feat(core): improve subagent result display

packages/cli/src/ui/components/messages/ShellToolMessage.test.tsx

@@ -191,49 +191,63 @@ describe('<ShellToolMessage />', () => {
         10,
         8,
         false,
+        true,
       ],
       [
         'uses ACTIVE_SHELL_MAX_LINES when availableTerminalHeight is large',
         100,
         ACTIVE_SHELL_MAX_LINES - 3,
         false,
+        true,
       ],
       [
         'uses full availableTerminalHeight when focused in alternate buffer mode',
         100,
         98, // 100 - 2
         true,
+        false,
       ],
       [
         'defaults to ACTIVE_SHELL_MAX_LINES in alternate buffer when availableTerminalHeight is undefined',
         undefined,
         ACTIVE_SHELL_MAX_LINES - 3,
         false,
+        false,
       ],
-    ])('%s', async (_, availableTerminalHeight, expectedMaxLines, focused) => {
-      const { lastFrame, waitUntilReady, unmount } = renderShell(
-        {
-          resultDisplay: LONG_OUTPUT,
-          renderOutputAsMarkdown: false,
-          availableTerminalHeight,
-          ptyId: 1,
-          status: CoreToolCallStatus.Executing,
-        },
-        {
-          useAlternateBuffer: true,
-          uiState: {
-            activePtyId: focused ? 1 : 2,
-            embeddedShellFocused: focused,
+    ])(
+      '%s',
+      async (
+        _,
+        availableTerminalHeight,
+        expectedMaxLines,
+        focused,
+        constrainHeight,
+      ) => {
+        const { lastFrame, waitUntilReady, unmount } = renderShell(
+          {
+            resultDisplay: LONG_OUTPUT,
+            renderOutputAsMarkdown: false,
+            availableTerminalHeight,
+            ptyId: 1,
+            status: CoreToolCallStatus.Executing,
           },
-        },
-      );
+          {
+            useAlternateBuffer: true,
+            uiState: {
+              activePtyId: focused ? 1 : 2,
+              embeddedShellFocused: focused,
+              constrainHeight,
+            },
+          },
+        );
 
-      await waitUntilReady();
-      const frame = lastFrame();
-      expect(frame.match(/Line \d+/g)?.length).toBe(expectedMaxLines);
-      expect(frame).toMatchSnapshot();
-      unmount();
-    });
+        await waitUntilReady();
+        const frame = lastFrame();
+        expect(frame.match(/Line \d+/g)?.length).toBe(expectedMaxLines);
+        expect(frame).toMatchSnapshot();
+        unmount();
+      },
+    );
 
     it('fully expands in standard mode when availableTerminalHeight is undefined', async () => {
       const { lastFrame, unmount } = renderShell(

packages/cli/src/ui/components/messages/ToolMessage.test.tsx

@@ -9,7 +9,11 @@ import { ToolMessage, type ToolMessageProps } from './ToolMessage.js';
 import { describe, it, expect, vi } from 'vitest';
 import { StreamingState } from '../../types.js';
 import { Text } from 'ink';
-import { type AnsiOutput, CoreToolCallStatus } from '@google/gemini-cli-core';
+import {
+  type AnsiOutput,
+  CoreToolCallStatus,
+  Kind,
+} from '@google/gemini-cli-core';
 import { renderWithProviders } from '../../../test-utils/render.js';
 import { tryParseJSON } from '../../../utils/jsonoutput.js';
 
@@ -435,4 +439,99 @@ describe('<ToolMessage />', () => {
     expect(output).toMatchSnapshot();
     unmount();
   });
+
+  describe('Truncation', () => {
+    it('applies truncation for Kind.Agent when availableTerminalHeight is provided', async () => {
+      const multilineString = Array.from(
+        { length: 30 },
+        (_, i) => `Line ${i + 1}`,
+      ).join('\n');
+
+      const { lastFrame, waitUntilReady, unmount } = renderWithProviders(
+        <ToolMessage
+          {...baseProps}
+          kind={Kind.Agent}
+          resultDisplay={multilineString}
+          renderOutputAsMarkdown={false}
+          availableTerminalHeight={40}
+        />,
+        {
+          uiActions,
+          uiState: {
+            streamingState: StreamingState.Idle,
+            constrainHeight: true,
+          },
+          width: 80,
+          useAlternateBuffer: false,
+        },
+      );
+      await waitUntilReady();
+      const output = lastFrame();
+
+      // Since kind=Kind.Agent and availableTerminalHeight is provided, it should truncate to SUBAGENT_MAX_LINES (15)
+      // and show the FIRST lines (overflowDirection='bottom')
+      expect(output).toContain('Line 1');
+      expect(output).toContain('Line 14');
+      expect(output).not.toContain('Line 16');
+      expect(output).not.toContain('Line 30');
+      unmount();
+    });
+
+    it('does NOT apply truncation for Kind.Agent when availableTerminalHeight is undefined', async () => {
+      const multilineString = Array.from(
+        { length: 30 },
+        (_, i) => `Line ${i + 1}`,
+      ).join('\n');
+
+      const { lastFrame, waitUntilReady, unmount } = renderWithProviders(
+        <ToolMessage
+          {...baseProps}
+          kind={Kind.Agent}
+          resultDisplay={multilineString}
+          renderOutputAsMarkdown={false}
+          availableTerminalHeight={undefined}
+        />,
+        {
+          uiActions,
+          uiState: { streamingState: StreamingState.Idle },
+          width: 80,
+          useAlternateBuffer: false,
+        },
+      );
+      await waitUntilReady();
+      const output = lastFrame();
+
+      expect(output).toContain('Line 1');
+      expect(output).toContain('Line 30');
+      unmount();
+    });
+
+    it('does NOT apply truncation for Kind.Read', async () => {
+      const multilineString = Array.from(
+        { length: 30 },
+        (_, i) => `Line ${i + 1}`,
+      ).join('\n');
+
+      const { lastFrame, waitUntilReady, unmount } = renderWithProviders(
+        <ToolMessage
+          {...baseProps}
+          kind={Kind.Read}
+          resultDisplay={multilineString}
+          renderOutputAsMarkdown={false}
+        />,
+        {
+          uiActions,
+          uiState: { streamingState: StreamingState.Idle },
+          width: 80,
+          useAlternateBuffer: false,
+        },
+      );
+      await waitUntilReady();
+      const output = lastFrame();
+
+      expect(output).toContain('Line 1');
+      expect(output).toContain('Line 30');
+      unmount();
+    });
+  });
 });

packages/cli/src/ui/components/messages/ToolMessage.tsx

@@ -21,8 +21,9 @@ import {
   useFocusHint,
   FocusHint,
 } from './ToolShared.js';
-import { type Config, CoreToolCallStatus } from '@google/gemini-cli-core';
+import { type Config, CoreToolCallStatus, Kind } from '@google/gemini-cli-core';
 import { ShellInputPrompt } from '../ShellInputPrompt.js';
+import { SUBAGENT_MAX_LINES } from '../../constants.js';
 
 export type { TextEmphasis };
 
@@ -45,6 +46,7 @@ export const ToolMessage: React.FC<ToolMessageProps> = ({
   description,
   resultDisplay,
   status,
+  kind,
   availableTerminalHeight,
   terminalWidth,
   emphasis = 'medium',
@@ -133,6 +135,12 @@ export const ToolMessage: React.FC<ToolMessageProps> = ({
           terminalWidth={terminalWidth}
           renderOutputAsMarkdown={renderOutputAsMarkdown}
           hasFocus={isThisShellFocused}
+          maxLines={
+            kind === Kind.Agent && availableTerminalHeight !== undefined
+              ? SUBAGENT_MAX_LINES
+              : undefined
+          }
+          overflowDirection={kind === Kind.Agent ? 'bottom' : 'top'}
         />
         {isThisShellFocused && config && (
           <Box paddingLeft={STATUS_INDICATOR_WIDTH} marginTop={1}>