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---

---
title: Try the MCP Python SDK v2 beta today
description: >-
  The 2026-07-28 MCP spec lands tomorrow and the Python SDK v2 beta already
  speaks it. What changed, multi-round-trip requests, and traces in Pydantic
  Logfire for free.
date: '2026-07-27'
authors:
  - Marcelo Trylesinski
categories:
  - MCP
  - Pydantic Logfire
  - Open Source
canonical: 'https://pydantic.dev/articles/mcp-python-sdk-v2-beta'
---

> Markdown version of [Try the MCP Python SDK v2 beta today](https://pydantic.dev/articles/mcp-python-sdk-v2-beta) — the canonical HTML page.
>
> By [Marcelo Trylesinski](https://pydantic.dev/authors/marcelo-trylesinski.md) · 2026-07-27 · MCP, Pydantic Logfire, Open Source
>
> Related: [You've built this agent before](https://pydantic.dev/articles/harness-week.md) · [Your traces already know how to fix your prompt](https://pydantic.dev/articles/logfire-prompt-optimization.md)
>
> All articles: [/articles.md](https://pydantic.dev/articles.md) · Site index: [/llms.txt](https://pydantic.dev/llms.txt)

---

Tomorrow the [2026-07-28 revision of the MCP specification](https://blog.modelcontextprotocol.io/posts/2026-07-28-release-candidate/)
lands. It's the largest revision of the protocol since launch: a stateless core, a first-class extensions framework,
MCP Apps, Tasks as an extension, and authorization hardening. We'll be marking the launch with a
[live release party on YouTube](https://www.youtube.com/live/T8OrdNOzvcU), so join us to celebrate and see what's new.

The Python SDK beta already speaks it. I help maintain that SDK, so let's walk through what changed, and what you get
for free when you point it at [Pydantic Logfire](https://pydantic.dev/logfire).

:::warn{title="This is a pre-release"}
v2 is a pre-release line and is not recommended for production. **Each pre-release may contain breaking changes from
the previous one**, so pin an exact version and expect to update your code when you bump the pin. v1.x stays the
stable line, and nothing about the 2026-07-28 spec release breaks it.

If you publish a package that depends on `mcp`, add a `<2` upper bound now (for example `mcp>=1.27,<2`) so stable v2
doesn't surprise your users.
:::

## Install

```bash
uv add "mcp[cli]==2.0.0rc1"         # or: pip install "mcp[cli]==2.0.0rc1"
```

The exact pin matters. `pip` and `uv` won't resolve to a pre-release unless you ask for one, so an unpinned install
gives you the latest v1.x instead.

## A server, and a client

Let's start with the smallest thing that works. Two type-hinted functions and a docstring:

```python
from mcp.server import MCPServer

mcp = MCPServer("Demo")


@mcp.tool()
def add(a: int, b: int) -> int:
    """Add two numbers."""
    return a + b


@mcp.resource("greeting://{name}")
def greeting(name: str) -> str:
    """Greet someone by name."""
    return f"Hello, {name}!"
```

That's a complete MCP server. You don't write JSON Schema, because `a: int, b: int` *is* the schema.

The same package is a full client. In v1 you had to stack three things: a transport context manager, a
`ClientSession` around it, and a hand-called `await session.initialize()`. Now it's one object:

```python
import asyncio

from mcp import Client

from server import mcp


async def main() -> None:
    async with Client(mcp) as client:
        result = await client.call_tool("add", {"a": 1, "b": 2})
        print(result.structured_content)  # {'result': 3}


asyncio.run(main())
```

`Client` takes a server object (in memory, no transport at all, which is how you should test), a URL for Streamable
HTTP, or any transport context manager. Swap `mcp` for `"http://localhost:8000/mcp"` and the same code talks to a
remote server.

## What changed in the SDK?

The renames are the first thing your v1 codebase hits, because the old import paths are gone rather than deprecated:

- **`FastMCP` is now `MCPServer`**, and everything under `mcp.server.fastmcp.*` moved to `mcp.server.mcpserver.*`.
  If you built your server with decorators, that rename is most of the port.
- **The wire types moved to their own distribution**, `mcp-types`, imported as `mcp_types`. It depends on nothing but
  Pydantic and `typing-extensions`, so a gateway or a proxy can consume MCP's wire shapes without installing an HTTP
  stack.
- **Every field is snake_case**: `result.is_error`, `tool.input_schema`, `listing.next_cursor`. The JSON on the wire
  is still camelCase, only the Python attribute spelling changed.
- **Transport configuration moved to `run()`** and the app builders. `MCPServer` is about what your server *is*, so
  `MCPServer("x", port=9000)` is a `TypeError` now.
- **The low-level `Server` was rebuilt, not renamed.** Handlers are constructor arguments with one uniform shape,
  `async (ctx, params) -> result`, and the ambient `server.request_context` ContextVar is gone.

Two changes don't announce themselves with an import error, so watch for them. Sync `def` tools now run on a worker
thread instead of blocking the event loop, which matters to thread-affine code. And the HTTP client is `httpx2`, which
verifies TLS through the operating system trust store instead of `certifi`'s bundle, so a minimal container with no
system CA store can suddenly start failing handshakes.

## What changed in the protocol?

v2 serves both revisions at once. The same `streamable_http_app()` answers a 2025-era client's `initialize` and a
2026-era client's requests, with no flag to flip and no separate deployment.

- **No handshake, no session.** Every request carries its protocol version, client info, and capabilities in `_meta`,
  and discovery is a plain `server/discover` request. Over Streamable HTTP there's no `Mcp-Session-Id` on the 2026
  path, so nothing ties a modern request to a worker, and any replica behind a round-robin load balancer can answer.
- **Roots, sampling, and MCP-level logging are deprecated** (SEP-2577) on every protocol version, and `ping` is
  removed outright. Expect an `MCPDeprecationWarning` on your first `ctx.info(...)` after upgrading.
- **Change notifications become one stream.** `subscriptions/listen` replaces the standalone GET stream and
  `resources/subscribe`.
- **Requests are routable without parsing bodies.** Modern HTTP requests carry `Mcp-Method` and, for tool-ish calls,
  `Mcp-Name` (SEP-2243), so gateways and rate limiters can route on headers alone.

And then there's the one that will actually change how you write tools.

## The server can't call you back anymore

This is the big one, so let's take it slowly.

Sometimes a tool can't finish in one round trip. It needs something only the user has: a choice, a confirmation, a
credential. Before 2026-07-28, the server got it by *calling back*. In the middle of handling your `tools/call`, it
opened its own request to the client: an elicitation, a sampling call, a `roots/list`.

The 2026-07-28 spec retires that back-channel. There is no channel for it, so `ctx.elicit()` and
`ctx.session.create_message()` raise `NoBackChannelError` on a modern connection.

Instead, **the server returns**. It answers `tools/call` with an `InputRequiredResult` carrying what it still needs
plus an opaque `request_state` token. The client fulfills the request, then calls the same tool *again*, with its
answers and the token attached. The server now has what it was missing and returns a normal `CallToolResult`.

That's the whole mechanism, and the nice part is that every leg is an ordinary client-to-server request. Nothing ever
flows the other way.

Now, you rarely build that by hand. You declare a dependency instead, and the SDK does the round trips for you:

```python
import asyncio
from typing import Annotated

from mcp_types import ElicitRequestParams, ElicitResult
from pydantic import BaseModel

from mcp import Client
from mcp.client import ClientRequestContext
from mcp.server import MCPServer
from mcp.server.mcpserver import AcceptedElicitation, Elicit, ElicitationResult, Resolve

mcp = MCPServer("Bookshop")


class Quantity(BaseModel):
    copies: int


async def ask_quantity() -> Elicit[Quantity]:
    """Resolver: ask the user how many copies to put aside."""
    return Elicit("How many copies?", Quantity)


@mcp.tool()
async def reserve(title: str, quantity: Annotated[ElicitationResult[Quantity], Resolve(ask_quantity)]) -> str:
    """Reserve copies of a book, asking the user how many."""
    if isinstance(quantity, AcceptedElicitation):
        return f"Reserved {quantity.data.copies} of {title!r}."
    return "Nothing reserved."


async def answer(context: ClientRequestContext, params: ElicitRequestParams) -> ElicitResult:
    return ElicitResult(action="accept", content={"copies": 2})


async def main() -> None:
    async with (
        Client(mcp, mode="legacy", elicitation_callback=answer) as legacy,
        Client(mcp, elicitation_callback=answer) as modern,
    ):
        for client in (legacy, modern):
            result = await client.call_tool("reserve", {"title": "Dune"})
            print(client.protocol_version, result.structured_content)


asyncio.run(main())
```

If you've used FastAPI, `Resolve(...)` is `Depends`. Same move, same reason.

The `quantity` parameter never appears in the tool's input schema, so the model is never told about it and can't
invent it. A parameter the model can't supply is a parameter the model can't get wrong.

Run that file and both clients get the same answer:

```
2025-11-25 {'result': "Reserved 2 of 'Dune'."}
2026-07-28 {'result': "Reserved 2 of 'Dune'."}
```

One tool body, two protocol eras. That's the part I like: you don't write the fork.

## Tracing is built in

Here's where it gets fun for me, because I work on [Logfire](https://pydantic.dev/logfire) too.

v2 depends on `opentelemetry-api` directly and ships an OpenTelemetry middleware **enabled by default**. Every server
emits a SERVER span per inbound message, and the client emits a CLIENT span per outbound request. It only depends on
the API half of OpenTelemetry, so with no exporter installed a span is a no-op and costs you basically nothing.

To see them, call `logfire.configure()`. That's the whole integration:

```python
import logfire
import uvicorn
from mcp.server import MCPServer

logfire.configure(service_name="mcp-server", distributed_tracing=True)

mcp = MCPServer("logfire-demo")


@mcp.tool()
def add(a: int, b: int) -> int:
    """Add two integers."""
    with logfire.span("add {a} + {b}", a=a, b=b) as span:
        result = a + b
        span.set_attribute("result", result)
        return result


uvicorn.run(mcp.streamable_http_app(), host="127.0.0.1", port=8000)
```

And the client:

```python
import asyncio

import logfire
from mcp import Client

logfire.configure(service_name="mcp-client")


async def main() -> None:
    with logfire.span("mcp session"):
        async with Client("http://127.0.0.1:8000/mcp") as client:
            tools = await client.list_tools()
            logfire.info("server exposes {names}", names=[t.name for t in tools.tools])

            result = await client.call_tool("add", {"a": 2, "b": 3})
            logfire.info("add(2, 3) -> {content}", content=result.content)


asyncio.run(main())
```

Those are two separate processes. In Logfire they arrive as one trace:

```
mcp session                    [mcp-client]
  MCP send server/discover     [mcp-client]
    server/discover            [mcp-server]
  MCP send tools/list          [mcp-client]
    tools/list                 [mcp-server]
  server exposes ['add']       [mcp-client]
  MCP send tools/call add      [mcp-client]
    tools/call add             [mcp-server]
      add 2 + 3                [mcp-server]
  add(2, 3) -> ...             [mcp-client]
```

The client injects W3C trace context into the request's `_meta` and the server extracts it (SEP-414), so a tool call
made by an agent on one machine and executed on another is a single connected tree, with your own `add 2 + 3` span
nested underneath. If an inbound message has no trace context, say from a client that isn't the SDK, the server span
parents to whatever is current instead of starting an orphan trace.

Query that `tools/call add` span back out and here's what it carries:

```
gen_ai.operation.name = execute_tool
gen_ai.tool.name      = add
jsonrpc.request.id    = 3
mcp.method.name       = tools/call
mcp.protocol.version  = 2026-07-28
```

`mcp.method.name` and `mcp.protocol.version` are on every span, `jsonrpc.request.id` on every request, and
`tools/call` spans follow OpenTelemetry's GenAI semantic conventions. That last one is why your tool calls group in a
tracing UI the way any other agent's do, without extra code. A handler that raises sets the span status to error, and
so does a tool result with `is_error=True`.

Now go back to that dual-era example and look at it in Logfire. The legacy client shows the old back-channel, with the
server's elicitation nested *inside* the tool call:

```
MCP send tools/call reserve        req=2
  tools/call reserve               req=2   proto=2025-11-25
    MCP send elicitation/create    req=1
```

The modern client shows two `tools/call reserve` spans instead, with different request ids:

```
tools/call reserve                 req=2   proto=2026-07-28
tools/call reserve                 req=3   proto=2026-07-28
```

That second one is the retry. No nested call back to the client, just the tool being asked again with the answer
attached. Multi-round-trip requests are the kind of thing that's hard to reason about from the spec text alone, and
much easier to believe when you can see both shapes side by side.

:::note{title="Two gotchas"}
Set `distributed_tracing=True` on the **server** so Logfire adopts the incoming trace context. Without it, Logfire
warns about a propagated trace context it decided to ignore, and you get two disconnected traces.

Don't use `logfire.instrument_mcp()` with v2. It targets the v1 SDK and patches symbols the v2 rework removed. You
don't need it, because the SDK emits the spans itself.
:::

To preview spans locally without sending them anywhere, run with `LOGFIRE_SEND_TO_LOGFIRE=false LOGFIRE_CONSOLE=true`.

## Try it today, and tell us what breaks

The spec lands tomorrow, and the point of a beta is the feedback. If you maintain an MCP server or client in Python,
the most useful thing you can do today is pin `2.0.0rc1`, port something real, and tell us what hurt.

- Read [What's new in v2](https://py.sdk.modelcontextprotocol.io/v2/whats-new/) for the full tour, and the
  [migration guide](https://py.sdk.modelcontextprotocol.io/v2/migration/) for every breaking change
- File feedback with the [v2 feedback template](https://github.com/modelcontextprotocol/python-sdk/issues/new?template=v2-feedback.yaml)
- Or come argue with me in `#python-sdk-dev` on the [MCP Contributors Discord](https://discord.gg/6CSzBmMkjX)

If you want those traces in a real UI, Logfire has a [free tier](https://pydantic.dev/pricing), and the
[getting started guide](https://pydantic.dev/docs/logfire/get-started/) takes about two minutes.