fix(cli): force UTF-8 stdout/stderr on Windows so Rich output never raises

Windows hands a redirected or legacy console stream the ANSI code page
(cp1252), which cannot encode Rich's panels or the model's text: one
check mark in a finding ended a headless run with UnicodeEncodeError, and
the error handler raised again rendering its own panel. Reconfigure both
streams to UTF-8 at startup on win32, and open the Go TUI's os.devnull
sink as UTF-8 so logging into it stops producing --- Logging error ---
reports under the same code page.

strix/interface/main.py

@@ -415,12 +415,30 @@ def _bootstrap_scan(args: argparse.Namespace) -> None:
     telemetry_start(args)
 
 
-def main() -> None:
-    configure_dependency_logging()
+def _force_utf8_streams() -> None:
+    """Give stdout/stderr a UTF-8 encoding, whatever the console code page.
 
+    Windows hands a redirected or legacy console stream the ANSI code page
+    (cp1252 and friends), which cannot encode Rich's output or the model's
+    text: one ``\u2713`` in a finding ends a headless run with
+    ``UnicodeEncodeError``. Streams without ``reconfigure`` (test captures,
+    replaced file objects) are left alone.
+    """
+    for stream in (sys.stdout, sys.stderr):
+        reconfigure = getattr(stream, "reconfigure", None)
+        if reconfigure is None:
+            continue
+        with contextlib.suppress(ValueError, OSError):
+            reconfigure(encoding="utf-8")
+
+
+def main() -> None:
     if sys.platform == "win32":
+        _force_utf8_streams()
         asyncio.set_event_loop_policy(asyncio.WindowsSelectorEventLoopPolicy())
 
+    configure_dependency_logging()
+
     if len(sys.argv) == 2 and sys.argv[1] in ("-h", "--help"):
         try:
             parse_arguments()

strix/interface/tui/runtime.py

@@ -11,7 +11,7 @@ import shutil
 import sys
 from copy import deepcopy
 from pathlib import Path
-from typing import TYPE_CHECKING, Any
+from typing import TYPE_CHECKING, Any, TextIO
 
 from strix.config import load_settings, persist_current
 from strix.core.agents import AgentCoordinator
@@ -58,6 +58,16 @@ class GoTuiPreActivationError(RuntimeError):
     """A sidecar failure raised before the Go TUI activates."""
 
 
+def _open_output_sink() -> TextIO:
+    """A UTF-8 ``os.devnull`` handle for Python's stdout/stderr while the Go TUI owns the terminal.
+
+    UTF-8 is explicit because the locale encoding (cp1252 on Windows) cannot take
+    the model's text, and a logging handler that fails to write there floods the
+    log with ``--- Logging error ---`` reports.
+    """
+    return Path(os.devnull).open("a", buffering=1, encoding="utf-8")
+
+
 class GoTuiRuntime:
     def __init__(self, args: argparse.Namespace) -> None:
         self.args = args
@@ -433,7 +443,7 @@ class GoTuiRuntime:
         # only the Python-level bindings change.
         original_stdout = sys.stdout
         original_stderr = sys.stderr
-        output_sink = Path(os.devnull).open("a", buffering=1)  # noqa: SIM115
+        output_sink = _open_output_sink()
         sys.stdout = output_sink
         sys.stderr = output_sink
         backend_socket: socket.socket | None = None

tests/test_utf8_streams.py

@@ -0,0 +1,65 @@
+"""Windows gives redirected stdout/stderr the ANSI code page; Strix forces UTF-8 (STR-742)."""
+
+from __future__ import annotations
+
+import io
+import sys
+
+import pytest
+from rich.console import Console
+
+from strix.interface.main import _force_utf8_streams
+from strix.interface.tui.runtime import _open_output_sink
+
+
+UNICODE_SAMPLE = "Reflected XSS in / \u2713 \u2192 \U0001f50d \u00b7 done"
+
+
+def _cp1252_stream() -> tuple[io.TextIOWrapper, io.BytesIO]:
+    raw = io.BytesIO()
+    return io.TextIOWrapper(raw, encoding="cp1252", line_buffering=True), raw
+
+
+def test_rich_output_raises_on_a_cp1252_stream(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setattr(sys, "stdout", _cp1252_stream()[0])
+    monkeypatch.setattr(sys, "stderr", _cp1252_stream()[0])
+
+    with pytest.raises(UnicodeEncodeError):
+        Console().print(UNICODE_SAMPLE)
+
+
+def test_force_utf8_streams_lets_rich_print_unicode(monkeypatch: pytest.MonkeyPatch) -> None:
+    stdout, stdout_bytes = _cp1252_stream()
+    stderr, stderr_bytes = _cp1252_stream()
+    monkeypatch.setattr(sys, "stdout", stdout)
+    monkeypatch.setattr(sys, "stderr", stderr)
+
+    _force_utf8_streams()
+
+    assert stdout.encoding == "utf-8"
+    assert stderr.encoding == "utf-8"
+    Console().print(UNICODE_SAMPLE)
+    Console(stderr=True).print(UNICODE_SAMPLE)
+    stdout.flush()
+    stderr.flush()
+    assert UNICODE_SAMPLE in stdout_bytes.getvalue().decode("utf-8")
+    assert UNICODE_SAMPLE in stderr_bytes.getvalue().decode("utf-8")
+
+
+def test_force_utf8_streams_skips_streams_without_reconfigure(
+    monkeypatch: pytest.MonkeyPatch,
+) -> None:
+    stdout = io.StringIO()
+    monkeypatch.setattr(sys, "stdout", stdout)
+    monkeypatch.setattr(sys, "stderr", io.StringIO())
+
+    _force_utf8_streams()
+
+    sys.stdout.write(UNICODE_SAMPLE)
+    assert stdout.getvalue() == UNICODE_SAMPLE
+
+
+def test_tui_output_sink_accepts_unicode() -> None:
+    with _open_output_sink() as sink:
+        assert sink.encoding == "utf-8"
+        sink.write(UNICODE_SAMPLE + "\n")