Switch to immutable types and add typing.Final for constants per PEP 591

src/mcp/cli/claude.py

@@ -5,13 +5,13 @@
 import shutil
 import sys
 from pathlib import Path
-from typing import Any
+from typing import Any, Final
 
 from mcp.server.fastmcp.utilities.logging import get_logger
 
 logger = get_logger(__name__)
 
-MCP_PACKAGE = "mcp[cli]"
+MCP_PACKAGE: Final = "mcp[cli]"
 
 
 def get_claude_config_path() -> Path | None:

src/mcp/client/stdio/__init__.py

@@ -3,7 +3,7 @@
 import sys
 from contextlib import asynccontextmanager
 from pathlib import Path
-from typing import Literal, TextIO
+from typing import Final, Literal, TextIO
 
 import anyio
 import anyio.lowlevel
@@ -25,8 +25,8 @@
 logger = logging.getLogger(__name__)
 
 # Environment variables to inherit by default
-DEFAULT_INHERITED_ENV_VARS = (
-    [
+DEFAULT_INHERITED_ENV_VARS: Final = (
+    (
         "APPDATA",
         "HOMEDRIVE",
         "HOMEPATH",
@@ -39,13 +39,20 @@
         "TEMP",
         "USERNAME",
         "USERPROFILE",
-    ]
+    )
     if sys.platform == "win32"
-    else ["HOME", "LOGNAME", "PATH", "SHELL", "TERM", "USER"]
+    else (
+        "HOME",
+        "LOGNAME",
+        "PATH",
+        "SHELL",
+        "TERM",
+        "USER",
+    )
 )
 
 # Timeout for process termination before falling back to force kill
-PROCESS_TERMINATION_TIMEOUT = 2.0
+PROCESS_TERMINATION_TIMEOUT: Final = 2.0
 
 
 def get_default_environment() -> dict[str, str]:

src/mcp/client/streamable_http.py

@@ -11,6 +11,7 @@
 from contextlib import asynccontextmanager
 from dataclasses import dataclass
 from datetime import timedelta
+from typing import Final
 
 import anyio
 import httpx
@@ -39,15 +40,15 @@
 StreamReader = MemoryObjectReceiveStream[SessionMessage]
 GetSessionIdCallback = Callable[[], str | None]
 
-MCP_SESSION_ID = "mcp-session-id"
-MCP_PROTOCOL_VERSION = "mcp-protocol-version"
-LAST_EVENT_ID = "last-event-id"
-CONTENT_TYPE = "content-type"
-ACCEPT = "accept"
+MCP_SESSION_ID: Final = "mcp-session-id"
+MCP_PROTOCOL_VERSION: Final = "mcp-protocol-version"
+LAST_EVENT_ID: Final = "last-event-id"
+CONTENT_TYPE: Final = "content-type"
+ACCEPT: Final = "accept"
 
 
-JSON = "application/json"
-SSE = "text/event-stream"
+JSON: Final = "application/json"
+SSE: Final = "text/event-stream"
 
 
 class StreamableHTTPError(Exception):

src/mcp/server/auth/routes.py

@@ -1,5 +1,5 @@
 from collections.abc import Awaitable, Callable
-from typing import Any
+from typing import Any, Final
 
 from pydantic import AnyHttpUrl
 from starlette.middleware.cors import CORSMiddleware
@@ -46,10 +46,10 @@ def validate_issuer_url(url: AnyHttpUrl):
         raise ValueError("Issuer URL must not have a query string")
 
 
-AUTHORIZATION_PATH = "/authorize"
-TOKEN_PATH = "/token"
-REGISTRATION_PATH = "/register"
-REVOCATION_PATH = "/revoke"
+AUTHORIZATION_PATH: Final = "/authorize"
+TOKEN_PATH: Final = "/token"
+REGISTRATION_PATH: Final = "/register"
+REVOCATION_PATH: Final = "/revoke"
 
 
 def cors_middleware(

src/mcp/server/streamable_http.py

@@ -15,6 +15,7 @@
 from contextlib import asynccontextmanager
 from dataclasses import dataclass
 from http import HTTPStatus
+from typing import Final
 
 import anyio
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
@@ -48,20 +49,20 @@
 
 
 # Header names
-MCP_SESSION_ID_HEADER = "mcp-session-id"
-MCP_PROTOCOL_VERSION_HEADER = "mcp-protocol-version"
-LAST_EVENT_ID_HEADER = "last-event-id"
+MCP_SESSION_ID_HEADER: Final = "mcp-session-id"
+MCP_PROTOCOL_VERSION_HEADER: Final = "mcp-protocol-version"
+LAST_EVENT_ID_HEADER: Final = "last-event-id"
 
 # Content types
-CONTENT_TYPE_JSON = "application/json"
-CONTENT_TYPE_SSE = "text/event-stream"
+CONTENT_TYPE_JSON: Final = "application/json"
+CONTENT_TYPE_SSE: Final = "text/event-stream"
 
 # Special key for the standalone GET stream
-GET_STREAM_KEY = "_GET_stream"
+GET_STREAM_KEY: Final = "_GET_stream"
 
 # Session ID validation pattern (visible ASCII characters ranging from 0x21 to 0x7E)
 # Pattern ensures entire string contains only valid characters by using ^ and $ anchors
-SESSION_ID_PATTERN = re.compile(r"^[\x21-\x7E]+$")
+SESSION_ID_PATTERN: Final = re.compile(r"^[\x21-\x7E]+$")
 
 # Type aliases
 StreamId = str

src/mcp/shared/version.py

@@ -1,3 +1,5 @@
+from typing import Final
+
 from mcp.types import LATEST_PROTOCOL_VERSION
 
-SUPPORTED_PROTOCOL_VERSIONS: list[str] = ["2024-11-05", "2025-03-26", LATEST_PROTOCOL_VERSION]
+SUPPORTED_PROTOCOL_VERSIONS: Final = ("2024-11-05", "2025-03-26", LATEST_PROTOCOL_VERSION)

src/mcp/types.py

@@ -1,5 +1,5 @@
 from collections.abc import Callable
-from typing import Annotated, Any, Generic, Literal, TypeAlias, TypeVar
+from typing import Annotated, Any, Final, Generic, Literal, TypeAlias, TypeVar
 
 from pydantic import BaseModel, ConfigDict, Field, FileUrl, RootModel
 from pydantic.networks import AnyUrl, UrlConstraints
@@ -23,15 +23,15 @@
   not separate types in the schema.
 """
 
-LATEST_PROTOCOL_VERSION = "2025-06-18"
+LATEST_PROTOCOL_VERSION: Final = "2025-06-18"
 
 """
 The default negotiated version of the Model Context Protocol when no version is specified.
 We need this to satisfy the MCP specification, which requires the server to assume a
 specific version if none is provided by the client. See section "Protocol Version Header" at
 https://modelcontextprotocol.io/specification
 """
-DEFAULT_NEGOTIATED_VERSION = "2025-03-26"
+DEFAULT_NEGOTIATED_VERSION: Final = "2025-03-26"
 
 ProgressToken = str | int
 Cursor = str
@@ -147,15 +147,14 @@ class JSONRPCResponse(BaseModel):
 
 
 # SDK error codes
-CONNECTION_CLOSED = -32000
+CONNECTION_CLOSED: Final = -32000
 # REQUEST_TIMEOUT = -32001  # the typescript sdk uses this
-
 # Standard JSON-RPC error codes
-PARSE_ERROR = -32700
-INVALID_REQUEST = -32600
-METHOD_NOT_FOUND = -32601
-INVALID_PARAMS = -32602
-INTERNAL_ERROR = -32603
+PARSE_ERROR: Final = -32700
+INVALID_REQUEST: Final = -32600
+METHOD_NOT_FOUND: Final = -32601
+INVALID_PARAMS: Final = -32602
+INTERNAL_ERROR: Final = -32603
 
 
 class ErrorData(BaseModel):