diff --git a/src/renderer/components/ChatPane.tsx b/src/renderer/components/ChatPane.tsx
index bf639ea..ff12b39 100644
--- a/src/renderer/components/ChatPane.tsx
+++ b/src/renderer/components/ChatPane.tsx
@@ -2,6 +2,7 @@ import { type KeyboardEvent, useEffect, useRef, useState } from 'react';
 import { AlertCircle, ArrowUp, Bot, Loader2, User } from 'lucide-react';
 
 import { MarkdownContent } from '@renderer/components/MarkdownContent';
+import { getAssistantMessagePhase } from '@renderer/lib/messagePhase';
 
 import type { PatternDefinition } from '@shared/domain/pattern';
 import type { ProjectRecord } from '@shared/domain/project';
@@ -96,8 +97,22 @@ export function ChatPane({ project, pattern, session, onSend }: ChatPaneProps) {
         ) : (
           <div className="mx-auto max-w-3xl px-6 py-4">
             <div className="space-y-1">
-              {session.messages.map((message) => {
+              {session.messages.map((message, index) => {
                 const isUser = message.role === 'user';
+                const phase = getAssistantMessagePhase(session, message, index);
+                const assistantContainerClass =
+                  phase === 'thinking'
+                    ? 'border-sky-500/20 bg-sky-500/5'
+                    : phase === 'final'
+                      ? 'border-emerald-500/20 bg-emerald-500/5'
+                      : 'border-zinc-800 bg-zinc-900/40';
+                const assistantBadgeClass =
+                  phase === 'thinking'
+                    ? 'border-sky-400/20 bg-sky-400/10 text-sky-300'
+                    : 'border-emerald-400/20 bg-emerald-400/10 text-emerald-300';
+                const phaseLabel =
+                  phase === 'thinking' ? 'Thinking' : phase === 'final' ? 'Final' : undefined;
+
                 return (
                   <div className="group py-3" key={message.id}>
                     <div className="flex gap-3">
@@ -111,10 +126,23 @@ export function ChatPane({ project, pattern, session, onSend }: ChatPaneProps) {
                         {isUser ? <User className="size-3.5" /> : <Bot className="size-3.5" />}
                       </div>
                       <div className="min-w-0 flex-1">
-                        <div className="mb-1 text-[12px] font-medium text-zinc-400">
-                          {message.authorName}
+                        <div className="mb-1 flex items-center gap-2 text-[12px] font-medium text-zinc-400">
+                          <span>{message.authorName}</span>
+                          {!isUser && phaseLabel && (
+                            <span
+                              className={`inline-flex items-center rounded-full border px-2 py-0.5 text-[10px] font-semibold uppercase tracking-[0.08em] ${assistantBadgeClass}`}
+                            >
+                              {phaseLabel}
+                            </span>
+                          )}
                         </div>
-                        <div className="text-[14px] leading-relaxed text-zinc-200">
+                        <div
+                          className={
+                            isUser
+                              ? 'text-[14px] leading-relaxed text-zinc-200'
+                              : `rounded-xl border px-4 py-3 text-[14px] leading-relaxed text-zinc-200 ${assistantContainerClass}`
+                          }
+                        >
                           <MarkdownContent content={message.content} />
                           {message.pending && message.content && (
                             <span className="mt-1 inline-block h-4 w-[2px] animate-pulse rounded-sm bg-zinc-400" />
diff --git a/src/renderer/lib/messagePhase.ts b/src/renderer/lib/messagePhase.ts
new file mode 100644
index 0000000..eab5306
--- /dev/null
+++ b/src/renderer/lib/messagePhase.ts
@@ -0,0 +1,35 @@
+import type { ChatMessageRecord, SessionRecord } from '@shared/domain/session';
+
+export type AssistantMessagePhase = 'default' | 'thinking' | 'final';
+
+export function getAssistantMessagePhase(
+  session: SessionRecord,
+  message: ChatMessageRecord,
+  index: number,
+): AssistantMessagePhase {
+  if (message.role !== 'assistant') {
+    return 'default';
+  }
+
+  if (message.pending) {
+    return 'thinking';
+  }
+
+  if (session.status === 'running') {
+    return 'default';
+  }
+
+  const lastCompletedAssistantIndex = findLastCompletedAssistantIndex(session.messages);
+  return index === lastCompletedAssistantIndex ? 'final' : 'default';
+}
+
+function findLastCompletedAssistantIndex(messages: ChatMessageRecord[]): number {
+  for (let index = messages.length - 1; index >= 0; index -= 1) {
+    const message = messages[index];
+    if (message.role === 'assistant' && !message.pending) {
+      return index;
+    }
+  }
+
+  return -1;
+}
diff --git a/tests/renderer/messagePhase.test.ts b/tests/renderer/messagePhase.test.ts
new file mode 100644
index 0000000..90cdaae
--- /dev/null
+++ b/tests/renderer/messagePhase.test.ts
@@ -0,0 +1,87 @@
+import { describe, expect, test } from 'bun:test';
+
+import { getAssistantMessagePhase } from '@renderer/lib/messagePhase';
+import type { SessionRecord } from '@shared/domain/session';
+
+function createSession(
+  messages: SessionRecord['messages'],
+  status: SessionRecord['status'] = 'idle',
+): SessionRecord {
+  return {
+    id: 'session-1',
+    projectId: 'project-1',
+    patternId: 'pattern-1',
+    title: 'Test session',
+    createdAt: '2026-03-23T00:00:00.000Z',
+    updatedAt: '2026-03-23T00:00:00.000Z',
+    status,
+    messages,
+  };
+}
+
+describe('assistant message phase', () => {
+  test('marks pending assistant messages as thinking', () => {
+    const session = createSession([
+      {
+        id: 'msg-1',
+        role: 'assistant',
+        authorName: 'Triage',
+        content: 'Draft',
+        createdAt: '2026-03-23T00:00:00.000Z',
+        pending: true,
+      },
+    ], 'running');
+
+    expect(getAssistantMessagePhase(session, session.messages[0], 0)).toBe('thinking');
+  });
+
+  test('marks the last completed assistant message as final when the session is idle', () => {
+    const session = createSession([
+      {
+        id: 'msg-1',
+        role: 'assistant',
+        authorName: 'Triage',
+        content: 'Earlier',
+        createdAt: '2026-03-23T00:00:00.000Z',
+      },
+      {
+        id: 'msg-2',
+        role: 'assistant',
+        authorName: 'UX Specialist',
+        content: 'Final answer',
+        createdAt: '2026-03-23T00:00:01.000Z',
+      },
+    ]);
+
+    expect(getAssistantMessagePhase(session, session.messages[0], 0)).toBe('default');
+    expect(getAssistantMessagePhase(session, session.messages[1], 1)).toBe('final');
+  });
+
+  test('does not mark completed assistant messages as final while the session is still running', () => {
+    const session = createSession([
+      {
+        id: 'msg-1',
+        role: 'assistant',
+        authorName: 'Triage',
+        content: 'In progress',
+        createdAt: '2026-03-23T00:00:00.000Z',
+      },
+    ], 'running');
+
+    expect(getAssistantMessagePhase(session, session.messages[0], 0)).toBe('default');
+  });
+
+  test('ignores non-assistant messages', () => {
+    const session = createSession([
+      {
+        id: 'msg-1',
+        role: 'user',
+        authorName: 'You',
+        content: 'Hello',
+        createdAt: '2026-03-23T00:00:00.000Z',
+      },
+    ]);
+
+    expect(getAssistantMessagePhase(session, session.messages[0], 0)).toBe('default');
+  });
+});