feat(core): infrastructure for event-driven subagent history (#23914)

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

@@ -8,7 +8,7 @@ import { renderWithProviders } from '../../../test-utils/render.js';
 import { SubagentGroupDisplay } from './SubagentGroupDisplay.js';
 import { Kind, CoreToolCallStatus } from '@google/gemini-cli-core';
 import type { IndividualToolCallDisplay } from '../../types.js';
-import { vi } from 'vitest';
+import { describe, it, expect, vi } from 'vitest';
 import { Text } from 'ink';
 
 vi.mock('../../utils/MarkdownDisplay.js', () => ({

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

@@ -191,8 +191,9 @@ export const SubagentGroupDisplay: React.FC<SubagentGroupDisplayProps> = ({
           }
         }
 
+        const history = toolCall.subagentHistory ?? progress.recentActivity;
         const lastActivity: SubagentActivityItem | undefined =
-          progress.recentActivity[progress.recentActivity.length - 1];
+          history[history.length - 1];
 
         // Collapsed View: Show single compact line per agent
         if (!isExpanded) {
@@ -260,6 +261,7 @@ export const SubagentGroupDisplay: React.FC<SubagentGroupDisplayProps> = ({
             <SubagentProgressDisplay
               progress={progress}
               terminalWidth={terminalWidth}
+              historyOverrides={toolCall.subagentHistory}
             />
           </Box>
         );

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

@@ -0,0 +1,85 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { describe, it, expect } from 'vitest';
+import { renderWithProviders } from '../../../test-utils/render.js';
+import { SubagentHistoryMessage } from './SubagentHistoryMessage.js';
+import type { HistoryItemSubagent } from '../../types.js';
+
+describe('SubagentHistoryMessage', () => {
+  const mockItem: HistoryItemSubagent = {
+    type: 'subagent',
+    agentName: 'research',
+    history: [
+      {
+        id: '1',
+        type: 'thought',
+        content: 'Thinking about the problem',
+        status: 'completed',
+      },
+      {
+        id: '2',
+        type: 'tool_call',
+        content: 'Calling search_web',
+        status: 'running',
+      },
+      {
+        id: '3',
+        type: 'tool_call',
+        content: 'Calling read_file fail',
+        status: 'error',
+      },
+    ],
+  };
+
+  it('renders header with agent name and item count', async () => {
+    const renderResult = await renderWithProviders(
+      <SubagentHistoryMessage item={mockItem} terminalWidth={80} />,
+    );
+    await renderResult.waitUntilReady();
+
+    const output = renderResult.lastFrame();
+    expect(output).toContain('research Trace (3 items)');
+    expect(output).toMatchSnapshot();
+    await expect(renderResult).toMatchSvgSnapshot();
+    renderResult.unmount();
+  });
+
+  it('renders thought activities with brain icon', async () => {
+    const renderResult = await renderWithProviders(
+      <SubagentHistoryMessage item={mockItem} terminalWidth={80} />,
+    );
+    await renderResult.waitUntilReady();
+
+    const output = renderResult.lastFrame();
+    expect(output).toContain('🧠 Thinking about the problem');
+    renderResult.unmount();
+  });
+
+  it('renders tool call activities with tool icon', async () => {
+    const renderResult = await renderWithProviders(
+      <SubagentHistoryMessage item={mockItem} terminalWidth={80} />,
+    );
+    await renderResult.waitUntilReady();
+
+    const output = renderResult.lastFrame();
+    expect(output).toContain('🛠️ Calling search_web');
+    renderResult.unmount();
+  });
+
+  it('renders status indicators correctly', async () => {
+    const renderResult = await renderWithProviders(
+      <SubagentHistoryMessage item={mockItem} terminalWidth={80} />,
+    );
+    await renderResult.waitUntilReady();
+
+    const output = renderResult.lastFrame();
+    expect(output).toContain('Calling search_web (Running...)');
+    expect(output).toContain('Thinking about the problem ✅');
+    expect(output).toContain('Calling read_file fail ❌');
+    renderResult.unmount();
+  });
+});

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

@@ -0,0 +1,38 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import type React from 'react';
+import { Box, Text } from 'ink';
+import type { HistoryItemSubagent } from '../../types.js';
+
+interface SubagentHistoryMessageProps {
+  item: HistoryItemSubagent;
+  terminalWidth: number;
+}
+
+export const SubagentHistoryMessage: React.FC<SubagentHistoryMessageProps> = ({
+  item,
+  terminalWidth,
+}) => (
+  <Box flexDirection="column" width={terminalWidth} marginBottom={1}>
+    <Box marginBottom={1}>
+      <Text bold color="cyan">
+        🤖 {item.agentName} Trace ({item.history.length} items)
+      </Text>
+    </Box>
+
+    {item.history.map((activity) => (
+      <Box key={activity.id} marginLeft={2} marginBottom={0}>
+        <Text color={activity.type === 'thought' ? 'gray' : 'white'}>
+          {activity.type === 'thought' ? '🧠' : '🛠️'} {activity.content}
+          {activity.status === 'running' && ' (Running...)'}
+          {activity.status === 'completed' && ' ✅'}
+          {activity.status === 'error' && ' ❌'}
+        </Text>
+      </Box>
+    ))}
+  </Box>
+);

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

@@ -20,6 +20,7 @@ import { safeJsonToMarkdown } from '@google/gemini-cli-core';
 export interface SubagentProgressDisplayProps {
   progress: SubagentProgress;
   terminalWidth: number;
+  historyOverrides?: SubagentActivityItem[];
 }
 
 export const formatToolArgs = (args?: string): string => {
@@ -57,7 +58,7 @@ export const formatToolArgs = (args?: string): string => {
 
 export const SubagentProgressDisplay: React.FC<
   SubagentProgressDisplayProps
-> = ({ progress, terminalWidth }) => {
+> = ({ progress, terminalWidth, historyOverrides }) => {
   let headerText: string | undefined;
   let headerColor = theme.text.secondary;
 
@@ -85,72 +86,77 @@ export const SubagentProgressDisplay: React.FC<
         </Box>
       )}
       <Box flexDirection="column" marginLeft={0} gap={0}>
-        {progress.recentActivity.map((item: SubagentActivityItem) => {
-          if (item.type === 'thought') {
-            const isCancellation = item.content === 'Request cancelled.';
-            const icon = isCancellation ? 'ℹ ' : '💭';
-            const color = isCancellation
-              ? theme.status.warning
-              : theme.text.secondary;
+        {(historyOverrides ?? progress.recentActivity).map(
+          (item: SubagentActivityItem) => {
+            if (item.type === 'thought') {
+              const isCancellation = item.content === 'Request cancelled.';
+              const icon = isCancellation ? 'ℹ ' : '💭';
+              const color = isCancellation
+                ? theme.status.warning
+                : theme.text.secondary;
 
-            return (
-              <Box key={item.id} flexDirection="row">
-                <Box minWidth={STATUS_INDICATOR_WIDTH}>
-                  <Text color={color}>{icon}</Text>
+              return (
+                <Box key={item.id} flexDirection="row">
+                  <Box minWidth={STATUS_INDICATOR_WIDTH}>
+                    <Text color={color}>{icon}</Text>
+                  </Box>
+                  <Box flexGrow={1}>
+                    <Text color={color}>{item.content}</Text>
+                  </Box>
                 </Box>
-                <Box flexGrow={1}>
-                  <Text color={color}>{item.content}</Text>
-                </Box>
-              </Box>
-            );
-          } else if (item.type === 'tool_call') {
-            const statusSymbol =
-              item.status === 'running' ? (
-                <Spinner type="dots" />
-              ) : item.status === 'completed' ? (
-                <Text color={theme.status.success}>{TOOL_STATUS.SUCCESS}</Text>
-              ) : item.status === 'cancelled' ? (
-                <Text color={theme.status.warning} bold>
-                  {TOOL_STATUS.CANCELED}
-                </Text>
-              ) : (
-                <Text color={theme.status.error}>{TOOL_STATUS.ERROR}</Text>
               );
-
-            const formattedArgs = item.description || formatToolArgs(item.args);
-            const displayArgs =
-              formattedArgs.length > 60
-                ? formattedArgs.slice(0, 60) + '...'
-                : formattedArgs;
-
-            return (
-              <Box key={item.id} flexDirection="row">
-                <Box minWidth={STATUS_INDICATOR_WIDTH}>{statusSymbol}</Box>
-                <Box flexDirection="row" flexGrow={1} flexWrap="wrap">
-                  <Text
-                    bold
-                    color={theme.text.primary}
-                    strikethrough={item.status === 'cancelled'}
-                  >
-                    {item.displayName || item.content}
+            } else if (item.type === 'tool_call') {
+              const statusSymbol =
+                item.status === 'running' ? (
+                  <Spinner type="dots" />
+                ) : item.status === 'completed' ? (
+                  <Text color={theme.status.success}>
+                    {TOOL_STATUS.SUCCESS}
                   </Text>
-                  {displayArgs && (
-                    <Box marginLeft={1}>
-                      <Text
-                        color={theme.text.secondary}
-                        wrap="truncate"
-                        strikethrough={item.status === 'cancelled'}
-                      >
-                        {displayArgs}
-                      </Text>
-                    </Box>
-                  )}
+                ) : item.status === 'cancelled' ? (
+                  <Text color={theme.status.warning} bold>
+                    {TOOL_STATUS.CANCELED}
+                  </Text>
+                ) : (
+                  <Text color={theme.status.error}>{TOOL_STATUS.ERROR}</Text>
+                );
+
+              const formattedArgs =
+                item.description || formatToolArgs(item.args);
+              const displayArgs =
+                formattedArgs.length > 60
+                  ? formattedArgs.slice(0, 60) + '...'
+                  : formattedArgs;
+
+              return (
+                <Box key={item.id} flexDirection="row">
+                  <Box minWidth={STATUS_INDICATOR_WIDTH}>{statusSymbol}</Box>
+                  <Box flexDirection="row" flexGrow={1} flexWrap="wrap">
+                    <Text
+                      bold
+                      color={theme.text.primary}
+                      strikethrough={item.status === 'cancelled'}
+                    >
+                      {item.displayName || item.content}
+                    </Text>
+                    {displayArgs && (
+                      <Box marginLeft={1}>
+                        <Text
+                          color={theme.text.secondary}
+                          wrap="truncate"
+                          strikethrough={item.status === 'cancelled'}
+                        >
+                          {displayArgs}
+                        </Text>
+                      </Box>
+                    )}
+                  </Box>
                 </Box>
-              </Box>
-            );
-          }
-          return null;
-        })}
+              );
+            }
+            return null;
+          },
+        )}
       </Box>
 
       {progress.result && (

packages/cli/src/ui/components/messages/__snapshots__/SubagentHistoryMessage-SubagentHistoryMessage-renders-header-with-agent-name-and-item-count.snap.svg

@@ -0,0 +1,12 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="920" height="105" viewBox="0 0 920 105">
+  <style>
+    text { font-family: Consolas, "Courier New", monospace; font-size: 14px; dominant-baseline: text-before-edge; white-space: pre; }
+  </style>
+  <rect width="920" height="105" fill="#000000" />
+  <g transform="translate(10, 10)">
+    <text x="0" y="2" fill="#00cdcd" textLength="234" lengthAdjust="spacingAndGlyphs" font-weight="bold">🤖 research Trace (3 items)</text>
+    <text x="18" y="36" fill="#7f7f7f" textLength="270" lengthAdjust="spacingAndGlyphs">🧠 Thinking about the problem ✅</text>
+    <text x="18" y="53" fill="#e5e5e5" textLength="297" lengthAdjust="spacingAndGlyphs">🛠️ Calling search_web (Running...)</text>
+    <text x="18" y="70" fill="#e5e5e5" textLength="234" lengthAdjust="spacingAndGlyphs">🛠️ Calling read_file fail ❌</text>
+  </g>
+</svg>

packages/cli/src/ui/components/messages/__snapshots__/SubagentHistoryMessage.test.tsx.snap

@@ -0,0 +1,19 @@
+// Vitest Snapshot v1, https://vitest.dev/guide/snapshot.html
+
+exports[`SubagentHistoryMessage > renders header with agent name and item count 1`] = `
+"🤖 research Trace (3 items)
+
+  🧠 Thinking about the problem ✅
+  🛠️ Calling search_web (Running...)
+  🛠️ Calling read_file fail ❌
+"
+`;
+
+exports[`SubagentHistoryMessage > renders header with agent name and item count 2`] = `
+"🤖 research Trace (3 items)
+
+  🧠 Thinking about the problem ✅
+  🛠️ Calling search_web (Running...)
+  🛠️ Calling read_file fail ❌
+"
+`;