diff --git a/src/tui/shared/state-write.ts b/src/tui/shared/state-write.ts
new file mode 100644
index 0000000..17af437
--- /dev/null
+++ b/src/tui/shared/state-write.ts
@@ -0,0 +1,29 @@
+import * as fs from 'fs/promises';
+import * as path from 'path';
+import { Selection } from './types.js';
+
+export async function writeSelection(
+  filePath: string,
+  projectDir: string,
+  selection: Selection
+): Promise<void> {
+  await fs.mkdir(path.dirname(filePath), { recursive: true });
+
+  const payload = {
+    version: 1,
+    updated_at: new Date().toISOString(),
+    project_dir: projectDir,
+    device: selection.device,
+    selection: {
+      kind: selection.pou.kind,
+      name: selection.pou.name,
+      path: selection.pou.relPath,
+      abs_path: selection.pou.absPath,
+    },
+    viewer_line: selection.viewerLine,
+  };
+
+  const tmp = `${filePath}.${process.pid}.tmp`;
+  await fs.writeFile(tmp, JSON.stringify(payload, null, 2), 'utf8');
+  await fs.rename(tmp, filePath);
+}
diff --git a/tests/tui/state-write.test.ts b/tests/tui/state-write.test.ts
new file mode 100644
index 0000000..3b14089
--- /dev/null
+++ b/tests/tui/state-write.test.ts
@@ -0,0 +1,58 @@
+import { describe, it, expect } from 'vitest';
+import * as fs from 'fs/promises';
+import * as path from 'path';
+import * as os from 'os';
+import { writeSelection } from '../../src/tui/shared/state-write.ts';
+import { Selection } from '../../src/tui/shared/types.ts';
+
+async function tmpDir(): Promise<string> {
+  return fs.mkdtemp(path.join(os.tmpdir(), 'phobics-tui-'));
+}
+
+const sampleSelection: Selection = {
+  device: 'CodesysRpi',
+  pou: {
+    name: 'FB_Test',
+    kind: 'FB',
+    relPath: 'Plc Logic/Application/FB_Test.st',
+    absPath: '/abs/Plc Logic/Application/FB_Test.st',
+    loc: 87,
+    mtimeMs: 0,
+  },
+  viewerLine: 12,
+};
+
+describe('writeSelection', () => {
+  it('writes a JSON file atomically with v1 envelope', async () => {
+    const dir = await tmpDir();
+    const target = path.join(dir, 'tui-state.json');
+    await writeSelection(target, '/abs/project', sampleSelection);
+
+    const text = await fs.readFile(target, 'utf8');
+    const parsed = JSON.parse(text);
+    expect(parsed.version).toBe(1);
+    expect(parsed.project_dir).toBe('/abs/project');
+    expect(parsed.device).toBe('CodesysRpi');
+    expect(parsed.selection.name).toBe('FB_Test');
+    expect(parsed.selection.kind).toBe('FB');
+    expect(parsed.viewer_line).toBe(12);
+    expect(typeof parsed.updated_at).toBe('string');
+    expect(() => new Date(parsed.updated_at)).not.toThrow();
+  });
+
+  it('creates parent directories as needed', async () => {
+    const dir = await tmpDir();
+    const target = path.join(dir, 'a', 'b', 'tui-state.json');
+    await writeSelection(target, '/abs/project', sampleSelection);
+    const stat = await fs.stat(target);
+    expect(stat.isFile()).toBe(true);
+  });
+
+  it('does not leave the .tmp file behind on success', async () => {
+    const dir = await tmpDir();
+    const target = path.join(dir, 'tui-state.json');
+    await writeSelection(target, '/abs/project', sampleSelection);
+    const entries = await fs.readdir(dir);
+    expect(entries.filter((e) => e.endsWith('.tmp'))).toEqual([]);
+  });
+});