diff --git a/packages/core/src/tools/exitPlanMode.test.ts b/packages/core/src/tools/exitPlanMode.test.ts
index fa592544cef..39f19babe97 100644
--- a/packages/core/src/tools/exitPlanMode.test.ts
+++ b/packages/core/src/tools/exitPlanMode.test.ts
@@ -91,6 +91,25 @@ describe('ExitPlanModeTool', () => {
     });
   });
 
+  describe('non-empty plan constraint in descriptions', () => {
+    it('should mention non-empty constraint in plan parameter description', () => {
+      const schema = tool.schema as {
+        parametersJsonSchema: {
+          properties: { plan: { description: string } };
+        };
+      };
+      expect(schema.parametersJsonSchema.properties.plan.description).toContain(
+        'empty strings will be rejected',
+      );
+    });
+
+    it('should mention non-empty constraint in tool description', () => {
+      expect(tool.schema.description).toContain(
+        'empty strings will be rejected',
+      );
+    });
+  });
+
   describe('validateToolParams', () => {
     it('should accept valid parameters', () => {
       const params: ExitPlanModeParams = {
diff --git a/packages/core/src/tools/exitPlanMode.ts b/packages/core/src/tools/exitPlanMode.ts
index 32aeb2d7989..689b09b426f 100644
--- a/packages/core/src/tools/exitPlanMode.ts
+++ b/packages/core/src/tools/exitPlanMode.ts
@@ -45,6 +45,7 @@ IMPORTANT: Only use this tool when the task requires planning the implementation
 ## Before Using This Tool
 Ensure your plan is complete and unambiguous:
 - If you have unresolved questions about requirements or approach, use AskUserQuestion first (in earlier phases)
+- The plan parameter MUST contain your actual plan content — empty strings will be rejected
 - Once your plan is finalized, use THIS tool to request approval
 
 **Important:** Do NOT use AskUserQuestion to ask "Is this plan okay?" or "Should I proceed?" - that's exactly what THIS tool does. ExitPlanMode inherently requests user approval of your plan.
@@ -64,7 +65,7 @@ const exitPlanModeToolSchemaData: FunctionDeclaration = {
       plan: {
         type: 'string',
         description:
-          'The plan you came up with, that you want to run by the user for approval. Supports markdown. The plan should be pretty concise.',
+          'The plan you came up with, that you want to run by the user for approval. Supports markdown. The plan should be pretty concise. Must contain your actual plan content — empty strings will be rejected.',
       },
       originalRequest: {
         type: 'string',