diff --git a/tests/tools/test_local_background_child_hang.py b/tests/tools/test_local_background_child_hang.py
index e8014e875c..a8cc0ba102 100644
--- a/tests/tools/test_local_background_child_hang.py
+++ b/tests/tools/test_local_background_child_hang.py
@@ -109,3 +109,46 @@ class TestBackgroundChildDoesNotHang:
         assert "日本語" in result["output"]
         assert "café" in result["output"]
         assert "résumé" in result["output"]
+
+    def test_utf8_multibyte_across_read_boundary(self, local_env):
+        """Multibyte UTF-8 characters straddling a 4096-byte ``os.read()`` boundary
+        must be decoded correctly via the incremental decoder — not lost to a
+        ``UnicodeDecodeError`` fallback.  Regression for a bug in the first draft
+        of the fix where a strict ``bytes.decode('utf-8')`` on each raw chunk
+        wiped the entire buffer as soon as any chunk split a multi-byte char.
+        """
+        # 10000 "日" chars = 30000 bytes — guaranteed to cross multiple 4096
+        # read boundaries, and most boundaries will land in the middle of the
+        # 3-byte UTF-8 encoding of U+65E5.
+        cmd = (
+            'python3 -c \'import sys; '
+            'sys.stdout.buffer.write(chr(0x65e5).encode("utf-8") * 10000); '
+            'sys.stdout.buffer.write(b"\\n")\''
+        )
+        result = local_env.execute(cmd, timeout=10)
+        assert result["returncode"] == 0
+        # All 10000 characters must survive the round-trip
+        assert result["output"].count("\u65e5") == 10000, (
+            f"lost multibyte chars across read boundaries: got "
+            f"{result['output'].count(chr(0x65e5))} / 10000"
+        )
+        # And the "[binary output detected ...]" fallback must NOT fire
+        assert "binary output detected" not in result["output"]
+
+    def test_invalid_utf8_uses_replacement_not_fallback(self, local_env):
+        """Truly invalid byte sequences must be substituted with U+FFFD (matching
+        the pre-fix ``errors='replace'`` behaviour of the old ``TextIOWrapper``
+        drain), not clobber the entire buffer with a fallback placeholder.
+        """
+        # Write a deliberate invalid UTF-8 lead byte sandwiched between valid ASCII
+        cmd = (
+            'python3 -c \'import sys; '
+            'sys.stdout.buffer.write(b"before "); '
+            'sys.stdout.buffer.write(b"\\xff\\xfe"); '
+            'sys.stdout.buffer.write(b" after\\n")\''
+        )
+        result = local_env.execute(cmd, timeout=5)
+        assert result["returncode"] == 0
+        assert "before" in result["output"]
+        assert "after" in result["output"]
+        assert "binary output detected" not in result["output"]
diff --git a/tools/environments/base.py b/tools/environments/base.py
index a7f0a849b5..cde78e1d41 100644
--- a/tools/environments/base.py
+++ b/tools/environments/base.py
@@ -6,6 +6,7 @@ re-sourced before each command. CWD persists via in-band stdout markers (remote)
 or a temp file (local).
 """
 
+import codecs
 import json
 import logging
 import os
@@ -453,37 +454,51 @@ class BaseEnvironment(ABC):
         # shortly after ``bash`` exits even if the pipe hasn't EOF'd yet.
         # Any output the grandchild writes after that point goes to an
         # orphaned pipe (harmless — the kernel reaps it when our end closes).
+        #
+        # Decoding: we ``os.read()`` raw bytes in fixed-size chunks (4096)
+        # so a single multibyte UTF-8 character can split across reads.  An
+        # incremental decoder buffers partial sequences across chunks, and
+        # ``errors="replace"`` mirrors the baseline ``TextIOWrapper`` (which
+        # was constructed with ``encoding="utf-8", errors="replace"`` on
+        # ``Popen``) so binary or mis-encoded output is preserved with
+        # U+FFFD substitution rather than clobbering the whole buffer.
+        decoder = codecs.getincrementaldecoder("utf-8")(errors="replace")
+
         def _drain():
             fd = proc.stdout.fileno()
             idle_after_exit = 0
-            while True:
-                try:
-                    ready, _, _ = select.select([fd], [], [], 0.1)
-                except (ValueError, OSError):
-                    break  # fd already closed
-                if ready:
+            try:
+                while True:
                     try:
-                        chunk = os.read(fd, 4096)
+                        ready, _, _ = select.select([fd], [], [], 0.1)
                     except (ValueError, OSError):
-                        break
-                    if not chunk:
-                        break  # true EOF — all writers closed
-                    try:
-                        output_chunks.append(chunk.decode("utf-8"))
-                    except UnicodeDecodeError:
-                        output_chunks.clear()
-                        output_chunks.append(
-                            "[binary output detected — raw bytes not displayable]"
-                        )
-                        break
-                    idle_after_exit = 0
-                elif proc.poll() is not None:
-                    # bash is gone and the pipe was idle for ~100ms.  Give
-                    # it two more cycles to catch any buffered tail, then
-                    # stop — otherwise we wait forever on a grandchild pipe.
-                    idle_after_exit += 1
-                    if idle_after_exit >= 3:
-                        break
+                        break  # fd already closed
+                    if ready:
+                        try:
+                            chunk = os.read(fd, 4096)
+                        except (ValueError, OSError):
+                            break
+                        if not chunk:
+                            break  # true EOF — all writers closed
+                        output_chunks.append(decoder.decode(chunk))
+                        idle_after_exit = 0
+                    elif proc.poll() is not None:
+                        # bash is gone and the pipe was idle for ~100ms.  Give
+                        # it two more cycles to catch any buffered tail, then
+                        # stop — otherwise we wait forever on a grandchild pipe.
+                        idle_after_exit += 1
+                        if idle_after_exit >= 3:
+                            break
+            finally:
+                # Flush any bytes buffered mid-sequence.  With ``errors="replace"``
+                # this emits U+FFFD for any final incomplete sequence rather than
+                # raising.
+                try:
+                    tail = decoder.decode(b"", final=True)
+                    if tail:
+                        output_chunks.append(tail)
+                except Exception:
+                    pass
 
         drain_thread = threading.Thread(target=_drain, daemon=True)
         drain_thread.start()