diff --git a/studio/frontend/package-lock.json b/studio/frontend/package-lock.json
index ed2ceb550..21d31d81e 100644
--- a/studio/frontend/package-lock.json
+++ b/studio/frontend/package-lock.json
@@ -12,6 +12,7 @@
         "@assistant-ui/react": "0.12.28",
         "@assistant-ui/react-markdown": "0.12.11",
         "@assistant-ui/react-streamdown": "0.1.11",
+        "@assistant-ui/tap": "0.5.10",
         "@base-ui/react": "^1.2.0",
         "@dagrejs/dagre": "^2.0.4",
         "@dagrejs/graphlib": "^3.0.4",
diff --git a/studio/frontend/package.json b/studio/frontend/package.json
index 22088c68a..463fbd926 100644
--- a/studio/frontend/package.json
+++ b/studio/frontend/package.json
@@ -20,6 +20,7 @@
     "@assistant-ui/react": "0.12.28",
     "@assistant-ui/react-markdown": "0.12.11",
     "@assistant-ui/react-streamdown": "0.1.11",
+    "@assistant-ui/tap": "0.5.10",
     "@base-ui/react": "^1.2.0",
     "@dagrejs/dagre": "^2.0.4",
     "@dagrejs/graphlib": "^3.0.4",
diff --git a/studio/frontend/src/components/assistant-ui/thread.tsx b/studio/frontend/src/components/assistant-ui/thread.tsx
index 31a1fb21e..dc3d1c21b 100644
--- a/studio/frontend/src/components/assistant-ui/thread.tsx
+++ b/studio/frontend/src/components/assistant-ui/thread.tsx
@@ -51,6 +51,7 @@ import {
   useAuiEvent,
   useAuiState,
 } from "@assistant-ui/react";
+import { flushResourcesSync } from "@assistant-ui/tap";
 import {
   ArrowDownIcon,
   ArrowUpIcon,
@@ -72,6 +73,8 @@ import {
 import { Copy01Icon, Delete02Icon, Edit03Icon, Tick02Icon } from "@hugeicons/core-free-icons";
 import { HugeiconsIcon } from "@hugeicons/react";
 import {
+  type ChangeEvent,
+  type CompositionEvent,
   type FC,
   type FormEvent,
   useCallback,
@@ -282,13 +285,15 @@ const PendingAudioChip: FC = () => {
 };
 
 const Composer: FC<{ disabled?: boolean }> = ({ disabled }) => {
+  const { inputProps, isComposing, isComposingRef } = useImeComposerInputHandlers();
+
   const handleSubmit = useCallback(
     (event: FormEvent<HTMLFormElement>) => {
-      if (disabled) {
+      if (disabled || isComposingRef.current) {
         event.preventDefault();
       }
     },
-    [disabled],
+    [disabled, isComposingRef],
   );
 
   const composerContent = (
@@ -304,8 +309,12 @@ const Composer: FC<{ disabled?: boolean }> = ({ disabled }) => {
         autoFocus={!disabled}
         disabled={disabled}
         aria-label="Message input"
+        {...inputProps}
+      />
+      <ComposerAction
+        disabled={disabled || isComposing}
+        blockSend={() => isComposingRef.current}
       />
-      <ComposerAction disabled={disabled} />
     </>
   );
 
@@ -330,6 +339,64 @@ const Composer: FC<{ disabled?: boolean }> = ({ disabled }) => {
   );
 };
 
+function isNativeComposing(event: Event) {
+  return "isComposing" in event && (event as InputEvent).isComposing === true;
+}
+
+function useImeComposerInputHandlers() {
+  const aui = useAui();
+  const composingRef = useRef(false);
+  const [isComposing, setIsComposing] = useState(false);
+
+  const setCompositionState = useCallback((next: boolean) => {
+    composingRef.current = next;
+    setIsComposing(next);
+  }, []);
+
+  const setComposerText = useCallback(
+    (value: string) => {
+      const composer = aui.composer();
+      if (!composer.getState().isEditing) {
+        return;
+      }
+      flushResourcesSync(() => {
+        composer.setText(value);
+      });
+    },
+    [aui],
+  );
+
+  const onCompositionStart = useCallback(() => {
+    setCompositionState(true);
+  }, [setCompositionState]);
+
+  const onCompositionEnd = useCallback(
+    (e: CompositionEvent<HTMLTextAreaElement>) => {
+      setCompositionState(false);
+      setComposerText(e.currentTarget.value);
+    },
+    [setComposerText, setCompositionState],
+  );
+
+  const onChange = useCallback(
+    (e: ChangeEvent<HTMLTextAreaElement>) => {
+      setCompositionState(isNativeComposing(e.nativeEvent));
+      setComposerText(e.target.value);
+    },
+    [setComposerText, setCompositionState],
+  );
+
+  return {
+    inputProps: {
+      onCompositionStart,
+      onCompositionEnd,
+      onChange,
+    },
+    isComposing,
+    isComposingRef: composingRef,
+  };
+}
+
 const ComposerAudioUpload: FC = () => {
   const audioInputRef = useRef<HTMLInputElement>(null);
   const setPendingAudio = useChatRuntimeStore((s) => s.setPendingAudio);
@@ -607,7 +674,10 @@ const ToolStatusDisplay: FC = () => {
   );
 };
 
-const ComposerAction: FC<{ disabled?: boolean }> = ({ disabled }) => {
+const ComposerAction: FC<{ disabled?: boolean; blockSend?: () => boolean }> = ({
+  disabled,
+  blockSend,
+}) => {
   return (
     <div className="aui-composer-action-wrapper composer-action-wrapper">
       <div className="flex items-center gap-1">
@@ -650,6 +720,11 @@ const ComposerAction: FC<{ disabled?: boolean }> = ({ disabled }) => {
               variant="default"
               size="icon"
               disabled={disabled}
+              onClick={(event) => {
+                if (blockSend?.()) {
+                  event.preventDefault();
+                }
+              }}
               className="aui-composer-send size-8 rounded-full"
               aria-label="Send message"
             >
@@ -903,6 +978,7 @@ const UserActionBar: FC = () => {
 
 const EditComposer: FC = () => {
   const aui = useAui();
+  const { inputProps, isComposingRef } = useImeComposerInputHandlers();
   const resendAfterCancelRef = useRef(false);
 
   useAuiEvent("thread.runEnd", () => {
@@ -919,16 +995,22 @@ const EditComposer: FC = () => {
         <ComposerPrimitive.Input
           className="aui-edit-composer-input min-h-14 w-full resize-none bg-transparent p-4 text-foreground text-sm font-[450] outline-none"
           autoFocus={true}
+          {...inputProps}
         />
         <div className="aui-edit-composer-footer mx-3 mb-3 flex items-center gap-2 self-end">
           <ComposerPrimitive.Cancel asChild={true}>
-            <Button variant="ghost" size="sm">
+            <Button type="button" variant="ghost" size="sm">
               Cancel
             </Button>
           </ComposerPrimitive.Cancel>
           <Button
+            type="button"
             size="sm"
-            onClick={() => {
+            onClick={(event) => {
+              if (isComposingRef.current) {
+                event.preventDefault();
+                return;
+              }
               const newText = aui.composer().getState().text;
               const originalText = aui.message().getCopyText();
 
diff --git a/studio/frontend/src/features/chat/shared-composer.tsx b/studio/frontend/src/features/chat/shared-composer.tsx
index c5544b29f..d48ffb293 100644
--- a/studio/frontend/src/features/chat/shared-composer.tsx
+++ b/studio/frontend/src/features/chat/shared-composer.tsx
@@ -20,6 +20,7 @@ import { toast } from "sonner";
 import { loadModel, validateModel } from "./api/chat-api";
 import { useChatRuntimeStore } from "./stores/chat-runtime-store";
 import {
+  type CompositionEvent,
   type KeyboardEvent,
   type MutableRefObject,
   type ReactElement,
@@ -52,6 +53,10 @@ export interface CompareHandle {
 const IMAGE_ACCEPT = "image/jpeg,image/png,image/webp,image/gif";
 const MAX_IMAGE_SIZE = 20 * 1024 * 1024;
 
+function isNativeComposing(event: Event) {
+  return "isComposing" in event && (event as InputEvent).isComposing === true;
+}
+
 function fileToBase64DataURL(file: File): Promise<string> {
   return new Promise((resolve, reject) => {
     const reader = new FileReader();
@@ -238,7 +243,9 @@ export function SharedComposer({
   const [pendingImages, setPendingImages] = useState<PendingImage[]>([]);
   const [pendingAudio, setPendingAudio] = useState<{ name: string; base64: string } | null>(null);
   const [dragging, setDragging] = useState(false);
+  const [isComposing, setIsComposing] = useState(false);
   const textareaRef = useRef<HTMLTextAreaElement>(null);
+  const composingRef = useRef(false);
   const fileInputRef = useRef<HTMLInputElement>(null);
   const audioInputRef = useRef<HTMLInputElement>(null);
 
@@ -323,7 +330,13 @@ export function SharedComposer({
     setPendingImages((prev) => prev.filter((p) => p.id !== id));
   }, []);
 
+  function setCompositionState(next: boolean) {
+    composingRef.current = next;
+    setIsComposing(next);
+  }
+
   async function send() {
+    if (composingRef.current) return;
     const msg = text.trim();
     if (!msg && pendingImages.length === 0 && !pendingAudio) return;
 
@@ -482,6 +495,9 @@ export function SharedComposer({
   const busy = running || comparing;
 
   function onKeyDown(e: KeyboardEvent) {
+    // IME composition (Japanese/Chinese/Korean): Enter commits the candidate.
+    // Don't hijack it. See issue #5318.
+    if (e.nativeEvent.isComposing || e.keyCode === 229) return;
     if (e.key === "Enter" && !e.shiftKey) {
       e.preventDefault();
       if (!busy) {
@@ -490,7 +506,7 @@ export function SharedComposer({
     }
   }
 
-  const canSend = (text.trim().length > 0 || pendingImages.length > 0 || pendingAudio !== null) && !busy;
+  const canSend = (text.trim().length > 0 || pendingImages.length > 0 || pendingAudio !== null) && !busy && !isComposing;
 
   return (
     <div
@@ -538,7 +554,23 @@ export function SharedComposer({
       <textarea
         ref={textareaRef}
         value={text}
-        onChange={(e) => setText(e.target.value)}
+        onChange={(e) => {
+          // ALWAYS mirror the DOM value into React state, even during IME
+          // composition. The controlled `value` prop must match the DOM at
+          // all times, otherwise any unrelated parent re-render reconciles
+          // the textarea back to the stored value mid-composition — wiping
+          // the IME preedit AND prior committed text (e.g. Tab cycling
+          // candidates erases earlier words). Issue #5318.
+          setCompositionState(isNativeComposing(e.nativeEvent));
+          setText(e.target.value);
+        }}
+        onCompositionStart={() => {
+          setCompositionState(true);
+        }}
+        onCompositionEnd={(e: CompositionEvent<HTMLTextAreaElement>) => {
+          setCompositionState(false);
+          setText(e.currentTarget.value);
+        }}
         onKeyDown={onKeyDown}
         placeholder="Send to both models..."
         className="composer-input"