feat: subscription types (T6)

core/src/main/scala/chimp/protocol/Subscriptions.scala

@@ -0,0 +1,34 @@
+package chimp.protocol
+
+import io.circe.{Codec, Json}
+
+/** The notification types a client opts into on a `subscriptions/listen` stream (2026-07-28). Each type is opt-in: the server MUST NOT send
+  * a type the client has not requested. `resourceSubscriptions` replaces the former `resources/subscribe` RPC.
+  */
+final case class SubscriptionFilter(
+    promptsListChanged: Option[Boolean] = None,
+    resourceSubscriptions: Option[List[String]] = None,
+    resourcesListChanged: Option[Boolean] = None,
+    toolsListChanged: Option[Boolean] = None
+) derives Codec
+
+/** Params of a `subscriptions/listen` request: the notification filter the client opts into on the stream. */
+final case class SubscriptionsListenParams(notifications: SubscriptionFilter, _meta: Option[Map[String, Json]] = None) derives Codec
+
+final case class SubscriptionsListenRequest(method: String = "subscriptions/listen", params: SubscriptionsListenParams) derives Codec
+
+/** Result of `subscriptions/listen`, sent only when the server tears the subscription down gracefully (e.g. on shutdown); the body is
+  * otherwise empty. The subscription id travels in `_meta` under `io.modelcontextprotocol/subscriptionId`.
+  */
+final case class SubscriptionsListenResult(resultType: ResultType = ResultType.Complete, _meta: Option[Map[String, Json]] = None)
+    derives Codec
+
+/** Params of the `notifications/subscriptions/acknowledged` notification: the subset of requested notification types the server will honor.
+  */
+final case class SubscriptionsAcknowledgedParams(notifications: SubscriptionFilter, _meta: Option[Map[String, Json]] = None) derives Codec
+
+/** The `notifications/subscriptions/acknowledged` notification the server sends first on a listen stream. */
+final case class SubscriptionsAcknowledgedNotification(
+    method: String = "notifications/subscriptions/acknowledged",
+    params: SubscriptionsAcknowledgedParams
+) derives Codec

core/src/main/scala/chimp/protocol/Versioning.scala

@@ -17,6 +17,7 @@ object ProtocolMeta:
   val ClientInfo: String = "io.modelcontextprotocol/clientInfo"
   val ClientCapabilities: String = "io.modelcontextprotocol/clientCapabilities"
   val ServerInfo: String = "io.modelcontextprotocol/serverInfo"
+  val SubscriptionId: String = "io.modelcontextprotocol/subscriptionId"
 
   /** An `UnsupportedProtocolVersion` error (`-32022`) naming the versions the server supports, so the client can retry with one of them. */
   def unsupportedVersionError(requested: String, supported: List[String]): JSONRPCErrorObject =

core/src/test/scala/chimp/protocol/Schema20260728ConformanceSpec.scala

@@ -34,3 +34,37 @@ class Schema20260728ConformanceSpec extends SchemaConformance:
     val msg: JSONRPCMessage =
       JSONRPCMessage.Error(id = RequestId(1), error = ProtocolMeta.unsupportedVersionError("1900-01-01", List("2026-07-28", "2025-11-25")))
     validate("UnsupportedProtocolVersionError", msg)
+
+  it should "produce a SubscriptionFilter that matches the spec schema" in:
+    validate(
+      "SubscriptionFilter",
+      SubscriptionFilter(resourceSubscriptions = Some(List("file:///x")), resourcesListChanged = Some(true), toolsListChanged = Some(false))
+    )
+
+  it should "produce SubscriptionsListenRequestParams that match the spec schema" in:
+    validate(
+      "SubscriptionsListenRequestParams",
+      SubscriptionsListenParams(
+        notifications = SubscriptionFilter(resourcesListChanged = Some(true)),
+        _meta = Some(Map(ProtocolMeta.ProtocolVersionKey -> "2026-07-28".asJson, ProtocolMeta.ClientCapabilities -> io.circe.Json.obj()))
+      )
+    )
+
+  it should "produce a SubscriptionsListenResult that matches the spec schema" in:
+    validate(
+      "SubscriptionsListenResult",
+      SubscriptionsListenResult(_meta =
+        Some(
+          Map(
+            ProtocolMeta.ServerInfo -> Implementation(name = "chimp", version = "1.0").asJson.deepDropNullValues,
+            ProtocolMeta.SubscriptionId -> "sub-1".asJson
+          )
+        )
+      )
+    )
+
+  it should "produce SubscriptionsAcknowledgedNotificationParams that match the spec schema" in:
+    validate(
+      "SubscriptionsAcknowledgedNotificationParams",
+      SubscriptionsAcknowledgedParams(notifications = SubscriptionFilter(promptsListChanged = Some(true)))
+    )