diff --git a/apps/client/src/routes/__tests__/builder.$databaseId.$tableId.test.tsx b/apps/client/src/routes/__tests__/builder.$databaseId.$tableId.test.tsx
new file mode 100644
index 0000000..e7fb1b0
--- /dev/null
+++ b/apps/client/src/routes/__tests__/builder.$databaseId.$tableId.test.tsx
@@ -0,0 +1,156 @@
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import { screen, waitFor } from '@testing-library/react';
+import userEvent from '@testing-library/user-event';
+import { QueryClient, QueryClientProvider } from '@tanstack/react-query';
+import StructureBuilder from '../builder.$databaseId.$tableId';
+import { apiClient } from '../../lib/apiClient';
+import { render } from '../../test/render';
+
+// Captures the options the page passes to useBlocker, so a test can ask the page the
+// same question the router would: "would leaving right now discard work?".
+let blockerOpts: { shouldBlockFn: () => boolean } | null = null;
+
+vi.mock('@tanstack/react-router', () => ({
+  createFileRoute: () => () => ({}),
+  useParams: () => ({ databaseId: 'db-1', tableId: 'tbl-1' }),
+  useBlocker: (opts: any) => {
+    blockerOpts = opts;
+    return { status: 'idle' };
+  },
+}));
+
+const queryClient = new QueryClient({
+  defaultOptions: {
+    queries: { retry: false },
+    mutations: { retry: false },
+  },
+});
+
+const mockDatabase = { id: 'db-1', name: 'Patrimoine', workspace_id: 'ws-1' };
+const mockTable = { id: 'tbl-1', name: 'Auteurs', database_id: 'db-1' };
+
+function renderBuilder() {
+  return render(
+    <QueryClientProvider client={queryClient}>
+      <StructureBuilder />
+    </QueryClientProvider>
+  );
+}
+
+describe('StructureBuilder save', () => {
+  beforeEach(() => {
+    queryClient.clear();
+    vi.restoreAllMocks();
+  });
+
+  it('does not re-POST a field created in the same session on a second save', async () => {
+    const user = userEvent.setup();
+    const serverFields: Record<string, unknown>[] = [];
+
+    vi.spyOn(apiClient, 'get').mockImplementation((url: string) => {
+      if (url.startsWith('/databases/db-1')) return Promise.resolve(mockDatabase);
+      if (url.startsWith('/tables/tbl-1')) return Promise.resolve(mockTable);
+      if (url.startsWith('/tables?database_id=db-1')) return Promise.resolve([mockTable]);
+      if (url.startsWith('/fields?table_id=tbl-1')) return Promise.resolve(serverFields);
+      return Promise.reject(new Error(`Unexpected URL: ${url}`));
+    });
+
+    const post = vi.spyOn(apiClient, 'post').mockImplementation((_url, data) => {
+      const created = { ...(data as Record<string, unknown>), id: 'field-created-1' };
+      serverFields.push(created);
+      return Promise.resolve(created);
+    });
+
+    const put = vi.spyOn(apiClient, 'put').mockImplementation((_url, data) => {
+      return Promise.resolve({ ...(data as Record<string, unknown>), id: 'field-created-1' });
+    });
+
+    renderBuilder();
+
+    await waitFor(() => {
+      expect(screen.getByTestId('add-field-text')).toBeInTheDocument();
+    });
+
+    await user.click(screen.getByTestId('add-field-text'));
+
+    const saveButton = screen.getByRole('button', { name: /save/i });
+
+    await user.click(saveButton);
+    await waitFor(() => {
+      expect(post).toHaveBeenCalledTimes(1);
+    });
+    expect(post).toHaveBeenCalledWith('/fields', expect.objectContaining({ type: 'text' }));
+
+    await user.click(saveButton);
+    await waitFor(() => {
+      expect(put).toHaveBeenCalledTimes(1);
+    });
+
+    // The field was created once and updated on the second save — never POSTed twice.
+    expect(post).toHaveBeenCalledTimes(1);
+    expect(put).toHaveBeenCalledWith(
+      '/fields/field-created-1',
+      expect.objectContaining({ type: 'text' })
+    );
+
+    // The saved field is no longer an unsaved change.
+    expect(blockerOpts?.shouldBlockFn()).toBe(false);
+  });
+
+  it('keeps a field whose save failed as new and unsaved', async () => {
+    const user = userEvent.setup();
+
+    vi.spyOn(apiClient, 'get').mockImplementation((url: string) => {
+      if (url.startsWith('/databases/db-1')) return Promise.resolve(mockDatabase);
+      if (url.startsWith('/tables/tbl-1')) return Promise.resolve(mockTable);
+      if (url.startsWith('/tables?database_id=db-1')) return Promise.resolve([mockTable]);
+      if (url.startsWith('/fields?table_id=tbl-1')) return Promise.resolve([]);
+      return Promise.reject(new Error(`Unexpected URL: ${url}`));
+    });
+
+    // The text field is accepted; the number field is rejected by the server.
+    const post = vi.spyOn(apiClient, 'post').mockImplementation((_url, data) => {
+      const payload = data as Record<string, unknown>;
+      if (payload.type === 'number') return Promise.reject(new Error('boom'));
+      return Promise.resolve({ ...payload, id: 'field-created-1' });
+    });
+
+    const put = vi.spyOn(apiClient, 'put').mockImplementation((_url, data) => {
+      return Promise.resolve({ ...(data as Record<string, unknown>), id: 'field-created-1' });
+    });
+
+    renderBuilder();
+
+    await waitFor(() => {
+      expect(screen.getByTestId('add-field-text')).toBeInTheDocument();
+    });
+
+    await user.click(screen.getByTestId('add-field-text'));
+    await user.click(screen.getByTestId('add-field-number'));
+
+    const saveButton = screen.getByRole('button', { name: /save/i });
+
+    await user.click(saveButton);
+    await waitFor(() => {
+      expect(post).toHaveBeenCalledTimes(2);
+    });
+
+    // The failed field never reached the server, so it stays an unsaved change.
+    await waitFor(() => {
+      expect(blockerOpts?.shouldBlockFn()).toBe(true);
+    });
+
+    await user.click(saveButton);
+    await waitFor(() => {
+      expect(put).toHaveBeenCalledTimes(1);
+    });
+
+    // The one that saved is updated; the one that failed is retried as a create.
+    expect(put).toHaveBeenCalledWith(
+      '/fields/field-created-1',
+      expect.objectContaining({ type: 'text' })
+    );
+    expect(post).toHaveBeenCalledTimes(3);
+    expect(post).toHaveBeenLastCalledWith('/fields', expect.objectContaining({ type: 'number' }));
+  });
+});
diff --git a/apps/client/src/routes/builder.$databaseId.$tableId.tsx b/apps/client/src/routes/builder.$databaseId.$tableId.tsx
index 9a197e0..c14e857 100644
--- a/apps/client/src/routes/builder.$databaseId.$tableId.tsx
+++ b/apps/client/src/routes/builder.$databaseId.$tableId.tsx
@@ -401,11 +401,37 @@ function StructureBuilder() {
     const newFields = fields.filter((f) => f.isNew);
     const existingFields = fields.filter((f) => !f.isNew);
 
-    Promise.all([
-      ...newFields.map((f) => saveFieldMutation.mutateAsync(f)),
-      ...existingFields.map((f) => saveFieldMutation.mutateAsync(f)),
-    ]).then(() => {
-      setSavedSnapshot(snapshotFields(fields));
+    const saveField = async (field: BuilderField): Promise<[string, Field | null]> => {
+      try {
+        return [field.id, await saveFieldMutation.mutateAsync(field)];
+      } catch {
+        return [field.id, null];
+      }
+    };
+
+    Promise.all([...newFields.map(saveField), ...existingFields.map(saveField)]).then((results) => {
+      const savedByDraftId = new Map(results);
+
+      // Rebase drafts onto what the server persisted: without this a field created in
+      // this session keeps isNew/persistedId=null and the next save POSTs it again,
+      // duplicating it on the table. Fields whose save failed keep their draft state.
+      setFields((prev) =>
+        prev.map((f) => {
+          const persistedId = savedByDraftId.get(f.id)?.id ?? f.persistedId;
+          if (!persistedId) return f;
+          return { ...f, isNew: false, persistedId };
+        })
+      );
+
+      // Only the fields that round-tripped are what the server now holds. Snapshot
+      // those and leave the rest alone, so a field whose save failed stays dirty and
+      // still trips the unsaved-changes guard.
+      const persisted = [...newFields, ...existingFields].filter((f) => savedByDraftId.get(f.id));
+      setSavedSnapshot((prev) => ({
+        ...prev,
+        ...Object.fromEntries(persisted.map((f) => [f.id, snapshotField(f)])),
+      }));
+
       queryClient.invalidateQueries({ queryKey: ['fields', tableId] });
     });
   }, [fields, queryClient, tableId, saveFieldMutation]);