fix: delegate CompositeBackend.id to default sandbox backend (#286)

* fix: delegate CompositeBackend.id to default sandbox backend

CompositeBackend failed isSandboxBackend check because it lacked the
required id property from SandboxBackendProtocol. Added a get id()
getter that delegates to the default backend's id when it is a sandbox,
or returns an empty string otherwise. This preserves the real sandbox
ID (e.g. from Daytona/Deno/Modal) for external lifecycle operations
like provider.delete({ sandboxId: sandbox.id }).

Fixes #275

* Fix delegation of CompositeBackend.id to sandbox backend

.changeset/great-dolphins-guess.md

@@ -0,0 +1,5 @@
+---
+"deepagents": patch
+---
+
+fix: delegate CompositeBackend.id to default sandbox backend

libs/deepagents/src/backends/composite.test.ts

@@ -18,6 +18,7 @@ import type {
   SandboxBackendProtocol,
   GrepMatch,
 } from "./protocol.js";
+import { isSandboxBackend } from "./protocol.js";
 
 /**
  * Mock sandbox backend for testing execute delegation
@@ -608,6 +609,38 @@ describe("CompositeBackend", () => {
         "doesn't support command execution",
       );
     });
+
+    it("should pass isSandboxBackend check when default backend supports execution", () => {
+      const mockSandbox = new MockSandboxBackend();
+      const { stateAndStore } = makeConfig();
+
+      const composite = new CompositeBackend(mockSandbox, {
+        "/store/": new StoreBackend(stateAndStore),
+      });
+
+      expect(isSandboxBackend(composite)).toBe(true);
+    });
+
+    it("should delegate id to default sandbox backend", () => {
+      const mockSandbox = new MockSandboxBackend();
+      const { stateAndStore } = makeConfig();
+
+      const composite = new CompositeBackend(mockSandbox, {
+        "/store/": new StoreBackend(stateAndStore),
+      });
+
+      expect(composite.id).toBe("mock-sandbox");
+    });
+
+    it("should return empty string id when default backend is not sandbox", () => {
+      const { stateAndStore } = makeConfig();
+
+      const composite = new CompositeBackend(new StateBackend(stateAndStore), {
+        "/store/": new StoreBackend(stateAndStore),
+      });
+
+      expect(composite.id).toBe("");
+    });
   });
 
   describe("uploadFiles", () => {

libs/deepagents/src/backends/composite.ts

@@ -42,6 +42,11 @@ export class CompositeBackend implements BackendProtocol {
     );
   }
 
+  /** Delegates to default backend's id if it is a sandbox, otherwise empty string. */
+  get id(): string {
+    return isSandboxBackend(this.default) ? this.default.id : "";
+  }
+
   /**
    * Determine which backend handles this key and strip prefix.
    *