diff --git a/src/tools/ast-grep/cli.ts b/src/tools/ast-grep/cli.ts
index 66f8f05..861d10c 100644
--- a/src/tools/ast-grep/cli.ts
+++ b/src/tools/ast-grep/cli.ts
@@ -1,5 +1,7 @@
 import { spawn } from "bun"
-import { SG_CLI_PATH } from "./constants"
+import { existsSync } from "fs"
+import { getSgCliPath, setSgCliPath, findSgCliPathSync } from "./constants"
+import { ensureAstGrepBinary } from "./downloader"
 import type { CliMatch, CliLanguage } from "./types"
 
 export interface RunOptions {
@@ -12,6 +14,65 @@ export interface RunOptions {
   updateAll?: boolean
 }
 
+let resolvedCliPath: string | null = null
+let initPromise: Promise<string | null> | null = null
+
+export async function getAstGrepPath(): Promise<string | null> {
+  if (resolvedCliPath !== null && existsSync(resolvedCliPath)) {
+    return resolvedCliPath
+  }
+
+  if (initPromise) {
+    return initPromise
+  }
+
+  initPromise = (async () => {
+    const syncPath = findSgCliPathSync()
+    if (syncPath && existsSync(syncPath)) {
+      resolvedCliPath = syncPath
+      setSgCliPath(syncPath)
+      return syncPath
+    }
+
+    const downloadedPath = await ensureAstGrepBinary()
+    if (downloadedPath) {
+      resolvedCliPath = downloadedPath
+      setSgCliPath(downloadedPath)
+      return downloadedPath
+    }
+
+    return null
+  })()
+
+  return initPromise
+}
+
+export function startBackgroundInit(): void {
+  if (!initPromise) {
+    initPromise = getAstGrepPath()
+    initPromise.catch(() => {})
+  }
+}
+
+interface SpawnResult {
+  stdout: string
+  stderr: string
+  exitCode: number
+}
+
+async function spawnSg(cliPath: string, args: string[]): Promise<SpawnResult> {
+  const proc = spawn([cliPath, ...args], {
+    stdout: "pipe",
+    stderr: "pipe",
+  })
+
+  const stdout = await new Response(proc.stdout).text()
+  const stderr = await new Response(proc.stderr).text()
+  const exitCode = await proc.exited
+
+  return { stdout, stderr, exitCode }
+}
+
 export async function runSg(options: RunOptions): Promise<CliMatch[]> {
   const args = ["run", "-p", options.pattern, "--lang", options.lang, "--json=compact"]
 
@@ -35,14 +96,45 @@ export async function runSg(options: RunOptions): Promise<CliMatch[]> {
   const paths = options.paths && options.paths.length > 0 ? options.paths : ["."]
   args.push(...paths)
 
-  const proc = spawn([SG_CLI_PATH, ...args], {
-    stdout: "pipe",
-    stderr: "pipe",
-  })
+  let cliPath = getSgCliPath()
 
-  const stdout = await new Response(proc.stdout).text()
-  const stderr = await new Response(proc.stderr).text()
-  const exitCode = await proc.exited
+  if (!existsSync(cliPath) && cliPath !== "sg") {
+    const downloadedPath = await getAstGrepPath()
+    if (downloadedPath) {
+      cliPath = downloadedPath
+    }
+  }
+
+  let result: SpawnResult
+  try {
+    result = await spawnSg(cliPath, args)
+  } catch (e) {
+    const error = e as NodeJS.ErrnoException
+    if (
+      error.code === "ENOENT" ||
+      error.message?.includes("ENOENT") ||
+      error.message?.includes("not found")
+    ) {
+      const downloadedPath = await ensureAstGrepBinary()
+      if (downloadedPath) {
+        resolvedCliPath = downloadedPath
+        setSgCliPath(downloadedPath)
+        result = await spawnSg(downloadedPath, args)
+      } else {
+        throw new Error(
+          `ast-grep CLI binary not found.\n\n` +
+            `Auto-download failed. Manual install options:\n` +
+            `  bun add -D @ast-grep/cli\n` +
+            `  cargo install ast-grep --locked\n` +
+            `  brew install ast-grep`
+        )
+      }
+    } else {
+      throw new Error(`Failed to spawn ast-grep: ${error.message}`)
+    }
+  }
+
+  const { stdout, stderr, exitCode } = result
 
   if (exitCode !== 0 && stdout.trim() === "") {
     if (stderr.includes("No files found")) {
@@ -64,3 +156,13 @@ export async function runSg(options: RunOptions): Promise<CliMatch[]> {
     return []
   }
 }
+
+export function isCliAvailable(): boolean {
+  const path = findSgCliPathSync()
+  return path !== null && existsSync(path)
+}
+
+export async function ensureCliAvailable(): Promise<boolean> {
+  const path = await getAstGrepPath()
+  return path !== null && existsSync(path)
+}
diff --git a/src/tools/ast-grep/constants.ts b/src/tools/ast-grep/constants.ts
index 24a43ff..68cbe50 100644
--- a/src/tools/ast-grep/constants.ts
+++ b/src/tools/ast-grep/constants.ts
@@ -1,6 +1,7 @@
 import { createRequire } from "module"
 import { dirname, join } from "path"
 import { existsSync } from "fs"
+import { getCachedBinaryPath } from "./downloader"
 
 type Platform = "darwin" | "linux" | "win32" | "unsupported"
 
@@ -21,30 +22,30 @@ function getPlatformPackageName(): string | null {
   return platformMap[`${platform}-${arch}`] ?? null
 }
 
-function findSgCliPath(): string {
-  // 1. Try to find from @ast-grep/cli package (installed via npm)
+export function findSgCliPathSync(): string | null {
+  const binaryName = process.platform === "win32" ? "sg.exe" : "sg"
+
   try {
     const require = createRequire(import.meta.url)
     const cliPkgPath = require.resolve("@ast-grep/cli/package.json")
     const cliDir = dirname(cliPkgPath)
-    const sgPath = join(cliDir, process.platform === "win32" ? "sg.exe" : "sg")
+    const sgPath = join(cliDir, binaryName)
 
     if (existsSync(sgPath)) {
       return sgPath
     }
   } catch {
-    // @ast-grep/cli not installed, try platform-specific package
+    // @ast-grep/cli not installed
   }
 
-  // 2. Try platform-specific package directly
   const platformPkg = getPlatformPackageName()
   if (platformPkg) {
     try {
       const require = createRequire(import.meta.url)
       const pkgPath = require.resolve(`${platformPkg}/package.json`)
       const pkgDir = dirname(pkgPath)
-      const binaryName = process.platform === "win32" ? "ast-grep.exe" : "ast-grep"
-      const binaryPath = join(pkgDir, binaryName)
+      const astGrepName = process.platform === "win32" ? "ast-grep.exe" : "ast-grep"
+      const binaryPath = join(pkgDir, astGrepName)
 
       if (existsSync(binaryPath)) {
         return binaryPath
@@ -54,12 +55,44 @@ function findSgCliPath(): string {
     }
   }
 
-  // 3. Fallback to system PATH
+  if (process.platform === "darwin") {
+    const homebrewPaths = ["/opt/homebrew/bin/sg", "/usr/local/bin/sg"]
+    for (const path of homebrewPaths) {
+      if (existsSync(path)) {
+        return path
+      }
+    }
+  }
+
+  const cachedPath = getCachedBinaryPath()
+  if (cachedPath) {
+    return cachedPath
+  }
+
+  return null
+}
+
+let resolvedCliPath: string | null = null
+
+export function getSgCliPath(): string {
+  if (resolvedCliPath !== null) {
+    return resolvedCliPath
+  }
+
+  const syncPath = findSgCliPathSync()
+  if (syncPath) {
+    resolvedCliPath = syncPath
+    return syncPath
+  }
+
   return "sg"
 }
 
-// ast-grep CLI path (auto-detected from node_modules or system PATH)
-export const SG_CLI_PATH = findSgCliPath()
+export function setSgCliPath(path: string): void {
+  resolvedCliPath = path
+}
+
+export const SG_CLI_PATH = getSgCliPath()
 
 // CLI supported languages (25 total)
 export const CLI_LANGUAGES = [
@@ -121,3 +154,99 @@ export const LANG_EXTENSIONS: Record<string, string[]> = {
   tsx: [".tsx"],
   yaml: [".yml", ".yaml"],
 }
+
+export interface EnvironmentCheckResult {
+  cli: {
+    available: boolean
+    path: string
+    error?: string
+  }
+  napi: {
+    available: boolean
+    error?: string
+  }
+}
+
+/**
+ * Check if ast-grep CLI and NAPI are available.
+ * Call this at startup to provide early feedback about missing dependencies.
+ */
+export function checkEnvironment(): EnvironmentCheckResult {
+  const result: EnvironmentCheckResult = {
+    cli: {
+      available: false,
+      path: SG_CLI_PATH,
+    },
+    napi: {
+      available: false,
+    },
+  }
+
+  // Check CLI availability
+  if (existsSync(SG_CLI_PATH)) {
+    result.cli.available = true
+  } else if (SG_CLI_PATH === "sg") {
+    // Fallback path - try which/where to find in PATH
+    try {
+      const { spawnSync } = require("child_process")
+      const whichResult = spawnSync(process.platform === "win32" ? "where" : "which", ["sg"], {
+        encoding: "utf-8",
+        timeout: 5000,
+      })
+      result.cli.available = whichResult.status === 0 && !!whichResult.stdout?.trim()
+      if (!result.cli.available) {
+        result.cli.error = "sg binary not found in PATH"
+      }
+    } catch {
+      result.cli.error = "Failed to check sg availability"
+    }
+  } else {
+    result.cli.error = `Binary not found: ${SG_CLI_PATH}`
+  }
+
+  // Check NAPI availability
+  try {
+    require("@ast-grep/napi")
+    result.napi.available = true
+  } catch (e) {
+    result.napi.available = false
+    result.napi.error = `@ast-grep/napi not installed: ${e instanceof Error ? e.message : String(e)}`
+  }
+
+  return result
+}
+
+/**
+ * Format environment check result as user-friendly message.
+ */
+export function formatEnvironmentCheck(result: EnvironmentCheckResult): string {
+  const lines: string[] = ["ast-grep Environment Status:", ""]
+
+  // CLI status
+  if (result.cli.available) {
+    lines.push(`✓ CLI: Available (${result.cli.path})`)
+  } else {
+    lines.push(`✗ CLI: Not available`)
+    if (result.cli.error) {
+      lines.push(`  Error: ${result.cli.error}`)
+    }
+    lines.push(`  Install: bun add -D @ast-grep/cli`)
+  }
+
+  // NAPI status
+  if (result.napi.available) {
+    lines.push(`✓ NAPI: Available`)
+  } else {
+    lines.push(`✗ NAPI: Not available`)
+    if (result.napi.error) {
+      lines.push(`  Error: ${result.napi.error}`)
+    }
+    lines.push(`  Install: bun add -D @ast-grep/napi`)
+  }
+
+  lines.push("")
+  lines.push(`CLI supports ${CLI_LANGUAGES.length} languages`)
+  lines.push(`NAPI supports ${NAPI_LANGUAGES.length} languages: ${NAPI_LANGUAGES.join(", ")}`)
+
+  return lines.join("\n")
+}
diff --git a/src/tools/ast-grep/downloader.ts b/src/tools/ast-grep/downloader.ts
new file mode 100644
index 0000000..dfad78f
--- /dev/null
+++ b/src/tools/ast-grep/downloader.ts
@@ -0,0 +1,151 @@
+import { spawn } from "bun"
+import { existsSync, mkdirSync, chmodSync, unlinkSync } from "fs"
+import { join } from "path"
+import { homedir } from "os"
+import { createRequire } from "module"
+
+const REPO = "ast-grep/ast-grep"
+
+// IMPORTANT: Update this when bumping @ast-grep/cli in package.json
+// This is only used as fallback when @ast-grep/cli package.json cannot be read
+const DEFAULT_VERSION = "0.40.0"
+
+function getAstGrepVersion(): string {
+  try {
+    const require = createRequire(import.meta.url)
+    const pkg = require("@ast-grep/cli/package.json")
+    return pkg.version
+  } catch {
+    return DEFAULT_VERSION
+  }
+}
+
+interface PlatformInfo {
+  arch: string
+  os: string
+}
+
+const PLATFORM_MAP: Record<string, PlatformInfo> = {
+  "darwin-arm64": { arch: "aarch64", os: "apple-darwin" },
+  "darwin-x64": { arch: "x86_64", os: "apple-darwin" },
+  "linux-arm64": { arch: "aarch64", os: "unknown-linux-gnu" },
+  "linux-x64": { arch: "x86_64", os: "unknown-linux-gnu" },
+  "win32-x64": { arch: "x86_64", os: "pc-windows-msvc" },
+  "win32-arm64": { arch: "aarch64", os: "pc-windows-msvc" },
+  "win32-ia32": { arch: "i686", os: "pc-windows-msvc" },
+}
+
+export function getCacheDir(): string {
+  if (process.platform === "win32") {
+    const localAppData = process.env.LOCALAPPDATA || process.env.APPDATA
+    const base = localAppData || join(homedir(), "AppData", "Local")
+    return join(base, "oh-my-opencode", "bin")
+  }
+
+  const xdgCache = process.env.XDG_CACHE_HOME
+  const base = xdgCache || join(homedir(), ".cache")
+  return join(base, "oh-my-opencode", "bin")
+}
+
+export function getBinaryName(): string {
+  return process.platform === "win32" ? "sg.exe" : "sg"
+}
+
+export function getCachedBinaryPath(): string | null {
+  const binaryPath = join(getCacheDir(), getBinaryName())
+  return existsSync(binaryPath) ? binaryPath : null
+}
+
+async function extractZip(archivePath: string, destDir: string): Promise<void> {
+  const proc =
+    process.platform === "win32"
+      ? spawn(
+          [
+            "powershell",
+            "-command",
+            `Expand-Archive -Path '${archivePath}' -DestinationPath '${destDir}' -Force`,
+          ],
+          { stdout: "pipe", stderr: "pipe" }
+        )
+      : spawn(["unzip", "-o", archivePath, "-d", destDir], { stdout: "pipe", stderr: "pipe" })
+
+  const exitCode = await proc.exited
+
+  if (exitCode !== 0) {
+    const stderr = await new Response(proc.stderr).text()
+    const toolHint =
+      process.platform === "win32"
+        ? "Ensure PowerShell is available on your system."
+        : "Please install 'unzip' (e.g., apt install unzip, brew install unzip)."
+    throw new Error(`zip extraction failed (exit ${exitCode}): ${stderr}\n\n${toolHint}`)
+  }
+}
+
+export async function downloadAstGrep(version: string = DEFAULT_VERSION): Promise<string | null> {
+  const platformKey = `${process.platform}-${process.arch}`
+  const platformInfo = PLATFORM_MAP[platformKey]
+
+  if (!platformInfo) {
+    console.error(`[oh-my-opencode] Unsupported platform for ast-grep: ${platformKey}`)
+    return null
+  }
+
+  const cacheDir = getCacheDir()
+  const binaryName = getBinaryName()
+  const binaryPath = join(cacheDir, binaryName)
+
+  if (existsSync(binaryPath)) {
+    return binaryPath
+  }
+
+  const { arch, os } = platformInfo
+  const assetName = `app-${arch}-${os}.zip`
+  const downloadUrl = `https://github.com/${REPO}/releases/download/${version}/${assetName}`
+
+  console.log(`[oh-my-opencode] Downloading ast-grep binary...`)
+
+  try {
+    if (!existsSync(cacheDir)) {
+      mkdirSync(cacheDir, { recursive: true })
+    }
+
+    const response = await fetch(downloadUrl, { redirect: "follow" })
+
+    if (!response.ok) {
+      throw new Error(`HTTP ${response.status}: ${response.statusText}`)
+    }
+
+    const archivePath = join(cacheDir, assetName)
+    const arrayBuffer = await response.arrayBuffer()
+    await Bun.write(archivePath, arrayBuffer)
+
+    await extractZip(archivePath, cacheDir)
+
+    if (existsSync(archivePath)) {
+      unlinkSync(archivePath)
+    }
+
+    if (process.platform !== "win32" && existsSync(binaryPath)) {
+      chmodSync(binaryPath, 0o755)
+    }
+
+    console.log(`[oh-my-opencode] ast-grep binary ready.`)
+
+    return binaryPath
+  } catch (err) {
+    console.error(
+      `[oh-my-opencode] Failed to download ast-grep: ${err instanceof Error ? err.message : err}`
+    )
+    return null
+  }
+}
+
+export async function ensureAstGrepBinary(): Promise<string | null> {
+  const cachedPath = getCachedBinaryPath()
+  if (cachedPath) {
+    return cachedPath
+  }
+
+  const version = getAstGrepVersion()
+  return downloadAstGrep(version)
+}
diff --git a/src/tools/ast-grep/index.ts b/src/tools/ast-grep/index.ts
index 0281566..109b4aa 100644
--- a/src/tools/ast-grep/index.ts
+++ b/src/tools/ast-grep/index.ts
@@ -1,14 +1,12 @@
-import {
-  ast_grep_search,
-  ast_grep_replace,
-} from "./tools"
+import { ast_grep_search, ast_grep_replace } from "./tools"
 
 export const builtinTools = {
   ast_grep_search,
   ast_grep_replace,
 }
 
-export {
-  ast_grep_search,
-  ast_grep_replace,
-}
+export { ast_grep_search, ast_grep_replace }
+export { ensureAstGrepBinary, getCachedBinaryPath, getCacheDir } from "./downloader"
+export { getAstGrepPath, isCliAvailable, ensureCliAvailable, startBackgroundInit } from "./cli"
+export { checkEnvironment, formatEnvironmentCheck } from "./constants"
+export type { EnvironmentCheckResult } from "./constants"
diff --git a/src/tools/ast-grep/napi.ts b/src/tools/ast-grep/napi.ts
index 62e178f..c8d3880 100644
--- a/src/tools/ast-grep/napi.ts
+++ b/src/tools/ast-grep/napi.ts
@@ -1,4 +1,5 @@
 import { parse, Lang } from "@ast-grep/napi"
+import { NAPI_LANGUAGES } from "./constants"
 import type { NapiLanguage, AnalyzeResult, MetaVariable, Range } from "./types"
 
 const LANG_MAP: Record<NapiLanguage, Lang> = {
@@ -10,7 +11,16 @@ const LANG_MAP: Record<NapiLanguage, Lang> = {
 }
 
 export function parseCode(code: string, lang: NapiLanguage) {
-  return parse(LANG_MAP[lang], code)
+  const parseLang = LANG_MAP[lang]
+  if (!parseLang) {
+    const supportedLangs = NAPI_LANGUAGES.join(", ")
+    throw new Error(
+      `Unsupported language for NAPI: "${lang}"\n` +
+        `Supported languages: ${supportedLangs}\n\n` +
+        `Use ast_grep_search for other languages (25 supported via CLI).`
+    )
+  }
+  return parse(parseLang, code)
 }
 
 export function findPattern(root: ReturnType<typeof parseCode>, pattern: string) {