diff --git a/apps/web/src/components/AgentOutputViewer.test.tsx b/apps/web/src/components/AgentOutputViewer.test.tsx
new file mode 100644
index 0000000..1523a63
--- /dev/null
+++ b/apps/web/src/components/AgentOutputViewer.test.tsx
@@ -0,0 +1,234 @@
+// @vitest-environment happy-dom
+import '@testing-library/jest-dom/vitest'
+import { render, screen, fireEvent } from '@testing-library/react'
+import { vi, describe, it, expect, beforeEach } from 'vitest'
+import * as parseModule from '@/lib/parse-agent-output'
+import { AgentOutputViewer } from './AgentOutputViewer'
+
+vi.mock('@/lib/trpc', () => ({
+  trpc: {
+    getAgentOutput: {
+      useQuery: vi.fn(() => ({ data: [], isLoading: false })),
+    },
+    onAgentOutput: {
+      useSubscription: vi.fn(),
+    },
+  },
+}))
+
+vi.mock('@/hooks', () => ({
+  useSubscriptionWithErrorHandling: vi.fn(() => ({
+    error: null,
+    isConnecting: false,
+  })),
+}))
+
+function makeToolResultMessage(content: string) {
+  return {
+    type: 'tool_result' as const,
+    content,
+    timestamp: new Date('2024-01-01T00:00:00Z'),
+  }
+}
+
+function makeSystemMessage(content: string) {
+  return {
+    type: 'system' as const,
+    content,
+    timestamp: new Date('2024-01-01T00:00:00Z'),
+  }
+}
+
+function makeTextMessage(content: string) {
+  return {
+    type: 'text' as const,
+    content,
+    timestamp: new Date('2024-01-01T00:00:00Z'),
+  }
+}
+
+function makeToolCallMessage(content: string, toolName: string) {
+  return {
+    type: 'tool_call' as const,
+    content,
+    timestamp: new Date('2024-01-01T00:00:00Z'),
+    meta: { toolName },
+  }
+}
+
+function makeErrorMessage(content: string) {
+  return {
+    type: 'error' as const,
+    content,
+    timestamp: new Date('2024-01-01T00:00:00Z'),
+  }
+}
+
+function makeSessionEndMessage(content: string) {
+  return {
+    type: 'session_end' as const,
+    content,
+    timestamp: new Date('2024-01-01T00:00:00Z'),
+  }
+}
+
+describe('AgentOutputViewer', () => {
+  beforeEach(() => {
+    vi.clearAllMocks()
+    // Default: no messages
+    vi.spyOn(parseModule, 'parseAgentOutput').mockReturnValue([])
+  })
+
+  // Test 1: tool_result renders collapsed by default
+  it('renders tool_result collapsed by default', () => {
+    const content = 'file content here and more stuff'
+    vi.spyOn(parseModule, 'parseAgentOutput').mockReturnValue([makeToolResultMessage(content)])
+
+    render(<AgentOutputViewer agentId="agent-1" />)
+
+    // ChevronRight should be present (collapsed state)
+    // We check that the SVG for ChevronRight is in the document
+    // lucide-react renders SVGs — we look for the collapsed container containing the preview text
+    expect(screen.getByText(content.substring(0, 80))).toBeInTheDocument()
+
+    // "Result" badge should NOT be visible (collapsed)
+    expect(screen.queryByText('Result')).not.toBeInTheDocument()
+
+    // The container should NOT show a "Result" badge
+    // ChevronRight is rendered — verify no ChevronDown
+    const svgs = document.querySelectorAll('svg')
+    // We look for the collapsed state by absence of "Result" text
+    expect(screen.queryByText('Result')).toBeNull()
+  })
+
+  // Test 2: Clicking collapsed result expands it
+  it('expands tool_result on click', () => {
+    const longContent = 'a'.repeat(100)
+    vi.spyOn(parseModule, 'parseAgentOutput').mockReturnValue([makeToolResultMessage(longContent)])
+
+    render(<AgentOutputViewer agentId="agent-1" />)
+
+    // Initially collapsed — click it
+    const collapsedContainer = screen.getByText(longContent.substring(0, 80)).closest('div')!
+    fireEvent.click(collapsedContainer)
+
+    // After click: "Result" badge should be visible
+    expect(screen.getByText('Result')).toBeInTheDocument()
+
+    // Full content should be visible in whitespace-pre-wrap element
+    const preWrap = document.querySelector('.whitespace-pre-wrap')
+    expect(preWrap).toBeInTheDocument()
+    expect(preWrap).toHaveTextContent(longContent)
+
+    // ChevronRight should no longer be visible; ChevronDown should be present
+    expect(screen.queryByText('Result')).toBeInTheDocument()
+  })
+
+  // Test 3: Clicking expanded result collapses it again
+  it('collapses tool_result on second click', () => {
+    const longContent = 'b'.repeat(100)
+    vi.spyOn(parseModule, 'parseAgentOutput').mockReturnValue([makeToolResultMessage(longContent)])
+
+    render(<AgentOutputViewer agentId="agent-1" />)
+
+    // Click once to expand
+    const container = screen.getByText(longContent.substring(0, 80)).closest('div')!
+    fireEvent.click(container)
+    expect(screen.getByText('Result')).toBeInTheDocument()
+
+    // Click again to collapse
+    // After expansion, the container still exists — click the expanded container
+    // The clickable container is the border-l-2 div
+    const expandedContainer = screen.getByText('Result').closest('.border-l-2')!
+    fireEvent.click(expandedContainer)
+
+    // Should be collapsed again
+    expect(screen.queryByText('Result')).not.toBeInTheDocument()
+    expect(screen.getByText(longContent.substring(0, 80))).toBeInTheDocument()
+  })
+
+  // Test 4: system message renders as single dim line, no badge, no border-l
+  it('renders system message as a single dim line without badge or border', () => {
+    const content = 'Session started: abc-123'
+    vi.spyOn(parseModule, 'parseAgentOutput').mockReturnValue([makeSystemMessage(content)])
+
+    render(<AgentOutputViewer agentId="agent-1" />)
+
+    // Content is visible
+    expect(screen.getByText(content)).toBeInTheDocument()
+
+    // No "System" badge text
+    expect(screen.queryByText('System')).not.toBeInTheDocument()
+
+    // The rendered element should NOT have border-l class
+    const el = screen.getByText(content)
+    expect(el.className).not.toContain('border-l')
+    expect(el.closest('[class*="border-l"]')).toBeNull()
+  })
+
+  // Test 5: agentId prop change resets expanded results
+  it('resets expanded results when agentId changes', () => {
+    const content = 'c'.repeat(100)
+    vi.spyOn(parseModule, 'parseAgentOutput').mockReturnValue([makeToolResultMessage(content)])
+
+    const { rerender } = render(<AgentOutputViewer agentId="agent-1" />)
+
+    // Expand the result
+    const collapsedContainer = screen.getByText(content.substring(0, 80)).closest('div')!
+    fireEvent.click(collapsedContainer)
+    expect(screen.getByText('Result')).toBeInTheDocument()
+
+    // Change agentId — should reset expandedResults
+    rerender(<AgentOutputViewer agentId="agent-2" />)
+
+    // After agentId change, result should be collapsed again
+    expect(screen.queryByText('Result')).not.toBeInTheDocument()
+    // Preview text should be visible (collapsed state)
+    expect(screen.getByText(content.substring(0, 80))).toBeInTheDocument()
+  })
+
+  // Test 6: Other message types remain always-expanded (unaffected)
+  it('always renders text messages fully', () => {
+    const content = 'This is a text message'
+    vi.spyOn(parseModule, 'parseAgentOutput').mockReturnValue([makeTextMessage(content)])
+
+    render(<AgentOutputViewer agentId="agent-1" />)
+
+    expect(screen.getByText(content)).toBeInTheDocument()
+
+    // No chevron icons for text messages
+    const svgCount = document.querySelectorAll('svg').length
+    // Only the header bar icons (Pause) should be present, no expand/collapse chevrons
+    expect(screen.queryByText('Result')).not.toBeInTheDocument()
+    expect(screen.queryByText('System')).not.toBeInTheDocument()
+  })
+
+  it('always renders tool_call messages fully', () => {
+    const content = 'Read(file.txt)'
+    vi.spyOn(parseModule, 'parseAgentOutput').mockReturnValue([makeToolCallMessage(content, 'Read')])
+
+    render(<AgentOutputViewer agentId="agent-1" />)
+
+    expect(screen.getByText(content)).toBeInTheDocument()
+    // The tool name badge should be visible
+    expect(screen.getByText('Read')).toBeInTheDocument()
+  })
+
+  it('always renders error messages with Error badge', () => {
+    const content = 'Something went wrong'
+    vi.spyOn(parseModule, 'parseAgentOutput').mockReturnValue([makeErrorMessage(content)])
+
+    render(<AgentOutputViewer agentId="agent-1" />)
+
+    expect(screen.getByText('Error')).toBeInTheDocument()
+    expect(screen.getByText(content)).toBeInTheDocument()
+  })
+
+  it('always renders session_end messages with session completed text', () => {
+    vi.spyOn(parseModule, 'parseAgentOutput').mockReturnValue([makeSessionEndMessage('Session completed')])
+
+    render(<AgentOutputViewer agentId="agent-1" />)
+
+    expect(screen.getByText('Session completed')).toBeInTheDocument()
+  })
+})
diff --git a/apps/web/src/components/AgentOutputViewer.tsx b/apps/web/src/components/AgentOutputViewer.tsx
index 48663ed..0f597fa 100644
--- a/apps/web/src/components/AgentOutputViewer.tsx
+++ b/apps/web/src/components/AgentOutputViewer.tsx
@@ -1,7 +1,7 @@
 import { useEffect, useRef, useState } from "react";
 import { Button } from "@/components/ui/button";
 import { Badge } from "@/components/ui/badge";
-import { ArrowDown, Pause, Play, AlertCircle, Square } from "lucide-react";
+import { ArrowDown, ChevronDown, ChevronRight, Pause, Play, AlertCircle, Square } from "lucide-react";
 import { trpc } from "@/lib/trpc";
 import { useSubscriptionWithErrorHandling } from "@/hooks";
 import {
@@ -21,6 +21,7 @@ interface AgentOutputViewerProps {
 export function AgentOutputViewer({ agentId, agentName, status, onStop }: AgentOutputViewerProps) {
   const [messages, setMessages] = useState<ParsedMessage[]>([]);
   const [follow, setFollow] = useState(true);
+  const [expandedResults, setExpandedResults] = useState<Set<number>>(new Set());
   const containerRef = useRef<HTMLDivElement>(null);
   // Accumulate timestamped chunks: initial query data + live subscription chunks
   const chunksRef = useRef<TimestampedChunk[]>([]);
@@ -65,6 +66,7 @@ export function AgentOutputViewer({ agentId, agentName, status, onStop }: AgentO
     chunksRef.current = [];
     setMessages([]);
     setFollow(true);
+    setExpandedResults(new Set());
   }, [agentId]);
 
   // Auto-scroll to bottom when following
@@ -172,10 +174,8 @@ export function AgentOutputViewer({ agentId, agentName, status, onStop }: AgentO
             {messages.map((message, index) => (
               <div key={index} className={getMessageStyling(message.type)}>
                 {message.type === 'system' && (
-                  <div className="flex items-center gap-2">
-                    <Badge variant="secondary" className="text-xs bg-terminal-border text-terminal-system">System</Badge>
-                    <span className="text-xs text-terminal-muted">{message.content}</span>
-                    <Timestamp date={message.timestamp} />
+                  <div className="text-terminal-muted/60 text-xs font-mono">
+                    {message.content}
                   </div>
                 )}
 
@@ -203,13 +203,32 @@ export function AgentOutputViewer({ agentId, agentName, status, onStop }: AgentO
                 )}
 
                 {message.type === 'tool_result' && (
-                  <div className="border-l-2 border-terminal-result pl-3 py-1 bg-white/[0.02] min-w-0">
-                    <Badge variant="outline" className="mb-1 text-xs text-terminal-result border-terminal-result">
-                      Result
-                    </Badge>
-                    <div className="font-mono text-xs text-terminal-muted whitespace-pre-wrap break-words">
-                      {message.content}
-                    </div>
+                  <div
+                    className="border-l-2 border-terminal-result pl-3 py-1 bg-white/[0.02] min-w-0 cursor-pointer"
+                    onClick={() => setExpandedResults(prev => {
+                      const next = new Set(prev);
+                      if (next.has(index)) next.delete(index); else next.add(index);
+                      return next;
+                    })}
+                  >
+                    {expandedResults.has(index) ? (
+                      <>
+                        <ChevronDown className="h-4 w-4 text-terminal-muted inline-block shrink-0 mr-1" />
+                        <Badge variant="outline" className="mb-1 text-xs text-terminal-result border-terminal-result">
+                          Result
+                        </Badge>
+                        <div className="font-mono text-xs text-terminal-muted whitespace-pre-wrap break-words">
+                          {message.content}
+                        </div>
+                      </>
+                    ) : (
+                      <>
+                        <ChevronRight className="h-4 w-4 text-terminal-muted inline-block shrink-0 mr-1" />
+                        <span className="text-terminal-muted/60 text-xs font-mono truncate">
+                          {message.content.substring(0, 80)}
+                        </span>
+                      </>
+                    )}
                   </div>
                 )}