[Schema][Server] Add the extensions framework SEP-2133 defines (#443)

* [Schema][Server] Add the extensions framework SEP-2133 defines

enableExtension() took any string and advertised it, with no way for an
extension to add a method: MessageFactory could not decode one, so nothing
downstream ever saw it.

Identifiers are now checked against the _meta key naming rules through
ExtensionIdentifier - a prefix is mandatory, since an unprefixed name has no
owner, and the modelcontextprotocol/mcp second labels the specification
reserves are recognised. An extension implementing
MethodProvidingExtensionInterface contributes both its message classes and
the handlers serving them.

MessageFactory::make() takes an $additional list of message classes, and
RequestHandlerInterface's result template is covariant so a handler
declaring a concrete result satisfies a collection typed by the interface.

* Fix stale @see reference in MethodProvidingExtensionInterface docblock

* [Schema][Server][Client] Make ExtensionIdentifier a value object

ExtensionInterface::getId() now returns ExtensionIdentifier, which
validates the SEP-2133 naming rules at construction instead of the
callers checking it. getMessages()/getRequestHandlers() move into
ExtensionInterface itself; AbstractExtension gives them empty
defaults for extensions that only announce a capability.
This commit is contained in:
Christopher Hertel 2026-08-18 20:47:27 +02:00 • committed by GitHub
commit 60711951d4
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@ -14,6 +14,7 @@ namespace Mcp\Client;
use Mcp\Client;
use Mcp\Client\Handler\Notification\NotificationHandlerInterface;
use Mcp\Client\Handler\Request\RequestHandlerInterface;
use Mcp\Exception\InvalidArgumentException;
use Mcp\Exception\LogicException;
use Mcp\Schema\ClientCapabilities;
use Mcp\Schema\Enum\ProtocolVersion;
@ -88,12 +89,13 @@ final class Builder
* Enable one or more MCP protocol extensions, announced to the server under
* `capabilities.extensions` in the initialize request.
*
* @throws LogicException if the same extension is enabled more than once
* @throws InvalidArgumentException if the identifier is not a valid `_meta` prefix
* @throws LogicException if the same extension is enabled more than once
*/
public function enableExtension(ExtensionInterface ...$extensions): self
{
foreach ($extensions as $extension) {
$id = $extension->getId();
$id = (string) $extension->getId();
if (isset($this->extensions[$id])) {
throw new LogicException(\sprintf('Extension "%s" is already enabled.', $id));