diff --git a/src/stores/chat.ts b/src/stores/chat.ts
index 574f561..4d6e6d7 100644
--- a/src/stores/chat.ts
+++ b/src/stores/chat.ts
@@ -230,8 +230,31 @@ function normalizeStreamingMessage(message: unknown): unknown {
     : rawMessage;
 }
 
+/**
+ * Strip Gateway-injected metadata that does NOT exist on the renderer's
+ * optimistic user message but is echoed back when the Gateway persists it:
+ *   - leading timestamp `[Wed 2026-04-22 10:30 GMT+8] `
+ *   - `[message_id: uuid]` tags sprinkled throughout the text
+ *   - `[media attached: path (mime) | path]` references appended when the
+ *     renderer sends attachments via `chat:sendWithMedia`
+ *   - Gateway-injected "Conversation info (untrusted metadata): ..." blocks
+ *
+ * Keeping this aligned with `cleanUserText` in `pages/Chat/message-utils.ts`
+ * is important: the user bubble renders the cleaned text, so the comparison
+ * used to dedupe optimistic vs server echoes must operate on the same
+ * cleaned form — otherwise the same visible message renders twice.
+ */
+function stripGatewayUserMetadata(text: string): string {
+  return text
+    .replace(/^\s*\[(?:Mon|Tue|Wed|Thu|Fri|Sat|Sun)\s+\d{4}-\d{2}-\d{2}\s+\d{2}:\d{2}\s+[^\]]+\]\s*/i, '')
+    .replace(/\s*\[media attached:[^\]]*\]/g, '')
+    .replace(/\s*\[message_id:\s*[^\]]+\]/g, '')
+    .replace(/^Conversation info\s*\([^)]*\):\s*```[a-z]*\n[\s\S]*?```\s*/i, '')
+    .replace(/^Conversation info\s*\([^)]*\):\s*\{[\s\S]*?\}\s*/i, '');
+}
+
 function normalizeComparableUserText(content: unknown): string {
-  return getMessageText(content)
+  return stripGatewayUserMetadata(getMessageText(content))
     .replace(/\s+/g, ' ')
     .trim();
 }
diff --git a/src/stores/chat/helpers.ts b/src/stores/chat/helpers.ts
index b317cc0..c3068f6 100644
--- a/src/stores/chat/helpers.ts
+++ b/src/stores/chat/helpers.ts
@@ -128,8 +128,31 @@ function normalizeStreamingMessage(message: unknown): unknown {
     : rawMessage;
 }
 
+/**
+ * Strip Gateway-injected metadata that does NOT exist on the renderer's
+ * optimistic user message but is echoed back when the Gateway persists it:
+ *   - leading timestamp `[Wed 2026-04-22 10:30 GMT+8] `
+ *   - `[message_id: uuid]` tags sprinkled throughout the text
+ *   - `[media attached: path (mime) | path]` references appended when the
+ *     renderer sends attachments via `chat:sendWithMedia`
+ *   - Gateway-injected "Conversation info (untrusted metadata): ..." blocks
+ *
+ * Keeping this aligned with `cleanUserText` in `pages/Chat/message-utils.ts`
+ * is important: the user bubble renders the cleaned text, so the comparison
+ * used to dedupe optimistic vs server echoes must operate on the same
+ * cleaned form — otherwise the same visible message renders twice.
+ */
+function stripGatewayUserMetadata(text: string): string {
+  return text
+    .replace(/^\s*\[(?:Mon|Tue|Wed|Thu|Fri|Sat|Sun)\s+\d{4}-\d{2}-\d{2}\s+\d{2}:\d{2}\s+[^\]]+\]\s*/i, '')
+    .replace(/\s*\[media attached:[^\]]*\]/g, '')
+    .replace(/\s*\[message_id:\s*[^\]]+\]/g, '')
+    .replace(/^Conversation info\s*\([^)]*\):\s*```[a-z]*\n[\s\S]*?```\s*/i, '')
+    .replace(/^Conversation info\s*\([^)]*\):\s*\{[\s\S]*?\}\s*/i, '');
+}
+
 function normalizeComparableUserText(content: unknown): string {
-  return getMessageText(content)
+  return stripGatewayUserMetadata(getMessageText(content))
     .replace(/\s+/g, ' ')
     .trim();
 }
diff --git a/tests/unit/chat-optimistic-match.test.ts b/tests/unit/chat-optimistic-match.test.ts
new file mode 100644
index 0000000..c7dff94
--- /dev/null
+++ b/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);
+  });
+});