diff --git a/src/app/stores/DatasetStore.test.ts b/src/app/stores/DatasetStore.test.ts
index a9297a8..4c19377 100644
--- a/src/app/stores/DatasetStore.test.ts
+++ b/src/app/stores/DatasetStore.test.ts
@@ -112,6 +112,21 @@ describe('URL dataset edits & refresh', () => {
     expect(ds.name).toBe('Remote'); // preserved
   });
 
+  test('a URL→inline conversion removes the url/fetchedAt keys (not left undefined)', () => {
+    const id = seedUrl();
+    store().startEdit();
+    store().updateForm({ source: 'inline', input: '[{"x":1},{"x":2}]' });
+    const later = new Date('2026-07-01T00:00:00Z');
+    expect(store().save(later)).toBe(true);
+    const ds = store().datasets.find((d) => d.id === id)!;
+    expect(ds.source).toBe('inline');
+    expect(ds.rowCount).toBe(2);
+    // The remote origin is shed entirely — the keys are gone, not present-but-undefined,
+    // so an inline record carries no stale url/fetchedAt.
+    expect('url' in ds).toBe(false);
+    expect('fetchedAt' in ds).toBe(false);
+  });
+
   test('refreshDataset is a no-op for an inline dataset', () => {
     store().add(
       createDataset({ name: 'Inline', data: [{ a: 1 }], format: 'json', source: 'inline', now: T }),
diff --git a/src/app/stores/DatasetStore.ts b/src/app/stores/DatasetStore.ts
index 02de3c5..7ef75bf 100644
--- a/src/app/stores/DatasetStore.ts
+++ b/src/app/stores/DatasetStore.ts
@@ -291,11 +291,9 @@ export const useDatasetStore = create<DatasetState>((set, get) => ({
           data: resolved.data,
           format: resolved.format,
           source: resolved.source,
-          // TODO: `update` shallow-merges, so these set the keys to `undefined`
-          // rather than removing them — a URL→inline record keeps `url`/`fetchedAt`
-          // present-but-undefined. Invisible today (rendering/profiling key off
-          // `source`/`data == null`, and JSON export drops undefined), but it breaks
-          // the "inline records carry no url/fetchedAt keys" invariant on this path.
+          // A URL→inline conversion must shed the remote origin; `update` deletes
+          // keys set to `undefined`, so these are removed rather than left
+          // present-but-undefined.
           url: undefined,
           fetchedAt: undefined,
           comment: form.comment,
@@ -415,7 +413,18 @@ export const useDatasetStore = create<DatasetState>((set, get) => ({
   update: (id, patch, now) => {
     const modified = patch.modified ?? (now ?? new Date()).toISOString();
     set((s) => ({
-      datasets: s.datasets.map((d) => (d.id === id ? { ...d, ...patch, modified } : d)),
+      datasets: s.datasets.map((d) => {
+        if (d.id !== id) return d;
+        const merged = { ...d, ...patch, modified };
+        // A patch key set to `undefined` removes it, rather than leaving a
+        // present-but-undefined key — so a URL→inline conversion drops `url`/
+        // `fetchedAt` cleanly, keeping the "inline records carry no remote-origin
+        // keys" invariant (docs/architecture/07 §6).
+        for (const key of Object.keys(patch) as (keyof Dataset)[]) {
+          if (patch[key] === undefined) delete merged[key];
+        }
+        return merged;
+      }),
     }));
   },