diff --git a/packages/cli/src/nonInteractiveCli.test.ts b/packages/cli/src/nonInteractiveCli.test.ts
index 70ede3ddfe..0b3a61e416 100644
--- a/packages/cli/src/nonInteractiveCli.test.ts
+++ b/packages/cli/src/nonInteractiveCli.test.ts
@@ -1519,6 +1519,9 @@ describe('runNonInteractive', () => {
         name: 'ShellTool',
         description: 'A shell tool',
         run: vi.fn(),
+        build: vi.fn().mockReturnValue({
+          getDescription: () => 'A shell tool',
+        }),
       }),
       getFunctionDeclarations: vi.fn().mockReturnValue([{ name: 'ShellTool' }]),
     } as unknown as ToolRegistry);
diff --git a/packages/cli/src/nonInteractiveCli.ts b/packages/cli/src/nonInteractiveCli.ts
index bf1f4e4d6d..c7e5d1c828 100644
--- a/packages/cli/src/nonInteractiveCli.ts
+++ b/packages/cli/src/nonInteractiveCli.ts
@@ -417,7 +417,9 @@ export async function runNonInteractive({
         return errToThrow;
       };
 
-      const runTerminalExitHandler = (handler: () => never): never => {
+      const runTerminalExitHandler = (
+        handler: () => void | never,
+      ): void | never => {
         terminalProcessExitHandled = true;
         return handler();
       };
diff --git a/packages/core/src/agent/types.ts b/packages/core/src/agent/types.ts
index 9bc3e81e0f..19837c138e 100644
--- a/packages/core/src/agent/types.ts
+++ b/packages/core/src/agent/types.ts
@@ -4,6 +4,8 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+import type { Kind } from '../tools/tools.js';
+
 export type WithMeta = { _meta?: Record<string, unknown> };
 
 export type Unsubscribe = () => void;
@@ -180,6 +182,16 @@ export interface ToolRequest {
   name: string;
   /** The arguments for the tool. */
   args: Record<string, unknown>;
+  /** UI specific metadata */
+  _meta?: {
+    legacyState?: {
+      displayName?: string;
+      isOutputMarkdown?: boolean;
+      description?: string;
+      kind?: Kind;
+    };
+    [key: string]: unknown;
+  };
 }
 
 /**
@@ -192,6 +204,18 @@ export interface ToolUpdate {
   displayContent?: ContentPart[];
   content?: ContentPart[];
   data?: Record<string, unknown>;
+  /** UI specific metadata */
+  _meta?: {
+    legacyState?: {
+      status?: string;
+      progressMessage?: string;
+      progress?: number;
+      progressTotal?: number;
+      pid?: number;
+      description?: string;
+    };
+    [key: string]: unknown;
+  };
 }
 
 export interface ToolResponse {
@@ -205,6 +229,13 @@ export interface ToolResponse {
   data?: Record<string, unknown>;
   /** When true, the tool call encountered an error that will be sent to the model. */
   isError?: boolean;
+  /** UI specific metadata */
+  _meta?: {
+    legacyState?: {
+      outputFile?: string;
+    };
+    [key: string]: unknown;
+  };
 }
 
 export type ElicitationRequest = {
diff --git a/packages/core/src/config/config.ts b/packages/core/src/config/config.ts
index 075c5439ad..4989308aa0 100644
--- a/packages/core/src/config/config.ts
+++ b/packages/core/src/config/config.ts
@@ -684,6 +684,7 @@ export interface ConfigParameters {
   adminSkillsEnabled?: boolean;
   experimentalJitContext?: boolean;
   experimentalMemoryManager?: boolean;
+  useAgentProtocol?: boolean;
   experimentalAgentHistoryTruncation?: boolean;
   experimentalAgentHistoryTruncationThreshold?: number;
   experimentalAgentHistoryRetainedMessages?: number;
@@ -922,6 +923,7 @@ export class Config implements McpContext, AgentLoopContext {
 
   private readonly experimentalJitContext: boolean;
   private readonly experimentalMemoryManager: boolean;
+  private readonly useAgentProtocol: boolean;
   private readonly experimentalAgentHistoryTruncation: boolean;
   private readonly experimentalAgentHistoryTruncationThreshold: number;
   private readonly experimentalAgentHistoryRetainedMessages: number;
@@ -1136,6 +1138,7 @@ export class Config implements McpContext, AgentLoopContext {
 
     this.experimentalJitContext = params.experimentalJitContext ?? true;
     this.experimentalMemoryManager = params.experimentalMemoryManager ?? false;
+    this.useAgentProtocol = params.useAgentProtocol ?? false;
     this.experimentalAgentHistoryTruncation =
       params.experimentalAgentHistoryTruncation ?? false;
     this.experimentalAgentHistoryTruncationThreshold =
@@ -2338,6 +2341,12 @@ export class Config implements McpContext, AgentLoopContext {
     return this.experimentalMemoryManager;
   }
 
+  getExperimentalUseAgentProtocol(): boolean {
+    return (
+      this.useAgentProtocol ||
+      process.env['GEMINI_CLI_USE_AGENT_PROTOCOL'] === 'true'
+    );
+  }
   isExperimentalAgentHistoryTruncationEnabled(): boolean {
     return this.experimentalAgentHistoryTruncation;
   }