Testing with an in-memory client

Goal

Write fast tests that exercise your tools, resources and prompts exactly as a client would.

Why not just test the handlers?

You could call the handler functions directly. But then you skip the parts that break most: schema conversion, validation, error mapping, and the JSON-RPC layer. Testing through a client covers all of it, and it is still fast.

A linked pair of transports

The SDK ships InMemoryTransport.createLinkedPair(). It returns two transports wired to each other. Connect the server to one and a Client to the other:

// server/test/helpers.ts
  const server = createJotterServer(store)
  const [clientTransport, serverTransport] = InMemoryTransport.createLinkedPair()
  const client = new Client({ name: 'jotter-tests', version: '0.0.0' })
  await Promise.all([server.connect(serverTransport), client.connect(clientTransport)])

Each test also gets its own temp folder and a store with predictable ids and times:

// server/test/helpers.ts
  const store = new NoteStore(path.join(dir, 'notes.json'), {
    // Predictable ids and times make the assertions readable.
    newId: () => (++n).toString(16).padStart(8, '0'),
    now: () => new Date(Date.UTC(2026, 0, 1, 0, n)),
  })

Because ids are 00000001, 00000002 and so on, assertions can compare exact strings.

What to test

For every tool, test the happy path and the bad input. From server/test/tools.test.ts:

// server/test/tools.test.ts
  it('refuses an id that is not 8 hex characters', async () => {
    const result = await call(h.client, 'delete_note', { id: '../../etc/passwd' })
    expect(result.isError).toBe(true)
    expect(textOf(result)).toContain('8 lowercase hex characters')
  })

A good MCP test suite also checks:

• the tool list, schemas and annotations (a renamed tool breaks every client config)
• resources: list, read, missing ids, completion
• prompts: arguments and the filled messages
• storage edge cases: missing file, broken file, overlapping writes
• that errors never leak paths or stack traces

The HTTP test in server/test/http.test.ts starts the real HTTP server on a random port and connects with StreamableHTTPClientTransport.

Check it works

npm test

Every test passes in about a second. Run npm run test:watch while you work.

Exercise

Write a test that adds a note, deletes it, and checks that notes://all no longer lists it. Then write one for the count_notes tool from chapter 3 if you built it.