fastmcp-testinglisted

# MCP server testing (in-memory client + recorded fixtures) > **The name is ambiguous — read this before you rename anything.** Two > different projects are called FastMCP, and they cannot be installed in the > same environment: > > | | package | server class | in-memory client | > |---|---|---|---| > | **(a)** the official SDK | `mcp` | `mcp.server.mcpserver.MCPServer` — **renamed** from `mcp.server.fastmcp.FastMCP` in the 2.0 break, which removed the old module with no shim | `mcp.client.client.Client` | > | **(b)** a separate project | `fastmcp` | `fastmcp.FastMCP` — **not** renamed, still current, on its own major line (3.x while the SDK is at 2.x) | `fastmcp.Client` | > > `fastmcp` still requires `mcp` 1.x, so in an environment holding `mcp` 2.x even > `import fastmcp` raises `ImportError: cannot import name 'McpError'`. Find out > which one the repo in front of you uses — `pyproject.toml` settles it — before > touching an import. A blind search-and-replace across the two does damage. > > This skill's name is kept for continuity; it covers both. - Use `pytest-asyncio`; mark async tests with `@pytest.mark.asyncio`. - NEVER hit live municipal APIs in tests. Intercept outbound `httpx` requests with `unittest.mock.AsyncMock` and inject recorded JSON fixtures from `tests/fixtures/`. - Prefer the **in-memory client of whichever SDK the server belongs to** for calling tools directly — `mcp.client.client.Client` for (a), `fastmcp.Client` for (b). Both take the server o