fastmcp/CLAUDE.md

FastMCP Development Guidelines

Testing and Investigation

In-Memory Transport - Always Preferred

When testing or investigating FastMCP servers, always prefer the in-memory transport unless you specifically need HTTP transport features. Pass a FastMCP server directly to a Client to eliminate separate processes and network complexity.

# Create your FastMCP server
mcp = FastMCP("TestServer")

@mcp.tool
def greet(name: str) -> str:
    return f"Hello, {name}!"

# Pass server directly to client - uses in-memory transport
async with Client(mcp) as client:
    result = await client.call_tool("greet", {"name": "World"})

When to Use HTTP Transport

Only use HTTP transport when testing network-specific features. Prefer StreamableHttp over SSE as it's the modern approach.

# Only when network testing is required
async with Client(transport=StreamableHttpTransport(server_url)) as client:
    result = await client.ping()

Development Workflow

• You must always run pre-commit if you open a PR, because it is run as part of a required check.
• When opening PRs, apply labels appropriately for bugs/breaking changes/enhancements/features. Generally, improvements are enhancements (not features) unless told otherwise.
• NEVER modify files in docs/python-sdk/**, as they are auto-generated.
• Use # type: ignore[attr-defined] in unit tests when accessing an MCP result of indeterminate type instead of asserting its type