feat(core): add native skill activation

packages/protocol/src/errors.ts

@@ -80,6 +80,15 @@ export class MessageNotFoundError extends Schema.TaggedErrorClass<MessageNotFoun
   { httpApiStatus: 404 },
 ) {}
 
+export class SkillNotFoundError extends Schema.TaggedErrorClass<SkillNotFoundError>()(
+  "SkillNotFoundError",
+  {
+    skill: Schema.String,
+    message: Schema.String,
+  },
+  { httpApiStatus: 404 },
+) {}
+
 export class InvalidCursorError extends Schema.TaggedErrorClass<InvalidCursorError>()(
   "InvalidCursorError",
   { message: Schema.String },

packages/protocol/src/groups/session.ts

@@ -15,6 +15,7 @@ import {
   ServiceUnavailableError,
   SessionBusyError,
   SessionNotFoundError,
+  SkillNotFoundError,
   UnknownError,
 } from "../errors"
 import { Agent } from "@opencode-ai/schema/agent"
@@ -256,6 +257,26 @@ export const makeSessionGroup = <I extends HttpApiMiddleware.AnyId, S>(sessionLo
           }),
         ),
     )
+    .add(
+      HttpApiEndpoint.post("session.skill", "/api/session/:sessionID/skill", {
+        params: { sessionID: Session.ID },
+        payload: Schema.Struct({
+          id: SessionMessage.ID.pipe(Schema.optional),
+          skill: Schema.String,
+          resume: Schema.Boolean.pipe(Schema.optional),
+        }),
+        success: HttpApiSchema.NoContent,
+        error: [SessionNotFoundError, SkillNotFoundError],
+      })
+        .middleware(sessionLocationMiddleware)
+        .annotateMerge(
+          OpenApi.annotations({
+            identifier: "v2.session.skill",
+            summary: "Activate skill",
+            description: "Activate a skill for a session by appending a skill message and resuming execution.",
+          }),
+        ),
+    )
     .add(
       HttpApiEndpoint.post("session.compact", "/api/session/:sessionID/compact", {
         params: { sessionID: Session.ID },