Stop advertising MCP_* env vars for MCPServer settings; drop pydantic-settings (#3170)

Author: maxisbey

docs/get-started/installation.md

@@ -34,7 +34,6 @@ You don't need to know any of this to use the SDK, but if you're wondering what
 * `mcp-types`: every protocol type (requests, results, content blocks) as its own package, versioned in lockstep with the SDK. Every `from mcp_types import ...` in these docs is this package.
 * [`anyio`](https://anyio.readthedocs.io/): the async runtime. The whole SDK is written against anyio, so it runs on either `asyncio` or `trio`.
 * [`pydantic`](https://docs.pydantic.dev/): what every `mcp_types` model is built on, plus all schema generation and validation.
-* [`pydantic-settings`](https://docs.pydantic.dev/latest/concepts/pydantic_settings/): server configuration via `MCP_*` environment variables and `.env` files.
 * [`httpx2`](https://pypi.org/project/httpx2/): the HTTP client behind the Streamable HTTP and SSE *client* transports, with server-sent events support built in.
 * [`starlette`](https://www.starlette.io/), [`uvicorn`](https://www.uvicorn.org/), [`sse-starlette`](https://pypi.org/project/sse-starlette/), and [`python-multipart`](https://pypi.org/project/python-multipart/): the HTTP *server* transports.
 * [`jsonschema`](https://pypi.org/project/jsonschema/): validates a tool's structured output against its declared output schema.

docs/migration.md

@@ -684,6 +684,22 @@ app = Starlette(routes=[Mount("/", app=mcp.streamable_http_app(json_response=Tru
 
 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.
 
+### `MCP_*` environment variables and `.env` files are no longer read
+
+The `Settings` docstring advertised configuration via `MCP_*` environment variables and a `.env` file (e.g. `MCP_DEBUG=true`), but constructor arguments have always taken precedence, so those environment variables never took effect. `Settings` is now a plain Pydantic model rather than a `pydantic-settings` `BaseSettings`, and `pydantic-settings` is no longer a dependency of the SDK.
+
+If you want environment-driven configuration, read the environment yourself and pass the values to the constructor:
+
+```python
+import os
+
+from mcp.server.mcpserver import MCPServer
+
+mcp = MCPServer("Demo", debug=os.environ.get("MCP_DEBUG") == "true")
+```
+
+If your own code uses `pydantic-settings`, add it to your project's dependencies directly.
+
 ### Streamable HTTP request bodies are limited to 4 MiB
 
 V2 applies a 4 MiB default limit to Streamable HTTP POST bodies and returns HTTP 413 before parsing

pyproject.toml

@@ -58,6 +58,9 @@ dev = [
     # We add mcp[cli] so `uv sync` considers the extras.
     "mcp[cli]",
     "mcp-example-stories",
+    # pydantic-settings is used only by examples (simple-auth, mcpserver/text_me);
+    # keep it reachable for pyright, which covers examples/servers.
+    "pydantic-settings>=2.5.2",
     "tomli>=2.0; python_version < '3.11'",
     "pyright>=1.1.400",
     "pytest>=8.4.0",
@@ -128,7 +131,6 @@ dependencies = [
     "starlette>=0.27; python_version < '3.14'",
     "python-multipart>=0.0.9",
     "sse-starlette>=3.0.0",
-    "pydantic-settings>=2.5.2",
     "uvicorn>=0.31.1; sys_platform != 'emscripten'",
     "jsonschema>=4.20.0",
     "pywin32>=311; sys_platform == 'win32'",

src/mcp/server/mcpserver/server.py

@@ -44,8 +44,8 @@
 from mcp_types import Resource as MCPResource
 from mcp_types import ResourceTemplate as MCPResourceTemplate
 from mcp_types import Tool as MCPTool
+from pydantic import BaseModel
 from pydantic.networks import AnyUrl
-from pydantic_settings import BaseSettings, SettingsConfigDict
 from starlette.applications import Starlette
 from starlette.middleware import Middleware
 from starlette.middleware.authentication import AuthenticationMiddleware
@@ -98,20 +98,8 @@
 _CallableT = TypeVar("_CallableT", bound=Callable[..., Any])
 
 
-class Settings(BaseSettings, Generic[LifespanResultT]):
-    """MCPServer settings.
-
-    All settings can be configured via environment variables with the prefix MCP_.
-    For example, MCP_DEBUG=true will set debug=True.
-    """
-
-    model_config = SettingsConfigDict(
-        env_prefix="MCP_",
-        env_file=".env",
-        env_nested_delimiter="__",
-        nested_model_default_partial_update=True,
-        extra="ignore",
-    )
+class Settings(BaseModel, Generic[LifespanResultT]):
+    """MCPServer settings, as passed to the `MCPServer` constructor."""
 
     # Server settings
     debug: bool