diff --git a/tests/hermes_cli/test_plugins_cmd.py b/tests/hermes_cli/test_plugins_cmd.py
index 1f38fca04749f..2c49b12441889 100644
--- a/tests/hermes_cli/test_plugins_cmd.py
+++ b/tests/hermes_cli/test_plugins_cmd.py
@@ -189,14 +189,14 @@ class TestReadManifest:
         assert result == {}
 
     def test_invalid_yaml_returns_empty_and_logs(self, tmp_path, caplog):
-        (tmp_path / "plugin.yaml").write_text(": : : bad yaml [[[")
+        (tmp_path / "plugin.yaml").write_text(": : : bad yaml [[[", encoding="utf-8")
         with caplog.at_level(logging.WARNING, logger="hermes_cli.plugins_cmd"):
             result = _read_manifest(tmp_path)
         assert result == {}
         assert any("Failed to read plugin.yaml" in r.message for r in caplog.records)
 
     def test_empty_file_returns_empty(self, tmp_path):
-        (tmp_path / "plugin.yaml").write_text("")
+        (tmp_path / "plugin.yaml").write_text("", encoding="utf-8")
         result = _read_manifest(tmp_path)
         assert result == {}
 
@@ -388,7 +388,7 @@ class TestCopyExampleFiles:
 
         # Create example file
         example_file = tmp_path / "config.yaml.example"
-        example_file.write_text("key: value")
+        example_file.write_text("key: value", encoding="utf-8")
 
         _copy_example_files(tmp_path, console)
 
@@ -404,7 +404,7 @@ class TestCopyExampleFiles:
 
         # Create example file
         example_file = tmp_path / "config.yaml.example"
-        example_file.write_text("key: value")
+        example_file.write_text("key: value", encoding="utf-8")
 
         # Mock shutil.copy2 to raise an error
         with patch(
@@ -497,10 +497,10 @@ class TestProviderDiscovery:
         """Saving a context engine persists to config.yaml."""
         monkeypatch.setenv("HERMES_HOME", str(tmp_path))
         config_file = tmp_path / "config.yaml"
-        config_file.write_text("context:\n  engine: compressor\n")
+        config_file.write_text("context:\n  engine: compressor\n", encoding="utf-8")
         from hermes_cli.plugins_cmd import _save_context_engine
         _save_context_engine("lcm")
-        content = yaml.safe_load(config_file.read_text())
+        content = yaml.safe_load(config_file.read_text(encoding="utf-8"))
         assert content["context"]["engine"] == "lcm"
 
 
@@ -525,7 +525,7 @@ class TestNoAutoActivation:
         # This tests the run_agent.py logic indirectly by checking that the
         # code path for default config doesn't call get_plugin_context_engine.
         import run_agent as ra_module
-        source = open(ra_module.__file__).read()
+        source = Path(ra_module.__file__).read_text(encoding="utf-8")
         # The old code had: "Even with default config, check if a plugin registered one"
         # The fix removes this. Verify it's gone.
         assert "Even with default config, check if a plugin registered one" not in source
@@ -545,16 +545,19 @@ class TestSubdirInstallE2E:
 
         repo_root.mkdir(parents=True, exist_ok=True)
         # Root-level noise: docs + tests that should NOT be installed.
-        (repo_root / "README.md").write_text("# Monorepo docs\n")
+        (repo_root / "README.md").write_text("# Monorepo docs\n", encoding="utf-8")
         (repo_root / "tests").mkdir()
-        (repo_root / "tests" / "test_x.py").write_text("def test_x():\n    pass\n")
+        (repo_root / "tests" / "test_x.py").write_text(
+            "def test_x():\n    pass\n", encoding="utf-8"
+        )
         # The actual plugin in a subdirectory.
         plugin_dir = repo_root / "my-plugin"
         plugin_dir.mkdir()
         (plugin_dir / "plugin.yaml").write_text(
-            "name: my-plugin\nmanifest_version: 1\ndescription: A subdir plugin\n"
+            "name: my-plugin\nmanifest_version: 1\ndescription: A subdir plugin\n",
+            encoding="utf-8",
         )
-        (plugin_dir / "__init__.py").write_text("# plugin entry\n")
+        (plugin_dir / "__init__.py").write_text("# plugin entry\n", encoding="utf-8")
 
         env = {
             **os.environ,
diff --git a/tests/stress/test_atypical_scenarios.py b/tests/stress/test_atypical_scenarios.py
index 936dbaf5baf61..ed537956ce855 100644
--- a/tests/stress/test_atypical_scenarios.py
+++ b/tests/stress/test_atypical_scenarios.py
@@ -705,7 +705,7 @@ def _idempotency_race_worker(hermes_home: str, key: str, result_file: str,
         )
     finally:
         conn.close()
-    with open(result_file, "w") as f:
+    with open(result_file, "w", encoding="utf-8") as f:
         f.write(tid)
 
 
@@ -731,12 +731,16 @@ def _(home, kb):
         p.start()
     time.sleep(0.1)  # let them hit the spin
     # Fire the gun
-    with open(barrier, "w") as f:
+    with open(barrier, "w", encoding="utf-8") as f:
         f.write("go")
     for p in procs:
         p.join(timeout=10)
 
-    tids = [open(r).read().strip() for r in results if os.path.exists(r)]
+    tids = [
+        Path(r).read_text(encoding="utf-8").strip()
+        for r in results
+        if os.path.exists(r)
+    ]
     assert len(tids) == 2, f"only {len(tids)} workers finished"
     assert tids[0] == tids[1], (
         f"idempotency key race produced two different tasks: {tids}"
diff --git a/tests/tools/test_web_tools_truncate.py b/tests/tools/test_web_tools_truncate.py
index b89466fe6540e..d1dab454fe7fb 100644
--- a/tests/tools/test_web_tools_truncate.py
+++ b/tests/tools/test_web_tools_truncate.py
@@ -6,6 +6,7 @@ _get_extract_char_limit, and the end-to-end web_extract_tool truncation behavior
 import asyncio
 import json
 import os
+from pathlib import Path
 from unittest.mock import patch
 
 import pytest
@@ -49,7 +50,7 @@ class TestTruncation:
         path_line = next(ln for ln in out.splitlines() if "Full text saved to:" in ln)
         stored_path = path_line.split("Full text saved to:", 1)[1].strip()
         assert os.path.exists(stored_path)
-        full = open(stored_path).read()
+        full = Path(stored_path).read_text(encoding="utf-8")
         assert "UNIQUE_MIDDLE_MARKER" in full
         assert "row 2500" in full  # the omitted-middle row is in the stored file