diff --git a/packages/collab-web/CHANGELOG.md b/packages/collab-web/CHANGELOG.md
index e57be1b78..d736435cc 100644
--- a/packages/collab-web/CHANGELOG.md
+++ b/packages/collab-web/CHANGELOG.md
@@ -2,6 +2,10 @@
 
 ## [Unreleased]
 
+### Fixed
+
+- Prevented IME composition (Korean/Japanese/Chinese) from duplicating the last character when pressing Enter to commit in the collab web composer. ([#6163](https://github.com/can1357/oh-my-pi/issues/6163))
+
 ## [17.0.1] - 2026-07-16
 
 ### Fixed
diff --git a/packages/collab-web/src/components/shell/Composer.tsx b/packages/collab-web/src/components/shell/Composer.tsx
index 5f3e45885..69b2ca033 100644
--- a/packages/collab-web/src/components/shell/Composer.tsx
+++ b/packages/collab-web/src/components/shell/Composer.tsx
@@ -1,5 +1,5 @@
 import { SendHorizontal, Square } from "lucide-react";
-import type { KeyboardEvent, ReactNode } from "react";
+import type { KeyboardEvent, ReactNode, RefObject } from "react";
 import { useCallback, useLayoutEffect, useRef, useState } from "react";
 import type { GuestClient, GuestSnapshot } from "../../lib/client";
 
@@ -21,6 +21,39 @@ function autosize(el: HTMLTextAreaElement | null): void {
 	el.style.overflowY = el.scrollHeight > max ? "auto" : "hidden";
 }
 
+/**
+ * Decides whether an Enter keydown should commit the composer. Returns `false` while an IME
+ * composition is active so the keystroke confirms the composition instead of submitting.
+ * `nativeEvent.isComposing` covers most browsers; `composing` bridges WebKit, which fires the
+ * confirming Enter keydown *after* `compositionend`.
+ */
+export function shouldSubmitOnEnter(e: KeyboardEvent<HTMLTextAreaElement>, composing: boolean): boolean {
+	if (e.key !== "Enter" || e.shiftKey) return false;
+	return !(e.nativeEvent.isComposing || composing);
+}
+
+/**
+ * Tracks IME composition state via a ref the keydown handler reads synchronously. The
+ * `compositionend` reset is deferred a tick because WebKit dispatches the confirming Enter
+ * keydown after `compositionend`, when `nativeEvent.isComposing` is already `false`.
+ */
+function useCompositionGuard(): {
+	composingRef: RefObject<boolean>;
+	onCompositionStart(): void;
+	onCompositionEnd(): void;
+} {
+	const composingRef = useRef(false);
+	const onCompositionStart = useCallback((): void => {
+		composingRef.current = true;
+	}, []);
+	const onCompositionEnd = useCallback((): void => {
+		setTimeout(() => {
+			composingRef.current = false;
+		}, 0);
+	}, []);
+	return { composingRef, onCompositionStart, onCompositionEnd };
+}
+
 interface AskEditorProps {
 	prefill: string | undefined;
 	onSubmit(value: string): void;
@@ -34,13 +67,14 @@ interface AskEditorProps {
 function AskEditor({ prefill, onSubmit }: AskEditorProps): ReactNode {
 	const [draft, setDraft] = useState(prefill ?? "");
 	const taRef = useRef<HTMLTextAreaElement | null>(null);
+	const { composingRef, onCompositionStart, onCompositionEnd } = useCompositionGuard();
 
 	useLayoutEffect(() => {
 		autosize(taRef.current);
 	}, [draft]);
 
 	const onKeyDown = (e: KeyboardEvent<HTMLTextAreaElement>): void => {
-		if (e.key === "Enter" && !e.shiftKey) {
+		if (shouldSubmitOnEnter(e, composingRef.current)) {
 			e.preventDefault();
 			onSubmit(draft);
 		}
@@ -54,6 +88,8 @@ function AskEditor({ prefill, onSubmit }: AskEditorProps): ReactNode {
 				value={draft}
 				onChange={e => setDraft(e.target.value)}
 				onKeyDown={onKeyDown}
+				onCompositionStart={onCompositionStart}
+				onCompositionEnd={onCompositionEnd}
 				placeholder="type your response…"
 				rows={1}
 				spellCheck={false}
@@ -75,6 +111,7 @@ function AskEditor({ prefill, onSubmit }: AskEditorProps): ReactNode {
 export function Composer({ client, snapshot }: ComposerProps): ReactNode {
 	const [text, setText] = useState("");
 	const taRef = useRef<HTMLTextAreaElement | null>(null);
+	const { composingRef, onCompositionStart, onCompositionEnd } = useCompositionGuard();
 
 	const live = snapshot.phase === "live";
 	const readOnly = snapshot.readOnly;
@@ -96,7 +133,7 @@ export function Composer({ client, snapshot }: ComposerProps): ReactNode {
 	}, [client, live, readOnly, text]);
 
 	const onKeyDown = (e: KeyboardEvent<HTMLTextAreaElement>): void => {
-		if (e.key === "Enter" && !e.shiftKey) {
+		if (shouldSubmitOnEnter(e, composingRef.current)) {
 			e.preventDefault();
 			send();
 		}
@@ -167,6 +204,8 @@ export function Composer({ client, snapshot }: ComposerProps): ReactNode {
 					value={text}
 					onChange={e => setText(e.target.value)}
 					onKeyDown={onKeyDown}
+					onCompositionStart={onCompositionStart}
+					onCompositionEnd={onCompositionEnd}
 					placeholder={
 						readOnly
 							? "read-only session — watching only"
diff --git a/packages/collab-web/test/composer.test.tsx b/packages/collab-web/test/composer.test.tsx
index 79c8a37e0..b25e2596f 100644
--- a/packages/collab-web/test/composer.test.tsx
+++ b/packages/collab-web/test/composer.test.tsx
@@ -1,8 +1,9 @@
 import { describe, expect, it } from "bun:test";
+import type { KeyboardEvent } from "react";
 import { renderToStaticMarkup } from "react-dom/server";
 import type { GuestSnapshot } from "../src/lib/client";
 import { GuestClient } from "../src/lib/client";
-import { Composer } from "../src/components/shell/Composer";
+import { Composer, shouldSubmitOnEnter } from "../src/components/shell/Composer";
 import { encodeBase64Url } from "../src/lib/link";
 
 const LINK = `roomroomroom1234#${encodeBase64Url(new Uint8Array(32))}`;
@@ -77,3 +78,35 @@ describe("Composer host UI requests", () => {
 		expect(submit.disabled).toBe(false);
 	});
 });
+
+type KeyEvt = KeyboardEvent<HTMLTextAreaElement>;
+
+function keydown(key: string, opts: { shiftKey?: boolean; isComposing?: boolean } = {}): KeyEvt {
+	return {
+		key,
+		shiftKey: opts.shiftKey ?? false,
+		nativeEvent: { isComposing: opts.isComposing ?? false },
+	} as KeyEvt;
+}
+
+describe("shouldSubmitOnEnter IME guard", () => {
+	it("submits on a plain Enter with no composition", () => {
+		expect(shouldSubmitOnEnter(keydown("Enter"), false)).toBe(true);
+	});
+
+	it("does not submit while nativeEvent.isComposing is true", () => {
+		expect(shouldSubmitOnEnter(keydown("Enter", { isComposing: true }), false)).toBe(false);
+	});
+
+	it("does not submit while the WebKit composing ref is still set", () => {
+		expect(shouldSubmitOnEnter(keydown("Enter"), true)).toBe(false);
+	});
+
+	it("does not submit on Shift+Enter (newline)", () => {
+		expect(shouldSubmitOnEnter(keydown("Enter", { shiftKey: true }), false)).toBe(false);
+	});
+
+	it("ignores non-Enter keys", () => {
+		expect(shouldSubmitOnEnter(keydown("a"), false)).toBe(false);
+	});
+});