fix(chat): dedupe optimistic user message against Gateway-prefixed echo (#887)

Co-authored-by: Cursor Agent <cursoragent@cursor.com>
Co-authored-by: Haze <hazeone@users.noreply.github.com>

tests/unit/chat-optimistic-match.test.ts

@@ -0,0 +1,68 @@
+import { describe, expect, it } from 'vitest';
+import { matchesOptimisticUserMessage } from '@/stores/chat/helpers';
+
+describe('matchesOptimisticUserMessage', () => {
+  it('matches when text is identical', () => {
+    const optimistic = { role: 'user', content: 'run github1', timestamp: 1_700_000_000 } as const;
+    const candidate = { role: 'user', content: 'run github1', timestamp: 1_700_000_000 } as const;
+
+    expect(matchesOptimisticUserMessage(candidate, optimistic, 1_700_000_000_000)).toBe(true);
+  });
+
+  it('matches when Gateway prefixes a weekday/timestamp prefix on the echoed user message', () => {
+    const optimistic = { role: 'user', content: 'run github1', timestamp: 1_700_000_000 } as const;
+    const candidate = {
+      role: 'user',
+      content: '[Wed 2026-04-22 10:30 GMT+8] run github1',
+      timestamp: 1_700_000_000,
+    } as const;
+
+    expect(matchesOptimisticUserMessage(candidate, optimistic, 1_700_000_000_000)).toBe(true);
+  });
+
+  it('matches when the server appends [media attached: ...] to the echoed user message', () => {
+    const optimistic = {
+      role: 'user',
+      content: 'Describe this image',
+      timestamp: 1_700_000_000,
+      _attachedFiles: [
+        {
+          fileName: 'shot.png',
+          mimeType: 'image/png',
+          fileSize: 123,
+          preview: null,
+          filePath: '/tmp/shot.png',
+        },
+      ],
+    } as const;
+    const candidate = {
+      role: 'user',
+      content: 'Describe this image\n\n[media attached: /tmp/shot.png (image/png) | /tmp/shot.png]',
+      timestamp: 1_700_000_000,
+    } as const;
+
+    expect(matchesOptimisticUserMessage(candidate, optimistic, 1_700_000_000_000)).toBe(true);
+  });
+
+  it('matches when the server strips a [message_id: ...] tag from the user message', () => {
+    const optimistic = { role: 'user', content: 'hello world', timestamp: 1_700_000_000 } as const;
+    const candidate = {
+      role: 'user',
+      content: 'hello world [message_id: 11111111-2222-3333-4444-555555555555]',
+      timestamp: 1_700_000_000,
+    } as const;
+
+    expect(matchesOptimisticUserMessage(candidate, optimistic, 1_700_000_000_000)).toBe(true);
+  });
+
+  it('still rejects unrelated user messages', () => {
+    const optimistic = { role: 'user', content: 'run github1', timestamp: 1_700_000_000 } as const;
+    const candidate = {
+      role: 'user',
+      content: '[Wed 2026-04-22 10:30 GMT+8] completely different text',
+      timestamp: 1_700_000_000,
+    } as const;
+
+    expect(matchesOptimisticUserMessage(candidate, optimistic, 1_700_000_000_000)).toBe(false);
+  });
+});