Snapshot testing

Manifest helpers return normalized, deep-sorted objects - stable across runs, with entry-level _meta and undefined values dropped - so vitest's built-in snapshots catch unintended API changes to your server.

A manifest records what the server actually reports over the wire, which includes descriptor fields your MCP SDK adds for you (for example execution or a $schema on generated schemas). An SDK upgrade can therefore move a snapshot without your server changing; review the diff and update it as you would any snapshot. Capabilities your server does not expose come back empty rather than failing, so a tools-only server snapshots cleanly.

import { expect } from "vitest";
import { createMcpTest } from "mcp-vitest";
import { capabilitiesManifest, toolManifest } from "mcp-vitest/snapshot";
import { createServer } from "./server.js";

const test = createMcpTest(() => createServer());

test("tool surface is unchanged", async ({ mcp }) => {
  expect(await toolManifest(mcp)).toMatchSnapshot();
});

test("capabilities are unchanged", async ({ mcp }) => {
  // { tools: string[], resources: string[], prompts: string[] }, names only
  expect(await capabilitiesManifest(mcp)).toMatchSnapshot();
});

toolManifest, resourceManifest, promptManifest, and capabilitiesManifest are available from mcp-vitest/snapshot or the package root.

Structured output

toMatchOutputSchema() validates a result's structuredContent against the schema the tool declared in tools/list. Pass a JSON Schema explicitly to validate against something else.

const result = await mcp.callTool("weather");
expect(result).toMatchOutputSchema();
expect(result).toMatchOutputSchema({
  type: "object",
  properties: { temperature: { type: "number" }, unit: { type: "string" } },
  required: ["temperature", "unit"],
});

Note that both SDK majors validate declared output schemas server-side: if a tool's output violates its own schema, the call comes back as a tool error (toBeToolError()) rather than delivering invalid structuredContent. The explicit-schema form is what you want for asserting a contract the tool does not declare.