diff --git a/agent/tools/bash/bash.py b/agent/tools/bash/bash.py
index a3652cac..3e2bf307 100644
--- a/agent/tools/bash/bash.py
+++ b/agent/tools/bash/bash.py
@@ -202,8 +202,12 @@ SAFETY:
             total_bytes = len(output.encode('utf-8'))
 
             if total_bytes > DEFAULT_MAX_BYTES:
-                # Save full output to temp file
-                with tempfile.NamedTemporaryFile(mode='w', delete=False, suffix='.log', prefix='bash-') as f:
+                # Save full output to temp file. encoding='utf-8' is required:
+                # the default text-mode encoding is the platform locale (e.g.
+                # cp936/GBK on Chinese Windows), which raises UnicodeEncodeError
+                # for output containing emoji or other non-locale characters and
+                # would discard an otherwise successful command result.
+                with tempfile.NamedTemporaryFile(mode='w', delete=False, suffix='.log', prefix='bash-', encoding='utf-8') as f:
                     f.write(output)
                     temp_file_path = f.name
 
diff --git a/tests/test_bash_output_encoding.py b/tests/test_bash_output_encoding.py
new file mode 100644
index 00000000..2c455ffd
--- /dev/null
+++ b/tests/test_bash_output_encoding.py
@@ -0,0 +1,59 @@
+# encoding:utf-8
+"""
+Regression test for the Bash tool spilling large output to a temp file.
+
+When a command's output exceeds DEFAULT_MAX_BYTES the full output is written to
+a temp file. That file must be opened with encoding='utf-8'; otherwise it falls
+back to the platform locale encoding (e.g. cp936/GBK on Chinese Windows), which
+raises UnicodeEncodeError for output containing emoji or other characters not
+representable in that codepage. The exception previously propagated out and
+turned an otherwise-successful command (exit code 0) into a tool error, losing
+all of its output.
+"""
+import os
+import sys
+import tempfile
+from unittest.mock import patch
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), ".."))
+
+from agent.tools.bash.bash import Bash
+
+
+def test_large_non_locale_output_is_saved_as_utf8(tmp_path):
+    tool = Bash({"cwd": str(tmp_path), "safety_mode": False})
+
+    # Emit ~80KB (> 50KB DEFAULT_MAX_BYTES) of an emoji so the tool spills the
+    # full output to a temp file. Raw UTF-8 bytes are written from the child so
+    # the command line stays pure ASCII and the child's own stdout encoding is
+    # irrelevant.
+    code = "import sys; sys.stdout.buffer.write((chr(0x1F389) * 20000).encode('utf-8'))"
+    command = f'"{sys.executable}" -c "{code}"'
+
+    real_named_temp_file = tempfile.NamedTemporaryFile
+    captured = {}
+
+    def spy(*args, **kwargs):
+        captured.update(kwargs)
+        return real_named_temp_file(*args, **kwargs)
+
+    temp_file_path = None
+    try:
+        with patch(
+            "agent.tools.bash.bash.tempfile.NamedTemporaryFile", side_effect=spy
+        ):
+            result = tool.execute({"command": command, "timeout": 60})
+
+        # Command succeeded, so the tool must not report an error.
+        assert result.status == "success", result.result
+
+        # The temp file must be opened as UTF-8 (the actual fix).
+        assert captured.get("encoding") == "utf-8"
+
+        # And the emoji must round-trip through the saved file.
+        temp_file_path = result.result["details"]["full_output_path"]
+        with open(temp_file_path, encoding="utf-8") as f:
+            assert "\U0001f389" in f.read()
+    finally:
+        if temp_file_path and os.path.exists(temp_file_path):
+            os.remove(temp_file_path)