diff --git a/hermes_cli/main.py b/hermes_cli/main.py
index 74b74f2725b..7795aa2dd4e 100644
--- a/hermes_cli/main.py
+++ b/hermes_cli/main.py
@@ -5549,6 +5549,8 @@ def _run_npm_install_deterministic(
             cwd=cwd,
             capture_output=capture_output,
             text=True,
+            encoding="utf-8",
+            errors="replace",
             check=False,
         )
         if ci_result.returncode == 0:
@@ -5561,6 +5563,8 @@ def _run_npm_install_deterministic(
         cwd=cwd,
         capture_output=capture_output,
         text=True,
+        encoding="utf-8",
+        errors="replace",
         check=False,
     )
 
@@ -5597,7 +5601,14 @@ def _build_web_ui(web_dir: Path, *, fatal: bool = False) -> bool:
         if fatal:
             print("  Run manually:  cd web && npm install && npm run build")
         return False
-    r2 = subprocess.run([npm, "run", "build"], cwd=web_dir, capture_output=True)
+    r2 = subprocess.run(
+        [npm, "run", "build"],
+        cwd=web_dir,
+        capture_output=True,
+        text=True,
+        encoding="utf-8",
+        errors="replace",
+    )
     if r2.returncode != 0:
         print(
             f"  {'✗' if fatal else '⚠'} Web UI build failed"
diff --git a/tests/hermes_cli/test_web_ui_build.py b/tests/hermes_cli/test_web_ui_build.py
index 47d3bb95a44..688cf7bdc29 100644
--- a/tests/hermes_cli/test_web_ui_build.py
+++ b/tests/hermes_cli/test_web_ui_build.py
@@ -13,7 +13,7 @@ from unittest.mock import patch
 
 import pytest
 
-from hermes_cli.main import _web_ui_build_needed, _build_web_ui
+from hermes_cli.main import _web_ui_build_needed, _build_web_ui, _run_npm_install_deterministic
 
 
 def _touch(path: Path, offset: float = 0.0) -> None:
@@ -119,3 +119,31 @@ class TestBuildWebUISkipsWhenFresh:
 
         assert result is True
         assert mock_run.call_count == 2  # npm install + npm run build
+
+    def test_npm_install_uses_utf8_replace_output_decoding(self, tmp_path):
+        web_dir, _ = _make_web_dir(tmp_path)
+        (web_dir / "package-lock.json").write_text("{}", encoding="utf-8")
+
+        mock_cp = __import__("subprocess").CompletedProcess([], 0, stdout="", stderr="")
+        with patch("hermes_cli.main.subprocess.run", return_value=mock_cp) as mock_run:
+            result = _run_npm_install_deterministic("/usr/bin/npm", web_dir)
+
+        assert result.returncode == 0
+        _, kwargs = mock_run.call_args
+        assert kwargs["text"] is True
+        assert kwargs["encoding"] == "utf-8"
+        assert kwargs["errors"] == "replace"
+
+    def test_web_build_uses_utf8_replace_output_decoding(self, tmp_path):
+        web_dir, _ = _make_web_dir(tmp_path)
+
+        mock_cp = __import__("subprocess").CompletedProcess([], 0, stdout="", stderr="")
+        with patch("hermes_cli.main.shutil.which", return_value="/usr/bin/npm"), \
+             patch("hermes_cli.main.subprocess.run", side_effect=[mock_cp, mock_cp]) as mock_run:
+            result = _build_web_ui(web_dir)
+
+        assert result is True
+        _, build_kwargs = mock_run.call_args_list[1]
+        assert build_kwargs["text"] is True
+        assert build_kwargs["encoding"] == "utf-8"
+        assert build_kwargs["errors"] == "replace"