diff --git a/src/app/components/ChartBuilderModal.test.tsx b/src/app/components/ChartBuilderModal.test.tsx
index b8b4b95..8201e60 100644
--- a/src/app/components/ChartBuilderModal.test.tsx
+++ b/src/app/components/ChartBuilderModal.test.tsx
@@ -53,7 +53,9 @@ describe('ChartBuilderModal', () => {
       now: T,
     });
     useDatasetStore.getState().add(ds);
-    useChartBuilderStore.getState().init(ds.id);
+    // `add` reassigns a collision-free id (it is the id authority), so read it back.
+    const id = useDatasetStore.getState().datasets[0].id;
+    useChartBuilderStore.getState().init(id);
     useChartBuilderStore.getState().setMark('bar');
     expect(useChartBuilderStore.getState().config.mark).toBe('bar');
 
diff --git a/src/app/stores/ChartBuilderStore.test.ts b/src/app/stores/ChartBuilderStore.test.ts
index a1f390c..3a8edaf 100644
--- a/src/app/stores/ChartBuilderStore.test.ts
+++ b/src/app/stores/ChartBuilderStore.test.ts
@@ -11,7 +11,9 @@ const T = new Date('2026-06-01T00:00:00Z');
 function seedDataset(name: string, data: unknown): number {
   const ds = createDataset({ name, data, format: 'json', source: 'inline', now: T });
   useDatasetStore.getState().add(ds);
-  return ds.id;
+  // `add` is the id authority and reassigns a collision-free id, so read it back
+  // rather than trusting the pre-add createDataset default.
+  return useDatasetStore.getState().datasets[0].id;
 }
 
 beforeEach(() => {
diff --git a/src/app/stores/DatasetStore.test.ts b/src/app/stores/DatasetStore.test.ts
index 773c9e9..2469162 100644
--- a/src/app/stores/DatasetStore.test.ts
+++ b/src/app/stores/DatasetStore.test.ts
@@ -186,3 +186,63 @@ describe('remove & view transitions', () => {
     expect(store().view).toBe('detail');
   });
 });
+
+describe('id assignment — the store is the collision-free id authority', () => {
+  test('add reassigns a fresh id even when two creates share a Date.now() id', () => {
+    // Simulate the tight-loop hazard: two datasets minted with the SAME injected id
+    // (what Date.now() yields within one millisecond). add must still give them
+    // distinct, monotonically increasing ids so they cannot collide in IndexedDB.
+    store().add(
+      createDataset({
+        name: 'A',
+        data: [{ a: 1 }],
+        format: 'json',
+        source: 'inline',
+        now: T,
+        id: 42,
+      }),
+    );
+    store().add(
+      createDataset({
+        name: 'B',
+        data: [{ a: 1 }],
+        format: 'json',
+        source: 'inline',
+        now: T,
+        id: 42,
+      }),
+    );
+    const ids = store().datasets.map((d) => d.id);
+    expect(new Set(ids).size).toBe(2);
+    // selectedId tracks the reassigned id of the most recent add, not the input 42.
+    expect(store().selectedId).toBe(store().datasets[0].id);
+  });
+
+  test('addDatasets gives a batch distinct ids past the existing maximum', () => {
+    store().add(
+      createDataset({ name: 'Seed', data: [{ a: 1 }], format: 'json', source: 'inline', now: T }),
+    );
+    const seedId = store().datasets[0].id;
+    store().addDatasets([
+      createDataset({
+        name: 'X',
+        data: [{ a: 1 }],
+        format: 'json',
+        source: 'inline',
+        now: T,
+        id: 42,
+      }),
+      createDataset({
+        name: 'Y',
+        data: [{ a: 1 }],
+        format: 'json',
+        source: 'inline',
+        now: T,
+        id: 42,
+      }),
+    ]);
+    const ids = store().datasets.map((d) => d.id);
+    expect(new Set(ids).size).toBe(3);
+    expect(Math.min(...ids.filter((i) => i !== seedId))).toBeGreaterThan(seedId);
+  });
+});
diff --git a/src/app/stores/DatasetStore.ts b/src/app/stores/DatasetStore.ts
index 13b2974..4064608 100644
--- a/src/app/stores/DatasetStore.ts
+++ b/src/app/stores/DatasetStore.ts
@@ -67,14 +67,19 @@ export interface DatasetState {
    */
   save: (now?: Date) => boolean;
 
-  /** Low-level: add a fully-formed dataset and select it. */
+  /**
+   * Low-level: add a fully-formed dataset and select it. The id is (re)assigned
+   * via `nextDatasetId`, so a `createDataset` default id (`Date.now()`) can never
+   * reach storage — see `nextDatasetId` for why that matters.
+   */
   add: (dataset: Dataset) => void;
   /**
    * Append imported datasets (spec §08 → datasets imported before snippets). Each
-   * is given a fresh monotonic numeric id so a batch never collides on the
-   * `Date.now()` default (the `add` TODO) nor with existing ids — safe because
-   * datasets are referenced by **name**, not id (docs/architecture/07 §1). Names
-   * are assumed already de-duped by the import service. Selection is unchanged.
+   * is given a fresh monotonic numeric id (`nextDatasetId`, the same authority as
+   * `add`) so a batch never collides on the `Date.now()` default nor with existing
+   * ids — safe because datasets are referenced by **name**, not id
+   * (docs/architecture/07 §1). Names are assumed already de-duped by the import
+   * service. Selection is unchanged.
    */
   addDatasets: (incoming: Dataset[]) => void;
   /** Low-level: merge a patch into a dataset, advancing `modified`. */
@@ -91,6 +96,20 @@ export function byModifiedDesc(a: Dataset, b: Dataset): number {
   return b.modified.localeCompare(a.modified);
 }
 
+/**
+ * Next free numeric id for a dataset: one past the current maximum. This store is
+ * the **single id authority** — both single (`add`) and batch (`addDatasets`)
+ * insertion route through it — so a `createDataset` default id (`Date.now()`)
+ * never reaches IndexedDB, where a tight creation loop (the documented import path)
+ * could repeat the same millisecond and collide on the numeric key. Dataset ids
+ * are an internal IndexedDB key only — snippets reference datasets by **name**
+ * (docs/architecture/07 §1) — so reusing an id freed by a deletion is harmless,
+ * which is why one-past-the-max suffices without a persistent counter.
+ */
+function nextDatasetId(datasets: ReadonlyArray<Dataset>): number {
+  return datasets.reduce((max, d) => Math.max(max, d.id), 0) + 1;
+}
+
 /**
  * Cheap form validation: returns an error message, or `null` when the form is
  * saveable. Deliberately does **not** `JSON.parse` the input — it only checks
@@ -256,18 +275,16 @@ export const useDatasetStore = create<DatasetState>((set, get) => ({
     return true;
   },
 
-  // TODO: createDataset ids default to `Date.now()` and `add` does not reassign
-  // on collision (despite that file's comment). Interactive create is safe, but
-  // the documented non-interactive import path (naming.ts) creates many datasets
-  // in a tight loop where `Date.now()` repeats — duplicate numeric keys would
-  // collide in IndexedDB. Give the store a monotonic id source (or reassign here)
-  // before wiring import.
-  add: (dataset) => set((s) => ({ datasets: [dataset, ...s.datasets], selectedId: dataset.id })),
+  add: (dataset) =>
+    set((s) => {
+      const withId = { ...dataset, id: nextDatasetId(s.datasets) };
+      return { datasets: [withId, ...s.datasets], selectedId: withId.id };
+    }),
 
   addDatasets: (incoming) => {
     if (incoming.length === 0) return;
     set((s) => {
-      let nextId = s.datasets.reduce((max, d) => Math.max(max, d.id), 0) + 1;
+      let nextId = nextDatasetId(s.datasets);
       const withIds = incoming.map((d) => ({ ...d, id: nextId++ }));
       return { datasets: [...withIds, ...s.datasets] };
     });