diff --git a/src/app/projects/[projectId]/components/chatForm/ChatInput.tsx b/src/app/projects/[projectId]/components/chatForm/ChatInput.tsx
index 3e8c37c..8d66b18 100644
--- a/src/app/projects/[projectId]/components/chatForm/ChatInput.tsx
+++ b/src/app/projects/[projectId]/components/chatForm/ChatInput.tsx
@@ -1,12 +1,10 @@
 import { AlertCircleIcon, LoaderIcon, SendIcon } from "lucide-react";
-import { type FC, useId, useRef, useState } from "react";
+import { type FC, useCallback, useId, useRef, useState } from "react";
 import { Button } from "../../../../../components/ui/button";
 import { Textarea } from "../../../../../components/ui/textarea";
-import {
-  CommandCompletion,
-  type CommandCompletionRef,
-} from "./CommandCompletion";
-import { FileCompletion, type FileCompletionRef } from "./FileCompletion";
+import type { CommandCompletionRef } from "./CommandCompletion";
+import type { FileCompletionRef } from "./FileCompletion";
+import { InlineCompletion } from "./InlineCompletion";
 
 export interface ChatInputProps {
   projectId: string;
@@ -33,8 +31,14 @@ export const ChatInput: FC<ChatInputProps> = ({
   disabled = false,
   buttonSize = "lg",
 }) => {
-  const textareaRef = useRef<HTMLTextAreaElement>(null);
   const [message, setMessage] = useState("");
+  const [cursorPosition, setCursorPosition] = useState<{
+    relative: { top: number; left: number };
+    absolute: { top: number; left: number };
+  }>({ relative: { top: 0, left: 0 }, absolute: { top: 0, left: 0 } });
+
+  const containerRef = useRef<HTMLDivElement>(null);
+  const textareaRef = useRef<HTMLTextAreaElement>(null);
   const commandCompletionRef = useRef<CommandCompletionRef>(null);
   const fileCompletionRef = useRef<FileCompletionRef>(null);
   const helpId = useId();
@@ -60,6 +64,65 @@ export const ChatInput: FC<ChatInputProps> = ({
     }
   };
 
+  const getCursorPosition = useCallback(() => {
+    const textarea = textareaRef.current;
+    const container = containerRef.current;
+    if (textarea === null || container === null) return undefined;
+
+    const cursorPos = textarea.selectionStart;
+    const textBeforeCursor = textarea.value.substring(0, cursorPos);
+    const textAfterCursor = textarea.value.substring(cursorPos);
+
+    const pre = document.createTextNode(textBeforeCursor);
+    const post = document.createTextNode(textAfterCursor);
+    const caret = document.createElement("span");
+    caret.innerHTML = "&nbsp;";
+
+    const mirrored = document.createElement("div");
+
+    mirrored.innerHTML = "";
+    mirrored.append(pre, caret, post);
+
+    const textareaStyles = window.getComputedStyle(textarea);
+    for (const property of [
+      "border",
+      "boxSizing",
+      "fontFamily",
+      "fontSize",
+      "fontWeight",
+      "letterSpacing",
+      "lineHeight",
+      "padding",
+      "textDecoration",
+      "textIndent",
+      "textTransform",
+      "whiteSpace",
+      "wordSpacing",
+      "wordWrap",
+    ] as const) {
+      mirrored.style[property] = textareaStyles[property];
+    }
+
+    mirrored.style.visibility = "hidden";
+    container.prepend(mirrored);
+
+    const caretRect = caret.getBoundingClientRect();
+    const containerRect = container.getBoundingClientRect();
+
+    container.removeChild(mirrored);
+
+    return {
+      relative: {
+        top: caretRect.top - containerRect.top - textarea.scrollTop,
+        left: caretRect.left - containerRect.left - textarea.scrollLeft,
+      },
+      absolute: {
+        top: caretRect.top - textarea.scrollTop,
+        left: caretRect.left - textarea.scrollLeft,
+      },
+    };
+  }, []);
+
   const handleCommandSelect = (command: string) => {
     setMessage(command);
     textareaRef.current?.focus();
@@ -80,11 +143,23 @@ export const ChatInput: FC<ChatInputProps> = ({
       )}
 
       <div className="space-y-3">
-        <div className="relative">
+        <div className="relative" ref={containerRef}>
           <Textarea
             ref={textareaRef}
             value={message}
-            onChange={(e) => setMessage(e.target.value)}
+            onChange={(e) => {
+              if (
+                e.target.value.endsWith("@") ||
+                e.target.value.endsWith("/")
+              ) {
+                const position = getCursorPosition();
+                if (position) {
+                  setCursorPosition(position);
+                }
+              }
+
+              setMessage(e.target.value);
+            }}
             onKeyDown={handleKeyDown}
             placeholder={placeholder}
             className={`${minHeight} resize-none`}
@@ -97,22 +172,15 @@ export const ChatInput: FC<ChatInputProps> = ({
             role="combobox"
             aria-autocomplete="list"
           />
-          <CommandCompletion
-            ref={commandCompletionRef}
+          <InlineCompletion
             projectId={projectId}
-            inputValue={message}
-            onCommandSelect={handleCommandSelect}
-            className="absolute top-full left-0 right-0"
+            message={message}
+            commandCompletionRef={commandCompletionRef}
+            fileCompletionRef={fileCompletionRef}
+            handleCommandSelect={handleCommandSelect}
+            handleFileSelect={handleFileSelect}
+            cursorPosition={cursorPosition}
           />
-          {
-            <FileCompletion
-              ref={fileCompletionRef}
-              projectId={projectId}
-              inputValue={message}
-              onFileSelect={handleFileSelect}
-              className="absolute top-full left-0 right-0"
-            />
-          }
         </div>
 
         <div className="flex items-center justify-between">
diff --git a/src/app/projects/[projectId]/components/chatForm/CommandCompletion.tsx b/src/app/projects/[projectId]/components/chatForm/CommandCompletion.tsx
index 8f67900..d36e64a 100644
--- a/src/app/projects/[projectId]/components/chatForm/CommandCompletion.tsx
+++ b/src/app/projects/[projectId]/components/chatForm/CommandCompletion.tsx
@@ -93,19 +93,19 @@ export const CommandCompletion = forwardRef<
   );
 
   // スクロール処理
-  const scrollToSelected = useCallback(() => {
-    if (selectedIndex >= 0 && listRef.current) {
-      const selectedElement = listRef.current.children[
-        selectedIndex + 1
-      ] as HTMLElement; // +1 for header
-      if (selectedElement) {
-        selectedElement.scrollIntoView({
+  const scrollToSelected = useCallback((index: number) => {
+    if (index >= 0 && listRef.current) {
+      // ボタン要素を直接検索
+      const buttons = listRef.current.querySelectorAll('button[role="option"]');
+      const selectedButton = buttons[index] as HTMLElement;
+      if (selectedButton) {
+        selectedButton.scrollIntoView({
           block: "nearest",
           behavior: "smooth",
         });
       }
     }
-  }, [selectedIndex]);
+  }, []);
 
   // メモ化されたキーボードナビゲーション処理
   const handleKeyboardNavigation = useCallback(
@@ -117,8 +117,8 @@ export const CommandCompletion = forwardRef<
           e.preventDefault();
           setSelectedIndex((prev) => {
             const newIndex = prev < filteredCommands.length - 1 ? prev + 1 : 0;
-            // スクロールを次のタイクで実行
-            setTimeout(scrollToSelected, 0);
+            // スクロールを次のフレームで実行
+            requestAnimationFrame(() => scrollToSelected(newIndex));
             return newIndex;
           });
           return true;
@@ -126,8 +126,8 @@ export const CommandCompletion = forwardRef<
           e.preventDefault();
           setSelectedIndex((prev) => {
             const newIndex = prev > 0 ? prev - 1 : filteredCommands.length - 1;
-            // スクロールを次のタイクで実行
-            setTimeout(scrollToSelected, 0);
+            // スクロールを次のフレームで実行
+            requestAnimationFrame(() => scrollToSelected(newIndex));
             return newIndex;
           });
           return true;
@@ -214,7 +214,7 @@ export const CommandCompletion = forwardRef<
                     variant="ghost"
                     size="sm"
                     className={cn(
-                      "w-full justify-start text-left font-mono text-sm h-8 px-2",
+                      "w-full justify-start text-left font-mono text-sm h-8 px-2 min-w-0",
                       index === selectedIndex &&
                         "bg-accent text-accent-foreground",
                     )}
@@ -223,11 +223,16 @@ export const CommandCompletion = forwardRef<
                     role="option"
                     aria-selected={index === selectedIndex}
                     aria-label={`Command: /${command}`}
+                    title={`/${command}`}
                   >
-                    <span className="text-muted-foreground mr-1">/</span>
-                    <span className="font-medium">{command}</span>
+                    <span className="text-muted-foreground mr-1 flex-shrink-0">
+                      /
+                    </span>
+                    <span className="font-medium truncate min-w-0">
+                      {command}
+                    </span>
                     {index === selectedIndex && (
-                      <CheckIcon className="w-3 h-3 ml-auto text-primary" />
+                      <CheckIcon className="w-3 h-3 ml-auto text-primary flex-shrink-0" />
                     )}
                   </Button>
                 ))}
diff --git a/src/app/projects/[projectId]/components/chatForm/FileCompletion.tsx b/src/app/projects/[projectId]/components/chatForm/FileCompletion.tsx
index 31e5e60..9e095d8 100644
--- a/src/app/projects/[projectId]/components/chatForm/FileCompletion.tsx
+++ b/src/app/projects/[projectId]/components/chatForm/FileCompletion.tsx
@@ -148,19 +148,19 @@ export const FileCompletion = forwardRef<
   );
 
   // Scroll to selected entry
-  const scrollToSelected = useCallback(() => {
-    if (selectedIndex >= 0 && listRef.current) {
-      const selectedElement = listRef.current.children[
-        selectedIndex + 1
-      ] as HTMLElement; // +1 for header
-      if (selectedElement) {
-        selectedElement.scrollIntoView({
+  const scrollToSelected = useCallback((index: number) => {
+    if (index >= 0 && listRef.current) {
+      // ボタン要素を直接検索
+      const buttons = listRef.current.querySelectorAll('button[role="option"]');
+      const selectedButton = buttons[index] as HTMLElement;
+      if (selectedButton) {
+        selectedButton.scrollIntoView({
           block: "nearest",
           behavior: "smooth",
         });
       }
     }
-  }, [selectedIndex]);
+  }, []);
 
   // Keyboard navigation
   const handleKeyboardNavigation = useCallback(
@@ -172,7 +172,7 @@ export const FileCompletion = forwardRef<
           e.preventDefault();
           setSelectedIndex((prev) => {
             const newIndex = prev < filteredEntries.length - 1 ? prev + 1 : 0;
-            setTimeout(scrollToSelected, 0);
+            requestAnimationFrame(() => scrollToSelected(newIndex));
             return newIndex;
           });
           return true;
@@ -180,7 +180,7 @@ export const FileCompletion = forwardRef<
           e.preventDefault();
           setSelectedIndex((prev) => {
             const newIndex = prev > 0 ? prev - 1 : filteredEntries.length - 1;
-            setTimeout(scrollToSelected, 0);
+            requestAnimationFrame(() => scrollToSelected(newIndex));
             return newIndex;
           });
           return true;
@@ -283,7 +283,7 @@ export const FileCompletion = forwardRef<
                     variant="ghost"
                     size="sm"
                     className={cn(
-                      "w-full justify-start text-left font-mono text-sm h-8 px-2",
+                      "w-full justify-start text-left font-mono text-sm h-8 px-2 min-w-0",
                       index === selectedIndex &&
                         "bg-accent text-accent-foreground",
                     )}
@@ -294,18 +294,23 @@ export const FileCompletion = forwardRef<
                     role="option"
                     aria-selected={index === selectedIndex}
                     aria-label={`${entry.type}: ${entry.name}`}
+                    title={entry.path}
                   >
                     {entry.type === "directory" ? (
-                      <FolderIcon className="w-3 h-3 mr-2 text-blue-500" />
+                      <FolderIcon className="w-3 h-3 mr-2 text-blue-500 flex-shrink-0" />
                     ) : (
-                      <FileIcon className="w-3 h-3 mr-2 text-gray-500" />
+                      <FileIcon className="w-3 h-3 mr-2 text-gray-500 flex-shrink-0" />
                     )}
-                    <span className="font-medium">{entry.name}</span>
+                    <span className="font-medium truncate min-w-0">
+                      {entry.name}
+                    </span>
                     {entry.type === "directory" && (
-                      <span className="text-muted-foreground ml-1">/</span>
+                      <span className="text-muted-foreground ml-1 flex-shrink-0">
+                        /
+                      </span>
                     )}
                     {index === selectedIndex && (
-                      <CheckIcon className="w-3 h-3 ml-auto text-primary" />
+                      <CheckIcon className="w-3 h-3 ml-auto text-primary flex-shrink-0" />
                     )}
                   </Button>
                 ))}
diff --git a/src/app/projects/[projectId]/components/chatForm/InlineCompletion.tsx b/src/app/projects/[projectId]/components/chatForm/InlineCompletion.tsx
new file mode 100644
index 0000000..dfd738f
--- /dev/null
+++ b/src/app/projects/[projectId]/components/chatForm/InlineCompletion.tsx
@@ -0,0 +1,135 @@
+import type { FC, RefObject } from "react";
+import { useMemo } from "react";
+import {
+  CommandCompletion,
+  type CommandCompletionRef,
+} from "./CommandCompletion";
+import { FileCompletion, type FileCompletionRef } from "./FileCompletion";
+
+interface PositionStyle {
+  top: number;
+  left: number;
+  placement: "above" | "below";
+}
+
+const calculateOptimalPosition = (
+  relativeCursorPosition: { top: number; left: number },
+  absoluteCursorPosition: { top: number; left: number },
+): PositionStyle => {
+  const viewportHeight =
+    typeof window !== "undefined" ? window.innerHeight : 800;
+  const viewportCenter = viewportHeight / 2;
+
+  // Estimated completion height (we'll measure actual height later if needed)
+  const estimatedCompletionHeight = 200;
+
+  // Determine preferred placement based on viewport position
+  console.log("debug cursor and viewport", {
+    absoluteCursorTop: absoluteCursorPosition.top,
+    viewportCenter,
+  });
+  const isInUpperHalf = absoluteCursorPosition.top < viewportCenter;
+
+  // Check if there's enough space for preferred placement
+  const spaceBelow = viewportHeight - absoluteCursorPosition.top;
+  const spaceAbove = absoluteCursorPosition.top;
+
+  let placement: "above" | "below";
+  let top: number;
+
+  if (isInUpperHalf && spaceBelow >= estimatedCompletionHeight) {
+    // Cursor in upper half and enough space below - place below
+    placement = "below";
+    top = relativeCursorPosition.top + 16;
+  } else if (!isInUpperHalf && spaceAbove >= estimatedCompletionHeight) {
+    // Cursor in lower half and enough space above - place above
+    placement = "above";
+    top = relativeCursorPosition.top - estimatedCompletionHeight - 8;
+  } else {
+    // Use whichever side has more space
+    if (spaceBelow > spaceAbove) {
+      placement = "below";
+      top = relativeCursorPosition.top + 16;
+    } else {
+      placement = "above";
+      top = relativeCursorPosition.top - estimatedCompletionHeight - 8;
+    }
+  }
+
+  // Ensure left position stays within viewport bounds
+  const estimatedCompletionWidth = 512; // Current w-lg width
+  const viewportWidth =
+    typeof window !== "undefined" ? window.innerWidth : 1200;
+  const maxLeft = viewportWidth - estimatedCompletionWidth - 16;
+  const adjustedLeft = Math.max(
+    16,
+    Math.min(relativeCursorPosition.left - 16, maxLeft),
+  );
+
+  return {
+    top,
+    left: adjustedLeft,
+    placement,
+  };
+};
+
+export const InlineCompletion: FC<{
+  projectId: string;
+  message: string;
+  commandCompletionRef: RefObject<CommandCompletionRef | null>;
+  fileCompletionRef: RefObject<FileCompletionRef | null>;
+  handleCommandSelect: (command: string) => void;
+  handleFileSelect: (filePath: string) => void;
+  cursorPosition: {
+    relative: { top: number; left: number };
+    absolute: { top: number; left: number };
+  };
+}> = ({
+  projectId,
+  message,
+  commandCompletionRef,
+  fileCompletionRef,
+  handleCommandSelect,
+  handleFileSelect,
+  cursorPosition,
+}) => {
+  const position = useMemo(() => {
+    return calculateOptimalPosition(
+      cursorPosition.relative,
+      cursorPosition.absolute,
+    );
+  }, [cursorPosition]);
+
+  return (
+    <div
+      className="absolute w-full max-w-sm sm:max-w-md lg:max-w-lg xl:max-w-xl"
+      style={{
+        top: position.top,
+        left: position.left,
+        maxWidth:
+          typeof window !== "undefined"
+            ? Math.min(512, window.innerWidth * 0.8)
+            : 512,
+      }}
+    >
+      <CommandCompletion
+        ref={commandCompletionRef}
+        projectId={projectId}
+        inputValue={message}
+        onCommandSelect={handleCommandSelect}
+        className={`absolute left-0 right-0 ${
+          position.placement === "above" ? "bottom-full mb-2" : "top-full mt-1"
+        }`}
+      />
+      <FileCompletion
+        ref={fileCompletionRef}
+        projectId={projectId}
+        inputValue={message}
+        onFileSelect={handleFileSelect}
+        className={`absolute left-0 right-0 ${
+          position.placement === "above" ? "bottom-full mb-2" : "top-full mt-1"
+        }`}
+      />
+    </div>
+  );
+};