diff --git a/pm/workspace.py b/pm/workspace.py
index 1f0f321592..2494eabd2c 100644
--- a/pm/workspace.py
+++ b/pm/workspace.py
@@ -382,6 +382,8 @@ def install_node_sidecar(
             cwd=str(plugin_dir),
             capture_output=True,
             text=True,
+            encoding="utf-8",
+            errors="replace",
             timeout=900,
         )
     except Exception as exc:
@@ -469,7 +471,8 @@ def lock_and_sync(
 
     if not frozen:
         lock = subprocess.run(
-            [uv_bin, "lock"], cwd=str(generated), env=run_env, capture_output=True, text=True, timeout=1800
+            [uv_bin, "lock"], cwd=str(generated), env=run_env,
+            capture_output=True, text=True, encoding="utf-8", errors="replace", timeout=1800,
         )
         if lock.returncode != 0:
             raise classify_uv_failure("lock", lock.returncode, lock.stderr or lock.stdout)
@@ -484,7 +487,10 @@ def lock_and_sync(
     cmd = [uv_bin, "sync", "--frozen", "--all-packages"]
     for extra in sorted(set(extras or [])):
         cmd += ["--extra", extra]
-    sync = subprocess.run(cmd, cwd=str(generated), env=run_env, capture_output=True, text=True, timeout=1800)
+    sync = subprocess.run(
+        cmd, cwd=str(generated), env=run_env,
+        capture_output=True, text=True, encoding="utf-8", errors="replace", timeout=1800,
+    )
     if sync.returncode != 0:
         # --frozen means the lock already resolved; a sync failure here is
         # install/download/tooling, never a NEW resolution conflict.
diff --git a/tests/pm/test_workspace_output_encoding.py b/tests/pm/test_workspace_output_encoding.py
new file mode 100644
index 0000000000..52d79340c8
--- /dev/null
+++ b/tests/pm/test_workspace_output_encoding.py
@@ -0,0 +1,97 @@
+"""Captured package-tool output must not depend on the host's ANSI code page."""
+
+import importlib
+import locale
+import os
+import subprocess
+import sys
+
+import pytest
+
+import pm.workspace as ws
+from pm.package import InstallError
+
+
+@pytest.fixture
+def legacy_locale_child(monkeypatch):
+    # The canonical runner enables UTF-8 mode. Pin only subprocess's default
+    # encoding seam to a legacy locale; keep the real host and pipe I/O.
+    monkeypatch.setattr(locale, "getencoding", lambda: "cp1252")
+    monkeypatch.setattr(subprocess, "_text_encoding", locale.getencoding)
+    real_run = subprocess.run
+
+    def run(*, stdout=b"", stderr=b"", returncode=0, **kwargs):
+        script = (
+            "import sys; "
+            f"sys.stdout.buffer.write({stdout!r}); "
+            f"sys.stderr.buffer.write({stderr!r}); "
+            f"sys.exit({returncode})"
+        )
+        return real_run([sys.executable, "-c", script], **kwargs)
+
+    return run
+
+
+@pytest.mark.parametrize("stage", ["lock", "sync"])
+@pytest.mark.parametrize("stream", ["stdout", "stderr"])
+@pytest.mark.parametrize("suffix", [b"", b"\xff"], ids=["utf8", "invalid-byte"])
+def test_uv_failure_retains_utf8_build_diagnostic(
+    tmp_path, monkeypatch, legacy_locale_child, stage, stream, suffix
+):
+    diagnostic = "🔍 cryptography: OpenSSL headers not found"
+    raw = diagnostic.encode("utf-8") + suffix + b"\n"
+    expected = diagnostic + ("�" if suffix else "")
+    monkeypatch.setattr(ws, "_generate_pyproject", lambda *a, **k: (tmp_path, False))
+    monkeypatch.setattr(
+        importlib.import_module("pm.ensure"), "uv",
+        lambda **kwargs: (sys.executable, dict(os.environ)),
+    )
+    completed = []
+
+    def run_uv(cmd, **kwargs):
+        # Successful lock output must also be decoded before sync can run.
+        output = {stream: raw} if cmd[1] == stage else {"stdout": raw, "stderr": raw}
+        result = legacy_locale_child(
+            **output, returncode=17 if cmd[1] == stage else 0, **kwargs
+        )
+        completed.append(result)
+        return result
+
+    monkeypatch.setattr(ws.subprocess, "run", run_uv)
+    with pytest.raises(InstallError) as excinfo:
+        ws.lock_and_sync([], venv_dir=tmp_path / "venv", root=tmp_path)
+
+    assert type(excinfo.value) is InstallError  # A build error is not a resolver conflict.
+    assert excinfo.value.cause == f"uv {stage} exited 17: {expected}"
+    assert getattr(completed[-1], stream) == expected + "\n"
+
+
+@pytest.mark.parametrize("install_cmd", ["ci", "install"])
+@pytest.mark.parametrize("stream", ["stdout", "stderr"])
+@pytest.mark.parametrize("returncode", [0, 17])
+def test_node_sidecar_retains_output_and_exit_status(
+    tmp_path, monkeypatch, legacy_locale_child, install_cmd, stream, returncode
+):
+    (tmp_path / "package.json").write_text("{}", encoding="utf-8")
+    if install_cmd == "ci":
+        (tmp_path / "package-lock.json").write_text("{}", encoding="utf-8")
+    monkeypatch.setattr(
+        importlib.import_module("pm.ensure"), "lazy_installs_allowed", lambda: True
+    )
+    diagnostic = "🔍 node-gyp: build toolchain unavailable"
+    raw = diagnostic.encode("utf-8") + b"\xff\n"
+    completed = []
+
+    def run_npm(cmd, **kwargs):
+        result = legacy_locale_child(**{stream: raw}, returncode=returncode, **kwargs)
+        completed.append(result)
+        return result
+
+    error = ws.install_node_sidecar(tmp_path, npm_bin=sys.executable, runner=run_npm)
+
+    expected = diagnostic + "�"
+    if returncode:
+        assert error == f"npm {install_cmd} exited {returncode}: {expected}"
+    else:
+        assert error is None
+    assert getattr(completed[-1], stream) == expected + "\n"
\ No newline at end of file