feat(core): Introduce AgentLoopContext.

This is the first PR, it introduces the new interface which will
be separated out of Config by the end of this series.

Author: joshualitt

packages/a2a-server/src/utils/testing_utils.ts

@@ -75,6 +75,14 @@ export function createMockConfig(
     validatePathAccess: vi.fn().mockReturnValue(undefined),
     ...overrides,
   } as unknown as Config;
+
+  // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
+  (mockConfig as unknown as { config: Config; promptId: string }).config =
+    mockConfig;
+  // eslint-disable-next-line @typescript-eslint/no-unsafe-type-assertion
+  (mockConfig as unknown as { config: Config; promptId: string }).promptId =
+    'test-prompt-id';
+
   mockConfig.getMessageBus = vi.fn().mockReturnValue(createMockMessageBus());
   mockConfig.getHookSystem = vi
     .fn()

packages/core/src/agents/agent-scheduler.test.ts

@@ -30,7 +30,7 @@ describe('agent-scheduler', () => {
     } as unknown as Mocked<ToolRegistry>;
     mockConfig = {
       getMessageBus: vi.fn().mockReturnValue(mockMessageBus),
-      getToolRegistry: vi.fn().mockReturnValue(mockToolRegistry),
+      toolRegistry: mockToolRegistry,
     } as unknown as Mocked<Config>;
   });
 
@@ -69,6 +69,6 @@ describe('agent-scheduler', () => {
 
     // Verify that the scheduler's config has the overridden tool registry
     const schedulerConfig = vi.mocked(Scheduler).mock.calls[0][0].config;
-    expect(schedulerConfig.getToolRegistry()).toBe(mockToolRegistry);
+    expect(schedulerConfig.toolRegistry).toBe(mockToolRegistry);
   });
 });

packages/core/src/config/agent-loop-context.ts

@@ -0,0 +1,27 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type { GeminiClient } from '../core/client.js';
+import type { MessageBus } from '../confirmation-bus/message-bus.js';
+import type { ToolRegistry } from '../tools/tool-registry.js';
+
+/**
+ * AgentLoopContext represents the execution-scoped view of the world for a single
+ * agent turn or sub-agent loop.
+ */
+export interface AgentLoopContext {
+  /** The unique ID for the current user turn or agent thought loop. */
+  readonly promptId: string;
+
+  /** The registry of tools available to the agent in this context. */
+  readonly toolRegistry: ToolRegistry;
+
+  /** The bus for user confirmations and messages in this context. */
+  readonly messageBus: MessageBus;
+
+  /** The client used to communicate with the LLM in this context. */
+  readonly geminiClient: GeminiClient;
+}