diff --git a/Dockerfile b/Dockerfile
index b20d507..3377783 100644
--- a/Dockerfile
+++ b/Dockerfile
@@ -1,4 +1,4 @@
-FROM python:alpine
+FROM python:3.13-alpine
 
 WORKDIR /app
 
diff --git a/dispatcher/dispatcher.py b/dispatcher/dispatcher.py
index 1be1a89..3590b86 100644
--- a/dispatcher/dispatcher.py
+++ b/dispatcher/dispatcher.py
@@ -5,8 +5,8 @@
 import requests
 import pathlib
 import queue
-import textwrap
 import shutil
+import tempfile
 from datetime import datetime
 
 from runner.submission import SubmissionRunner
@@ -293,11 +293,6 @@ def create_container(
             finally:
                 self.dec_container()
         logger().info(f'finish task {submission_id}/{case_no}')
-        # truncate long stdout/stderr
-        _res = res.copy()
-        for k in ('Stdout', 'Stderr'):
-            _res[k] = textwrap.shorten(_res.get(k, ''), 37, placeholder='...')
-        logger().debug(f'runner result: {_res}')
         with self.locks[submission_id]:
             self.on_case_complete(
                 submission_id=submission_id,
@@ -378,16 +373,26 @@ def on_submission_complete(self, submission_id: str):
         assert [*submission_result.keys()] == [*range(len(submission_result))]
         submission_result = [*submission_result.values()]
         # post data
-        submission_data = {
-            'tasks': submission_result,
-            'token': config.SANDBOX_TOKEN
-        }
-        self.release(submission_id)
-        logger().info(f'send to BE [submission_id={submission_id}]')
-        resp = requests.put(
-            f'{config.BACKEND_API}/submission/{submission_id}/complete',
-            json=submission_data,
-        )
+        with tempfile.NamedTemporaryFile("w") as tmpf:
+            submission_data = {
+                'tasks': submission_result,
+                'token': config.SANDBOX_TOKEN
+            }
+            # write payload to file
+            json.dump(submission_data, tmpf)
+            tmpf.flush()
+            # release resources
+            del submission_data
+            self.release(submission_id)
+
+            logger().info(f'send to BE [submission_id={submission_id}]')
+            # open in binary mode as requests needs a binary stream
+            with open(tmpf.name, "rb") as payload:
+                resp = requests.put(
+                    f'{config.BACKEND_API}/submission/{submission_id}/complete',
+                    data=payload,
+                    headers={'Content-Type': 'application/json'},
+                )
         logger().debug(f'get BE response: [{resp.status_code}] {resp.text}', )
         # clear
         if resp.ok:
diff --git a/runner/sandbox.py b/runner/sandbox.py
index 05f52ec..f4fbe93 100644
--- a/runner/sandbox.py
+++ b/runner/sandbox.py
@@ -3,8 +3,8 @@
 import tarfile
 import tempfile
 from dataclasses import dataclass
-from io import BytesIO
 from typing import Optional
+from pathlib import Path
 import docker
 
 
@@ -115,21 +115,11 @@ def run(self):
             raise JudgeError
         # retrive result
         try:
-            result = self.get(
-                container=container,
-                path='/result/',
-                filename='result',
-            ).split('\n')
-            stdout = self.get(
-                container=container,
-                path='/result/',
-                filename='stdout',
-            )
-            stderr = self.get(
-                container=container,
-                path='/result/',
-                filename='stderr',
+            result, stdout, stderr = self.get_result(
+                container,
+                ['result', 'stdout', 'stderr'],
             )
+            result = result.split('\n')
         except Exception as e:
             self.client.remove_container(container, v=True, force=True)
             logging.error(e)
@@ -147,16 +137,21 @@ def run(self):
             DockerExitCode=exit_status['StatusCode'],
         )
 
-    def get(self, container, path, filename):
-        bits, _ = self.client.get_archive(container, f'{path}{filename}')
-        tarbits = b''.join(bits)
-        tar = tarfile.open(fileobj=BytesIO(tarbits))
-        with tempfile.TemporaryDirectory() as extract_path:
-            tar.extract(filename, extract_path)
-            with open(
-                    f'{extract_path}/{filename}',
+    def get_result(self, container, filenames: list[str]) -> list[str]:
+        result_dir = '/result'
+        bits, _ = self.client.get_archive(container, result_dir)
+        with (tempfile.NamedTemporaryFile() as
+              tarball, tempfile.TemporaryDirectory() as extract_path):
+            for chunk in bits:
+                tarball.write(chunk)
+            tarball.flush()
+            tarball.seek(0)
+            with tarfile.open(fileobj=tarball) as tar:
+                tar.extractall(extract_path)
+            return [
+                open(
+                    Path(extract_path) / result_dir.lstrip('/') / filename,
                     'r',
                     errors='ignore',
-            ) as f:
-                contents = f.read()
-        return contents
+                ).read() for filename in filenames
+            ]