From f86e0ee418e34927058dce3798d1bb9fc50fc628 Mon Sep 17 00:00:00 2001
From: Aishanee Shah <aishaneeshah@google.com>
Date: Fri, 8 May 2026 14:36:39 -0400
Subject: [PATCH] fix(core): throw explicit error on dropped tool responses
 (#26668)

---
 .../core/src/agents/local-executor.test.ts    | 73 ++++++++++++++++++-
 packages/core/src/agents/local-executor.ts    | 26 ++++---
 2 files changed, 87 insertions(+), 12 deletions(-)

diff --git a/packages/core/src/agents/local-executor.test.ts b/packages/core/src/agents/local-executor.test.ts
index cb328e1125..a35dc580b7 100644
--- a/packages/core/src/agents/local-executor.test.ts
+++ b/packages/core/src/agents/local-executor.test.ts
@@ -2227,6 +2227,69 @@ describe('LocalAgentExecutor', () => {
       // Agent should terminate with ABORTED status
       expect(output.terminate_reason).toBe(AgentTerminateMode.ABORTED);
     });
+
+    it('should throw a critical error when a tool response is dropped by the scheduler', async () => {
+      const definition = createTestDefinition([LS_TOOL_NAME]);
+      const executor = await LocalAgentExecutor.create(
+        definition,
+        mockConfig,
+        onActivity,
+      );
+
+      // Turn 1: Model calls two tools
+      mockModelResponse([
+        { name: LS_TOOL_NAME, args: { path: 'dir1' }, id: 'call1' },
+        { name: LS_TOOL_NAME, args: { path: 'dir2' }, id: 'call2' },
+      ]);
+
+      // Simulate scheduler returning only ONE result for TWO calls (dropped response)
+      mockScheduleAgentTools.mockResolvedValueOnce([
+        {
+          status: 'success',
+          request: { callId: 'call1', name: LS_TOOL_NAME },
+          response: {
+            responseParts: [
+              {
+                functionResponse: {
+                  name: LS_TOOL_NAME,
+                  id: 'call1',
+                  response: { ok: true },
+                },
+              },
+            ],
+          },
+        },
+      ]);
+
+      await expect(
+        executor.run({ goal: 'Protocol test' }, signal),
+      ).rejects.toThrow(
+        'Critical System Failure: Tool execution result was lost/dropped by the scheduler',
+      );
+    });
+
+    it('should throw a critical error when all scheduler results are missing/dropped', async () => {
+      const definition = createTestDefinition([LS_TOOL_NAME]);
+      const executor = await LocalAgentExecutor.create(
+        definition,
+        mockConfig,
+        onActivity,
+      );
+
+      // Turn 1: Model calls one tool
+      mockModelResponse([
+        { name: LS_TOOL_NAME, args: { path: 'dir1' }, id: 'call1' },
+      ]);
+
+      // Simulate scheduler returning NO results (dropped response)
+      mockScheduleAgentTools.mockResolvedValueOnce([]);
+
+      await expect(
+        executor.run({ goal: 'Protocol test 2' }, signal),
+      ).rejects.toThrow(
+        'Critical System Failure: Tool execution result was lost/dropped by the scheduler',
+      );
+    });
   });
 
   describe('Model Routing', () => {
@@ -2334,7 +2397,15 @@ describe('LocalAgentExecutor', () => {
           },
           response: {
             resultDisplay: 'ls result',
-            responseParts: [],
+            responseParts: [
+              {
+                functionResponse: {
+                  name: LS_TOOL_NAME,
+                  id: 'call1',
+                  response: { ok: true },
+                },
+              },
+            ],
             data: {},
           },
         },
diff --git a/packages/core/src/agents/local-executor.ts b/packages/core/src/agents/local-executor.ts
index b05a80f0b7..8780325ab8 100644
--- a/packages/core/src/agents/local-executor.ts
+++ b/packages/core/src/agents/local-executor.ts
@@ -1287,25 +1287,29 @@ export class LocalAgentExecutor<TOutput extends z.ZodTypeAny> {
       }
     }
 
-    // Reconstruct toolResponseParts in the original order
+    // Ensure exactly one response per function call to satisfy the Gemini API protocol.
     const toolResponseParts: Part[] = [];
     for (const [index, functionCall] of functionCalls.entries()) {
       const callId = functionCall.id ?? `${promptId}-${index}`;
       const part = syncResults.get(callId);
+
       if (part) {
         toolResponseParts.push(part);
+        continue;
       }
-    }
 
-    // If all authorized tool calls failed (and task isn't complete), provide a generic error.
-    if (
-      functionCalls.length > 0 &&
-      toolResponseParts.length === 0 &&
-      !taskCompleted
-    ) {
-      toolResponseParts.push({
-        text: 'All tool calls failed or were unauthorized. Please analyze the errors and try an alternative approach.',
-      });
+      const isAborted = signal.aborted;
+      const isTaskComplete =
+        functionCall.name === COMPLETE_TASK_TOOL_NAME && taskCompleted;
+
+      // Safely skip missing responses if the run was interrupted or the turn won't be sent back.
+      if (isAborted || isTaskComplete) {
+        continue;
+      }
+
+      throw new Error(
+        `[LocalAgentExecutor] Critical System Failure: Tool execution result was lost/dropped by the scheduler for callId ${callId} (${functionCall.name}). This indicates an internal race condition or scheduler bug.`,
+      );
     }
 
     return {