When code changes affect what the app *sends* (RPC/HTTP payloads/options) or what the app *decides* based on the environment, add targeted tests that (1) assert the exact externally observable behavior and (2) make the inputs deterministic. Practical rules: - Client/server contract: If a function changes a request field (e.g., RPC `session.create`), write a regression test that triggers the path (e.g., via the hook) and asserts the exact payload contains the new field/value. - API wrapper options: If you add wrapper-level configuration (e.g., `timeoutMs`), add a focused unit test that calls the wrapper (or mounts the relevant layer) and asserts the options passed to the underlying API client. - Environment detection: If logic depends on host/system files or runtime environment, refactor to inject the relevant values (like sysfs/DMI fields) so unit tests can deterministically cover QEMU/virtualization/non-match and read-failure branches. Example (Vitest-style contract assertion): ```ts import { describe, it, expect, vi } from 'vitest' import { renderHook } from '@testing-library/react' import { useSessionLifecycle } from './useSessionLifecycle' vi.mock('../rpc', () => ({ rpc: vi.fn(), })) it('sends launch cwd in session.create payload', async () => { process.env.HERMES_CWD = '/expected/launch/dir' const { rpc } = await import('../rpc') ;(rpc as unknown as ReturnType).mockResolvedValue({ /* ... */ }) const { result } = renderHook(() => useSessionLifecycle({ /* required opts */ }) ) await result.current.newSession(/* args that lead to startNewSession */) expect(rpc).toHaveBeenCalledWith('session.create', expect.objectContaining({ cwd: '/expected/launch/dir', })) }) ``` Apply the same pattern to wrapper options (assert `timeoutMs`) and to environment detection (inject sysfs/DMI inputs rather than reading the host).