diff --git a/src/app/components/DatasetsModal.tsx b/src/app/components/DatasetsModal.tsx
index 0c85820..d1c1158 100644
--- a/src/app/components/DatasetsModal.tsx
+++ b/src/app/components/DatasetsModal.tsx
@@ -19,7 +19,12 @@ import { detectFormat, detectFormatFromUrl, type DataFormat } from '@core/format
 import { closeModal, openModal, resnapshot } from '../modals/ModalCoordinator';
 import { confirm } from '../stores/ConfirmStore';
 import { notify } from '../stores/NotificationStore';
-import { selectSelectedDataset, useDatasetStore, byModifiedDesc } from '../stores/DatasetStore';
+import {
+  selectCanSave,
+  selectSelectedDataset,
+  useDatasetStore,
+  byModifiedDesc,
+} from '../stores/DatasetStore';
 import { useSnippetStore } from '../stores/SnippetStore';
 import { SegmentedControl, type SegmentedOption } from './SegmentedControl';
 import { Icon } from './Icon';
@@ -338,6 +343,8 @@ function DatasetFormView({ editing }: { editing: boolean }) {
   const updateForm = useDatasetStore((s) => s.updateForm);
   const cancelForm = useDatasetStore((s) => s.cancelForm);
   const save = useDatasetStore((s) => s.save);
+  // Save stays disabled until a name and valid data/URL are present (spec §05).
+  const canSave = useDatasetStore(selectCanSave);
 
   // Live format/source hint from the current input (spec §05 → Auto-detection).
   const detected =
@@ -433,7 +440,12 @@ function DatasetFormView({ editing }: { editing: boolean }) {
         <button type="button" className={styles.action} onClick={handleCancel}>
           Cancel
         </button>
-        <button type="button" className={`${styles.action} ${styles.primary}`} onClick={handleSave}>
+        <button
+          type="button"
+          className={`${styles.action} ${styles.primary}`}
+          disabled={!canSave}
+          onClick={handleSave}
+        >
           {editing ? 'Save changes' : 'Create dataset'}
         </button>
       </div>
diff --git a/src/app/stores/DatasetStore.test.ts b/src/app/stores/DatasetStore.test.ts
index 3642da9..773c9e9 100644
--- a/src/app/stores/DatasetStore.test.ts
+++ b/src/app/stores/DatasetStore.test.ts
@@ -1,7 +1,12 @@
 import { beforeEach, describe, expect, test } from 'vitest';
 import { createDataset } from '@core/dataset';
 import { createSnippet } from '@core/snippet';
-import { selectSelectedDataset, useDatasetStore, type DatasetForm } from './DatasetStore';
+import {
+  selectCanSave,
+  selectSelectedDataset,
+  useDatasetStore,
+  type DatasetForm,
+} from './DatasetStore';
 import { useSnippetStore } from './SnippetStore';
 
 const store = () => useDatasetStore.getState();
@@ -82,6 +87,45 @@ describe('save — validation', () => {
   });
 });
 
+describe('selectCanSave — drives the Save button disabled state (§05)', () => {
+  const canSaveWith = (patch: Partial<DatasetForm>) => {
+    store().startCreate();
+    store().updateForm(patch);
+    return selectCanSave(useDatasetStore.getState());
+  };
+
+  test('false until a name and valid data are present', () => {
+    store().startCreate();
+    expect(selectCanSave(useDatasetStore.getState())).toBe(false); // empty form
+    expect(canSaveWith({ name: 'Sales' })).toBe(false); // no data
+    expect(canSaveWith({ name: '  ', input: '[{"a":1}]' })).toBe(false); // blank name
+    expect(canSaveWith({ name: 'Sales', input: 'not data' })).toBe(false); // unrecognized
+  });
+
+  test('true for valid inline data and valid URLs', () => {
+    expect(canSaveWith({ name: 'Sales', input: '[{"a":1,"b":2}]' })).toBe(true);
+    expect(canSaveWith({ name: 'Regions', input: 'city,pop\nA,10' })).toBe(true);
+    expect(canSaveWith({ name: 'Remote', source: 'url', input: 'https://x/y.csv' })).toBe(true);
+  });
+
+  test('false for a non-http URL', () => {
+    expect(canSaveWith({ name: 'Bad', source: 'url', input: 'ftp://x/y.csv' })).toBe(false);
+  });
+
+  test('false on a duplicate name, true once the editing dataset keeps its own name', () => {
+    store().add(
+      createDataset({ name: 'Sales', data: [{ a: 1 }], format: 'json', source: 'inline', now: T }),
+    );
+    const id = store().datasets[0].id;
+    // Creating another "Sales" is blocked.
+    expect(canSaveWith({ name: 'Sales', input: '[{"a":1}]' })).toBe(false);
+    // Editing the existing one may keep its own name (excludeId).
+    store().select(id);
+    store().startEdit();
+    expect(selectCanSave(useDatasetStore.getState())).toBe(true);
+  });
+});
+
 describe('save — edit', () => {
   test('updating inline data re-profiles and advances modified', () => {
     store().add(
diff --git a/src/app/stores/DatasetStore.ts b/src/app/stores/DatasetStore.ts
index 855e800..13b2974 100644
--- a/src/app/stores/DatasetStore.ts
+++ b/src/app/stores/DatasetStore.ts
@@ -91,40 +91,59 @@ export function byModifiedDesc(a: Dataset, b: Dataset): number {
   return b.modified.localeCompare(a.modified);
 }
 
+/**
+ * Cheap form validation: returns an error message, or `null` when the form is
+ * saveable. Deliberately does **not** `JSON.parse` the input — it only checks
+ * name/input presence, name uniqueness, and that a format is detectable — so it
+ * is safe to run on every render to drive the Save button's disabled state
+ * (spec §05). Error copy follows the council resolution (docs/architecture/10
+ * §error copy → GOV.UK error-message): action-oriented, specific, says how to fix.
+ */
+function validateForm(
+  form: DatasetForm,
+  datasets: Dataset[],
+  excludeId: number | undefined,
+): string | null {
+  const name = form.name.trim();
+  if (name === '') return 'Enter a dataset name.';
+  if (isNameTaken(name, datasets, excludeId)) {
+    return `A dataset named "${name}" already exists. Choose a different name.`;
+  }
+
+  const input = form.input.trim();
+  if (input === '') {
+    return form.source === 'url' ? 'Enter a URL.' : 'Paste JSON, CSV, or TSV data to save.';
+  }
+
+  if (form.source === 'url') {
+    if (!/^https?:\/\//i.test(input)) return 'Enter a URL starting with http:// or https://.';
+    return null;
+  }
+
+  // Inline: the data must auto-detect to a known format (spec §05 → Auto-detection).
+  if (!detectFormat(form.input).format) return 'Enter valid JSON, CSV, or TSV data.';
+  return null;
+}
+
 /** Validate a form into a saveable shape, or return an error message. */
 function resolveForm(
   form: DatasetForm,
   datasets: Dataset[],
   excludeId: number | undefined,
 ): { name: string; data: unknown; format: DataFormat; source: DataSource } | { error: string } {
-  // Error copy follows the council resolution (docs/architecture/10 §error copy →
-  // GOV.UK error-message): action-oriented, specific, and says how to fix it.
-  const name = form.name.trim();
-  if (name === '') return { error: 'Enter a dataset name.' };
-  if (isNameTaken(name, datasets, excludeId)) {
-    return { error: `A dataset named "${name}" already exists. Choose a different name.` };
-  }
+  const error = validateForm(form, datasets, excludeId);
+  if (error) return { error };
 
+  const name = form.name.trim();
   const input = form.input.trim();
-  if (input === '') {
-    return {
-      error: form.source === 'url' ? 'Enter a URL.' : 'Paste JSON, CSV, or TSV data to save.',
-    };
-  }
 
   if (form.source === 'url') {
-    if (!/^https?:\/\//i.test(input)) {
-      return { error: 'Enter a URL starting with http:// or https://.' };
-    }
     // Format is inferred from the extension; default to JSON when unknown (§05).
     return { name, data: input, format: detectFormatFromUrl(input) ?? 'json', source: 'url' };
   }
 
-  // Inline: the data must auto-detect to a known format (spec §05 → Auto-detection).
-  const { format } = detectFormat(form.input);
-  if (!format) {
-    return { error: 'Enter valid JSON, CSV, or TSV data.' };
-  }
+  // validateForm guaranteed a detectable format above.
+  const format = detectFormat(form.input).format as DataFormat;
   // JSON/TopoJSON are stored parsed; CSV/TSV keep their raw text (data model §09B).
   const data =
     format === 'json' || format === 'topojson' ? (JSON.parse(form.input) as unknown) : form.input;
@@ -276,3 +295,13 @@ export const useDatasetStore = create<DatasetState>((set, get) => ({
 /** Selector: the selected dataset record, or null. Derive — never store. */
 export const selectSelectedDataset = (s: DatasetState): Dataset | null =>
   s.datasets.find((d) => d.id === s.selectedId) ?? null;
+
+/**
+ * Selector: whether the current form is saveable, powering the Save button's
+ * disabled state (spec §05 → Save is disabled until a name and valid data/URL
+ * are present). Mirrors the `save()` action's `excludeId` derivation so an edit
+ * can keep its own name.
+ */
+export const selectCanSave = (s: DatasetState): boolean =>
+  validateForm(s.form, s.datasets, s.view === 'edit' ? (s.selectedId ?? undefined) : undefined) ===
+  null;