fix(core): keep MCP quiet while a location shuts down (#60668)

packages/core/src/mcp/index.ts

@@ -184,6 +184,11 @@ export const layer = (options?: Options) =>
       // Server scopes close in parallel on shutdown, so one slow server's stop grace does not delay
       // the rest past a service restart's kill deadline and leave them orphaned.
       const servers = yield* Scope.fork(root, "parallel")
+      // Connection callbacks run here. Created after `servers`, so the layer scope closes this set first
+      // (finalizers run in reverse): in-flight callbacks are interrupted and later ones never start, so
+      // connections closing with the Location publish nothing. Clients re-fetch on MCP events, which
+      // would boot the Location straight back up.
+      const callbacks = yield* FiberSet.makeRuntime<never, void, never>()
 
       const entries = new Map<ServerName, ServerEntry>()
       const attached = new Map<ServerName, AttachedEntry[]>()
@@ -352,7 +357,7 @@ export const layer = (options?: Options) =>
       const whenLive =
         (name: ServerName, entry: ServerEntry, connection: McpClient.Connection) =>
         <E>(effect: Effect.Effect<void, E>) =>
-          fork(
+          callbacks(
             Effect.suspend(() => (entry.client === connection ? effect : Effect.void)).pipe(
               locks.withLock(name),
               Effect.ignore,

packages/core/test/mcp.test.ts

@@ -1883,6 +1883,39 @@ test("isolates nested configured MCP mutations and reconciles them", async () =>
   )
 })
 
+test("publishes MCP changes while live but not while the location shuts down", async () => {
+  const published: string[] = []
+  await Effect.runPromise(
+    Effect.scoped(
+      Effect.gen(function* () {
+        const server = yield* resourceServer({ listChanged: true })
+        const scope = yield* Scope.make()
+        const context = yield* Layer.buildWithScope(
+          resourceMcpLayer(server.url, undefined, undefined, { published }),
+          scope,
+        )
+        yield* Effect.gen(function* () {
+          const service = yield* Mcp.Service
+          expect(yield* settled(service)).toEqual({ status: "connected" })
+          published.length = 0
+          yield* Effect.promise(server.sendResourceListChanged)
+          yield* Effect.sync(() => published).pipe(
+            Effect.filterOrFail(
+              (types) => types.includes(McpEvent.ResourcesChanged.type),
+              () => new Error("resource change was not published"),
+            ),
+            Effect.retry({ times: 200, schedule: Schedule.spaced("10 millis") }),
+          )
+        }).pipe(Effect.provide(context))
+        published.length = 0
+        // Location shutdown closes plugin registrations under State.shutdown, so config does not reconcile.
+        yield* State.shutdown(Scope.close(scope, Exit.void))
+        expect(published.filter((type) => type.startsWith("mcp."))).toEqual([])
+      }),
+    ),
+  )
+})
+
 testEffect(Layer.empty).live("batches MCP transforms without connecting intermediate configurations", () =>
   Effect.gen(function* () {
     const server = yield* resourceServer()