chore: let agent refuse to perform (#38803)

Author: pavelfeldman

package-lock.json

@@ -55,7 +55,7 @@
     "@eslint/compat": "^1.3.2",
     "@eslint/eslintrc": "^3.3.1",
     "@eslint/js": "^9.34.0",
-    "@lowire/loop": "^0.0.23",
+    "@lowire/loop": "^0.0.24",
     "@modelcontextprotocol/sdk": "^1.25.2",
     "@octokit/graphql-schema": "^15.26.0",
     "@stylistic/eslint-plugin": "^5.2.3",

package.json

@@ -5,7 +5,7 @@ THIRD-PARTY SOFTWARE NOTICES AND INFORMATION
 This project incorporates components from the projects listed below. The original copyright notices and the licenses under which Microsoft received such components are set forth below. Microsoft reserves all rights not expressly granted herein, whether by implication, estoppel or otherwise.
 
 -	@hono/node-server@1.19.8 (https://github.com/honojs/node-server)
--	@lowire/loop@0.0.23 (https://github.com/pavelfeldman/lowire)
+-	@lowire/loop@0.0.24 (https://github.com/pavelfeldman/lowire)
 -	@modelcontextprotocol/sdk@1.25.2 (https://github.com/modelcontextprotocol/typescript-sdk)
 -	accepts@2.0.0 (https://github.com/jshttp/accepts)
 -	agent-base@7.1.4 (https://github.com/TooTallNate/proxy-agents)
@@ -500,7 +500,7 @@ MIT
 =========================================
 END OF @hono/node-server@1.19.8 AND INFORMATION
 
-%% @lowire/loop@0.0.23 NOTICES AND INFORMATION BEGIN HERE
+%% @lowire/loop@0.0.24 NOTICES AND INFORMATION BEGIN HERE
 =========================================
 Apache License
                            Version 2.0, January 2004
@@ -704,7 +704,7 @@ Apache License
    See the License for the specific language governing permissions and
    limitations under the License.
 =========================================
-END OF @lowire/loop@0.0.23 AND INFORMATION
+END OF @lowire/loop@0.0.24 AND INFORMATION
 
 %% @modelcontextprotocol/sdk@1.25.2 NOTICES AND INFORMATION BEGIN HERE
 =========================================

packages/playwright-core/ThirdPartyNotices.txt

@@ -3,7 +3,7 @@
   "version": "0.0.1",
   "private": true,
   "dependencies": {
-    "@lowire/loop": "^0.0.23",
+    "@lowire/loop": "^0.0.24",
     "@modelcontextprotocol/sdk": "^1.25.2",
     "zod": "^4.3.5",
     "zod-to-json-schema": "^3.25.1"

packages/playwright-core/bundles/mcp/package-lock.json

@@ -95,7 +95,7 @@ async function runLoop(progress: Progress, context: Context, toolDefinitions: To
     throw new Error(`This action requires API key to be set on the page agent.`);
 
   const { full } = await page.snapshotForAI(progress);
-  const { tools, callTool, reportedResult } = toolsForLoop(progress, context, toolDefinitions, { resultSchema });
+  const { tools, callTool, reportedResult, refusedToPerformReason } = toolsForLoop(progress, context, toolDefinitions, { resultSchema, refuseToPerform: 'allow' });
   const secrets = Object.fromEntries((context.agentParams.secrets || [])?.map(s => ([s.name, s.value])));
 
   const apiCacheTextBefore = context.agentParams.apiCacheFile ?
@@ -151,10 +151,13 @@ async function runLoop(progress: Progress, context: Context, toolDefinitions: To
     }
   }
 
+  if (refusedToPerformReason())
+    throw new Error(`Agent refused to perform action: ${refusedToPerformReason()}`);
+
   if (error)
     throw new Error(`Agentic loop failed: ${error}`);
 
-  return { result: resultSchema ? reportedResult() : undefined };
+  return { result: reportedResult ? reportedResult() : undefined };
 }
 
 async function cachedPerform(progress: Progress, context: Context, cacheKey: string): Promise<actions.ActionWithCode[] | undefined> {
@@ -202,8 +205,8 @@ const allCaches = new Map<string, Cache>();
 async function cachedActions(cacheFile: string): Promise<Cache> {
   let cache = allCaches.get(cacheFile);
   if (!cache) {
-    const text = await fs.promises.readFile(cacheFile, 'utf-8').catch(() => '{}');
-    const parsed = actions.cachedActionsSchema.safeParse(JSON.parse(text));
+    const json = await fs.promises.readFile(cacheFile, 'utf-8').then(text => JSON.parse(text)).catch(() => ({}));
+    const parsed = actions.cachedActionsSchema.safeParse(json);
     if (parsed.error)
       throw new Error(`Failed to parse cache file ${cacheFile}:\n${zod.prettifyError(parsed.error)}`);
     cache = { actions: parsed.data, newActions: {} };

packages/playwright-core/bundles/mcp/package.json

@@ -34,7 +34,14 @@ export function defineTool<Input extends zod.Schema>(tool: ToolDefinition<Input>
   return tool;
 }
 
-export function toolsForLoop(progress: Progress, context: Context, toolDefinitions: ToolDefinition[], options: { resultSchema?: loopTypes.Schema } = {}): { tools: loopTypes.Tool[], callTool: loopTypes.ToolCallback, reportedResult: () => any } {
+type ToolsForLoop = {
+  tools: loopTypes.Tool[];
+  callTool: loopTypes.ToolCallback;
+  reportedResult?: () => any;
+  refusedToPerformReason: () => string | undefined;
+};
+
+export function toolsForLoop(progress: Progress, context: Context, toolDefinitions: ToolDefinition[], options: { resultSchema?: loopTypes.Schema, refuseToPerform?: 'allow' | 'deny' } = {}): ToolsForLoop {
   const tools = toolDefinitions.map(tool => {
     const result: loopTypes.Tool = {
       name: tool.schema.name,
@@ -43,6 +50,7 @@ export function toolsForLoop(progress: Progress, context: Context, toolDefinitio
     };
     return result;
   });
+
   if (options.resultSchema) {
     tools.push({
       name: 'report_result',
@@ -51,7 +59,25 @@ export function toolsForLoop(progress: Progress, context: Context, toolDefinitio
     });
   }
 
+  if (options.refuseToPerform === 'allow') {
+    tools.push({
+      name: 'refuse_to_perform',
+      description: 'Refuse to perform action.',
+      inputSchema: {
+        type: 'object',
+        properties: {
+          reason: {
+            type: 'string',
+            description: `Call this when you believe that you can't perform the action because something is wrong with the page. The reason will be reported to the user.`,
+          },
+        },
+        required: ['reason'],
+      },
+    });
+  }
+
   let reportedResult: any;
+  let refusedToPerformReason: string | undefined;
 
   const callTool: loopTypes.ToolCallback = async params => {
     if (params.name === 'report_result') {
@@ -62,6 +88,14 @@ export function toolsForLoop(progress: Progress, context: Context, toolDefinitio
       };
     }
 
+    if (params.name === 'refuse_to_perform') {
+      refusedToPerformReason = params.arguments.reason;
+      return {
+        content: [{ type: 'text', text: 'Done' }],
+        isError: false,
+      };
+    }
+
     const tool = toolDefinitions.find(t => t.schema.name === params.name);
     if (!tool) {
       return {
@@ -85,6 +119,7 @@ export function toolsForLoop(progress: Progress, context: Context, toolDefinitio
   return {
     tools,
     callTool,
-    reportedResult: () => reportedResult,
+    reportedResult: options.resultSchema ? () => reportedResult : undefined,
+    refusedToPerformReason: () => refusedToPerformReason,
   };
 }