Add Documentation for FastMCP Server Testing (#2244)

* Add doc with recommendations for server testing

* Updates to testing doc

* Apply suggestion from @Copilot

Co-authored-by: Copilot <[email protected]>

* add to sidebar under patterns

* Update docs/patterns/testing.mdx

Co-authored-by: Copilot <[email protected]>

---------

Co-authored-by: Copilot <[email protected]>

Author: strawgate

docs/docs.json

@@ -236,7 +236,8 @@
               "patterns/tool-transformation",
               "patterns/decorating-methods",
               "patterns/cli",
-              "patterns/contrib"
+              "patterns/contrib",
+              "patterns/testing"
             ]
           },
           {

docs/patterns/testing.mdx

@@ -0,0 +1,85 @@
+---
+title: Testing your FastMCP Server
+sidebarTitle: Testing
+description: How to test your FastMCP server.
+icon: vial
+---
+
+The best way to ensure a reliable and maintainable FastMCP Server is to test it! The FastMCP Client combined with Pytest provides a simple and powerful way to test your FastMCP servers. 
+
+Using Pytest Fixtures, you can wrap your FastMCP Server in a Client instance that makes interacting with your server fast and easy. This is especially useful when building your own MCP Servers and enables a tight development loop by allowing you to avoid using a separate tool like MCP Inspector during development:
+
+```python
+import pytest
+from fastmcp.client import Client
+from fastmcp.client.transports import FastMCPTransport
+
+from my_project.main import mcp
+
+@pytest.fixture
+async def main_mcp_client():
+    async with Client(transport=mcp) as mcp_client:
+        yield mcp_client
+
+async def test_list_tools(main_mcp_client: Client[FastMCPTransport]):
+    list_tools = await main_mcp_client.list_tools()
+
+    assert len(list_tools) == 5
+```
+
+We recommend the [inline-snapshot library](https://github.com/15r10nk/inline-snapshot) for asserting complex data structures coming from your MCP Server. This library allows you to write tests that are easy to read and understand, and are also easy to update when the data structure changes. 
+
+```python
+from inline_snapshot import snapshot
+
+async def test_list_tools(main_mcp_client: Client[FastMCPTransport]):
+    list_tools = await main_mcp_client.list_tools()
+
+    assert list_tools == snapshot()
+```
+
+Simply run `pytest --inline-snapshot=fix,create` to fill in the `snapshot()` with actual data.
+
+<Tip>
+For values that change you can leverage the [dirty-equals](https://github.com/samuelcolvin/dirty-equals) library to perform flexible equality assertions on dynamic or non-deterministic values.
+</Tip>
+
+Using the pytest `parametrize` decorator, you can easily test your tools with a wide variety of inputs.
+
+```python
+import pytest
+from my_project.main import mcp
+
+from fastmcp.client import Client
+from fastmcp.client.transports import FastMCPTransport
+@pytest.fixture
+async def main_mcp_client():
+    async with Client(mcp) as client:
+        yield client
+
+
+@pytest.mark.parametrize(
+    "first_number, second_number, expected",
+    [
+        (1, 2, 3),
+        (2, 3, 5),
+        (3, 4, 7),
+    ],
+)
+async def test_add(
+    first_number: int,
+    second_number: int,
+    expected: int,
+    main_mcp_client: Client[FastMCPTransport],
+):
+    result = await main_mcp_client.call_tool(
+        name="add", arguments={"x": first_number, "y": second_number}
+    )
+    assert result.data is not None
+    assert isinstance(result.data, int)
+    assert result.data == expected
+```
+
+<Tip>
+The [FastMCP Repository contains thousands of tests](https://github.com/jlowin/fastmcp/tree/main/tests) for the FastMCP Client and Server. Everything from connecting to remote MCP servers, to testing tools, resources, and prompts is covered, take a look for inspiration!
+</Tip>