feat(core): relax extension() Local constraint from JSONObject to object

Interfaces have no implicit index signature (TS#15300), so capability
shapes declared as `interface` were rejected. JSONObject did not actually
enforce serializability; the cast moves to one place inside the SDK
instead of every consumer.

Author: felixweinberger

packages/client/src/client/client.ts

@@ -321,21 +321,21 @@ export class Client extends Protocol<ClientContext> {
      * {@linkcode ExtensionHandle.getPeerSettings | handle.getPeerSettings()} returns the server's
      * `capabilities.extensions[id]` blob (validated against `peerSchema` if provided).
      */
-    public extension<L extends JSONObject>(id: string, settings: L): ExtensionHandle<L, JSONObject, ClientContext>;
-    public extension<L extends JSONObject, P extends AnySchema>(
+    public extension<L extends object>(id: string, settings: L): ExtensionHandle<L, JSONObject, ClientContext>;
+    public extension<L extends object, P extends AnySchema>(
         id: string,
         settings: L,
         opts: ExtensionOptions<P>
     ): ExtensionHandle<L, SchemaOutput<P>, ClientContext>;
-    public extension<L extends JSONObject, P extends AnySchema>(
+    public extension<L extends object, P extends AnySchema>(
         id: string,
         settings: L,
         opts?: ExtensionOptions<P>
     ): ExtensionHandle<L, SchemaOutput<P> | JSONObject, ClientContext> {
         if (this.transport) {
             throw new SdkError(SdkErrorCode.AlreadyConnected, 'Cannot register extension after connecting to transport');
         }
-        this._capabilities.extensions = { ...this._capabilities.extensions, [id]: settings };
+        this._capabilities.extensions = { ...this._capabilities.extensions, [id]: settings as JSONObject };
         return new ExtensionHandle(
             this,
             id,

packages/core/src/shared/extensionHandle.ts

@@ -56,7 +56,7 @@ export interface ExtensionOptions<P extends AnySchema> {
  * did not advertise the same extension ID; when lax (the default), sends proceed regardless and
  * {@linkcode getPeerSettings} returns `undefined`.
  */
-export class ExtensionHandle<Local extends JSONObject, Peer = JSONObject, ContextT extends BaseContext = BaseContext> {
+export class ExtensionHandle<Local extends object, Peer = JSONObject, ContextT extends BaseContext = BaseContext> {
     private _peerSettingsCache?: { value: Peer | undefined };
 
     /**

packages/core/test/shared/extensionHandle.test.ts

@@ -27,7 +27,7 @@ function makeHandle(opts: { peer?: JSONObject | undefined; strict?: boolean; pee
     host: MockHost;
     handle: ExtensionHandle<JSONObject, unknown, BaseContext>;
 } {
-    const host = makeMockHost();
+    const host = makeMockHost() as unknown as ExtensionHost<BaseContext>;
     const handle = new ExtensionHandle(
         host as unknown as ExtensionHost<BaseContext>,
         'io.example/ui',
@@ -146,3 +146,15 @@ describe('ExtensionHandle — id and settings', () => {
         expect(handle.settings).toEqual({ local: true });
     });
 });
+
+describe('ExtensionHandle — accepts interface-typed settings', () => {
+    test('compiles with an interface (no implicit index signature)', () => {
+        interface MyCaps {
+            featureA: boolean;
+        }
+        const caps: MyCaps = { featureA: true };
+        const host = makeMockHost() as unknown as ExtensionHost<BaseContext>;
+        const handle = new ExtensionHandle(host, 'test/iface', caps, () => undefined, false);
+        expect(handle.settings.featureA).toBe(true);
+    });
+});

packages/server/src/server/server.ts

@@ -235,21 +235,21 @@ export class Server extends Protocol<ServerContext> {
      * {@linkcode ExtensionHandle.getPeerSettings | handle.getPeerSettings()} returns the client's
      * `capabilities.extensions[id]` blob (validated against `peerSchema` if provided).
      */
-    public extension<L extends JSONObject>(id: string, settings: L): ExtensionHandle<L, JSONObject, ServerContext>;
-    public extension<L extends JSONObject, P extends AnySchema>(
+    public extension<L extends object>(id: string, settings: L): ExtensionHandle<L, JSONObject, ServerContext>;
+    public extension<L extends object, P extends AnySchema>(
         id: string,
         settings: L,
         opts: ExtensionOptions<P>
     ): ExtensionHandle<L, SchemaOutput<P>, ServerContext>;
-    public extension<L extends JSONObject, P extends AnySchema>(
+    public extension<L extends object, P extends AnySchema>(
         id: string,
         settings: L,
         opts?: ExtensionOptions<P>
     ): ExtensionHandle<L, SchemaOutput<P> | JSONObject, ServerContext> {
         if (this.transport) {
             throw new SdkError(SdkErrorCode.AlreadyConnected, 'Cannot register extension after connecting to transport');
         }
-        this._capabilities.extensions = { ...this._capabilities.extensions, [id]: settings };
+        this._capabilities.extensions = { ...this._capabilities.extensions, [id]: settings as JSONObject };
         return new ExtensionHandle(
             this,
             id,