fix(core): ensure subagents use qualified MCP tool names (#20801)

packages/core/src/agents/local-executor.ts

@@ -16,10 +16,7 @@ import type {
   Schema,
 } from '@google/genai';
 import { ToolRegistry } from '../tools/tool-registry.js';
-import {
-  DiscoveredMCPTool,
-  MCP_QUALIFIED_NAME_SEPARATOR,
-} from '../tools/mcp-tool.js';
+import { DiscoveredMCPTool } from '../tools/mcp-tool.js';
 import { CompressionStatus } from '../core/turn.js';
 import { type ToolCallRequestInfo } from '../scheduler/types.js';
 import { ChatCompressionService } from '../services/chatCompressionService.js';
@@ -142,15 +139,14 @@ export class LocalAgentExecutor<TOutput extends z.ZodTypeAny> {
       // registry and register it with the agent's isolated registry.
       const tool = parentToolRegistry.getTool(toolName);
       if (tool) {
-        if (
-          tool instanceof DiscoveredMCPTool &&
-          !toolName.includes(MCP_QUALIFIED_NAME_SEPARATOR)
-        ) {
-          throw new Error(
-            `MCP tool '${toolName}' must be requested with its server prefix (e.g., '${tool.serverName}${MCP_QUALIFIED_NAME_SEPARATOR}${toolName}') in agent '${definition.name}'.`,
-          );
+        if (tool instanceof DiscoveredMCPTool) {
+          // Subagents MUST use fully qualified names for MCP tools to ensure
+          // unambiguous tool calls and to comply with policy requirements.
+          // We automatically "upgrade" any MCP tool to its qualified version.
+          agentToolRegistry.registerTool(tool.asFullyQualifiedTool());
+        } else {
+          agentToolRegistry.registerTool(tool);
         }
-        agentToolRegistry.registerTool(tool);
       }
     };