diff --git a/package-lock.json b/package-lock.json
index 0c6c449d32..0df4109d84 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -10839,6 +10839,18 @@
       "integrity": "sha512-xHjhDr3cNBK0BzdUJSPXZntQUx/mwMS5Rw4A7lPJ90XGAO6ISP/ePDNuo0vhqOZU+UD5JoodwCAAoZQd3FeAKw==",
       "license": "MIT"
     },
+    "node_modules/isbinaryfile": {
+      "version": "5.0.7",
+      "resolved": "https://registry.npmjs.org/isbinaryfile/-/isbinaryfile-5.0.7.tgz",
+      "integrity": "sha512-gnWD14Jh3FzS3CPhF0AxNOJ8CxqeblPTADzI38r0wt8ZyQl5edpy75myt08EG2oKvpyiqSqsx+Wkz9vtkbTqYQ==",
+      "license": "MIT",
+      "engines": {
+        "node": ">= 18.0.0"
+      },
+      "funding": {
+        "url": "https://github.com/sponsors/gjtorikian/"
+      }
+    },
     "node_modules/isexe": {
       "version": "2.0.0",
       "resolved": "https://registry.npmjs.org/isexe/-/isexe-2.0.0.tgz",
@@ -18187,6 +18199,7 @@
         "https-proxy-agent": "^7.0.6",
         "ignore": "^7.0.0",
         "ipaddr.js": "^1.9.1",
+        "isbinaryfile": "^5.0.7",
         "js-yaml": "^4.1.1",
         "json-stable-stringify": "^1.3.0",
         "marked": "^15.0.12",
diff --git a/packages/core/package.json b/packages/core/package.json
index 90010084f7..00c663690d 100644
--- a/packages/core/package.json
+++ b/packages/core/package.json
@@ -68,6 +68,7 @@
     "https-proxy-agent": "^7.0.6",
     "ignore": "^7.0.0",
     "ipaddr.js": "^1.9.1",
+    "isbinaryfile": "^5.0.7",
     "js-yaml": "^4.1.1",
     "json-stable-stringify": "^1.3.0",
     "marked": "^15.0.12",
diff --git a/packages/core/src/utils/fileUtils.test.ts b/packages/core/src/utils/fileUtils.test.ts
index c31688e44e..5e7c6d3df2 100644
--- a/packages/core/src/utils/fileUtils.test.ts
+++ b/packages/core/src/utils/fileUtils.test.ts
@@ -286,6 +286,25 @@ describe('fileUtils', () => {
       }
       expect(await isBinaryFile(filePathForBinaryTest)).toBe(false);
     });
+
+    it('should return false for a source file containing literal U+FFFD (replacement character)', async () => {
+      const content =
+        '// Rust-style source\npub const UNICODE_REPLACEMENT_CHAR: char = \'\uFFFD\';\nlet s = "\uFFFD\uFFFD\uFFFD";\n';
+      actualNodeFs.writeFileSync(filePathForBinaryTest, content, 'utf8');
+      expect(await isBinaryFile(filePathForBinaryTest)).toBe(false);
+    });
+
+    it('should return false for a file with mixed CJK, emoji, and U+FFFD content', async () => {
+      const content = '\uFFFD\uFFFD hello \u4e16\u754c \uD83D\uDE00\n';
+      actualNodeFs.writeFileSync(filePathForBinaryTest, content, 'utf8');
+      expect(await isBinaryFile(filePathForBinaryTest)).toBe(false);
+    });
+
+    it('should return true for a file with dense invalid UTF-8 byte sequences', async () => {
+      const binaryContent = Buffer.alloc(128, 0x80);
+      actualNodeFs.writeFileSync(filePathForBinaryTest, binaryContent);
+      expect(await isBinaryFile(filePathForBinaryTest)).toBe(true);
+    });
   });
 
   describe('BOM detection and encoding', () => {
diff --git a/packages/core/src/utils/fileUtils.ts b/packages/core/src/utils/fileUtils.ts
index 9fb3bf3e3f..52191171aa 100644
--- a/packages/core/src/utils/fileUtils.ts
+++ b/packages/core/src/utils/fileUtils.ts
@@ -8,6 +8,7 @@ import fs from 'node:fs';
 import fsPromises from 'node:fs/promises';
 import path from 'node:path';
 import type { PartUnion } from '@google/genai';
+import { isBinaryFile as isBinaryFileCheck } from 'isbinaryfile';
 import mime from 'mime/lite';
 import type { FileSystemService } from '../services/fileSystemService.js';
 import { ToolErrorType } from '../tools/tool-error.js';
@@ -345,53 +346,17 @@ export async function isEmpty(filePath: string): Promise<boolean> {
 
 /**
  * Heuristic: determine if a file is likely binary.
- * Now BOM-aware: if a Unicode BOM is detected, we treat it as text.
- * For non-BOM files, retain the existing null-byte and non-printable ratio checks.
+ * Delegates to the `isbinaryfile` package for UTF-8-aware detection.
  */
 export async function isBinaryFile(filePath: string): Promise<boolean> {
-  let fh: fs.promises.FileHandle | null = null;
   try {
-    fh = await fs.promises.open(filePath, 'r');
-    const stats = await fh.stat();
-    const fileSize = stats.size;
-    if (fileSize === 0) return false; // empty is not binary
-
-    // Sample up to 4KB from the head (previous behavior)
-    const sampleSize = Math.min(4096, fileSize);
-    const buf = Buffer.alloc(sampleSize);
-    const { bytesRead } = await fh.read(buf, 0, sampleSize, 0);
-    if (bytesRead === 0) return false;
-
-    // BOM → text (avoid false positives for UTF‑16/32 with nulls)
-    const bom = detectBOM(buf.subarray(0, Math.min(4, bytesRead)));
-    if (bom) return false;
-
-    let nonPrintableCount = 0;
-    for (let i = 0; i < bytesRead; i++) {
-      if (buf[i] === 0) return true; // strong indicator of binary when no BOM
-      if (buf[i] < 9 || (buf[i] > 13 && buf[i] < 32)) {
-        nonPrintableCount++;
-      }
-    }
-    // If >30% non-printable characters, consider it binary
-    return nonPrintableCount / bytesRead > 0.3;
+    return await isBinaryFileCheck(filePath);
   } catch (error) {
     debugLogger.warn(
       `Failed to check if file is binary: ${filePath}`,
       error instanceof Error ? error.message : String(error),
     );
     return false;
-  } finally {
-    if (fh) {
-      try {
-        await fh.close();
-      } catch (closeError) {
-        debugLogger.warn(
-          `Failed to close file handle for: ${filePath}`,
-          closeError instanceof Error ? closeError.message : String(closeError),
-        );
-      }
-    }
   }
 }