diff --git a/apps/desktop/src/lib/tauri-api.test.ts b/apps/desktop/src/lib/tauri-api.test.ts
new file mode 100644
index 0000000..11ab4d2
--- /dev/null
+++ b/apps/desktop/src/lib/tauri-api.test.ts
@@ -0,0 +1,113 @@
+// Contract tests for the renderer↔Rust IPC boundary (tauri-api.ts).
+//
+// These lock the command names and the snake_case↔camelCase mapping that the
+// Rust #[tauri::command] handlers expect. HANDOFF §8a: casing mismatches across
+// this boundary shipped real bugs twice. The Rust side is guarded by
+// src-tauri/src/tools.rs casing_tests; this guards the TS side.
+//
+// `invoke` is mocked so no Tauri runtime is needed.
+
+import { beforeEach, describe, expect, it, vi } from 'vitest';
+import { invoke } from '@tauri-apps/api/core';
+import {
+  appendAllowMatcher,
+  getAppInfo,
+  loadSettingsFile,
+  readCredentials,
+  saveCredentials,
+  saveSettingsFile,
+  sessionAppend,
+  sessionCreate,
+} from './tauri-api.js';
+
+vi.mock('@tauri-apps/api/core', () => ({ invoke: vi.fn() }));
+const invokeMock = vi.mocked(invoke);
+
+beforeEach(() => {
+  invokeMock.mockReset();
+});
+
+describe('readCredentials', () => {
+  it('maps Rust snake_case → renderer camelCase (the §8a direction)', async () => {
+    invokeMock.mockResolvedValue({
+      api_key: 'sk-123',
+      auth_token: 'tok-9',
+      base_url: 'https://api.deepseek.com/v1',
+    });
+    const creds = await readCredentials();
+    expect(invokeMock).toHaveBeenCalledWith('read_credentials');
+    expect(creds).toEqual({
+      apiKey: 'sk-123',
+      authToken: 'tok-9',
+      baseURL: 'https://api.deepseek.com/v1',
+    });
+  });
+
+  it('leaves missing fields undefined (does not invent empty strings)', async () => {
+    invokeMock.mockResolvedValue({ api_key: 'only-key' });
+    const creds = await readCredentials();
+    expect(creds).toEqual({ apiKey: 'only-key', authToken: undefined, baseURL: undefined });
+  });
+});
+
+describe('saveCredentials', () => {
+  it('sends snake_case under `creds` (matches the Rust input struct)', async () => {
+    invokeMock.mockResolvedValue(undefined);
+    await saveCredentials({ apiKey: 'sk-x', authToken: 'tok', baseURL: 'https://h/v1' });
+    expect(invokeMock).toHaveBeenCalledWith('save_credentials', {
+      creds: { api_key: 'sk-x', auth_token: 'tok', base_url: 'https://h/v1' },
+    });
+  });
+
+  it('round-trips with readCredentials (save shape decodes back to the same camelCase)', async () => {
+    invokeMock.mockResolvedValue(undefined);
+    const input = { apiKey: 'a', authToken: 'b', baseURL: 'c' };
+    await saveCredentials(input);
+    const sent = invokeMock.mock.calls[0]![1] as { creds: Record<string, string> };
+    // Simulate the backend echoing those stored fields back on read.
+    invokeMock.mockResolvedValue(sent.creds);
+    expect(await readCredentials()).toEqual(input);
+  });
+});
+
+describe('command name + argument contracts', () => {
+  it('getAppInfo → get_app_info (no args)', async () => {
+    invokeMock.mockResolvedValue({ version: '1.0.0', platform: 'darwin', home_dir: '/Users/x' });
+    await getAppInfo();
+    expect(invokeMock).toHaveBeenCalledWith('get_app_info');
+  });
+
+  it('saveSettingsFile → save_settings_file with { value }', async () => {
+    invokeMock.mockResolvedValue(undefined);
+    await saveSettingsFile({ effortLevel: 'high' });
+    expect(invokeMock).toHaveBeenCalledWith('save_settings_file', {
+      value: { effortLevel: 'high' },
+    });
+  });
+
+  it('loadSettingsFile → load_settings_file', async () => {
+    invokeMock.mockResolvedValue({});
+    await loadSettingsFile();
+    expect(invokeMock).toHaveBeenCalledWith('load_settings_file');
+  });
+
+  it('appendAllowMatcher → append_allow_matcher with { matcher }', async () => {
+    invokeMock.mockResolvedValue(undefined);
+    await appendAllowMatcher('Write');
+    expect(invokeMock).toHaveBeenCalledWith('append_allow_matcher', { matcher: 'Write' });
+  });
+
+  it('sessionCreate → session_create with { cwd } and returns the id', async () => {
+    invokeMock.mockResolvedValue('sess-abc');
+    const id = await sessionCreate('/proj');
+    expect(invokeMock).toHaveBeenCalledWith('session_create', { cwd: '/proj' });
+    expect(id).toBe('sess-abc');
+  });
+
+  it('sessionAppend → session_append with { id, message }', async () => {
+    invokeMock.mockResolvedValue(undefined);
+    const msg = { type: 'message', role: 'user', content: [] };
+    await sessionAppend('sess-abc', msg);
+    expect(invokeMock).toHaveBeenCalledWith('session_append', { id: 'sess-abc', message: msg });
+  });
+});