) {
           return true;
         }
@@ -1211,7 +1211,7 @@ export class IngestBundleRunner {
         ].sort();
         for (const connectionId of touchedConnections) {
           try {
-            const allSources = await this.deps.semanticLayerService.loadAllSources(connectionId);
+            const { sources: allSources } = await this.deps.semanticLayerService.loadAllSources(connectionId);
             await this.deps.slSearchService.indexSources(connectionId, allSources);
           } catch (err) {
             this.logger.warn(

packages/context/src/ingest/wiki-sl-ref-repair.test.ts

@@ -44,23 +44,26 @@ describe('repairWikiSlRefs', () => {
       })),
     };
     const semanticLayerService = {
-      loadAllSources: vi.fn(async () => [
+      loadAllSources: vi.fn(async () => ({
-        {
+        sources: [
-          name: 'mart_customer_health',
+          {
-          grain: [],
+            name: 'mart_customer_health',
-          columns: [],
+            grain: [],
-          joins: [],
+            columns: [],
-          measures: [{ name: 'high_risk_account_count', expr: 'count(*)' }],
+            joins: [],
-          segments: [{ name: 'high_risk', expr: "risk_level = 'high'" }],
+            measures: [{ name: 'high_risk_account_count', expr: 'count(*)' }],
-        },
+            segments: [{ name: 'high_risk', expr: "risk_level = 'high'" }],
-        {
+          },
-          name: 'int_procurement_qualifying_actions',
+          {
-          grain: [],
+            name: 'int_procurement_qualifying_actions',
-          columns: [],
+            grain: [],
-          joins: [],
+            columns: [],
-          measures: [],
+            joins: [],
-        },
+            measures: [],
-      ]),
+          },
+        ],
+        loadErrors: [],
+      })),
     };
 
     const result = await repairWikiSlRefs({

packages/context/src/ingest/wiki-sl-ref-repair.ts

@@ -56,7 +56,8 @@ async function loadVisibleSlRefs(
   const warnings: string[] = [];
   for (const connectionId of connectionIds) {
     try {
-      for (const source of await semanticLayerService.loadAllSources(connectionId)) {
+      const { sources } = await semanticLayerService.loadAllSources(connectionId);
+      for (const source of sources) {
         for (const ref of entityRefsForSource(source)) {
           refs.add(ref);
         }

packages/context/src/memory/memory-agent.service.ingest.test.ts

@@ -89,7 +89,7 @@ const buildMocks = (overrides: Partial<BuiltMocks> = {}): BuiltMocks => {
     embeddingService: { computeEmbedding: vi.fn() },
     semanticLayerService: {
       forWorktree: vi.fn().mockReturnThis(),
-      loadAllSources: vi.fn().mockResolvedValue([]),
+      loadAllSources: vi.fn().mockResolvedValue({ sources: [], loadErrors: [] }),
       readSourceFile: vi.fn(),
     },
     slSearchService: { indexSources: vi.fn(), buildSearchText: vi.fn() },

packages/context/src/memory/memory-agent.service.ts

@@ -308,7 +308,7 @@ export class MemoryAgentService {
     // Reindex SL search if any SL actions actually landed on main.
     if (hasSL && finalActions.some((a) => a.target === 'sl')) {
       try {
-        const allSources = await this.deps.semanticLayerService.loadAllSources(input.connectionId!);
+        const { sources: allSources } = await this.deps.semanticLayerService.loadAllSources(input.connectionId!);
         await this.deps.slSearchService.indexSources(input.connectionId!, allSources);
       } catch (e) {
         this.logger.warn(
@@ -610,7 +610,7 @@ export class MemoryAgentService {
 
   private async buildSlIndex(connectionId: string): Promise<string> {
     const [sources, warehouseLine] = await Promise.all([
-      this.deps.semanticLayerService.loadAllSources(connectionId),
+      this.deps.semanticLayerService.loadAllSources(connectionId).then((result) => result.sources),
       this.buildWarehouseLine(connectionId),
     ]);
     const indexLines =

packages/context/src/scan/description-generation.ts

@@ -371,7 +371,7 @@ export class KtxDescriptionGenerator {
         connectorId: input.connector.id,
         table: input.table.name,
       });
-      return 'Table not found';
+      return null;
     }
 
     try {
@@ -397,7 +397,7 @@ export class KtxDescriptionGenerator {
       return description;
     } catch (error) {
       this.logger?.error(`Error generating table description: ${errorMessage(error)}`);
-      return 'Table not found';
+      return null;
     }
   }
 

packages/context/src/sl/local-sl.test.ts

@@ -392,6 +392,26 @@ describe('local semantic-layer helpers', () => {
     ).rejects.toThrow('Invalid semantic-layer source');
   });
 
+  it('reports legacy overlay column patches with a file-attributed migration hint', async () => {
+    const invalidYaml = [
+      'name: orders',
+      'columns:',
+      '  - name: status',
+      '    descriptions:',
+      '      user: Order status.',
+      '',
+    ].join('\n');
+
+    await expect(
+      validateLocalSlSource(invalidYaml, { project, connectionId: 'warehouse', sourceName: 'orders' }),
+    ).resolves.toEqual({
+      valid: false,
+      errors: [
+        "semantic-layer/warehouse/orders.yaml: column 'status' patches a manifest column but is in 'columns:' — move it to 'column_overrides:'",
+      ],
+    });
+  });
+
   it('rejects unsafe source paths', async () => {
     await expect(
       readLocalSlSource(project, {

packages/context/src/sl/local-sl.ts

@@ -12,6 +12,7 @@ import {
   type ManifestTableEntry,
   projectManifestEntry,
   SemanticLayerService,
+  toResolvedWire,
 } from './semantic-layer.service.js';
 import type { PgliteSlSearchPrototypeOwnerOptions } from './pglite-sl-search-prototype.js';
 import { loadLatestSlDictionaryEntries } from './sl-dictionary-profile.js';
@@ -240,7 +241,12 @@ export async function loadLocalSlSourceRecords(
     if (!base) {
       continue;
     }
-    const source = composeOverlay(base.source, parsed);
+    let source: SemanticLayerSource;
+    try {
+      source = composeOverlay(base.source, parsed);
+    } catch (error) {
+      throw new Error(`${path}: ${error instanceof Error ? error.message : String(error)}`);
+    }
     sources.set(name, {
       ...summarizeSemanticSource({ connectionId, path, source }),
       yaml: sourceToYaml(source),
@@ -253,11 +259,28 @@ export async function loadLocalSlSourceRecords(
 
 export async function validateLocalSlSource(
   rawYaml: string,
-  options?: { project?: KtxLocalProject; connectionId?: string },
+  options?: { project?: KtxLocalProject; connectionId?: string; sourceName?: string },
 ): Promise<LocalSlValidationResult> {
   try {
     const parsed = parseYamlRecord(rawYaml);
     const schema = parsed.table || parsed.sql ? sourceDefinitionSchema : sourceOverlaySchema;
+    if (schema === sourceOverlaySchema && Array.isArray(parsed.columns)) {
+      const sourceName = options?.sourceName ?? (typeof parsed.name === 'string' ? parsed.name : 'source');
+      const path =
+        options?.connectionId && isSafeConnectionId(options.connectionId)
+          ? `semantic-layer/${options.connectionId}/${sourceName}.yaml`
+          : `${sourceName}.yaml`;
+      const legacyColumnPatchErrors = parsed.columns
+        .filter((column): column is Record<string, unknown> => isRecord(column))
+        .filter((column) => typeof column.name === 'string' && (!column.expr || !column.type))
+        .map(
+          (column) =>
+            `${path}: column '${column.name}' patches a manifest column but is in 'columns:' — move it to 'column_overrides:'`,
+        );
+      if (legacyColumnPatchErrors.length > 0) {
+        return { valid: false, errors: legacyColumnPatchErrors };
+      }
+    }
     const result = schema.parse(parsed);
     const errors: string[] = [];
 
@@ -268,6 +291,10 @@ export async function validateLocalSlSource(
       );
     }
 
+    if ('table' in result || 'sql' in result) {
+      toResolvedWire(result as SemanticLayerSource);
+    }
+
     return { valid: errors.length === 0, errors };
   } catch (error) {
     return { valid: false, errors: validationErrors(error) };

packages/context/src/sl/ports.ts

@@ -1,4 +1,4 @@
-import type { SemanticLayerQueryInput, SemanticLayerSource } from './types.js';
+import type { ResolvedSemanticLayerSource, SemanticLayerQueryInput } from './types.js';
 
 export interface KtxConnectionInfo {
   id: string;
@@ -20,7 +20,7 @@ export interface SlConnectionCatalogPort {
 
 export interface SlPythonPort {
   validateSources(input: {
-    sources: SemanticLayerSource[];
+    sources: ResolvedSemanticLayerSource[];
     dialect: string;
     recently_touched?: string[];
   }): Promise<{
@@ -28,7 +28,7 @@ export interface SlPythonPort {
     error?: unknown;
   }>;
   query(input: {
-    sources: SemanticLayerSource[];
+    sources: ResolvedSemanticLayerSource[];
     query: SemanticLayerQueryInput;
     dialect: string;
   }): Promise<{ data?: { sql?: string; plan?: Record<string, unknown> } | null; error?: unknown }>;

packages/context/src/sl/schemas.contract.test.ts

@@ -0,0 +1,60 @@
+import { execFileSync } from 'node:child_process';
+import { Ajv2020 } from 'ajv/dist/2020.js';
+import { describe, expect, it } from 'vitest';
+
+import { resolvedSourceSchema } from './schemas.js';
+import { toResolvedWire } from './semantic-layer.service.js';
+import type { SemanticLayerSource } from './types.js';
+
+const sourceDefinitionJsonSchema = JSON.parse(
+  execFileSync('uv', ['run', 'python', '-m', 'semantic_layer', 'dump-schema'], {
+    cwd: new URL('../../../../', import.meta.url),
+    encoding: 'utf8',
+  }),
+) as Record<string, unknown>;
+
+const fixtures: SemanticLayerSource[] = [
+  {
+    name: 'orders',
+    table: 'public.orders',
+    grain: ['id'],
+    columns: [
+      { name: 'id', type: 'number' },
+      {
+        name: 'status',
+        type: 'string',
+        descriptions: { dbt: 'Order lifecycle status.' },
+        constraints: { dbt: { not_null: true } },
+        enum_values: { dbt: ['placed', 'shipped'] },
+        tests: { dbt: [{ name: 'accepted_values', package: 'dbt' }] },
+      },
+    ],
+    joins: [{ to: 'customers', on: 'orders.customer_id = customers.id', relationship: 'many_to_one' }],
+    measures: [{ name: 'order_count', expr: 'count(id)' }],
+    segments: [{ name: 'paid', expr: "status = 'paid'" }],
+    default_time_dimension: { dbt: 'created_at' },
+    tags: { dbt: ['mart'] },
+    freshness: { dbt: { loaded_at_field: 'updated_at' } },
+  },
+  {
+    name: 'aav_orders',
+    sql: 'select id, status from public.orders where status = paid',
+    grain: ['id'],
+    columns: [{ name: 'id', type: 'number' }],
+    joins: [],
+    measures: [],
+  },
+];
+
+describe('resolved source JSON Schema contract', () => {
+  it('keeps TS resolved-source fixtures accepted by the Python SourceDefinition schema', () => {
+    const ajv = new Ajv2020({ allErrors: true, strict: false });
+    const validate = ajv.compile(sourceDefinitionJsonSchema);
+
+    for (const fixture of fixtures) {
+      const wire = toResolvedWire(fixture);
+      expect(resolvedSourceSchema.safeParse(wire).success).toBe(true);
+      expect(validate(wire), JSON.stringify(validate.errors, null, 2)).toBe(true);
+    }
+  });
+});

packages/context/src/sl/schemas.ts

@@ -78,6 +78,8 @@ const joinDeclarationSchema = z.object({
   alias: z.string().optional(),
 });
 
+const resolvedJoinDeclarationSchema = joinDeclarationSchema.strict();
+
 const sourceColumnSchema = z.object({
   name: unqualifiedNameSchema,
   // type/descriptions optional on standalone sources: compose-time enrichment fills them
@@ -89,24 +91,39 @@ const sourceColumnSchema = z.object({
   visibility: z.enum(columnVisibilityValues).optional(),
   descriptions: descriptionsSchema.optional(),
   expr: z.string().optional(),
+  natural_granularity: z.string().optional(),
   constraints: sourceKeyedColumnConstraintsSchema.optional(),
   enum_values: sourceKeyedStringArraySchema.optional(),
   tests: dbtColumnTestsSchema.optional(),
 });
 
-/** Overlay column: type requires expr (structural types are inherited from manifest). */
+const resolvedSourceColumnSchema = sourceColumnSchema.extend({
+  type: z.enum(columnTypeValues),
+}).strict();
+
+/** Overlay column: computed columns only. Structural columns live in the manifest. */
 const overlayColumnSchema = z
   .object({
     name: unqualifiedNameSchema,
-    type: z.enum(columnTypeValues).optional(),
+    type: z.enum(columnTypeValues),
     role: z.enum(columnRoleValues).optional(),
     visibility: z.enum(columnVisibilityValues).optional(),
     descriptions: descriptionsSchema.optional(),
-    expr: z.string().optional(),
+    expr: z.string().min(1),
   })
-  .refine((col) => !col.type || col.expr, {
+  .strict();
-    message: "Overlay column with 'type' must also have 'expr' (only computed columns may specify a type)",
+
-  });
+const columnOverrideSchema = z
+  .object({
+    name: unqualifiedNameSchema,
+    role: z.enum(columnRoleValues).optional(),
+    visibility: z.enum(columnVisibilityValues).optional(),
+    descriptions: descriptionsSchema.optional(),
+    constraints: sourceKeyedColumnConstraintsSchema.optional(),
+    enum_values: sourceKeyedStringArraySchema.optional(),
+    tests: dbtColumnTestsSchema.optional(),
+  })
+  .strict();
 
 /** Standalone source: has `table` or `sql`, requires grain + columns. */
 export const sourceDefinitionSchema = z
@@ -143,6 +160,26 @@ export const sourceDefinitionSchema = z
     message: "Standalone source must have exactly one of 'table' or 'sql' (not both)",
   });
 
+export const resolvedSourceSchema = z
+  .object({
+    name: z.string().min(1),
+    descriptions: descriptionsSchema.optional(),
+    table: z.string().optional(),
+    sql: z.string().optional(),
+    grain: z.array(unqualifiedNameSchema).min(1),
+    columns: z.array(resolvedSourceColumnSchema).min(1),
+    joins: z.array(resolvedJoinDeclarationSchema).default([]),
+    measures: z.array(slMeasureDefinitionSchema).default([]),
+    segments: z.array(segmentDefinitionSchema).optional(),
+    default_time_dimension: defaultTimeDimensionDbtSchema.optional(),
+    tags: sourceKeyedStringArraySchema.optional(),
+    freshness: sourceFreshnessSchema.optional(),
+  })
+  .strict()
+  .refine((s) => (s.table || s.sql) && !(s.table && s.sql), {
+    message: "Resolved source must have exactly one of 'table' or 'sql' (not both)",
+  });
+
 /** Overlay source: no table/sql, all fields optional except name. */
 export const sourceOverlaySchema = z
   .object({
@@ -150,6 +187,7 @@ export const sourceOverlaySchema = z
     descriptions: z.record(z.string(), z.string()).optional(),
     grain: z.array(unqualifiedNameSchema).optional(),
     columns: z.array(overlayColumnSchema).optional(),
+    column_overrides: z.array(columnOverrideSchema).optional(),
     joins: z.array(joinDeclarationSchema).optional(),
     measures: z.array(slMeasureDefinitionSchema).optional(),
     segments: z.array(segmentDefinitionSchema).optional(),

packages/context/src/sl/semantic-layer.service.test.ts

@@ -2,13 +2,17 @@ import type { Mock } from 'vitest';
 import { beforeEach, describe, expect, it, vi } from 'vitest';
 
 import {
+  ColumnNameCollisionError,
   composeOverlay,
+  ConflictingExcludeAndOverrideError,
   enrichColumnsFromManifest,
   findDanglingSegmentRefs,
   projectManifestEntry,
   SemanticLayerService,
+  toResolvedWire,
+  UnknownColumnOverrideError,
 } from './semantic-layer.service.js';
-import { sourceDefinitionSchema } from './schemas.js';
+import { resolvedSourceSchema, sourceDefinitionSchema, sourceOverlaySchema } from './schemas.js';
 import type { SemanticLayerSource } from './types.js';
 
 const pythonPort = {
@@ -139,10 +143,10 @@ describe('composeOverlay', () => {
     expect(composed.measures).toHaveLength(1);
   });
 
-  it('merges overlay columns onto same-named manifest columns, preserving manifest type when overlay omits it', () => {
+  it('applies column_overrides to same-named manifest columns', () => {
     const overlay = {
       name: 'fct_labs',
-      columns: [
+      column_overrides: [
         { name: 'lab_order_id', descriptions: { user: 'Primary key' } },
         { name: 'admin_user_id', descriptions: { user: 'FK to admin_users' } },
       ],
@@ -158,11 +162,13 @@ describe('composeOverlay', () => {
     expect(adminUser?.descriptions).toEqual({ user: 'FK to admin_users' });
   });
 
-  it('still appends new overlay computed columns alongside merged same-name columns', () => {
+  it('appends computed columns alongside column overrides', () => {
     const overlay = {
       name: 'fct_labs',
-      columns: [
+      column_overrides: [
         { name: 'lab_order_id', descriptions: { user: 'PK doc' } },
+      ],
+      columns: [
         { name: 'is_byol', type: 'boolean', expr: "lab_type = 'byol'" },
       ],
     };
@@ -172,6 +178,34 @@ describe('composeOverlay', () => {
     expect(composed.columns.find((c) => c.name === 'lab_order_id')?.type).toBe('string');
   });
 
+  it('rejects column_overrides that target unknown manifest columns', () => {
+    expect(() =>
+      composeOverlay(baseTable, {
+        name: 'fct_labs',
+        column_overrides: [{ name: 'missing', descriptions: { user: 'Nope' } }],
+      }),
+    ).toThrow(UnknownColumnOverrideError);
+  });
+
+  it('rejects computed columns whose names collide with manifest columns', () => {
+    expect(() =>
+      composeOverlay(baseTable, {
+        name: 'fct_labs',
+        columns: [{ name: 'lab_order_id', type: 'string', expr: 'lab_order_id' }],
+      }),
+    ).toThrow(ColumnNameCollisionError);
+  });
+
+  it('rejects exclude/override conflicts before applying exclusions', () => {
+    expect(() =>
+      composeOverlay(baseTable, {
+        name: 'fct_labs',
+        exclude_columns: ['lab_order_id'],
+        column_overrides: [{ name: 'lab_order_id', descriptions: { user: 'Hidden PK' } }],
+      }),
+    ).toThrow(ConflictingExcludeAndOverrideError);
+  });
+
   it('merges overlay descriptions (plural) with base descriptions keyed by source', () => {
     const baseWithDescriptions: SemanticLayerSource = {
       ...baseTable,
@@ -451,6 +485,62 @@ describe('sourceDefinitionSchema', () => {
   });
 });
 
+describe('sourceOverlaySchema', () => {
+  it('accepts column_overrides and keeps columns computed-only', () => {
+    const result = sourceOverlaySchema.safeParse({
+      name: 'orders',
+      column_overrides: [{ name: 'status', descriptions: { user: 'Lifecycle status' } }],
+      columns: [{ name: 'is_paid', type: 'boolean', expr: "status = 'paid'" }],
+    });
+    expect(result.success).toBe(true);
+  });
+
+  it('rejects typeless overlay columns and singular description on overrides', () => {
+    const result = sourceOverlaySchema.safeParse({
+      name: 'orders',
+      column_overrides: [{ name: 'status', description: 'Lifecycle status' }],
+      columns: [{ name: 'status', descriptions: { user: 'Lifecycle status' } }],
+    });
+    expect(result.success).toBe(false);
+    if (!result.success) {
+      const paths = result.error.issues.map((issue) => issue.path.join('.'));
+      expect(paths).toContain('column_overrides.0');
+      expect(paths).toContain('columns.0.type');
+      expect(paths).toContain('columns.0.expr');
+    }
+  });
+});
+
+describe('toResolvedWire', () => {
+  it('strips TS-only authoring and provenance fields before the Python boundary', () => {
+    const wire = toResolvedWire({
+      name: 'orders',
+      table: 'public.orders',
+      inherits_columns_from: 'orders',
+      grain: ['id'],
+      columns: [{ name: 'id', type: 'string' }],
+      joins: [{ to: 'customers', on: 'orders.customer_id = customers.id', relationship: 'many_to_one', source: 'formal' }],
+      measures: [],
+      usage: {
+        narrative: 'Frequently queried orders.',
+        frequencyTier: 'high',
+        commonFilters: ['status'],
+        commonJoins: [],
+      },
+    });
+
+    expect(wire).toEqual({
+      name: 'orders',
+      table: 'public.orders',
+      grain: ['id'],
+      columns: [{ name: 'id', type: 'string' }],
+      joins: [{ to: 'customers', on: 'orders.customer_id = customers.id', relationship: 'many_to_one' }],
+      measures: [],
+    });
+    expect(resolvedSourceSchema.parse(wire)).toEqual(wire);
+  });
+});
+
 describe('projectManifestEntry', () => {
   it('projects manifest usage onto the semantic-layer source', () => {
     const source = projectManifestEntry('orders', {
@@ -570,7 +660,8 @@ describe('loadAllSources — standalone enrichment via inherits_columns_from', (
       ].join('\n'),
     });
 
-    const sources = await service.loadAllSources('conn-1');
+    const { sources, loadErrors } = await service.loadAllSources('conn-1');
+    expect(loadErrors).toEqual([]);
 
     expect(sources[0]).toMatchObject({
       name: 'orders',
@@ -634,7 +725,8 @@ describe('loadAllSources — standalone enrichment via inherits_columns_from', (
       return Promise.reject(new Error(`Unexpected readFile: ${path}`));
     });
 
-    const sources = await service.loadAllSources('conn-1');
+    const { sources, loadErrors } = await service.loadAllSources('conn-1');
+    expect(loadErrors).toEqual([]);
     const aav = sources.find((s) => s.name === 'aav_consignments');
     expect(aav).toBeDefined();
     expect(aav?.columns).toEqual([
@@ -679,7 +771,8 @@ describe('loadAllSources — standalone enrichment via inherits_columns_from', (
       });
     });
 
-    const sources = await service.loadAllSources('conn-1');
+    const { sources, loadErrors } = await service.loadAllSources('conn-1');
+    expect(loadErrors).toEqual([]);
     const aav = sources.find((s) => s.name === 'aav_consignments');
     expect(aav?.columns[0].type).toBe('string');
   });
@@ -703,7 +796,8 @@ describe('loadAllSources — standalone enrichment via inherits_columns_from', (
       ].join('\n'),
     });
 
-    const sources = await service.loadAllSources('conn-1');
+    const { sources, loadErrors } = await service.loadAllSources('conn-1');
+    expect(loadErrors).toEqual([]);
     const aav = sources.find((s) => s.name === 'aav_consignments');
     expect(aav?.columns).toEqual([{ name: 'FOO', type: 'string' }]);
   });
@@ -726,7 +820,8 @@ describe('loadAllSources — standalone enrichment via inherits_columns_from', (
       ].join('\n'),
     });
 
-    const sources = await service.loadAllSources('conn-1');
+    const { sources, loadErrors } = await service.loadAllSources('conn-1');
+    expect(loadErrors).toEqual([]);
 
     expect(sources[0]).toMatchObject({
       name: 'orders',
@@ -734,6 +829,33 @@ describe('loadAllSources — standalone enrichment via inherits_columns_from', (
       columns: [{ name: 'id', type: 'string', descriptions: { user: 'Stable order identifier.' } }],
     });
   });
+
+  it('reports file-attributed errors for legacy overlay column patches', async () => {
+    const schemaPath = 'semantic-layer/conn-1/_schema/marts.yaml';
+    const overlayPath = 'semantic-layer/conn-1/orders.yaml';
+    configService.listFiles.mockResolvedValue({ files: [schemaPath, overlayPath] });
+    configService.readFile.mockImplementation((path: string) => {
+      if (path === schemaPath) {
+        return Promise.resolve({
+          content: [
+            'tables:',
+            '  orders:',
+            '    table: public.orders',
+            '    columns:',
+            '      - { name: id, type: string, pk: true }',
+          ].join('\n'),
+        });
+      }
+      return Promise.resolve({
+        content: ['name: orders', 'columns:', '  - name: id', '    descriptions: { user: "Stable id." }'].join('\n'),
+      });
+    });
+
+    const { loadErrors } = await service.loadAllSources('conn-1');
+
+    expect(loadErrors.join('\n')).toContain(overlayPath);
+    expect(loadErrors.join('\n')).toContain("move it to 'column_overrides:'");
+  });
 });
 
 describe('validateWithProposedSource', () => {

packages/context/src/sl/semantic-layer.service.ts

@@ -4,8 +4,14 @@ import { noopLogger } from '../core/index.js';
 import type { TableUsageOutput } from '../ingest/adapters/historic-sql/skill-schemas.js';
 import type { SlConnectionCatalogPort, SlPythonPort } from './ports.js';
 import { normalizeSemanticLayerDescriptions } from './description-normalization.js';
-import { isOverlaySource, sourceDefinitionSchema, sourceOverlaySchema } from './schemas.js';
+import { isOverlaySource, resolvedSourceSchema, sourceDefinitionSchema, sourceOverlaySchema } from './schemas.js';
-import type { SemanticLayerQueryExecutionResult, SemanticLayerQueryInput, SemanticLayerSource } from './types.js';
+import type {
+  ResolvedSemanticLayerSource,
+  SemanticLayerColumnOverride,
+  SemanticLayerQueryExecutionResult,
+  SemanticLayerQueryInput,
+  SemanticLayerSource,
+} from './types.js';
 
 interface WriteSourceOptions {
   skipValidation?: boolean;
@@ -14,6 +20,30 @@ interface WriteSourceOptions {
 const SL_DIR_PREFIX = 'semantic-layer';
 const CONNECTION_ID_PATTERN = /^[a-zA-Z0-9][a-zA-Z0-9_-]*$/;
 
+export interface LoadAllSourcesResult {
+  sources: SemanticLayerSource[];
+  loadErrors: string[];
+}
+
+export class UnknownColumnOverrideError extends Error {}
+export class ColumnNameCollisionError extends Error {}
+export class ConflictingExcludeAndOverrideError extends Error {}
+class ComposeContractError extends Error {}
+
+function isComposeError(error: unknown): boolean {
+  return (
+    error instanceof UnknownColumnOverrideError ||
+    error instanceof ColumnNameCollisionError ||
+    error instanceof ConflictingExcludeAndOverrideError ||
+    error instanceof ComposeContractError
+  );
+}
+
+function formatComposeError(filePath: string, error: unknown): string {
+  const message = error instanceof Error ? error.message : String(error);
+  return `${filePath}: ${message}`;
+}
+
 function formatPortError(error: unknown, fallback: string): string {
   if (typeof error === 'string') {
     return error;
@@ -37,6 +67,24 @@ function formatPortError(error: unknown, fallback: string): string {
   return fallback;
 }
 
+export function toResolvedWire(source: SemanticLayerSource): ResolvedSemanticLayerSource {
+  const stripped = {
+    ...source,
+    columns: source.columns.map((column) => ({ ...column })),
+    joins: source.joins.map(({ source: _source, ...join }) => join),
+  } as Record<string, unknown>;
+  delete stripped.inherits_columns_from;
+  delete stripped.usage;
+  delete stripped.source_type;
+
+  const parsed = resolvedSourceSchema.safeParse(stripped);
+  if (!parsed.success) {
+    const issues = parsed.error.issues.map((issue) => `${issue.path.join('.')}: ${issue.message}`).join('; ');
+    throw new ComposeContractError(`resolved source '${source.name}' violates the TS/Python contract: ${issues}`);
+  }
+  return parsed.data as ResolvedSemanticLayerSource;
+}
+
 export class SemanticLayerService {
   constructor(
     private readonly configService: KtxFileStorePort,
@@ -158,16 +206,17 @@ export class SemanticLayerService {
     }
   }
 
-  async loadAllSources(connectionId: string): Promise<SemanticLayerSource[]> {
+  async loadAllSources(connectionId: string): Promise<LoadAllSourcesResult> {
     const dir = `${SL_DIR_PREFIX}/${connectionId}`;
     const schemaDir = `${dir}/_schema`;
+    const loadErrors: string[] = [];
 
     let allFiles: string[];
     try {
       const result = await this.configService.listFiles(dir);
       allFiles = result.files.filter((f) => f.endsWith('.yaml'));
     } catch {
-      return [];
+      return { sources: [], loadErrors };
     }
 
     // 1. Load manifest shards from _schema/*.yaml → project to sources
@@ -184,7 +233,9 @@ export class SemanticLayerService {
           }
         }
       } catch (e) {
-        this.logger.warn(`Failed to parse manifest shard ${filePath}: ${e}`);
+        const message = `Failed to parse manifest shard ${filePath}: ${e instanceof Error ? e.message : String(e)}`;
+        loadErrors.push(message);
+        this.logger.warn(message);
       }
     }
 
@@ -227,6 +278,7 @@ export class SemanticLayerService {
               );
             }
           }
+          toResolvedWire(standalone);
           sources.set(name, standalone);
         } else {
           // Overlay — compose with manifest entry if present
@@ -238,11 +290,15 @@ export class SemanticLayerService {
           }
         }
       } catch (e) {
-        this.logger.warn(`Failed to parse YAML file ${filePath}: ${e}`);
+        const message = isComposeError(e)
+          ? formatComposeError(filePath, e)
+          : `Failed to parse YAML file ${filePath}: ${e instanceof Error ? e.message : String(e)}`;
+        loadErrors.push(message);
+        this.logger.warn(message);
       }
     }
 
-    return Array.from(sources.values());
+    return { sources: Array.from(sources.values()), loadErrors };
   }
 
   /**
@@ -622,8 +678,10 @@ export class SemanticLayerService {
     connectionId: string,
     proposedSource: SemanticLayerSource,
   ): Promise<{ errors: string[]; warnings: string[]; perSourceWarnings: Record<string, string[]> }> {
-    const existing = await this.loadAllSources(connectionId);
+    const loaded = await this.loadAllSources(connectionId);
+    const existing = loaded.sources;
     const merged = existing.filter((s) => s.name !== proposedSource.name);
+    const loadErrors = [...loaded.loadErrors];
 
     // Overlays (no table/sql) must be composed with their manifest base before
     // validation, otherwise the filter below drops them and the edited source
@@ -641,11 +699,27 @@ export class SemanticLayerService {
           perSourceWarnings: {},
         };
       }
-      toPush = composeOverlay(base, { ...proposedSource });
+      try {
+        toPush = composeOverlay(base, { ...proposedSource });
+      } catch (error) {
+        return {
+          errors: [...loadErrors, formatComposeError(`${proposedSource.name}.yaml`, error)],
+          warnings: [],
+          perSourceWarnings: {},
+        };
+      }
     } else if (proposedSource.inherits_columns_from) {
       const base = await this.findManifestEntryByTableRef(connectionId, proposedSource.inherits_columns_from);
       if (base) {
-        toPush = enrichColumnsFromManifest(proposedSource, base);
+        try {
+          toPush = enrichColumnsFromManifest(proposedSource, base);
+        } catch (error) {
+          return {
+            errors: [...loadErrors, formatComposeError(`${proposedSource.name}.yaml`, error)],
+            warnings: [],
+            perSourceWarnings: {},
+          };
+        }
       }
       // Miss is non-fatal — the source ships unenriched, validator will surface
       // any column-without-type errors via the warehouse probe.
@@ -654,37 +728,37 @@ export class SemanticLayerService {
 
     const validatable = merged.filter((s) => s.table != null || s.sql != null);
     if (validatable.length === 0) {
-      return { errors: [], warnings: [], perSourceWarnings: {} };
+      return { errors: loadErrors, warnings: [], perSourceWarnings: {} };
     }
 
     const dialect = await this.getDialectForConnection(connectionId);
 
     try {
       const { data, error } = await this.python.validateSources({
-        sources: validatable,
+        sources: validatable.map(toResolvedWire),
         dialect,
         recently_touched: [proposedSource.name],
       });
       if (error) {
         const errorMsg = formatPortError(error, 'Unknown validation error');
-        return { errors: [errorMsg], warnings: [], perSourceWarnings: {} };
+        return { errors: [...loadErrors, errorMsg], warnings: [], perSourceWarnings: {} };
       }
       if (!data) {
         return {
-          errors: await this.validatePhysicalTableReferences(connectionId, validatable),
+          errors: [...loadErrors, ...(await this.validatePhysicalTableReferences(connectionId, validatable))],
           warnings: [],
           perSourceWarnings: {},
         };
       }
       const physicalErrors = await this.validatePhysicalTableReferences(connectionId, validatable);
       return {
-        errors: [...(data.errors ?? []), ...physicalErrors],
+        errors: [...loadErrors, ...(data.errors ?? []), ...physicalErrors],
         warnings: data.warnings ?? [],
         perSourceWarnings: data.per_source_warnings ?? {},
       };
     } catch (e) {
       return {
-        errors: [`Validation call failed: ${e instanceof Error ? e.message : String(e)}`],
+        errors: [...loadErrors, `Validation call failed: ${e instanceof Error ? e.message : String(e)}`],
         warnings: [],
         perSourceWarnings: {},
       };
@@ -692,23 +766,23 @@ export class SemanticLayerService {
   }
 
   async validateSourcesForConnection(connectionId: string): Promise<{ errors: string[]; warnings: string[] }> {
-    const allSources = await this.loadAllSources(connectionId);
+    const { sources: allSources, loadErrors } = await this.loadAllSources(connectionId);
     const sources = allSources.filter((source) => source.table != null || source.sql != null);
     if (sources.length === 0) {
-      return { errors: [], warnings: [] };
+      return { errors: loadErrors, warnings: [] };
     }
 
     const dialect = await this.getDialectForConnection(connectionId);
-    const { data, error } = await this.python.validateSources({ sources, dialect });
+    const { data, error } = await this.python.validateSources({ sources: sources.map(toResolvedWire), dialect });
     if (error) {
-      return { errors: [formatPortError(error, 'Unknown validation error')], warnings: [] };
+      return { errors: [...loadErrors, formatPortError(error, 'Unknown validation error')], warnings: [] };
     }
     if (!data) {
-      return { errors: await this.validatePhysicalTableReferences(connectionId, sources), warnings: [] };
+      return { errors: [...loadErrors, ...(await this.validatePhysicalTableReferences(connectionId, sources))], warnings: [] };
     }
     const physicalErrors = await this.validatePhysicalTableReferences(connectionId, sources);
     return {
-      errors: [...(data.errors ?? []), ...physicalErrors],
+      errors: [...loadErrors, ...(data.errors ?? []), ...physicalErrors],
       warnings: data.warnings ?? [],
     };
   }
@@ -802,6 +876,7 @@ export class SemanticLayerService {
         } else {
           // Overlay — check references against manifest
           const excludeColumns = (data.exclude_columns as string[]) ?? [];
+          const columnOverrides = (data.column_overrides as Array<{ name: string }> | undefined) ?? [];
           const disableJoins = (data.disable_joins as string[]) ?? [];
           const cols = manifestColumns.get(name);
           const joins = manifestJoins.get(name);
@@ -817,6 +892,16 @@ export class SemanticLayerService {
             }
           }
 
+          const excluded = new Set(excludeColumns);
+          for (const override of columnOverrides) {
+            if (!cols.has(override.name)) {
+              warnings.push(`${name}: column_overrides references non-existent column '${override.name}'`);
+            }
+            if (excluded.has(override.name)) {
+              warnings.push(`${name}: column '${override.name}' appears in both exclude_columns and column_overrides`);
+            }
+          }
+
           for (const joinOn of disableJoins) {
             const normalized = joinOn.replace(/\s+/g, ' ').trim();
             if (!joins?.has(normalized)) {
@@ -999,7 +1084,10 @@ export class SemanticLayerService {
    */
   async executeQuery(connectionId: string, query: SemanticLayerQueryInput): Promise<SemanticLayerQueryExecutionResult> {
     // 1. Load sources, filtering out sources with no table or sql
-    const allSources = await this.loadAllSources(connectionId);
+    const { sources: allSources, loadErrors } = await this.loadAllSources(connectionId);
+    if (loadErrors.length > 0) {
+      throw new Error(`Semantic layer source load failed: ${loadErrors.join('; ')}`);
+    }
     const sources = allSources.filter((s) => {
       if (!s.table && !s.sql) {
         this.logger.warn(`Skipping source "${s.name}" with no table or sql defined`);
@@ -1021,7 +1109,7 @@ export class SemanticLayerService {
 
     // 3. Generate SQL via python SL engine
     const { data: slResult, error: slError } = await this.python.query({
-      sources,
+      sources: sources.map(toResolvedWire),
       query,
       dialect,
     });
@@ -1092,18 +1180,20 @@ export function projectManifestEntry(name: string, entry: ManifestTableEntry): S
   const grain = pkColumns.length > 0 ? pkColumns : entry.columns.map((c) => c.name);
 
   // Table-level dbt config from manifest shards is surfaced on the source for search / tools.
-  return {
+  const source: SemanticLayerSource = {
     name,
     table: entry.table,
     descriptions: entry.descriptions,
     grain,
     columns,
-    joins: (entry.joins ?? []).map((j) => ({ to: j.to, on: j.on, relationship: j.relationship, source: j.source })),
+    joins: (entry.joins ?? []).map((j) => ({ to: j.to, on: j.on, relationship: j.relationship })),
     measures: [],
     ...(entry.tags?.dbt?.length ? { tags: entry.tags } : {}),
     ...(entry.freshness?.dbt ? { freshness: entry.freshness } : {}),
     ...(entry.usage ? { usage: entry.usage } : {}),
   };
+  toResolvedWire(source);
+  return source;
 }
 
 function normalizeWs(s: string): string {
@@ -1331,6 +1421,7 @@ const COMPOSE_KNOWN_KEYS = new Set([
   'descriptions',
   'grain',
   'columns',
+  'column_overrides',
   'joins',
   'measures',
   'segments',
@@ -1365,27 +1456,48 @@ export function composeOverlay(base: SemanticLayerSource, overlay: Record<string
     result.usage = normalizedOverlay.usage as SemanticLayerSource['usage'];
   }
 
-  // Filter out excluded columns
   const excluded = new Set((normalizedOverlay.exclude_columns as string[] | undefined) ?? []);
-  const baseColumns = result.columns.filter((c) => !excluded.has(c.name));
+  const columnOverrides = (normalizedOverlay.column_overrides as SemanticLayerColumnOverride[] | undefined) ?? [];
-
+  const overrideNames = columnOverrides.map((column) => column.name);
-  // Overlay columns matched by name merge onto the base column (overlay fields win, but
+  const conflictingOverrides = overrideNames.filter((name) => excluded.has(name));
-  // the base column's type/role/etc are preserved when the overlay omits them — dbt-style
+  if (conflictingOverrides.length > 0) {
-  // overlays often declare a column only to attach descriptions). New names append.
+    throw new ConflictingExcludeAndOverrideError(
-  const overlayColumns = (normalizedOverlay.columns as SemanticLayerSource['columns'] | undefined) ?? [];
+      `column_overrides conflict with exclude_columns for '${base.name}': ${conflictingOverrides.join(', ')}`,
-  const baseByName = new Map(baseColumns.map((c) => [c.name.toLowerCase(), c]));
+    );
-  const mergedAppended: SemanticLayerSource['columns'] = [];
-  const mergedByName = new Map<string, SemanticLayerSource['columns'][number]>();
-  for (const overlay of overlayColumns) {
-    const key = overlay.name.toLowerCase();
-    const base = baseByName.get(key);
-    if (base) {
-      mergedByName.set(key, mergeOverlayColumn(base, overlay));
-    } else {
-      mergedAppended.push(overlay);
-    }
   }
-  result.columns = [...baseColumns.map((c) => mergedByName.get(c.name.toLowerCase()) ?? c), ...mergedAppended];
+
+  const baseByLowerName = new Map(base.columns.map((column) => [column.name.toLowerCase(), column]));
+  const columnsByLowerName = new Map(
+    result.columns.filter((column) => !excluded.has(column.name)).map((column) => [column.name.toLowerCase(), column]),
+  );
+
+  for (const override of columnOverrides) {
+    const key = override.name.toLowerCase();
+    const baseColumn = baseByLowerName.get(key);
+    if (!baseColumn) {
+      throw new UnknownColumnOverrideError(
+        `column '${override.name}' in column_overrides does not exist on manifest source '${base.name}'`,
+      );
+    }
+    const baseDescriptions = baseColumn.descriptions ?? {};
+    const overrideDescriptions = override.descriptions ?? {};
+    const merged = { ...baseColumn, ...override };
+    if (Object.keys(baseDescriptions).length > 0 || Object.keys(overrideDescriptions).length > 0) {
+      merged.descriptions = { ...baseDescriptions, ...overrideDescriptions };
+    }
+    columnsByLowerName.set(key, merged);
+  }
+
+  const computedColumns = (normalizedOverlay.columns as SemanticLayerSource['columns'] | undefined) ?? [];
+  for (const column of computedColumns) {
+    if (baseByLowerName.has(column.name.toLowerCase())) {
+      throw new ColumnNameCollisionError(
+        `column '${column.name}' in columns patches a manifest column on '${base.name}' — move it to 'column_overrides:'`,
+      );
+    }
+    columnsByLowerName.set(column.name.toLowerCase(), column);
+  }
+  result.columns = [...columnsByLowerName.values()];
 
   // Measures from overlay only
   result.measures = (normalizedOverlay.measures as SemanticLayerSource['measures'] | undefined) ?? [];
@@ -1414,6 +1526,12 @@ export function composeOverlay(base: SemanticLayerSource, overlay: Record<string
   const newJoins = overlayJoins.filter((j) => !existingKeys.has(`${j.to}::${normalizeWs(j.on)}`));
   result.joins = [...manifestJoins, ...newJoins];
 
+  const overlayParse = sourceOverlaySchema.safeParse(normalizedOverlay);
+  if (!overlayParse.success) {
+    const issues = overlayParse.error.issues.map((issue) => `${issue.path.join('.')}: ${issue.message}`).join('; ');
+    throw new ComposeContractError(`overlay for '${base.name}' violates the authoring schema: ${issues}`);
+  }
+  toResolvedWire(result);
   return result;
 }
 
@@ -1445,32 +1563,6 @@ function parseJoinOn(
   return { fromColumn: leftCol, toColumn: rightCol };
 }
 
-/**
- * Merge an overlay column declaration onto a matching manifest column. Overlay fields
- * win, except descriptions (plural) which merge per source key. Manifest values are
- * preserved when the overlay omits them — this lets dbt/metabase emit description-only
- * overlay column entries without redeclaring `type:` (which would have to mirror the
- * scan column and rot when the schema changes).
- */
-function mergeOverlayColumn(
-  base: SemanticLayerSource['columns'][number],
-  overlay: SemanticLayerSource['columns'][number],
-): SemanticLayerSource['columns'][number] {
-  const merged: SemanticLayerSource['columns'][number] = { ...base, ...overlay };
-  if (!overlay.type && base.type) {
-    merged.type = base.type;
-  }
-  if (!overlay.role && base.role) {
-    merged.role = base.role;
-  }
-  const baseDescriptions = base.descriptions ?? {};
-  const overlayDescriptions = overlay.descriptions ?? {};
-  if (Object.keys(baseDescriptions).length > 0 || Object.keys(overlayDescriptions).length > 0) {
-    merged.descriptions = { ...baseDescriptions, ...overlayDescriptions };
-  }
-  return merged;
-}
-
 /**
  * Fill any blank `type`, `descriptions`, or `role` on the source's columns from the
  * matching manifest column (by name). Local values always win. Columns absent from
@@ -1503,5 +1595,7 @@ export function enrichColumnsFromManifest(
     }
     return merged;
   });
-  return { ...source, columns: enrichedColumns };
+  const enriched = { ...source, columns: enrichedColumns };
+  toResolvedWire(enriched);
+  return enriched;
 }

packages/context/src/sl/tools/sl-discover.tool.test.ts

@@ -7,7 +7,7 @@ import { SlDiscoverTool } from './sl-discover.tool.js';
 function makeTool() {
   const semanticLayerService = {
     listConnectionIdsWithNames: vi.fn(async () => [] as Array<{ id: string; name: string; connectionType: string }>),
-    loadAllSources: vi.fn(async () => [] as SemanticLayerSource[]),
+    loadAllSources: vi.fn(async () => ({ sources: [] as SemanticLayerSource[], loadErrors: [] })),
   };
   const slSearchService = {
     search: vi.fn(async () => []),
@@ -53,7 +53,8 @@ describe('SlDiscoverTool - session-scoped reads', () => {
       listConnectionIdsWithNames: vi.fn().mockResolvedValue([
         { id: 'warehouse', name: 'warehouse', connectionType: 'postgres' },
       ]),
-      loadAllSources: vi.fn().mockResolvedValue([
+      loadAllSources: vi.fn().mockResolvedValue({
+        sources: [
         {
           name: 'orders',
           table: 'public.orders',
@@ -62,7 +63,9 @@ describe('SlDiscoverTool - session-scoped reads', () => {
           measures: [],
           joins: [],
         },
-      ]),
+        ],
+        loadErrors: [],
+      }),
     };
 
     const result = await tool.call({}, makeContext({ session: makeSession(sessionSemanticLayerService) }));

packages/context/src/sl/tools/sl-discover.tool.ts

@@ -101,7 +101,7 @@ Use this to understand what data is available before querying through the semant
     // If inspecting a specific source — show the SL interface (columns, measures, joins)
     // without the raw SQL. Use `sl_read_source` to see the full YAML including SQL.
     if (sourceName) {
-      const sources = await semanticLayerService.loadAllSources(connectionId);
+      const { sources } = await semanticLayerService.loadAllSources(connectionId);
       const source = sources.find((s) => s.name === sourceName);
       if (!source) {
         return {
@@ -151,7 +151,7 @@ Use this to understand what data is available before querying through the semant
     // Load sources from all connections in parallel
     const results = await Promise.all(
       connections.map(async (conn) => {
-        const sources = await semanticLayerService.loadAllSources(conn.id);
+        const { sources } = await semanticLayerService.loadAllSources(conn.id);
         let filtered = sources;
         if (query) {
           filtered = await this.filterByQuery(conn.id, sources, query);
@@ -213,7 +213,7 @@ Use this to understand what data is available before querying through the semant
     connectionName: string,
     query?: string,
   ): Promise<ToolOutput<SlDiscoverStructured>> {
-    const sources = await semanticLayerService.loadAllSources(connectionId);
+    const { sources } = await semanticLayerService.loadAllSources(connectionId);
 
     if (sources.length === 0) {
       return {

packages/context/src/sl/tools/sl-edit-source.tool.test.ts

@@ -11,7 +11,7 @@ function makeTool(overrides: any = {}) {
     }),
     validateWithProposedSource: vi.fn().mockResolvedValue({ errors: [], warnings: [] }),
     writeSource: vi.fn().mockResolvedValue({ commitHash: 'c1' }),
-    loadAllSources: vi.fn().mockResolvedValue([]),
+    loadAllSources: vi.fn().mockResolvedValue({ sources: [], loadErrors: [] }),
     deleteSource: vi.fn().mockResolvedValue(undefined),
     isManifestBacked: vi.fn().mockResolvedValue(false),
     ...overrides.semanticLayerService,
@@ -44,7 +44,7 @@ function makeSession(overrides: Partial<ToolSession> = {}): ToolSession {
       }),
       validateWithProposedSource: vi.fn().mockResolvedValue({ errors: [], warnings: [] }),
       writeSource: vi.fn().mockResolvedValue({ commitHash: 'c1' }),
-      loadAllSources: vi.fn().mockResolvedValue([]),
+      loadAllSources: vi.fn().mockResolvedValue({ sources: [], loadErrors: [] }),
     } as any,
     wikiService: {} as any,
     configService: {} as any,
@@ -191,9 +191,10 @@ describe('SlEditSourceTool — manifest-backed source without overlay', () => {
     expect(joinedErrors).toContain('manifest');
     expect(joinedErrors).toContain('sl_write_source');
     expect(joinedErrors).toContain('overlay');
-    // Overlay shape: only name + measures/segments/description
+    // Overlay shape: name plus overlay-only fields.
     expect(joinedErrors).toContain('measures');
     expect(joinedErrors).toContain('segments');
+    expect(joinedErrors).toContain('column_overrides');
   });
 
   it('still returns the plain "Source not found" error for truly-missing names', async () => {

packages/context/src/sl/tools/sl-edit-source.tool.ts

@@ -127,7 +127,8 @@ If no source exists yet, use sl_write_source instead — this tool will reject t
               `    - name: <measure_name>`,
               `      expr: "<expression>"`,
               `      description: "<what it measures>"`,
-              `Overlay shape: "name:" plus any of "measures:", "segments:", "descriptions:". Do NOT include "sql:", "table:", "grain:", "columns:", or "joins:" — those are inherited from the manifest.`,
+              `Overlay shape: "name:" plus any of "measures:", "segments:", "descriptions:", "joins:", "disable_joins:", "exclude_columns:", "column_overrides:", or computed-only "columns:" entries with expr + type.`,
+              `Do NOT include "sql:", "table:", "grain:", or base-table "columns:" — those are inherited from the manifest.`,
             ].join('\n'),
           ],
           sourceName,
@@ -181,7 +182,7 @@ If no source exists yet, use sl_write_source instead — this tool will reject t
       const result = await semanticLayerService.writeSource(connectionId, source, author, authorEmail, commitMessage);
 
       if (!skipIndex) {
-        const allSources = await semanticLayerService.loadAllSources(connectionId);
+        const { sources: allSources } = await semanticLayerService.loadAllSources(connectionId);
         await this.slSearchService.indexSources(connectionId, allSources).catch(() => {});
       }
 

packages/context/src/sl/tools/sl-validate.tool.test.ts

@@ -34,7 +34,7 @@ describe('SlValidateTool — session-aware touched-set filtering', () => {
       { name: 'customers', table: 'x.customers', grain: ['id'], columns: [], joins: [], measures: [] },
     ];
     const serviceMock = {
-      loadAllSources: vi.fn().mockResolvedValue(sources),
+      loadAllSources: vi.fn().mockResolvedValue({ sources, loadErrors: [] }),
       validateSourcesForConnection: vi.fn().mockResolvedValue({
         errors: ['orders: missing join target', 'customers: invalid grain'],
         warnings: ['orders: disconnected-components warning'],

packages/context/src/sl/tools/sl-validate.tool.ts

@@ -62,7 +62,7 @@ Checks: all join targets exist, grain is valid, no missing references.
 
     const semanticLayerService = context.session?.semanticLayerService ?? this.semanticLayerService;
 
-    const sources = await semanticLayerService.loadAllSources(connectionId);
+    const { sources } = await semanticLayerService.loadAllSources(connectionId);
     if (sources.length === 0) {
       return this.buildOutput(true, [], '(all)', {
         validationErrors: ['No sources found for this connection.'],

packages/context/src/sl/tools/sl-warehouse-validation.test.ts

@@ -8,7 +8,7 @@ function makeDeps(opts: { sourceYaml: string; executeQuery: ReturnType<typeof vi
       isManifestBacked: vi.fn().mockResolvedValue(false),
       listManifestSourceNames: vi.fn().mockResolvedValue([]),
       loadSource: vi.fn().mockResolvedValue(null),
-      loadAllSources: vi.fn().mockResolvedValue([]),
+      loadAllSources: vi.fn().mockResolvedValue({ sources: [], loadErrors: [] }),
       validatePhysicalTableReferences: vi.fn().mockResolvedValue([]),
     } as never,
     connections: {

packages/context/src/sl/tools/sl-warehouse-validation.ts

@@ -88,8 +88,9 @@ export async function validateSingleSource(
       errors.push(
         `${sourceName}.yaml: standalone source shadows an existing manifest entry — ` +
           `writing it as-is drops the manifest's columns and joins. ` +
-          `Remove "sql:", "table:", "grain:", "columns:", and "joins:" and keep only ` +
+          `Remove "sql:", "table:", "grain:", and base-table "columns:" and keep only ` +
-          `"name:" plus "measures:"/"segments:"/"descriptions:" to write an overlay ` +
+          `"name:" plus overlay fields such as "measures:", "segments:", "descriptions:", ` +
+          `"joins:", "column_overrides:", or computed-only "columns:" to write an overlay ` +
           `that inherits the manifest schema. Call sl_read_source to inspect the existing source first.`,
       );
       return { errors, warnings };
@@ -108,7 +109,7 @@ export async function validateSingleSource(
     }
     if (errorPaths.has('columns')) {
       warnings.push(
-        `${sourceName}.yaml: hint — overlay columns must be computed: {name, expr, type}. Do NOT include base table columns.`,
+        `${sourceName}.yaml: hint — overlay columns must be computed: {name, expr, type}. Use column_overrides for manifest column descriptions or metadata.`,
       );
     }
     if (errorPaths.has('measures')) {
@@ -240,7 +241,8 @@ async function probeOverlayMeasures(
       }
     | undefined;
   try {
-    const all = await deps.semanticLayerService.loadAllSources(connectionId);
+    const { sources: all, loadErrors } = await deps.semanticLayerService.loadAllSources(connectionId);
+    errors.push(...loadErrors);
     composed = all.find((s) => s.name === sourceName);
   } catch (e) {
     errors.push(

packages/context/src/sl/tools/sl-write-source.tool.test.ts

@@ -8,7 +8,7 @@ function makeTool(overrides: Partial<Record<string, any>> = {}) {
     listManifestSourceNames: vi.fn().mockResolvedValue(['ACCOUNTS', 'ORDERS']),
     isManifestBacked: vi.fn().mockResolvedValue(false),
     loadSource: vi.fn().mockResolvedValue(null),
-    loadAllSources: vi.fn().mockResolvedValue([]),
+    loadAllSources: vi.fn().mockResolvedValue({ sources: [], loadErrors: [] }),
     validateWithProposedSource: vi.fn().mockResolvedValue({ errors: [], warnings: [] }),
     writeSource: vi.fn().mockResolvedValue({ commitHash: 'c1' }),
     deleteSource: vi.fn().mockResolvedValue(undefined),
@@ -59,7 +59,7 @@ describe('SlWriteSourceTool — session gating', () => {
       actions: [],
       semanticLayerService: {
         loadSource: vi.fn().mockResolvedValue(null),
-        loadAllSources: vi.fn().mockResolvedValue([]),
+        loadAllSources: vi.fn().mockResolvedValue({ sources: [], loadErrors: [] }),
         validateWithProposedSource: vi.fn().mockResolvedValue({ errors: [], warnings: [] }),
         writeSource: vi.fn().mockResolvedValue({ commitHash: 'c1' }),
         deleteSource: vi.fn().mockResolvedValue(undefined),
@@ -213,7 +213,7 @@ describe('SlWriteSourceTool — session gating', () => {
       ingest: { runId: 'run-1', jobId: 'job-1', syncId: 'sync-1', sourceKey: 'metabase' },
       semanticLayerService: {
         loadSource: vi.fn().mockResolvedValue(null),
-        loadAllSources: vi.fn().mockResolvedValue([]),
+        loadAllSources: vi.fn().mockResolvedValue({ sources: [], loadErrors: [] }),
         validateWithProposedSource: vi.fn().mockResolvedValue({ errors: [], warnings: [] }),
         writeSource: vi.fn().mockResolvedValue({ commitHash: 'c1' }),
         deleteSource: vi.fn().mockResolvedValue(undefined),

packages/context/src/sl/tools/sl-write-source.tool.ts

@@ -23,7 +23,9 @@ const slWriteSourceInputSchema = z.object({
     .describe('Name of the source to create, edit, or delete'),
   source: sourceInputSchema
     .optional()
-    .describe('Source definition (standalone with table/sql) or overlay (measures, computed columns, etc.)'),
+    .describe(
+      'Source definition (standalone with table/sql) or overlay (measures, column_overrides, computed columns, etc.)',
+    ),
   delete: z.boolean().optional().describe('Set to true to delete this source entirely'),
   rawPaths: z
     .array(z.string().min(1))
@@ -73,7 +75,8 @@ If the source already exists, this tool will overwrite it with the new definitio
 - table: For physical table/view sources (e.g., "public.orders"). Mutually exclusive with sql.
 - sql: For SQL-based sources (the SQL query). Mutually exclusive with table.
 - grain: What one row represents (e.g., ["id"], ["customer_id", "product_id"])
-- columns: All columns with type (string/number/time/boolean) and optional descriptions
+- columns: All columns with type (string/number/time/boolean) and optional descriptions. On overlays, columns are computed-only and require expr + type.
+- column_overrides: Overlay-only metadata patches for existing manifest columns (descriptions, role, visibility, constraints, enum_values, tests). Do not include type or expr.
 - joins: Relationships to other sources (to, on, relationship: many_to_one/one_to_many/one_to_one)
 - measures: Pre-defined aggregations (name, expr like "sum(amount)", optional filter, optional segments — bare names of segments defined on the same source, optional description)
 - segments: Named, reusable boolean predicates scoped to this source (name, expr — a SQL boolean over this source's columns, optional description). A measure references one with \`segments: [name]\`; a query references one with the dotted form \`source.segment_name\`. Use when the same predicate appears on 3+ measures — e.g. extract \`is_paid = true and is_refunded = '0'\` as \`segments: [{name: paid_non_refunded, expr: "..."}]\` and have each measure use \`segments: [paid_non_refunded]\` instead of re-typing the predicate inside \`sum(case when ... then x end)\`. Segments are predicates only — they cannot be selected as dimensions or grouped by; if you need to group by the predicate, add a \`columns[]\` entry instead.
@@ -113,7 +116,7 @@ Do NOT join back to a table that the SQL already aggregates from if the grain co
       try {
         await semanticLayerService.deleteSource(connectionId, sourceName, author, authorEmail);
         if (!skipIndex) {
-          const allSources = await semanticLayerService.loadAllSources(connectionId);
+          const { sources: allSources } = await semanticLayerService.loadAllSources(connectionId);
           await this.slSearchService.indexSources(connectionId, allSources).catch(() => {});
         }
         if (context.session) {
@@ -210,7 +213,7 @@ Do NOT join back to a table that the SQL already aggregates from if the grain co
       );
 
       if (!skipIndex) {
-        const allSources = await semanticLayerService.loadAllSources(connectionId);
+        const { sources: allSources } = await semanticLayerService.loadAllSources(connectionId);
         await this.slSearchService.indexSources(connectionId, allSources).catch(() => {});
       }
 
@@ -317,8 +320,9 @@ Do NOT join back to a table that the SQL already aggregates from if the grain co
       `Error: cannot write "${sourceName}" as a standalone source — a manifest entry with that name already exists.`,
       `  Writing standalone would drop the manifest's columns and joins, leaving only what you list here.`,
       `To add measures/segments on top of the manifest, rewrite this YAML as an overlay:`,
-      `  - Remove "sql:", "table:", "grain:", "columns:", and "joins:".`,
+      `  - Remove "sql:", "table:", "grain:", and base-table "columns:".`,
-      `  - Keep only "name:", plus "measures:", "segments:", and/or "descriptions:".`,
+      `  - Keep "name:" plus "measures:", "segments:", "descriptions:", "joins:", "disable_joins:",`,
+      `    "exclude_columns:", "column_overrides:", and/or computed-only "columns:" entries with expr + type.`,
       `  - The manifest's schema is inherited automatically.`,
       `If you really need a different base table, use a different source name.`,
     ].join('\n');

packages/context/src/sl/types.ts

@@ -47,6 +47,32 @@ export interface SemanticLayerSource {
   usage?: TableUsageOutput;
 }
 
+type SemanticLayerColumn = SemanticLayerSource['columns'][number];
+type SemanticLayerJoin = SemanticLayerSource['joins'][number];
+
+export interface SemanticLayerColumnOverride {
+  name: string;
+  role?: string;
+  visibility?: string;
+  descriptions?: Record<string, string>;
+  constraints?: { dbt?: { not_null?: boolean; unique?: boolean } };
+  enum_values?: { dbt?: string[] };
+  tests?: {
+    dbt?: Array<{ name: string; package: string; kwargs?: Record<string, unknown> }>;
+    dbt_by_package?: Record<string, string[]>;
+  };
+}
+
+export type ResolvedSemanticLayerSource = Omit<
+  SemanticLayerSource,
+  'inherits_columns_from' | 'usage' | 'joins'
+> & {
+  table?: string;
+  sql?: string;
+  columns: Array<SemanticLayerColumn & { type: string }>;
+  joins: Array<Omit<SemanticLayerJoin, 'source'>>;
+};
+
 export interface SemanticLayerQueryInput {
   measures: Array<string | { expr: string; name: string }>;
   dimensions: Array<string | { field: string; granularity?: string }>;

pnpm-lock.yaml

@@ -376,6 +376,9 @@ importers:
       '@vitest/coverage-v8':
         specifier: ^4.1.6
         version: 4.1.6(vitest@4.1.6)
+      ajv:
+        specifier: 8.20.0
+        version: 8.20.0
       typescript:
         specifier: ^6.0.3
         version: 6.0.3

python/ktx-sl/semantic_layer/__main__.py

@@ -1,3 +1,22 @@
-from semantic_layer.cli import main
+from __future__ import annotations
 
-main()
+import json
+import sys
+
+from semantic_layer.cli import main as cli_main
+from semantic_layer.models import SourceDefinition
+
+
+def dump_schema() -> None:
+    json.dump(
+        SourceDefinition.model_json_schema(), sys.stdout, indent=2, sort_keys=True
+    )
+    sys.stdout.write("\n")
+
+
+if __name__ == "__main__":
+    if len(sys.argv) > 1 and sys.argv[1] in {"dump-schema", "schema"}:
+        sys.argv.pop(1)
+        dump_schema()
+    else:
+        cli_main()

python/ktx-sl/semantic_layer/loader.py

@@ -87,18 +87,23 @@ class SourceLoader:
                 sources[name] = SourceDefinition(**data)
             else:
                 # Overlay — validate and compose with matching manifest entry
-                errors = validate_overlay(data)
-                if errors:
-                    raise ValueError(
-                        f"Invalid overlay '{name}' in {path}: {'; '.join(errors)}"
-                    )
                 base = sources.get(name)
                 if base:
+                    errors = validate_overlay(data, {c.name for c in base.columns})
+                    if errors:
+                        raise ValueError(
+                            f"Invalid overlay '{name}' in {path}: {'; '.join(errors)}"
+                        )
                     (
                         sources[name],
                         description_sources[name],
                     ) = self._compose(base, data, description_sources.get(name))
                 else:
+                    errors = validate_overlay(data)
+                    if errors:
+                        raise ValueError(
+                            f"Invalid overlay '{name}' in {path}: {'; '.join(errors)}"
+                        )
                     logger.warning(
                         "Orphan overlay '%s' in %s: no matching manifest entry, skipping",
                         name,
@@ -149,12 +154,55 @@ class SourceLoader:
                 description_sources or None,
             )
 
-        # Filter columns
+        excluded = set(overlay.get("exclude_columns", []))
+        overrides = overlay.get("column_overrides", [])
+        override_names = {override.get("name") for override in overrides}
+        conflicts = sorted(name for name in override_names if name in excluded)
+        if conflicts:
+            raise ValueError(
+                "column_overrides conflict with exclude_columns: "
+                + ", ".join(conflicts)
+            )
+
+        base_by_name = {column.name: column for column in base.columns}
+
+        for override in overrides:
+            name = override.get("name")
+            base_column = base_by_name.get(name)
+            if base_column is None:
+                raise ValueError(
+                    f"column '{name}' in column_overrides does not exist on manifest source '{base.name}'"
+                )
+
         excluded = set(overlay.get("exclude_columns", []))
         source.columns = [c for c in source.columns if c.name not in excluded]
 
-        # Append computed columns (overlay columns with expr)
+        columns_by_name = {column.name: column for column in source.columns}
+
+        for override in overrides:
+            name = override["name"]
+            base_column = base_by_name[name]
+            merged = base_column.model_dump(mode="python", exclude_none=True)
+            base_descriptions = merged.get("descriptions") or {}
+            override_data = dict(override)
+            override_descriptions = override_data.get("descriptions") or {}
+            merged.update(override_data)
+            if base_descriptions or override_descriptions:
+                merged["descriptions"] = {
+                    **base_descriptions,
+                    **override_descriptions,
+                }
+            columns_by_name[name] = SourceColumn(**merged)
+        source.columns = list(columns_by_name.values())
+
+        # Append computed columns. Manifest column names cannot be reused here;
+        # use column_overrides for metadata patches.
         for col in overlay.get("columns", []):
+            name = col.get("name")
+            if name in base_by_name:
+                raise ValueError(
+                    f"column '{name}' in columns patches a manifest column on '{base.name}' — move it to 'column_overrides:'"
+                )
             source.columns.append(SourceColumn(**col))
 
         # Set measures
@@ -181,6 +229,11 @@ class SourceLoader:
         ]
         source.joins = manifest_joins + new_joins
 
+        if not source.table and not source.sql:
+            raise ValueError("resolved source must have 'table' or 'sql'")
+        if source.table and source.sql:
+            raise ValueError("'table' and 'sql' are mutually exclusive")
+
         return source, (description_sources or None)
 
     def _validate_cross_references(self, sources: dict[str, SourceDefinition]) -> None:

python/ktx-sl/semantic_layer/manifest.py

@@ -143,7 +143,9 @@ class Manifest(BaseModel):
 # ── Projection ──────────────────────────────────────────────────────
 
 
-def validate_overlay(data: dict) -> list[str]:
+def validate_overlay(
+    data: dict, manifest_column_names: set[str] | None = None
+) -> list[str]:
     """Validate that overlay data doesn't contain structural fields.
 
     Returns a list of error messages (empty if valid).
@@ -162,11 +164,26 @@ def validate_overlay(data: dict) -> list[str]:
             errors.append(
                 f"Overlay column '{col.get('name', '?')}' must use 'descriptions'"
             )
-        if "type" in col and "expr" not in col:
+        if "expr" not in col:
             errors.append(
-                f"Overlay column '{col.get('name', '?')}' specifies 'type' without 'expr' "
+                f"Overlay column '{col.get('name', '?')}' in 'columns' must define "
-                f"(structural types are inherited from manifest — only computed columns may specify a type)"
+                f"'expr' and 'type' (use 'column_overrides' to patch manifest columns)"
             )
+        if "type" not in col:
+            errors.append(
+                f"Overlay column '{col.get('name', '?')}' in 'columns' must define "
+                f"'type' and 'expr' (use 'column_overrides' to patch manifest columns)"
+            )
+    for col in data.get("column_overrides", []):
+        name = col.get("name", "?")
+        if "description" in col:
+            errors.append(f"Column override '{name}' must use 'descriptions'")
+        if "type" in col:
+            errors.append(f"Column override '{name}' must not contain 'type'")
+        if "expr" in col:
+            errors.append(f"Column override '{name}' must not contain 'expr'")
+        if manifest_column_names is not None and name not in manifest_column_names:
+            errors.append(f"Column override '{name}' does not match a manifest column")
     return errors
 
 

python/ktx-sl/semantic_layer/models.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 from enum import Enum
 from typing import Any, Literal
 
-from pydantic import BaseModel, Field, model_validator
+from pydantic import BaseModel, ConfigDict, Field, model_validator
 
 
 # ── Source Definition Models ──────────────────────────────────────────
@@ -105,6 +105,8 @@ class DefaultTimeDimensionDbt(BaseModel):
 
 
 class SourceDefinition(BaseModel):
+    model_config = ConfigDict(extra="forbid")
+
     name: str
     description: str | None = None
     descriptions: dict[str, str] | None = None
@@ -123,6 +125,8 @@ class SourceDefinition(BaseModel):
     def validate_source(self) -> SourceDefinition:
         if self.description is None:
             self.description = _resolve_description_map(self.descriptions)
+        if not self.table and not self.sql:
+            raise ValueError("resolved source must have 'table' or 'sql'")
         if self.table and self.sql:
             raise ValueError("'table' and 'sql' are mutually exclusive")
         if not self.grain:

python/ktx-sl/tests/test_loader.py

@@ -148,11 +148,21 @@ class TestLoaderEdgeCases:
             with open(Path(tmpdir) / "test.yaml", "w") as f:
                 yaml.dump(data, f)
             loader = SourceLoader(tmpdir)
-            try:
+            with pytest.raises(Exception, match="unknown_field"):
-                sources = loader.load_all()
+                loader.load_all()
-                assert "test" in sources
+
-            except Exception:
+    def test_source_requires_table_or_sql(self):
-                pass
+        with tempfile.TemporaryDirectory() as tmpdir:
+            data = {
+                "name": "test",
+                "grain": ["id"],
+                "columns": [{"name": "id", "type": "number"}],
+            }
+            with open(Path(tmpdir) / "test.yaml", "w") as f:
+                yaml.dump(data, f)
+            loader = SourceLoader(tmpdir)
+            with pytest.raises(Exception, match="table.*sql"):
+                loader.load_file(Path(tmpdir) / "test.yaml")
 
     def test_subdirectory_sources(self):
         with tempfile.TemporaryDirectory() as tmpdir:

python/ktx-sl/tests/test_manifest.py

@@ -205,12 +205,15 @@ class TestValidateOverlay:
             "descriptions": {"user": "Revenue-bearing orders"},
             "grain": ["id"],
             "measures": [{"name": "revenue", "expr": "sum(total)"}],
+            "column_overrides": [
+                {"name": "status", "descriptions": {"user": "Order lifecycle status"}}
+            ],
             "columns": [
                 {"name": "is_high_value", "expr": "total > 1000", "type": "boolean"}
             ],
             "exclude_columns": ["status"],
         }
-        errors = validate_overlay(data)
+        errors = validate_overlay(data, {"status", "total"})
         assert errors == []
 
     def test_validate_overlay_rejects_table(self):
@@ -225,14 +228,13 @@ class TestValidateOverlay:
         assert len(errors) == 1
         assert "sql" in errors[0].lower()
 
-    def test_validate_overlay_rejects_type_without_expr(self):
+    def test_validate_overlay_rejects_column_without_expr(self):
         data = {
             "name": "orders",
             "columns": [{"name": "status", "type": "string"}],
         }
         errors = validate_overlay(data)
         assert len(errors) == 1
-        assert "type" in errors[0].lower()
         assert "expr" in errors[0].lower()
 
     def test_validate_overlay_allows_type_with_expr(self):
@@ -243,6 +245,33 @@ class TestValidateOverlay:
         errors = validate_overlay(data)
         assert errors == []
 
+    def test_validate_overlay_rejects_column_override_structural_fields(self):
+        data = {
+            "name": "orders",
+            "column_overrides": [
+                {
+                    "name": "status",
+                    "description": "Status",
+                    "type": "string",
+                    "expr": "status",
+                }
+            ],
+        }
+        errors = validate_overlay(data, {"status"})
+        assert len(errors) == 3
+        assert "descriptions" in errors[0]
+        assert "type" in errors[1]
+        assert "expr" in errors[2]
+
+    def test_validate_overlay_rejects_unknown_column_override(self):
+        data = {
+            "name": "orders",
+            "column_overrides": [{"name": "missing", "descriptions": {"user": "Nope"}}],
+        }
+        errors = validate_overlay(data, {"status"})
+        assert len(errors) == 1
+        assert "does not match" in errors[0]
+
 
 # ── Two-Tier Loading Tests ─────────────────────────────────────────
 
@@ -502,6 +531,77 @@ class TestTwoTierLoading:
         assert hv.expr == "total > 1000"
         assert hv.type == "boolean"
 
+    def test_overlay_column_overrides_patch_manifest_columns(self, tmp_path: Path):
+        schema_dir = tmp_path / "_schema"
+        _write_yaml(schema_dir / "public.yaml", _manifest_tables())
+
+        overlay = {
+            "name": "orders",
+            "column_overrides": [
+                {"name": "status", "descriptions": {"user": "Order lifecycle status"}}
+            ],
+        }
+        _write_yaml(tmp_path / "orders.yaml", overlay)
+        _write_yaml(tmp_path / "customers.yaml", {"name": "customers"})
+
+        loader = SourceLoader(tmp_path)
+        sources = loader.load_all()
+
+        status = next(c for c in sources["orders"].columns if c.name == "status")
+        assert status.type == "string"
+        assert status.description == "Order lifecycle status"
+        assert status.descriptions == {"user": "Order lifecycle status"}
+
+    def test_overlay_rejects_unknown_column_override(self, tmp_path: Path):
+        schema_dir = tmp_path / "_schema"
+        _write_yaml(schema_dir / "public.yaml", _manifest_tables())
+
+        overlay = {
+            "name": "orders",
+            "column_overrides": [
+                {"name": "missing", "descriptions": {"user": "No such column"}}
+            ],
+        }
+        _write_yaml(tmp_path / "orders.yaml", overlay)
+        _write_yaml(tmp_path / "customers.yaml", {"name": "customers"})
+
+        loader = SourceLoader(tmp_path)
+        with pytest.raises(ValueError, match="Column override 'missing'"):
+            loader.load_all()
+
+    def test_overlay_rejects_computed_column_name_collision(self, tmp_path: Path):
+        schema_dir = tmp_path / "_schema"
+        _write_yaml(schema_dir / "public.yaml", _manifest_tables())
+
+        overlay = {
+            "name": "orders",
+            "columns": [{"name": "status", "type": "string", "expr": "status"}],
+        }
+        _write_yaml(tmp_path / "orders.yaml", overlay)
+        _write_yaml(tmp_path / "customers.yaml", {"name": "customers"})
+
+        loader = SourceLoader(tmp_path)
+        with pytest.raises(ValueError, match="move it to 'column_overrides:'"):
+            loader.load_all()
+
+    def test_overlay_rejects_exclude_override_conflict(self, tmp_path: Path):
+        schema_dir = tmp_path / "_schema"
+        _write_yaml(schema_dir / "public.yaml", _manifest_tables())
+
+        overlay = {
+            "name": "orders",
+            "exclude_columns": ["status"],
+            "column_overrides": [
+                {"name": "status", "descriptions": {"user": "Hidden status"}}
+            ],
+        }
+        _write_yaml(tmp_path / "orders.yaml", overlay)
+        _write_yaml(tmp_path / "customers.yaml", {"name": "customers"})
+
+        loader = SourceLoader(tmp_path)
+        with pytest.raises(ValueError, match="conflict with exclude_columns"):
+            loader.load_all()
+
     def test_overlay_measures_set(self, tmp_path: Path):
         schema_dir = tmp_path / "_schema"
         _write_yaml(schema_dir / "public.yaml", _manifest_tables())