diff --git a/packages/core/src/core/coreToolScheduler.test.ts b/packages/core/src/core/coreToolScheduler.test.ts
index 36df456f9ba..0705d878e4d 100644
--- a/packages/core/src/core/coreToolScheduler.test.ts
+++ b/packages/core/src/core/coreToolScheduler.test.ts
@@ -6233,9 +6233,18 @@ describe('CoreToolScheduler plan mode with ask_user_question', () => {
       expect(completedCalls[0].response.resultDisplay).toBe(
         'Plan mode blocked a non-read-only tool call.',
       );
-      expect(
-        JSON.stringify(completedCalls[0].response.responseParts),
-      ).toContain('exit_plan_mode tool');
+      // Response must use error key (not output) so LLM recognizes it as a failure
+      const responseParts = completedCalls[0].response.responseParts;
+      const responseJson = JSON.stringify(responseParts);
+      expect(responseJson).toContain('"error"');
+      expect(responseJson).toContain('Tool blocked by plan mode');
+      expect(responseJson).toContain('write_file');
+      // Plan-required teammates get exit_plan_mode hint
+      expect(responseJson).toContain('exit_plan_mode');
+      expect(completedCalls[0].response.error).toBeInstanceOf(Error);
+      expect(completedCalls[0].response.errorType).toBe(
+        ToolErrorType.EXECUTION_DENIED,
+      );
     }
   });
 
@@ -6302,11 +6311,18 @@ describe('CoreToolScheduler plan mode with ask_user_question', () => {
         .calls[0][0] as ToolCall[];
       expect(completedCalls[0].status).toBe('error');
       if (completedCalls[0].status === 'error') {
-        const responseText = JSON.stringify(
-          completedCalls[0].response.responseParts,
+        // SDK, subagent, and teammate paths all get the same error format
+        // but different guidance: SDK/subagents get "Present your plan directly"
+        const responseParts = completedCalls[0].response.responseParts;
+        const responseJson = JSON.stringify(responseParts);
+        expect(responseJson).toContain('"error"');
+        expect(responseJson).toContain('Tool blocked by plan mode');
+        expect(responseJson).toContain('Present your plan directly');
+        expect(responseJson).not.toContain('exit_plan_mode');
+        expect(completedCalls[0].response.error).toBeInstanceOf(Error);
+        expect(completedCalls[0].response.errorType).toBe(
+          ToolErrorType.EXECUTION_DENIED,
         );
-        expect(responseText).toContain('Present your plan directly');
-        expect(responseText).not.toContain('exit_plan_mode tool');
       }
     },
   );
diff --git a/packages/core/src/core/coreToolScheduler.ts b/packages/core/src/core/coreToolScheduler.ts
index b4718dc46cc..cfbea8d5fff 100644
--- a/packages/core/src/core/coreToolScheduler.ts
+++ b/packages/core/src/core/coreToolScheduler.ts
@@ -112,7 +112,6 @@ import {
 } from '../tools/modifiable-tool.js';
 import * as Diff from 'diff';
 import levenshtein from 'fast-levenshtein';
-import { getPlanModeSystemReminder } from './prompts.js';
 import { ShellToolInvocation } from '../tools/shell.js';
 import { IdeClient } from '../ide/ide-client.js';
 import {
@@ -2458,22 +2457,27 @@ export class CoreToolScheduler {
                 isEnterPlanModeTool,
               )
             ) {
+              // SDK and ordinary subagent-like callers should return plans
+              // directly; they do not have exit_plan_mode available. Plan-required
+              // teammates have a dedicated exit_plan_mode approval path.
+              const isPlanRequiredTeammate =
+                !shouldUsePlanOnlyReminderInSubagentContext() &&
+                !this.config.getSdkMode();
+              const planModeError = new Error(
+                `Tool blocked by plan mode: "${reqInfo.name}" is not a read-only tool. ` +
+                  `Only read-only tools (read_file, grep_search, glob, list_directory, ` +
+                  `web_fetch, etc.) are allowed in plan mode.` +
+                  (isPlanRequiredTeammate
+                    ? ` Call exit_plan_mode to exit plan mode and execute this tool.`
+                    : ` Present your plan directly to the caller instead of executing this tool.`),
+              );
               this.setStatusInternal(reqInfo.callId, 'error', {
-                callId: reqInfo.callId,
-                responseParts: convertToFunctionResponse(
-                  reqInfo.name,
-                  reqInfo.callId,
-                  // SDK callers and ordinary subagent-like callers should
-                  // return plans directly. Plan-required teammates have a
-                  // dedicated exit_plan_mode approval path instead.
-                  getPlanModeSystemReminder(
-                    shouldUsePlanOnlyReminderInSubagentContext() ||
-                      this.config.getSdkMode(),
-                  ),
+                ...createErrorResponse(
+                  reqInfo,
+                  planModeError,
+                  ToolErrorType.EXECUTION_DENIED,
                 ),
                 resultDisplay: 'Plan mode blocked a non-read-only tool call.',
-                error: undefined,
-                errorType: undefined,
               });
               setToolSpanFailure(
                 toolSpan,