To run a Python MCP server, install the official MCP SDK v2 with its CLI extra, create a server file, and start it with the development command. The SDK requires Python 3.10 or newer. For local clients that launch your program, use stdio; for clients connecting to a network endpoint, use Streamable HTTP and configure its host security deliberately.
Install the official Python MCP SDK
The MCP Python SDK documentation lists v2 as its current stable release line and specifies Python 3.10+. The cli extra provides the mcp command used for development.
- Check your interpreter:
python --version. Use Python 3.10 or later. - Install the package in your project with one of these options:
uv add "mcp[cli]"
orpip install "mcp[cli]"
Use the same Python environment for installation and execution. With uv, run commands through uv run so the project environment is used. With pip, activate the virtual environment where you installed the package before running the CLI.
Create a minimal server with one tool
Save this complete example as server.py. It registers a named server and a tool called add, which accepts two integers and returns their sum.
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from mcp.server.fastmcp import FastMCP
mcp = FastMCP("Example Python Server")
@mcp.tool()
def add(a: int, b: int) -> int:
"""Add two integers and return the result."""
return a + b
if __name__ == "__main__":
mcp.run()The function’s name, parameters, type hints, and docstring give an MCP client the tool’s identity and description. Keep the example’s mcp.run() default for the usual local stdio launch; choose another transport explicitly when serving network clients. This uses the v2-style FastMCP API rather than assuming older SDK examples or imports still apply.
Run and inspect it during development
From the directory containing server.py, run:
uv run mcp dev server.pyThe official quickstart uses this command to start the server file in the SDK’s development workflow, allowing you to inspect and exercise its registered tools. It is a development path, not by itself a production deployment recipe. The quickstart also documents testing through an in-process SDK client; use the SDK’s current quickstart for the version-specific client setup: Python server quickstart.
If you installed with pip rather than uv, activate the environment containing the CLI and run mcp dev server.py. If the shell cannot find mcp, verify that the CLI extra was installed in the active environment.
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Choose a transport for the client you have
The SDK supports stdio, Streamable HTTP, and SSE. These are different connection models, not interchangeable launch flags: choose according to how the MCP client reaches the server.
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| Transport | Connection model | When it fits | Operational consideration |
|---|---|---|---|
stdio |
A local host launches the server as a subprocess and exchanges protocol messages through standard input and output. | Local developer tools or other hosts configured to start a command on the same machine. | Standard output is reserved for protocol traffic; send logs and diagnostics to standard error. |
streamable-http |
A client reaches a server over HTTP. | Web or remote-client integrations where the server is hosted as an ASGI application or process. | Configure accepted host values for a real hostname; plan process scaling and session handling as part of deployment. |
sse |
An additional network transport supported by the SDK. | Use when the client and deployment you target require the SDK’s SSE transport. | Check client compatibility and the SDK’s current transport documentation instead of assuming it behaves like stdio or Streamable HTTP. |
The current MCPServer.run() API supports all three names and defaults to stdio. For a client that launches a local process, the default is generally the natural choice. For a client that connects to an HTTP endpoint, select Streamable HTTP and serve the HTTP app or run the matching transport directly.
Keep stdio protocol traffic clean
With stdio, the server reads MCP messages from stdin and writes them to stdout. Any ordinary text printed to stdout can be mistaken for a protocol message and break the connection. Avoid print() for diagnostics in this mode; configure logging to stderr instead.
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import logging
import sys
logging.basicConfig(stream=sys.stderr, level=logging.INFO)
logging.info("Server starting")Tool results should be returned through the SDK’s tool mechanism, not emitted as ad hoc terminal output. This distinction matters even when the server appears to run normally in a terminal: the MCP host communicates through those same streams.
Run the server over Streamable HTTP
Use the SDK ASGI app
For integration with a web application, the SDK’s mcp.streamable_http_app() returns a Starlette ASGI app and includes the /mcp route. An ASGI host such as Uvicorn can serve it. A basic local host can be assembled as follows, assuming the server object from the earlier example:
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if __name__ == "__main__":
app = mcp.streamable_http_app()
uvicorn.run(app, host="127.0.0.1", port=8000)Install Uvicorn in the same environment if it is not already present. Start the file with that environment’s Python, then the MCP endpoint is http://127.0.0.1:8000/mcp. This binds to the local machine for development; it does not make the server publicly reachable.
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Do not expose localhost defaults unchanged
The ASGI helper is localhost-oriented and enables DNS-rebinding protections. A real hostname must be explicitly included in the transport security settings’ accepted host values. Configure that allowlist to match the hostname through which clients actually reach the service, and retain the protections rather than treating them as optional cosmetics. The exact configuration interface can change with SDK releases, so use the version-matched deployment guide: MCP remote server deployment guide.
Do not assume that changing Uvicorn’s bind address is sufficient. Binding controls where the process listens; accepted-host security controls which host headers the MCP transport will accept. For a public deployment, account separately for TLS termination, authentication and authorization, firewall or proxy configuration, and the identity of permitted clients. The SDK’s transport helper does not by itself establish those application-specific policies.
Plan scaling and session behavior
The SDK deployment guide notes that mcp.run("streamable-http") starts one Uvicorn process. Production worker counts, process supervision, and multi-worker behavior depend on the ASGI/process architecture and session handling. Before adding workers or replicas, confirm that sessions and any in-memory state are handled correctly across processes; a successful single-process local run does not prove that a multi-worker topology will behave correctly.
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Build beyond the first tool
Once the client can discover and invoke add, extend the server in small steps:
- Give each tool a narrow, descriptive name and a docstring that tells a model or user what it does.
- Use accurate parameter types and validate values that need constraints beyond their Python type.
- Return useful errors for invalid inputs without exposing secrets or internal implementation details.
- Keep side effects explicit. A tool that writes files, sends messages, or changes remote data should have an understandable scope and appropriate access controls.
- Choose stdio or HTTP based on the client’s actual connection model before designing deployment around it.
These are application-design practices rather than transport guarantees. The SDK makes tools available through MCP; it does not decide which users should be authorized to invoke a sensitive operation.
Troubleshoot common startup and connection problems
| Symptom | Likely cause | What to check |
|---|---|---|
mcp: command not found |
The CLI extra is missing, or the command is being run outside the environment where it was installed. | Install mcp[cli] and activate that environment, or use uv run mcp dev server.py from the project. |
| Import or syntax errors during startup | The interpreter may be too old, the wrong environment may be active, or code may follow a different SDK API version. | Check python --version, verify the installed package environment, and follow the v2 documentation rather than copying an older example. |
| The local host cannot connect in stdio mode | The host may be launching a different executable or working directory, or the server may emit non-protocol text to stdout. | Check the host’s configured command and file path, then move diagnostic prints and logging to stderr. |
| The HTTP route returns a host-related rejection | The hostname is not included in the transport’s accepted-host settings. | Configure the host allowlist for the actual hostname and review the deployment guide’s DNS-rebinding security guidance. |
| A remote client cannot reach a locally working endpoint | The process may be bound only to loopback, or a proxy, firewall, TLS, or routing layer may not forward the endpoint. | Check the bind address and network path separately from host allowlisting; expose the service only behind the access controls your deployment requires. |
| Behavior changes after adding workers | Sessions or in-memory server state may not be shared across worker processes. | Review the ASGI deployment architecture and session strategy before scaling the process count. |
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Frequently Asked Questions
Does mcp.run() use stdio by default?
Yes. The current Python SDK API defaults to stdio; select another supported transport when your client requires it.
Quick Recap
What URL does the SDK’s Streamable HTTP app expose?
The ASGI helper includes the /mcp route.
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