Move the SEP-2663 wire models to mcp.shared.tasks and complete the task lifecycle

- The wire models (Task, CreateTaskResult, request params, typed request
  wrappers, and a lenient GetTaskResult parser) move to mcp.shared.tasks
  so client code can import them without reaching into the server tier;
  mcp.server.tasks re-exports the public names.
- Tasks(augment=...) scopes augmentation per request (SEP-2663: the
  server decides at its own discretion); None keeps augment-everything.
- A JSON-RPC error under augmentation now records a task born "failed"
  (error inlined on tasks/get with no result key, statusMessage carrying
  the diagnostic) and returns CreateTaskResult(status="failed"). Errors
  on every non-augmented path propagate unchanged.
- require_client_extension moves to mcp.server.extension so the extension
  tier no longer reaches into the composition tier; mcp.server.mcpserver
  keeps re-exporting it.

Author: maxisbey

docs/migration.md

@@ -481,7 +481,8 @@ another handler raises at construction. A `MethodBinding` may set
 `protocol_versions` to scope an extension method to specific wire versions
 (`frozenset()` is rejected — use `None` to admit every version); a request at
 any other version is `METHOD_NOT_FOUND`. An
-extension handler can call `mcp.server.mcpserver.require_client_extension(ctx, identifier)`
+extension handler can call `mcp.server.extension.require_client_extension(ctx, identifier)`
+(also re-exported from `mcp.server.mcpserver`)
 to reject a request with the `-32021` (missing required client capability) error
 when the client did not declare the extension.
 

src/mcp/server/extension.py

@@ -24,11 +24,17 @@
 from dataclasses import dataclass, field
 from typing import TYPE_CHECKING, Any
 
-from mcp_types import CallToolRequestParams
+from mcp_types import (
+    MISSING_REQUIRED_CLIENT_CAPABILITY,
+    CallToolRequestParams,
+    ClientCapabilities,
+    MissingRequiredClientCapabilityErrorData,
+)
 from mcp_types.methods import SPEC_CLIENT_METHODS
 from pydantic import BaseModel
 
 from mcp.server.context import CallNext, HandlerResult, ServerMiddleware, ServerRequestContext
+from mcp.shared.exceptions import MCPError
 
 if TYPE_CHECKING:
     from mcp.server.mcpserver.resources import Resource
@@ -167,6 +173,36 @@ async def intercept_tool_call(
         return await call_next(ctx)
 
 
+def require_client_extension(ctx: ServerRequestContext[Any, Any], identifier: str) -> None:
+    """Assert the connected client declared support for `identifier`.
+
+    Call this from an extension's handler or `intercept_tool_call` before
+    offering extension-specific behaviour. Raises `MCPError` with the
+    `-32021` (missing required client capability) code and a
+    `requiredCapabilities` payload when the client did not declare the
+    extension, per SEP-2133.
+
+    Args:
+        ctx: The current request context.
+        identifier: The extension identifier the client must have declared.
+
+    Raises:
+        MCPError: With code `MISSING_REQUIRED_CLIENT_CAPABILITY` if the client
+            did not advertise `identifier`.
+    """
+    client_params = ctx.session.client_params
+    declared = client_params.capabilities.extensions if client_params else None
+    if not declared or identifier not in declared:
+        data = MissingRequiredClientCapabilityErrorData(
+            required_capabilities=ClientCapabilities(extensions={identifier: {}})
+        )
+        raise MCPError(
+            code=MISSING_REQUIRED_CLIENT_CAPABILITY,
+            message=f"Client did not declare required extension {identifier!r}",
+            data=data.model_dump(by_alias=True, mode="json", exclude_none=True),
+        )
+
+
 def compose_tool_call_interceptor(extensions: Sequence[Extension]) -> ServerMiddleware[Any]:
     """Fold every extension's `intercept_tool_call` into one `ServerMiddleware`.
 

src/mcp/server/mcpserver/server.py

@@ -14,12 +14,10 @@
     INTERNAL_ERROR,
     INVALID_PARAMS,
     METHOD_NOT_FOUND,
-    MISSING_REQUIRED_CLIENT_CAPABILITY,
     Annotations,
     BlobResourceContents,
     CallToolRequestParams,
     CallToolResult,
-    ClientCapabilities,
     CompleteRequestParams,
     CompleteResult,
     Completion,
@@ -31,7 +29,6 @@
     ListResourcesResult,
     ListResourceTemplatesResult,
     ListToolsResult,
-    MissingRequiredClientCapabilityErrorData,
     PaginatedRequestParams,
     ReadResourceRequestParams,
     ReadResourceResult,
@@ -67,6 +64,9 @@
     compose_tool_call_interceptor,
     validate_extension_identifier,
 )
+from mcp.server.extension import (
+    require_client_extension as require_client_extension,
+)
 from mcp.server.lowlevel.helper_types import ReadResourceContents
 from mcp.server.lowlevel.server import LifespanResultT, Server
 from mcp.server.lowlevel.server import lifespan as default_lifespan
@@ -1256,33 +1256,3 @@ async def gated(ctx: ServerRequestContext[Any, Any], params: Any) -> HandlerResu
         return await method.handler(ctx, params)
 
     return gated
-
-
-def require_client_extension(ctx: ServerRequestContext[Any, Any], identifier: str) -> None:
-    """Assert the connected client declared support for `identifier`.
-
-    Call this from an extension's handler or `intercept_tool_call` before
-    offering extension-specific behaviour. Raises `MCPError` with the
-    `-32021` (missing required client capability) code and a
-    `requiredCapabilities` payload when the client did not declare the
-    extension, per SEP-2133.
-
-    Args:
-        ctx: The current request context.
-        identifier: The extension identifier the client must have declared.
-
-    Raises:
-        MCPError: With code `MISSING_REQUIRED_CLIENT_CAPABILITY` if the client
-            did not advertise `identifier`.
-    """
-    client_params = ctx.session.client_params
-    declared = client_params.capabilities.extensions if client_params else None
-    if not declared or identifier not in declared:
-        data = MissingRequiredClientCapabilityErrorData(
-            required_capabilities=ClientCapabilities(extensions={identifier: {}})
-        )
-        raise MCPError(
-            code=MISSING_REQUIRED_CLIENT_CAPABILITY,
-            message=f"Client did not declare required extension {identifier!r}",
-            data=data.model_dump(by_alias=True, mode="json", exclude_none=True),
-        )