api-testing

## Overview Tests target handler behavior directly — call `handler(input, ctx)`, assert on the return value or thrown error. The framework's handler factory (try/catch, formatting, telemetry) is not involved. Use `createMockContext` from `@cyanheads/mcp-ts-core/testing` to construct the `ctx` argument. **Additional exports from `/testing`:** `createMockSession()` binds a mock handler context to an HTTP session; `createFetchMock()` provides a strict upstream HTTP fake; `runToolContract()` executes a definition through schema, handler, formatting, enrichment/content, and production-shaped error-envelope checks. `createMockLogger()` returns a standalone `MockContextLogger`, `createInMemoryStorage(options?)` provides a real `StorageService` backed by `InMemoryProvider`, and `expectInputRequired(run)` returns the `input_required` result a multi-round-trip handler asked for (see [Mock inputs](#mock-inputs)). **Other testing subpaths:** `/testing/vitest` (fixtures and conformance suites, below), `/testing/fuzz` (see [Fuzz testing](#fuzz-testing)), and `/testing/apps`, whose `renderAppTool` renders an app tool's `ui://` view against your server (a stdio command or an HTTP URL) in a headless MCP Apps host and reports initialization, errors, CSP violations, the view↔host messages, and screenshots. It needs the optional peers `@modelcontextprotocol/client` and `@modelcontextprotocol/ext-apps` plus a `chrome-headless-shell` build; the `field-test` skill covers installing one and reading the report. **Philosophy:** Test behavior, not implementation. Refactors should not break tests. Match the repo's existing test layout: fresh scaffolds use `tests/`, while colocated `src/**/*.test.ts` files are also supported. Integration tests at I/O boundaries over unit tests of internals. --- ## `mcpTest` — fixture-based Vitest test `mcpTest` is a `test.extend`-based Vitest test that provides `ctx`, `session`, `fetchMock`, and `storage` as **per-test fixtures** — fresh instances for every test, eliminating boilerplate and enforcing isolation automatically. `fetchMock` is installed as `globalThis.fetch` only when requested by a test and restored afterward. ```ts import { mcpTest } from '@cyanheads/mcp-ts-core/testing/vitest'; mcpTest('echoes the message', async ({ ctx }) => { const result = await echoTool.handler(echoTool.input.parse({ message: 'hi' }), ctx); expect(result.message).toBe('hi'); }); mcpTest('uses storage fixture', async ({ ctx, storage }) => { const svc = new MyService(config, storage); const result = await svc.doWork(ctx); expect(result).toBeDefined(); }); mcpTest('stubs an upstream HTTP boundary', async ({ fetchMock }) => { fetchMock.route({ match: 'https://api.example.test/items/42', respond: Response.json({ id: '42' }), }); await expect(loadItem('42')).resolves.toMatchObject({ id: '42' }); }); ``` ### Fixtures | Fixture | Type | Per-test? | Notes | |:--------|:-----|:----------|:------| | `ctx` | `Context` | Yes | Fresh `createMockContext()` each test | | `session` | `MockSession` | Yes | Fresh `{ sessionId, tenantId, ctx }` from `createMockSession()` | | `fetchMock` | `FetchMockHarness` | Yes | Strict fetch fake installed/restored around the requesting test | | `storage` | `StorageService` | Yes | Fresh `createInMemoryStorage()` each test | ### Extending with the function form Override fixtures using the **function form** (`async ({}, use) => { ... }`) to preserve per-test freshness. A bare-value override shares one mutable instance across the entire file — defeating the fixture's isolation guarantee. ```ts import { createMockContext } from '@cyanheads/mcp-ts-core/testing/vitest'; // Correct — function form gives each test a fresh context: const tenantTest = mcpTest.extend({ ctx: async ({}, use) => { await use(createMockContext({ tenantId: 'test-tenant' })); }, }); // Wrong — bare value shares one ctx across every test in the file: // const tenantTest = mcpTest.extend({ ctx: createMockContext({ tenantId: 'test-tenant' }) }); ``` The portable `/testing` helpers are re-exported from `@cyanheads/mcp-ts-core/testing/vitest` so fixture overrides don't need a second import. --- ## Upstream HTTP testing with `createFetchMock` Use the fetch harness at real outbound I/O boundaries. Stub the external service, not server-owned services or handlers. ```ts import { createFetchMock } from '@cyanheads/mcp-ts-core/testing'; const http = createFetchMock([ { method: 'GET', match: 'https://api.example.test/items/42', respond: Response.json({ id: '42', name: 'Example' }), }, ]); http.install(); try { await expect(loadItem('42')).resolves.toEqual({ id: '42', name: 'Example' }); expect(http.calls[0]?.request.url).toBe('https://api.example.test/items/42'); } finally { http.restore(); } ``` Routes match in registration order. `match` accepts an exact URL, `RegExp`, or request predicate; `respond` accepts a static `Response` or a response factory. A static response's body is read once, on the route's first match, and every call is served a fresh `Response` over those bytes with the same `status`, `statusText`, and headers — so a consumer that cancels the body, or an error-body reader like `httpErrorFromResponse` that stops past its cap, settles on Node as on Bun. Set `once: true` for one-shot behavior. Unmatched requests throw unless `onUnhandled` is provided. **A request predicate routes on the URL's origin, never a prefix.** `req.url.startsWith(BASE_URL)` also matches a lookalike host (`https://api.example.test.evil.com/...`), which CodeQL reports as high-severity incomplete URL substring sanitization — it scans test files as readily as `src/`, so a suite that is green locally still fails the security check on a pull request. Parse the URL and compare origins, matching the path separately: ```ts match: (req) => { const url = new URL(req.url); return url.origin === new URL(BASE_URL).origin && url.pathname.startsWith('/items/'); }, ``` --- ## Tool conformance with `toolContractSuite` Point the reusable suite at a definition plus representative success and failure inputs. It checks input/output schemas, invokes the real handler, applies formatting/enrichment/content, and validates both public error surfaces. ```ts import { JsonRpcErrorCode } from '@cyanheads/mcp-ts-core/errors'; import { toolContractSuite } from '@cyanheads/mcp-ts-core/testing/vitest'; toolContractSuite(searchTool, { success: [{ name: 'returns matches', input: { query: 'mcp' } }], errors: [{ name: 'reports an empty query', input: { query: '' }, code: JsonRpcErrorCode.InvalidParams, reason: 'empty_query', }], }); ``` Use `runToolContract(definition, input, { context })` from `/testing` when a custom test runner or an imperative assertion is a better fit. It intentionally skips transport auth and telemetry; those belong in transport/integration tests. A declared reason thrown without a hint — a bare `ctx.fail('reason')` or a service throw carrying `{ reason }` — comes back with the entry's `recovery` as `data.recovery.hint` and a `Recovery:` line in `content[]`, as in production. The one production field it leaves out is `data.requestId` (and the `request <id>` term closing `content[]`), since there is no real request; a test asserting the factory's envelope instead expects both. Calling `definition.handler(...)` directly returns the `McpError` exactly as the throw site built it — no fill, no request id. Arguments that fail the `input` schema are rejected the way the production handler factory rejects them: `InvalidParams` (`-32602`), with a message naming the tool and every failing field. That is the code a client sees on the wire, so assert it — not `ValidationError` (`-32007`), which stays the classification for a `ZodError` a handler throws itself. A result that breaks the tool's own `output` or `enrichment` schema is the definition's bug, so it returns `InternalError` (`-32603`) with a message naming that contract, exactly as in production. Cancellation settles as it does in production. Pass `context: { signal }` and abort it: once the signal has fired, whatever the handler — or the output validation, `format()`, and enrichment after it — throws comes back as `RequestCancelled` (`-32011`), whether that is the signal's `AbortError`, its reason string, a `withRetry` backoff that stopped, or an `McpError` of the handler's own. A throw while the signal is still live keeps its own classification, and argument parsing stays outside the settle, so schema-invalid arguments on an aborted signal still return `InvalidParams`. A `toolContractSuite` error case with an aborted `context.signal` asserts `code: JsonRpcErrorCode.RequestCancelled` the same way. **Resources have no contract runner.** A resource definition's `handler(...)` called directly returns the throw site's `McpError` with no recovery fill, so asserting a resource's declared `errors[]` hints needs a path through the resource factory. Serve the definition from `createWorkerHandler({ name, title, resources: [def] })` (`/worker`) and `fetch` it a `resources/read` JSON-RPC request carrying the current protocol revision in both the `MCP-Protocol-Version` header and `params._meta`. The response is plain JSON or an SSE `data:` frame, and its `error` carries `code`, `data.reason`, and the filled `data.recovery.hint`. --- ## `createMockContext` options ```ts import { createMockContext } from '@cyanheads/mcp-ts-core/testing'; createMockContext() // working ctx.state on tenant 'default' createMockContext({ tenantId: 'test-tenant' }) // explicit tenant scope for ctx.state createMockContext({ errors: myTool.errors }) // attaches typed ctx.fail keyed by the contract reasons createMockContext({ inputResponses: { confirm: { action: 'accept', content: { ok: true } } } }) // second round of a multi-round-trip handler createMockContext({ requestState: 'opaque-state' }) // seeds ctx.inputs.state() createMockContext({ clientCapabilities: { roots: {} } }) // seeds ctx.clientCapabilities and filters inputResponses to declared kinds createMockContext({ requestId: 'my-id' }) // override request ID (default: 'test-request-id') createMockContext({ notifyResourceListChanged: () => {} }) // with resource-list change notifier createMockContext({ notifyResourceUpdated: (_uri) => {} }) // with resource update notifier createMockContext({ signal: controller.signal }) // custom AbortSignal createMockContext({ auth: { clientId: 'test', scopes: [], sub: 'test-user' } }) // with auth context createMockContext({ uri: new URL('myscheme://item/123') }) // for resource handler testing ``` `MockContextOptions` interface: ```ts interface MockContextOptions<TErrors extends readonly ErrorContract[] | undefined> { auth?: AuthContext; clientCapabilities?: ClientCapabilities; errors?: TErrors | undefined; inputResponses?: InputResponses | Record<string, unknown>; notifyPromptListChanged?: () => void; notifyResourceListChanged?: () => void; notifyResourceUpdated?: (uri: string) => void; notifyToolListChanged?: () => void; requestId?: string; requestState?: unknown; sessionId?: string; signal?: AbortSignal; tenantId?: string; uri?: URL; } ``` | Option | Effect | |:-------|:-------| | _(none)_ | Working `ctx.state` on tenant `'default'`; `ctx.inputs` is empty (first round) | | `auth` | Sets `ctx.auth` for scope-checking tests | | `clientCapabilities` | Sets `ctx.clientCapabilities` (`undefined` when omitted) and applies the production filter to `inputResponses`: only the answers these capabilities cover reach `ctx.inputs` (elicit → `elicitation`, and `elicitation.form` when it carries `content`; sampling → `sampling`, and `sampling.tools` when it holds a `tool_use` / `tool_result` block; roots → `roots`). Omitted, every seeded response reaches `ctx.inputs`, so existing `{ inputResponses }` tests are unaffected. The mock's `ctx.requestInput` stays ungated either way | | `errors` | Attaches a typed `ctx.fail` against the contract — same wiring the production handler factory uses. Pass `myTool.errors` directly; the return type narrows to `HandlerContext<ReasonOf<…>>`, so the context is assignable to that definition's handler parameter. | | `inputResponses` | Seeds `ctx.inputs` with the responses a retried request would carry, keyed by the identifiers the handler's `ctx.requestInput(...)` assigned (see below) | | `notifyPromptListChanged` | Assigns `ctx.notifyPromptListChanged` for prompt-list change notification tests | | `notifyResourceListChanged` | Assigns `ctx.notifyResourceListChanged` for resource notification tests | | `notifyResourceUpdated` | Assigns `ctx.notifyResourceUpdated` for resource update notification tests | | `notifyToolListChanged` | Assigns `ctx.notifyToolListChanged` for tool-list change notification tests | | `requestId` | Overrides `ctx.requestId` (default: `'test-request-id'`) | | `requestState` | Seeds `ctx.inputs.state()` — the opaque state a prior round attached | | `sessionId` | Sets `ctx.sessionId` for handlers that branch on session ID | | `signal` | Overrides `ctx.signal` — useful for cancellation testing | | `tenantId` | Scopes `ctx.state` to a specific tenant. Defaults to `'default'` — the value stdio (and HTTP with `MCP_AUTH_MODE=none`) resolves | | `uri` | Sets `ctx.uri` for resource handler testing | ### Mock state `ctx.state` is a real `StorageService` over an `InMemoryProvider` — the production storage path, not a `Map`. A test therefore sees the same rules a deployed server enforces: - **Keys** match `^[a-zA-Z0-9_.\-/]+$` and may not contain `..`. Colons are rejected, so `cache:v1:abc` throws `McpError(ValidationError)` in the test exactly as it would in a deployment; use `cache/v1/abc`. - **Values** round-trip as JSON, as on every persistent provider. A read returns a fresh object in its JSON form — a `Date` reads back as its ISO string — so a test cannot pass on identity or on a `Date`/`Map` surviving storage. A value JSON cannot encode (`bigint`, a cyclic reference, a top-level `undefined`, function, or symbol) rejects with `McpError(SerializationError)`. - **TTL** is honored. An entry written with `{ ttl: 30 }` reads back as `null` once 30 seconds elapse — drive the clock with `vi.useFakeTimers()` to assert expiry. - **`getMany` / `setMany` / `deleteMany` / `list`** validate every key and prefix, and `list` paginates with the same opaque cursors.