FazBrowse GitHub Viewer | Trending |
URL:
| Home
Tools: [Download Repo ZIP]   [Original HTTPS Page]

docs: fill migration guide gaps surfaced by automated upgrade eval by maxisbey · Pull Request #2412 · modelcontextprotocol/python-sdk · GitHub

/ python-sdk Public

docs: fill migration guide gaps surfaced by automated upgrade eval #2412

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Merged
maxisbey merged 2 commits into from
Apr 9, 2026
Conversation 3 Commits 2 Checks 31 Files changed
Merged

docs: fill migration guide gaps surfaced by automated upgrade eval #2412

Changes from all commits
Commits
Show all changes
2 commits
Select commit Hold shift + click to select a range
3a52324
docs: fill migration guide gaps surfaced by upgrade eval
maxisbey Apr 9, 2026
409f98b
docs: address minor migration guide gaps from upgrade eval
maxisbey Apr 9, 2026
File filter

Filter by extension

Filter by extension .md  (1) All 1 file type selected
Viewed files
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Unified
Split
Hide whitespace
Diff view
Unified
Split
Hide whitespace
240 changes: 235 additions & 5 deletions docs/migration.md
Show comments Show annotations
View file
Open in desktop
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters. Learn more about bidirectional Unicode characters
Show hidden characters
Original file line number Diff line number Diff line change
Expand Up @@ -38,6 +38,7 @@
headers={"Authorization": "Bearer token"},
timeout=httpx.Timeout(30, read=300),
auth=my_auth,
follow_redirects=True,
)

async with http_client:
Expand All @@ -48,6 +49,8 @@
...
```

v1's internal client set `follow_redirects=True`; set it explicitly when supplying your own `httpx.AsyncClient` to preserve that behavior.

### `get_session_id` callback removed from `streamable_http_client`

The `get_session_id` callback (third element of the returned tuple) has been removed from `streamable_http_client`. The function now returns a 2-tuple `(read_stream, write_stream)` instead of a 3-tuple.
Expand Down Expand Up @@ -100,6 +103,8 @@

The `headers`, `timeout`, `sse_read_timeout`, and `auth` parameters have been removed from `StreamableHTTPTransport`. Configure these on the `httpx.AsyncClient` instead (see example above).

Note: `sse_client` retains its `headers`, `timeout`, `sse_read_timeout`, and `auth` parameters — only the streamable HTTP transport changed.

### Removed type aliases and classes

The following deprecated type aliases and classes have been removed from `mcp.types`:
Expand All @@ -126,6 +131,52 @@
# Use `str` instead of `Cursor` for pagination cursors
```

### Field names changed from camelCase to snake_case

All Pydantic model fields in `mcp.types` now use snake_case names for Python attribute access. The JSON wire format is unchanged — serialization still uses camelCase via Pydantic aliases.

**Before (v1):**

```python
result = await session.call_tool("my_tool", {"x": 1})
if result.isError:
...

tools = await session.list_tools()
cursor = tools.nextCursor
schema = tools.tools[0].inputSchema
```

**After (v2):**

```python
result = await session.call_tool("my_tool", {"x": 1})
if result.is_error:
...

tools = await session.list_tools()
cursor = tools.next_cursor
schema = tools.tools[0].input_schema
```

Common renames:

| v1 (camelCase) | v2 (snake_case) |
|----------------|-----------------|
| `inputSchema` | `input_schema` |
| `outputSchema` | `output_schema` |
| `isError` | `is_error` |
| `nextCursor` | `next_cursor` |
| `mimeType` | `mime_type` |
| `structuredContent` | `structured_content` |
| `serverInfo` | `server_info` |
| `protocolVersion` | `protocol_version` |
| `uriTemplate` | `uri_template` |
| `listChanged` | `list_changed` |
| `progressToken` | `progress_token` |

Because `populate_by_name=True` is set, the old camelCase names still work as constructor kwargs (e.g., `Tool(inputSchema={...})` is accepted), but attribute access must use snake_case (`tool.input_schema`).

### `args` parameter removed from `ClientSessionGroup.call_tool()`

The deprecated `args` parameter has been removed from `ClientSessionGroup.call_tool()`. Use `arguments` instead.
Expand Down Expand Up @@ -225,6 +276,28 @@
from mcp import MCPError
```

The constructor signature also changed — it now takes `code`, `message`, and optional `data` directly instead of wrapping an `ErrorData`:

**Before (v1):**

```python
from mcp.shared.exceptions import McpError
from mcp.types import ErrorData, INVALID_REQUEST

raise McpError(ErrorData(code=INVALID_REQUEST, message="bad input"))
```

**After (v2):**

```python
from mcp.shared.exceptions import MCPError
from mcp.types import INVALID_REQUEST

raise MCPError(INVALID_REQUEST, "bad input")
# or, if you already have an ErrorData:
raise MCPError.from_error_data(error_data)
```

### `FastMCP` renamed to `MCPServer`

The `FastMCP` class has been renamed to `MCPServer` to better reflect its role as the main server class in the SDK. This is a simple rename with no functional changes to the class itself.
Expand All @@ -240,11 +313,19 @@
**After (v2):**

```python
from mcp.server.mcpserver import MCPServer
from mcp.server.mcpserver import MCPServer, Context

mcp = MCPServer("Demo")
```

`Context` is the type annotation for the `ctx` parameter injected into tools, resources, and prompts (see [`get_context()` removed](#mcpserverget_context-removed) below).

All submodules under `mcp.server.fastmcp.*` are now under `mcp.server.mcpserver.*` with the same structure. Common imports:

- `Image`, `Audio` — from `mcp.server.mcpserver` (or `.utilities.types`)
- `UserMessage`, `AssistantMessage` — from `mcp.server.mcpserver.prompts.base`
- `ToolError`, `ResourceError` — from `mcp.server.mcpserver.exceptions`

### `mount_path` parameter removed from MCPServer

The `mount_path` parameter has been removed from `MCPServer.__init__()`, `MCPServer.run()`, `MCPServer.run_sse_async()`, and `MCPServer.sse_app()`. It was also removed from the `Settings` class.
Expand Down Expand Up @@ -312,6 +393,8 @@

**Note:** DNS rebinding protection is automatically enabled when `host` is `127.0.0.1`, `localhost`, or `::1`. This now happens in `sse_app()` and `streamable_http_app()` instead of the constructor.

If you were mutating these via `mcp.settings` after construction (e.g., `mcp.settings.port = 9000`), pass them to `run()` / `sse_app()` / `streamable_http_app()` instead — these fields no longer exist on `Settings`. The `debug` and `log_level` parameters remain on the constructor.

### `MCPServer.get_context()` removed

`MCPServer.get_context()` has been removed. Context is now injected by the framework and passed explicitly — there is no ambient ContextVar to read from.
Expand All @@ -331,6 +414,8 @@
**After (v2):**

```python
from mcp.server.mcpserver import Context

@mcp.tool()
async def my_tool(x: int, ctx: Context) -> str:
await ctx.info("Processing...")
Expand All @@ -343,6 +428,45 @@

The internal layers (`ToolManager.call_tool`, `Tool.run`, `Prompt.render`, `ResourceTemplate.create_resource`, etc.) now require `context` as a positional argument.

### Registering lowlevel handlers on `MCPServer` (workaround)

`MCPServer` does not expose public APIs for `subscribe_resource`, `unsubscribe_resource`, or `set_logging_level` handlers. In v1, the workaround was to reach into the private lowlevel server and use its decorator methods:

**Before (v1):**

```python
@mcp._mcp_server.set_logging_level() # pyright: ignore[reportPrivateUsage]
async def handle_set_logging_level(level: str) -> None:
...

mcp._mcp_server.subscribe_resource()(handle_subscribe) # pyright: ignore[reportPrivateUsage]
```

In v2, the lowlevel `Server` no longer has decorator methods (handlers are constructor-only), so the equivalent workaround is `_add_request_handler`:

**After (v2):**

```python
from mcp.server import ServerRequestContext
from mcp.types import EmptyResult, SetLevelRequestParams, SubscribeRequestParams


async def handle_set_logging_level(ctx: ServerRequestContext, params: SetLevelRequestParams) -> EmptyResult:
...
return EmptyResult()


async def handle_subscribe(ctx: ServerRequestContext, params: SubscribeRequestParams) -> EmptyResult:
...
return EmptyResult()


mcp._lowlevel_server._add_request_handler("logging/setLevel", handle_set_logging_level) # pyright: ignore[reportPrivateUsage]
mcp._lowlevel_server._add_request_handler("resources/subscribe", handle_subscribe) # pyright: ignore[reportPrivateUsage]
```

This is a private API and may change. A public way to register these handlers on `MCPServer` is planned; until then, use this workaround or use the lowlevel `Server` directly.

### Replace `RootModel` by union types with `TypeAdapter` validation

The following union types are no longer `RootModel` subclasses:
Expand Down Expand Up @@ -383,6 +507,22 @@
# No .root access needed - notification is the actual type
```

The same applies when constructing values — the wrapper call is no longer needed:

**Before (v1):**

```python
await session.send_notification(ClientNotification(InitializedNotification()))
await session.send_request(ClientRequest(PingRequest()), EmptyResult)
```

**After (v2):**

```python
await session.send_notification(InitializedNotification())
await session.send_request(PingRequest(), EmptyResult)
```

**Available adapters:**

| Union Type | Adapter |
Expand Down Expand Up @@ -428,6 +568,8 @@

### `RequestContext` type parameters simplified

The `mcp.shared.context` module has been removed. `RequestContext` is now split into `ClientRequestContext` (in `mcp.client.context`) and `ServerRequestContext` (in `mcp.server.context`).

The `RequestContext` class has been split to separate shared fields from server-specific fields. The shared `RequestContext` now only takes 1 type parameter (the session type) instead of 3.

**`RequestContext` changes:**
Expand Down Expand Up @@ -455,17 +597,33 @@
ctx: ClientRequestContext

# For server-specific context with lifespan and request types
server_ctx: ServerRequestContext[LifespanContextT, RequestT]
```

The high-level `Context` class (injected into `@mcp.tool()` etc.) similarly dropped its `ServerSessionT` parameter: `Context[ServerSessionT, LifespanContextT, RequestT]` → `Context[LifespanContextT, RequestT]`. Both remaining parameters have defaults, so bare `Context` is usually sufficient:

**Before (v1):**

```python
async def my_tool(ctx: Context[ServerSession, None]) -> str: ...
```

**After (v2):**

```python
async def my_tool(ctx: Context) -> str: ...
# or, with an explicit lifespan type:
async def my_tool(ctx: Context[MyLifespanState]) -> str: ...
```

### `ProgressContext` and `progress()` context manager removed

The `mcp.shared.progress` module (`ProgressContext`, `Progress`, and the `progress()` context manager) has been removed. This module had no real-world adoption — all users send progress notifications via `Context.report_progress()` or `session.send_progress_notification()` directly.

**Before:**
**Before (v1):**

```python
from mcp.shared.progress import progress

Check warning on line 626 in docs/migration.md

View check run for this annotation

Claude / Claude Code Review

Context generic migration loses LifespanContextT=None precision 

The migration guide shows `Context[ServerSession, None]` → bare `Context`, but this silently changes `LifespanContextT` from `None` to `dict[str, Any]` for type-checker users. The type-accurate v2 equivalent of `Context[ServerSession, None]` is `Context[None]`, not bare `Context`.
Comment on lines 600 to 626

claude Bot Apr 9, 2026

Copy link
Copy Markdown
Contributor

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

🟡 The migration guide shows Context[ServerSession, None] → bare Context, but this silently changes LifespanContextT from None to dict[str, Any] for type-checker users. The type-accurate v2 equivalent of Context[ServerSession, None] is Context[None], not bare Context.

Extended reasoning...

What the bug is: In the migration guide's "Context generic params" section (lines 600–626), the before example is Context[ServerSession, None] where the second type param is LifespanContextT=None. The after example shows bare Context, which in v2 resolves to Context[dict[str, Any], Any] because LifespanContextT has a default of dict[str, Any].

The specific code path: In src/mcp/server/context.py, LifespanContextT is defined as TypeVar('LifespanContextT', default=dict[str, Any]). So bare Context expands to Context[dict[str, Any], Any] under a type checker, not Context[None, Any].

Why existing doc text doesn't prevent it: The guide says bare Context is "usually sufficient", which partially mitigates the concern, but the direct before/after pairing of Context[ServerSession, None] → Context implies they are type-equivalent when they are not. Users following this one-to-one mapping will silently change LifespanContextT from None to dict[str, Any].

Impact: Runtime behavior is completely unaffected — TypeVar defaults only matter to static type checkers. However, users who had Context[ServerSession, None] specifically to signal "no lifespan" and follow this guide will end up with Context (i.e., Context[dict[str, Any], Any]), which could cause false type-checker positives or negatives if they try to access ctx.request_context.lifespan_context and expect it to be None.

How to fix: Change the after example to show Context[None] as the direct type-preserving equivalent, and note bare Context separately as a convenience when lifespan type precision is not required:

# After (v2) — type-preserving equivalent:
async def my_tool(ctx: Context[None]) -> str: ...
# or, if lifespan type precision is not needed:
async def my_tool(ctx: Context) -> str: ...

Step-by-step proof:

  1. v1 code: async def my_tool(ctx: Context[ServerSession, None]) -> str: ... — here LifespanContextT = None
  2. Migration guide says: migrate to async def my_tool(ctx: Context) -> str: ...
  3. In v2, Context without type params uses TypeVar defaults: LifespanContextT = TypeVar('LifespanContextT', default=dict[str, Any])
  4. So bare Context = Context[dict[str, Any], Any], not Context[None, Any]
  5. The type-preserving equivalent is Context[None] (keeping LifespanContextT=None), which the guide does not show as an option

with progress(ctx, total=100) as p:
await p.progress(25)
Expand All @@ -490,6 +648,46 @@
)
```

### `create_connected_server_and_client_session` removed

The `create_connected_server_and_client_session` helper in `mcp.shared.memory` has been removed. Use `mcp.client.Client` instead — it accepts a `Server` or `MCPServer` instance directly and handles the in-memory transport and session setup for you.

**Before (v1):**

```python
from mcp.shared.memory import create_connected_server_and_client_session

async with create_connected_server_and_client_session(server) as session:
result = await session.call_tool("my_tool", {"x": 1})
```

**After (v2):**

```python
from mcp.client import Client

async with Client(server) as client:
result = await client.call_tool("my_tool", {"x": 1})
```

`Client` accepts the same callback parameters the old helper did (`sampling_callback`, `list_roots_callback`, `logging_callback`, `message_handler`, `elicitation_callback`, `client_info`) plus `raise_exceptions` to surface server-side errors.

If you need direct access to the underlying `ClientSession` and memory streams (e.g., for low-level transport testing), `create_client_server_memory_streams` is still available in `mcp.shared.memory`:

```python
import anyio
from mcp.client.session import ClientSession
from mcp.shared.memory import create_client_server_memory_streams

async with create_client_server_memory_streams() as (client_streams, server_streams):
async with anyio.create_task_group() as tg:
tg.start_soon(lambda: server.run(*server_streams, server.create_initialization_options()))
async with ClientSession(*client_streams) as session:
await session.initialize()
...
tg.cancel_scope.cancel()
```

### Resource URI type changed from `AnyUrl` to `str`

The `uri` field on resource-related types now uses `str` instead of Pydantic's `AnyUrl`. This aligns with the [MCP specification schema](https://github.com/modelcontextprotocol/modelcontextprotocol/blob/main/schema/draft/schema.ts) which defines URIs as plain strings (`uri: string`) without strict URL validation. This change allows relative paths like `users/me` that were previously rejected.
Expand Down Expand Up @@ -593,6 +791,8 @@
server = Server("my-server", on_list_tools=handle_list_tools)
```

If you need to check whether a handler is registered, track this yourself — there is currently no public introspection API.

### Lowlevel `Server`: decorator-based handlers replaced with constructor `on_*` params

The lowlevel `Server` class no longer uses decorator methods for handler registration. Instead, handlers are passed as `on_*` keyword arguments to the constructor.
Expand Down Expand Up @@ -645,6 +845,29 @@
- Handlers return the full result type (e.g. `ListToolsResult`) rather than unwrapped values (e.g. `list[Tool]`).
- The automatic `jsonschema` input/output validation that the old `call_tool()` decorator performed has been removed. There is no built-in replacement — if you relied on schema validation in the lowlevel server, you will need to validate inputs yourself in your handler.

**Complete handler reference:**

All handlers receive `ctx: ServerRequestContext` as the first argument. The second argument and return type are:

| v1 decorator | v2 constructor kwarg | `params` type | return type |
|---|---|---|---|
| `@server.list_tools()` | `on_list_tools` | `PaginatedRequestParams \| None` | `ListToolsResult` |
| `@server.call_tool()` | `on_call_tool` | `CallToolRequestParams` | `CallToolResult \| CreateTaskResult` |
| `@server.list_resources()` | `on_list_resources` | `PaginatedRequestParams \| None` | `ListResourcesResult` |
| `@server.list_resource_templates()` | `on_list_resource_templates` | `PaginatedRequestParams \| None` | `ListResourceTemplatesResult` |
| `@server.read_resource()` | `on_read_resource` | `ReadResourceRequestParams` | `ReadResourceResult` |
| `@server.subscribe_resource()` | `on_subscribe_resource` | `SubscribeRequestParams` | `EmptyResult` |
| `@server.unsubscribe_resource()` | `on_unsubscribe_resource` | `UnsubscribeRequestParams` | `EmptyResult` |
| `@server.list_prompts()` | `on_list_prompts` | `PaginatedRequestParams \| None` | `ListPromptsResult` |
| `@server.get_prompt()` | `on_get_prompt` | `GetPromptRequestParams` | `GetPromptResult` |
| `@server.completion()` | `on_completion` | `CompleteRequestParams` | `CompleteResult` |
| `@server.set_logging_level()` | `on_set_logging_level` | `SetLevelRequestParams` | `EmptyResult` |
| — | `on_ping` | `RequestParams \| None` | `EmptyResult` |
| `@server.progress_notification()` | `on_progress` | `ProgressNotificationParams` | `None` |
| — | `on_roots_list_changed` | `NotificationParams \| None` | `None` |

All `params` and return types are importable from `mcp.types`.

**Notification handlers:**

```python
Expand Down Expand Up @@ -694,10 +917,17 @@

**`read_resource()` — content type wrapping removed:**

The old decorator auto-wrapped `str` into `TextResourceContents` and `bytes` into `BlobResourceContents` (with base64 encoding), and applied a default mime type of `text/plain`:
The old decorator auto-wrapped `Iterable[ReadResourceContents]` (and the deprecated `str`/`bytes` shorthand) into `TextResourceContents`/`BlobResourceContents`, handling base64 encoding and mime-type defaulting:

```python
# Before (v1) — str/bytes auto-wrapped with mime type defaulting
# Before (v1) — Iterable[ReadResourceContents] auto-wrapped
from mcp.server.lowlevel.helper_types import ReadResourceContents

@server.read_resource()
async def handle(uri: AnyUrl) -> Iterable[ReadResourceContents]:
return [ReadResourceContents(content="file contents", mime_type="text/plain")]

# Before (v1) — str/bytes shorthand (already deprecated in v1)
@server.read_resource()
async def handle(uri: str) -> str:
return "file contents"
Expand Down Expand Up @@ -849,7 +1079,7 @@
params = CallToolRequestParams(
name="my_tool",
arguments={},
_meta={"progressToken": "tok", "customField": "value"}, # OK
_meta={"my_custom_key": "value", "another": 123}, # OK
)
```

Expand Down
Loading
Back | FazBrowse Home | New Git URL