Add documentation about testing (#1426)

docs/testing.md

@@ -0,0 +1,78 @@
+# Testing MCP Servers
+
+If you call yourself a developer, you will want to test your MCP server.
+The Python SDK offers the `create_connected_server_and_client_session` function to create a session
+using an in-memory transport. I know, I know, the name is too long... We are working on improving it.
+
+Anyway, let's assume you have a simple server with a single tool:
+
+```python title="server.py"
+from mcp.server import FastMCP
+
+app = FastMCP("Calculator")
+
+@app.tool()
+def add(a: int, b: int) -> int:
+    """Add two numbers."""  # (1)!
+    return a + b
+```
+
+1. The docstring is automatically added as the description of the tool.
+
+To run the below test, you'll need to install the following dependencies:
+
+=== "pip"
+    ```bash
+    pip install inline-snapshot pytest
+    ```
+
+=== "uv"
+    ```bash
+    uv add inline-snapshot pytest
+    ```
+
+!!! info
+    I think [`pytest`](https://docs.pytest.org/en/stable/) is a pretty standard testing framework,
+    so I won't go into details here.
+
+    The [`inline-snapshot`](https://15r10nk.github.io/inline-snapshot/latest/) is a library that allows
+    you to take snapshots of the output of your tests. Which makes it easier to create tests for your
+    server - you don't need to use it, but we are spreading the word for best practices.
+
+```python title="test_server.py"
+from collections.abc import AsyncGenerator
+
+import pytest
+from inline_snapshot import snapshot
+from mcp.client.session import ClientSession
+from mcp.shared.memory import create_connected_server_and_client_session
+from mcp.types import CallToolResult, TextContent
+
+from server import app
+
+
+@pytest.fixture
+def anyio_backend():  # (1)!
+    return "asyncio"
+
+
+@pytest.fixture
+async def client_session() -> AsyncGenerator[ClientSession]:
+    async with create_connected_server_and_client_session(app, raise_exceptions=True) as _session:
+        yield _session
+
+
+@pytest.mark.anyio
+async def test_call_add_tool(client_session: ClientSession):
+    result = await client_session.call_tool("add", {"a": 1, "b": 2})
+    assert result == snapshot(
+        CallToolResult(
+            content=[TextContent(type="text", text="3")],
+            structuredContent={"result": 3},
+        )
+    )
+```
+
+1. If you are using `trio`, you should set `"trio"` as the `anyio_backend`. Check more information in the [anyio documentation](https://anyio.readthedocs.io/en/stable/testing.html#specifying-the-backends-to-run-on).
+
+There you go! You can now extend your tests to cover more scenarios.