3 files changed, 298 insertions, 10 deletions
diff --git a/packages/cli/src/ui/hooks/useCommandCompletion.test.ts b/packages/cli/src/ui/hooks/useCommandCompletion.test.ts
index a3c96935..00bc8ac3 100644
--- a/packages/cli/src/ui/hooks/useCommandCompletion.test.ts
+++ b/packages/cli/src/ui/hooks/useCommandCompletion.test.ts
@@ -84,7 +84,9 @@ const setupMocks = ({
 
 describe('useCommandCompletion', () => {
   const mockCommandContext = {} as CommandContext;
-  const mockConfig = {} as Config;
+  const mockConfig = {
+    getEnablePromptCompletion: () => false,
+  } as Config;
   const testDirs: string[] = [];
   const testRootDir = '/';
 
@@ -511,7 +513,7 @@ describe('useCommandCompletion', () => {
       });
 
       expect(result.current.textBuffer.text).toBe(
-        '@src/file1.txt  is a good file',
+        '@src/file1.txt is a good file',
       );
     });
   });
diff --git a/packages/cli/src/ui/hooks/useCommandCompletion.tsx b/packages/cli/src/ui/hooks/useCommandCompletion.tsx
index 07d0e056..166f03c5 100644
--- a/packages/cli/src/ui/hooks/useCommandCompletion.tsx
+++ b/packages/cli/src/ui/hooks/useCommandCompletion.tsx
@@ -15,6 +15,11 @@ import { isSlashCommand } from '../utils/commandUtils.js';
 import { toCodePoints } from '../utils/textUtils.js';
 import { useAtCompletion } from './useAtCompletion.js';
 import { useSlashCompletion } from './useSlashCompletion.js';
+import {
+  usePromptCompletion,
+  PromptCompletion,
+  PROMPT_COMPLETION_MIN_LENGTH,
+} from './usePromptCompletion.js';
 import { Config } from '@google/gemini-cli-core';
 import { useCompletion } from './useCompletion.js';
 
@@ -22,6 +27,7 @@ export enum CompletionMode {
   IDLE = 'IDLE',
   AT = 'AT',
   SLASH = 'SLASH',
+  PROMPT = 'PROMPT',
 }
 
 export interface UseCommandCompletionReturn {
@@ -37,6 +43,7 @@ export interface UseCommandCompletionReturn {
   navigateUp: () => void;
   navigateDown: () => void;
   handleAutocomplete: (indexToUse: number) => void;
+  promptCompletion: PromptCompletion;
 }
 
 export function useCommandCompletion(
@@ -93,12 +100,7 @@ export function useCommandCompletion(
             backslashCount++;
           }
           if (backslashCount % 2 === 0) {
-            return {
-              completionMode: CompletionMode.IDLE,
-              query: null,
-              completionStart: -1,
-              completionEnd: -1,
-            };
+            break;
           }
         } else if (char === '@') {
           let end = codePoints.length;
@@ -125,13 +127,33 @@ export function useCommandCompletion(
           };
         }
       }
+
+      // Check for prompt completion - only if enabled
+      const trimmedText = buffer.text.trim();
+      const isPromptCompletionEnabled =
+        config?.getEnablePromptCompletion() ?? false;
+
+      if (
+        isPromptCompletionEnabled &&
+        trimmedText.length >= PROMPT_COMPLETION_MIN_LENGTH &&
+        !trimmedText.startsWith('/') &&
+        !trimmedText.includes('@')
+      ) {
+        return {
+          completionMode: CompletionMode.PROMPT,
+          query: trimmedText,
+          completionStart: 0,
+          completionEnd: trimmedText.length,
+        };
+      }
+
       return {
         completionMode: CompletionMode.IDLE,
         query: null,
         completionStart: -1,
         completionEnd: -1,
       };
-    }, [cursorRow, cursorCol, buffer.lines]);
+    }, [cursorRow, cursorCol, buffer.lines, buffer.text, config]);
 
   useAtCompletion({
     enabled: completionMode === CompletionMode.AT,
@@ -152,6 +174,12 @@ export function useCommandCompletion(
     setIsPerfectMatch,
   });
 
+  const promptCompletion = usePromptCompletion({
+    buffer,
+    config,
+    enabled: completionMode === CompletionMode.PROMPT,
+  });
+
   useEffect(() => {
     setActiveSuggestionIndex(suggestions.length > 0 ? 0 : -1);
     setVisibleStartIndex(0);
@@ -202,7 +230,11 @@ export function useCommandCompletion(
         }
       }
 
-      suggestionText += ' ';
+      const lineCodePoints = toCodePoints(buffer.lines[cursorRow] || '');
+      const charAfterCompletion = lineCodePoints[end];
+      if (charAfterCompletion !== ' ') {
+        suggestionText += ' ';
+      }
 
       buffer.replaceRangeByOffset(
         logicalPosToOffset(buffer.lines, cursorRow, start),
@@ -234,5 +266,6 @@ export function useCommandCompletion(
     navigateUp,
     navigateDown,
     handleAutocomplete,
+    promptCompletion,
   };
 }
diff --git a/packages/cli/src/ui/hooks/usePromptCompletion.ts b/packages/cli/src/ui/hooks/usePromptCompletion.ts
new file mode 100644
index 00000000..466d020b
--- /dev/null
+++ b/packages/cli/src/ui/hooks/usePromptCompletion.ts
@@ -0,0 +1,253 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+import { useState, useCallback, useRef, useEffect, useMemo } from 'react';
+import {
+  Config,
+  DEFAULT_GEMINI_FLASH_LITE_MODEL,
+  getResponseText,
+} from '@google/gemini-cli-core';
+import { Content, GenerateContentConfig } from '@google/genai';
+import { TextBuffer } from '../components/shared/text-buffer.js';
+
+export const PROMPT_COMPLETION_MIN_LENGTH = 5;
+export const PROMPT_COMPLETION_DEBOUNCE_MS = 250;
+
+export interface PromptCompletion {
+  text: string;
+  isLoading: boolean;
+  isActive: boolean;
+  accept: () => void;
+  clear: () => void;
+  markSelected: (selectedText: string) => void;
+}
+
+export interface UsePromptCompletionOptions {
+  buffer: TextBuffer;
+  config?: Config;
+  enabled: boolean;
+}
+
+export function usePromptCompletion({
+  buffer,
+  config,
+  enabled,
+}: UsePromptCompletionOptions): PromptCompletion {
+  const [ghostText, setGhostText] = useState<string>('');
+  const [isLoadingGhostText, setIsLoadingGhostText] = useState<boolean>(false);
+  const abortControllerRef = useRef<AbortController | null>(null);
+  const [justSelectedSuggestion, setJustSelectedSuggestion] =
+    useState<boolean>(false);
+  const lastSelectedTextRef = useRef<string>('');
+  const lastRequestedTextRef = useRef<string>('');
+
+  const isPromptCompletionEnabled =
+    enabled && (config?.getEnablePromptCompletion() ?? false);
+
+  const clearGhostText = useCallback(() => {
+    setGhostText('');
+    setIsLoadingGhostText(false);
+  }, []);
+
+  const acceptGhostText = useCallback(() => {
+    if (ghostText && ghostText.length > buffer.text.length) {
+      buffer.setText(ghostText);
+      setGhostText('');
+      setJustSelectedSuggestion(true);
+      lastSelectedTextRef.current = ghostText;
+    }
+  }, [ghostText, buffer]);
+
+  const markSuggestionSelected = useCallback((selectedText: string) => {
+    setJustSelectedSuggestion(true);
+    lastSelectedTextRef.current = selectedText;
+  }, []);
+
+  const generatePromptSuggestions = useCallback(async () => {
+    const trimmedText = buffer.text.trim();
+    const geminiClient = config?.getGeminiClient();
+
+    if (trimmedText === lastRequestedTextRef.current) {
+      return;
+    }
+
+    if (abortControllerRef.current) {
+      abortControllerRef.current.abort();
+    }
+
+    if (
+      trimmedText.length < PROMPT_COMPLETION_MIN_LENGTH ||
+      !geminiClient ||
+      trimmedText.startsWith('/') ||
+      trimmedText.includes('@') ||
+      !isPromptCompletionEnabled
+    ) {
+      clearGhostText();
+      lastRequestedTextRef.current = '';
+      return;
+    }
+
+    lastRequestedTextRef.current = trimmedText;
+    setIsLoadingGhostText(true);
+
+    abortControllerRef.current = new AbortController();
+    const signal = abortControllerRef.current.signal;
+
+    try {
+      const contents: Content[] = [
+        {
+          role: 'user',
+          parts: [
+            {
+              text: `You are a professional prompt engineering assistant. Complete the user's partial prompt with expert precision and clarity. User's input: "${trimmedText}" Continue this prompt by adding specific, actionable details that align with the user's intent. Focus on: clear, precise language; structured requirements; professional terminology; measurable outcomes. Length Guidelines: Keep suggestions concise (ideally 10-20 characters); prioritize brevity while maintaining clarity; use essential keywords only; avoid redundant phrases. Start your response with the exact user text ("${trimmedText}") followed by your completion. Provide practical, implementation-focused suggestions rather than creative interpretations. Format: Plain text only. Single completion. Match the user's language. Emphasize conciseness over elaboration.`,
+            },
+          ],
+        },
+      ];
+
+      const generationConfig: GenerateContentConfig = {
+        temperature: 0.3,
+        maxOutputTokens: 16000,
+        thinkingConfig: {
+          thinkingBudget: 0,
+        },
+      };
+
+      const response = await geminiClient.generateContent(
+        contents,
+        generationConfig,
+        signal,
+        DEFAULT_GEMINI_FLASH_LITE_MODEL,
+      );
+
+      if (signal.aborted) {
+        return;
+      }
+
+      if (response) {
+        const responseText = getResponseText(response);
+
+        if (responseText) {
+          const suggestionText = responseText.trim();
+
+          if (
+            suggestionText.length > 0 &&
+            suggestionText.startsWith(trimmedText)
+          ) {
+            setGhostText(suggestionText);
+          } else {
+            clearGhostText();
+          }
+        }
+      }
+    } catch (error) {
+      if (
+        !(
+          signal.aborted ||
+          (error instanceof Error && error.name === 'AbortError')
+        )
+      ) {
+        console.error('prompt completion error:', error);
+        // Clear the last requested text to allow retry only on real errors
+        lastRequestedTextRef.current = '';
+      }
+      clearGhostText();
+    } finally {
+      if (!signal.aborted) {
+        setIsLoadingGhostText(false);
+      }
+    }
+  }, [buffer.text, config, clearGhostText, isPromptCompletionEnabled]);
+
+  const isCursorAtEnd = useCallback(() => {
+    const [cursorRow, cursorCol] = buffer.cursor;
+    const totalLines = buffer.lines.length;
+    if (cursorRow !== totalLines - 1) {
+      return false;
+    }
+
+    const lastLine = buffer.lines[cursorRow] || '';
+    return cursorCol === lastLine.length;
+  }, [buffer.cursor, buffer.lines]);
+
+  const handlePromptCompletion = useCallback(() => {
+    if (!isCursorAtEnd()) {
+      clearGhostText();
+      return;
+    }
+
+    const trimmedText = buffer.text.trim();
+
+    if (justSelectedSuggestion && trimmedText === lastSelectedTextRef.current) {
+      return;
+    }
+
+    if (trimmedText !== lastSelectedTextRef.current) {
+      setJustSelectedSuggestion(false);
+      lastSelectedTextRef.current = '';
+    }
+
+    generatePromptSuggestions();
+  }, [
+    buffer.text,
+    generatePromptSuggestions,
+    justSelectedSuggestion,
+    isCursorAtEnd,
+    clearGhostText,
+  ]);
+
+  // Debounce prompt completion
+  useEffect(() => {
+    const timeoutId = setTimeout(
+      handlePromptCompletion,
+      PROMPT_COMPLETION_DEBOUNCE_MS,
+    );
+    return () => clearTimeout(timeoutId);
+  }, [buffer.text, buffer.cursor, handlePromptCompletion]);
+
+  // Ghost text validation - clear if it doesn't match current text or cursor not at end
+  useEffect(() => {
+    const currentText = buffer.text.trim();
+
+    if (ghostText && !isCursorAtEnd()) {
+      clearGhostText();
+      return;
+    }
+
+    if (
+      ghostText &&
+      currentText.length > 0 &&
+      !ghostText.startsWith(currentText)
+    ) {
+      clearGhostText();
+    }
+  }, [buffer.text, buffer.cursor, ghostText, clearGhostText, isCursorAtEnd]);
+
+  // Cleanup on unmount
+  useEffect(() => () => abortControllerRef.current?.abort(), []);
+
+  const isActive = useMemo(() => {
+    if (!isPromptCompletionEnabled) return false;
+
+    if (!isCursorAtEnd()) return false;
+
+    const trimmedText = buffer.text.trim();
+    return (
+      trimmedText.length >= PROMPT_COMPLETION_MIN_LENGTH &&
+      !trimmedText.startsWith('/') &&
+      !trimmedText.includes('@')
+    );
+  }, [buffer.text, isPromptCompletionEnabled, isCursorAtEnd]);
+
+  return {
+    text: ghostText,
+    isLoading: isLoadingGhostText,
+    isActive,
+    accept: acceptGhostText,
+    clear: clearGhostText,
+    markSelected: markSuggestionSelected,
+  };
+}