diff --git a/examples/custom_sink/agentevals_example_custom_sink/sink.py b/examples/custom_sink/agentevals_example_custom_sink/sink.py
index de666cf..0332a32 100644
--- a/examples/custom_sink/agentevals_example_custom_sink/sink.py
+++ b/examples/custom_sink/agentevals_example_custom_sink/sink.py
@@ -38,7 +38,7 @@ def __init__(self, path: Path) -> None:
     async def _write(self, payload: dict) -> None:
         async with self._lock:
             self._path.parent.mkdir(parents=True, exist_ok=True)
-            with self._path.open("a") as f:  # noqa: ASYNC230
+            with self._path.open("a", encoding="utf-8") as f:  # noqa: ASYNC230
                 f.write(json.dumps(payload) + "\n")
 
     async def emit_partial(self, run_id: UUID, results: list[Result], attempt: int) -> None:
diff --git a/src/agentevals/api/debug_routes.py b/src/agentevals/api/debug_routes.py
index 4a3d8e6..c040536 100644
--- a/src/agentevals/api/debug_routes.py
+++ b/src/agentevals/api/debug_routes.py
@@ -114,7 +114,7 @@ def _collect_temp_files(session_ids: set[str] | None = None) -> dict[str, str]:
                 if sid not in session_ids:
                     continue
             try:
-                with open(path) as f:
+                with open(path, encoding="utf-8") as f:
                     files[basename] = f.read()
             except OSError:
                 logger.debug("Could not read temp file %s", path)
diff --git a/src/agentevals/api/routes.py b/src/agentevals/api/routes.py
index 29de1d2..b54c3cf 100644
--- a/src/agentevals/api/routes.py
+++ b/src/agentevals/api/routes.py
@@ -129,7 +129,7 @@ def _load_eval_set_dict(path: str | None) -> dict | None:
     if not path:
         return None
     try:
-        with open(path) as f:
+        with open(path, encoding="utf-8") as f:
             return json.load(f)
     except (OSError, json.JSONDecodeError):
         logger.warning("could not re-read eval_set file at %s for persistence", path)
diff --git a/src/agentevals/api/streaming_routes.py b/src/agentevals/api/streaming_routes.py
index 22746c2..831fcb5 100644
--- a/src/agentevals/api/streaming_routes.py
+++ b/src/agentevals/api/streaming_routes.py
@@ -225,7 +225,7 @@ async def evaluate_sessions(
 
         import tempfile
 
-        eval_set_file = tempfile.NamedTemporaryFile(mode="w", suffix=".json", delete=False)
+        eval_set_file = tempfile.NamedTemporaryFile(mode="w", suffix=".json", delete=False, encoding="utf-8")
         json.dump(eval_set_response.data.eval_set, eval_set_file)
         eval_set_file.close()
 
@@ -334,7 +334,7 @@ async def prepare_evaluation(
         temp_dir = tempfile.gettempdir()
 
         eval_set_file = os.path.join(temp_dir, f"eval_set_{request.golden_session_id}.json")
-        with open(eval_set_file, "w") as f:  # noqa: ASYNC230
+        with open(eval_set_file, "w", encoding="utf-8") as f:  # noqa: ASYNC230
             json.dump(eval_set_response.data.eval_set, f)
 
         trace_files = []
@@ -415,7 +415,7 @@ async def get_trace(
         # evaluate-sessions path. Serializing spans here independently would
         # drop the resource attribute and lose agent identity on the run.
         trace_file = await manager._save_spans_to_temp_file(session)
-        with open(trace_file) as f:  # noqa: ASYNC230
+        with open(trace_file, encoding="utf-8") as f:  # noqa: ASYNC230
             trace_content = f.read()
         num_spans = sum(1 for line in trace_content.splitlines() if line.strip())
 
diff --git a/src/agentevals/cli.py b/src/agentevals/cli.py
index 1a9cd89..da26055 100644
--- a/src/agentevals/cli.py
+++ b/src/agentevals/cli.py
@@ -840,8 +840,8 @@ def migrate_create(name: str, output_dir: str | None) -> None:
         f"-- Migration {next_version:06d}: {name}\n"
         "-- Once tagged in a release this file is immutable. Fix bugs by adding a NEW migration.\n\n"
     )
-    up_path.write_text(header)
-    down_path.write_text(header)
+    up_path.write_text(header, encoding="utf-8")
+    down_path.write_text(header, encoding="utf-8")
     click.echo(f"Created {up_path}")
     click.echo(f"Created {down_path}")
 
diff --git a/src/agentevals/eval_config_loader.py b/src/agentevals/eval_config_loader.py
index 822264b..45518a6 100644
--- a/src/agentevals/eval_config_loader.py
+++ b/src/agentevals/eval_config_loader.py
@@ -58,7 +58,7 @@ def load_eval_config(path: str | Path) -> EvalRunConfig:
     if not path.exists():
         raise FileNotFoundError(f"Eval config file not found: {path}")
 
-    with open(path) as f:
+    with open(path, encoding="utf-8") as f:
         data = yaml.safe_load(f)
 
     if not isinstance(data, dict):
diff --git a/src/agentevals/evaluator/sources.py b/src/agentevals/evaluator/sources.py
index 8fe022c..1ebae26 100644
--- a/src/agentevals/evaluator/sources.py
+++ b/src/agentevals/evaluator/sources.py
@@ -66,7 +66,7 @@ def _read_cache(key: str, ttl: int = _CACHE_TTL_SECONDS) -> list[EvaluatorInfo]
     if not cache_file.exists():
         return None
     try:
-        data = json.loads(cache_file.read_text())
+        data = json.loads(cache_file.read_text(encoding="utf-8"))
         if time.time() - data.get("ts", 0) > ttl:
             return None
         return [EvaluatorInfo(**item) for item in data["evaluators"]]
@@ -83,7 +83,8 @@ def _write_cache(key: str, evaluators: list[EvaluatorInfo]) -> None:
                     "ts": time.time(),
                     "evaluators": [asdict(g) for g in evaluators],
                 }
-            )
+            ),
+            encoding="utf-8",
         )
     except Exception:
         pass
diff --git a/src/agentevals/evaluator/venv.py b/src/agentevals/evaluator/venv.py
index fcbfc88..85232cf 100644
--- a/src/agentevals/evaluator/venv.py
+++ b/src/agentevals/evaluator/venv.py
@@ -42,7 +42,11 @@ def _venv_key(evaluator_path: Path) -> str:
 
 def _is_venv_valid(venv_dir: Path, req_hash: str) -> bool:
     hash_file = venv_dir / _HASH_FILE
-    return _venv_python(venv_dir).exists() and hash_file.exists() and hash_file.read_text().strip() == req_hash
+    return (
+        _venv_python(venv_dir).exists()
+        and hash_file.exists()
+        and hash_file.read_text(encoding="utf-8").strip() == req_hash
+    )
 
 
 def _create_venv(venv_dir: Path, uv: str | None) -> None:
@@ -104,7 +108,7 @@ def ensure_venv(evaluator_path: Path) -> Path | None:
         stderr = exc.stderr.decode() if isinstance(exc.stderr, bytes) else (exc.stderr or "")
         raise RuntimeError(f"Failed to set up environment for evaluator '{evaluator_path.stem}': {stderr}") from exc
 
-    (venv_dir / _HASH_FILE).write_text(req_hash)
+    (venv_dir / _HASH_FILE).write_text(req_hash, encoding="utf-8")
     logger.info("Environment ready for '%s'", evaluator_path.stem)
     return _venv_python(venv_dir)
 
diff --git a/src/agentevals/loader/auto.py b/src/agentevals/loader/auto.py
index 3ac3ddb..c50f3df 100644
--- a/src/agentevals/loader/auto.py
+++ b/src/agentevals/loader/auto.py
@@ -51,7 +51,7 @@ def detect_format(path: str) -> str | None:
     load can fall back to a default.
     """
     try:
-        with open(path) as f:
+        with open(path, encoding="utf-8") as f:
             if path.lower().endswith(".jsonl"):
                 return OTLP_JSON
             data = json.load(f)
diff --git a/src/agentevals/loader/jaeger.py b/src/agentevals/loader/jaeger.py
index cb02338..14a27fd 100644
--- a/src/agentevals/loader/jaeger.py
+++ b/src/agentevals/loader/jaeger.py
@@ -43,7 +43,7 @@ def format_name(self) -> str:
         return "jaeger-json"
 
     def load(self, source: str) -> list[Trace]:
-        with open(source) as f:
+        with open(source, encoding="utf-8") as f:
             raw = json.load(f)
 
         if not isinstance(raw, dict) or "data" not in raw:
diff --git a/src/agentevals/loader/otlp.py b/src/agentevals/loader/otlp.py
index 8155e02..ed22156 100644
--- a/src/agentevals/loader/otlp.py
+++ b/src/agentevals/loader/otlp.py
@@ -35,7 +35,7 @@ def format_name(self) -> str:
 
     def load(self, source: str) -> list[Trace]:
         """Load OTLP JSON file or JSONL (one span per line)."""
-        with open(source) as f:
+        with open(source, encoding="utf-8") as f:
             content = f.read().strip()
 
         if not content:
diff --git a/src/agentevals/mcp_server.py b/src/agentevals/mcp_server.py
index 6ed3c4b..4025ea0 100644
--- a/src/agentevals/mcp_server.py
+++ b/src/agentevals/mcp_server.py
@@ -378,7 +378,7 @@ async def summarize_session(session_id: str) -> SummarizeSessionResponse:
 
         raw = await _post("/api/streaming/get-trace", {"session_id": session_id})
 
-        with tempfile.NamedTemporaryFile(mode="w", suffix=".jsonl", delete=False) as f:
+        with tempfile.NamedTemporaryFile(mode="w", suffix=".jsonl", delete=False, encoding="utf-8") as f:
             f.write(raw["traceContent"])
             tmp_path = f.name
 
diff --git a/src/agentevals/run/fetcher.py b/src/agentevals/run/fetcher.py
index 34f8bae..d5c20d8 100644
--- a/src/agentevals/run/fetcher.py
+++ b/src/agentevals/run/fetcher.py
@@ -37,7 +37,7 @@ class InlineTraceFetcher:
     async def fetch(self, target: TraceTarget, context: dict) -> list[Trace]:
         if not target.inline:
             raise ValueError("InlineTraceFetcher requires target.inline to be set")
-        with tempfile.NamedTemporaryFile(mode="w", suffix=".json", delete=False) as f:
+        with tempfile.NamedTemporaryFile(mode="w", suffix=".json", delete=False, encoding="utf-8") as f:
             json.dump(target.inline, f)
             path = Path(f.name)
         try:
@@ -61,7 +61,7 @@ async def fetch(self, target: TraceTarget, context: dict) -> list[Trace]:
             resp = await client.get(url, headers=headers)
             resp.raise_for_status()
             payload = resp.json()
-        with tempfile.NamedTemporaryFile(mode="w", suffix=".json", delete=False) as f:
+        with tempfile.NamedTemporaryFile(mode="w", suffix=".json", delete=False, encoding="utf-8") as f:
             json.dump(payload, f)
             path = Path(f.name)
         try:
diff --git a/src/agentevals/run/sinks.py b/src/agentevals/run/sinks.py
index 06fd6c7..db228b3 100644
--- a/src/agentevals/run/sinks.py
+++ b/src/agentevals/run/sinks.py
@@ -77,7 +77,7 @@ def __init__(self, path: str | Path) -> None:
     async def _write(self, payload: dict) -> None:
         async with self._lock:
             self._path.parent.mkdir(parents=True, exist_ok=True)
-            with self._path.open("a") as f:  # noqa: ASYNC230
+            with self._path.open("a", encoding="utf-8") as f:  # noqa: ASYNC230
                 f.write(json.dumps(payload) + "\n")
 
     async def emit_partial(self, run_id: UUID, results: list[Result], attempt: int) -> None:
diff --git a/src/agentevals/runner.py b/src/agentevals/runner.py
index ebca577..eae4066 100644
--- a/src/agentevals/runner.py
+++ b/src/agentevals/runner.py
@@ -64,7 +64,7 @@ class RunResult(BaseModel):
 
 
 def load_eval_set(path: str) -> EvalSet:
-    with open(path) as f:
+    with open(path, encoding="utf-8") as f:
         data = json.load(f)
     return EvalSet.model_validate(data)
 
diff --git a/src/agentevals/storage/config.py b/src/agentevals/storage/config.py
index fcf5ea4..6947747 100644
--- a/src/agentevals/storage/config.py
+++ b/src/agentevals/storage/config.py
@@ -88,7 +88,7 @@ def _read_dsn_from_env() -> str | None:
     file_path = os.environ.get("AGENTEVALS_DATABASE_URL_FILE")
     if file_path:
         try:
-            with open(file_path) as f:
+            with open(file_path, encoding="utf-8") as f:
                 return f.read().strip() or None
         except OSError as exc:
             raise ValueError(f"AGENTEVALS_DATABASE_URL_FILE={file_path!r} is unreadable: {exc}") from exc
diff --git a/src/agentevals/streaming/ws_server.py b/src/agentevals/streaming/ws_server.py
index f7448fc..966c599 100644
--- a/src/agentevals/streaming/ws_server.py
+++ b/src/agentevals/streaming/ws_server.py
@@ -664,7 +664,7 @@ async def _save_spans_to_temp_file(self, session: TraceSession) -> Path:
         # span here; otherwise it's lost on reload and runs can't group by agent.
         service_name = (session.metadata or {}).get(OTEL_SERVICE_NAME)
 
-        with open(temp_file, "w") as f:  # noqa: ASYNC230
+        with open(temp_file, "w", encoding="utf-8") as f:  # noqa: ASYNC230
             for span in enriched_spans:
                 span_copy = span.copy()
                 span_copy["traceId"] = session.trace_id
@@ -695,7 +695,7 @@ async def _extract_invocations(self, session: TraceSession) -> list[dict]:
                 - modelInfo: Model metadata (model name, tokens, etc.)
         """
         try:
-            temp_file = tempfile.NamedTemporaryFile(mode="w", suffix=".jsonl", delete=False)
+            temp_file = tempfile.NamedTemporaryFile(mode="w", suffix=".jsonl", delete=False, encoding="utf-8")
 
             has_genai_spans = any(
                 span.get("attributes", [])
diff --git a/tests/run/test_sinks.py b/tests/run/test_sinks.py
index e0a3bd4..73a6a65 100644
--- a/tests/run/test_sinks.py
+++ b/tests/run/test_sinks.py
@@ -77,7 +77,7 @@ async def test_emits_partial_and_final(self, tmp_path):
         await sink.emit_partial(run_id, [_result(run_id)], attempt=1)
         await sink.emit_final(run_id, {"trace_count": 1}, attempt=1)
         await sink.emit_error(run_id, "boom", attempt=1)
-        lines = path.read_text().strip().splitlines()
+        lines = path.read_text(encoding="utf-8").strip().splitlines()
         assert len(lines) == 3
         partial = json.loads(lines[0])
         assert partial["phase"] == "partial"
@@ -249,7 +249,7 @@ def _factory(spec: dict) -> FileSink:
         fanout = build_sinks([{"kind": "plugin_file", "path": str(path)}])
         run_id = uuid4()
         await fanout.emit_final(run_id, {"ok": True}, attempt=1)
-        assert json.loads(path.read_text().strip())["summary"] == {"ok": True}
+        assert json.loads(path.read_text(encoding="utf-8").strip())["summary"] == {"ok": True}
 
     async def test_programmatic_registration_overrides_builtin(self, isolated_sink_plugins):
         finals: list[dict] = []
@@ -299,7 +299,7 @@ def file_factory(spec: dict) -> FileSink:
         ep.load.assert_called_once_with()
         run_id = uuid4()
         await fanout.emit_final(run_id, {"via": "ep"}, attempt=1)
-        assert json.loads(path.read_text().strip())["summary"] == {"via": "ep"}
+        assert json.loads(path.read_text(encoding="utf-8").strip())["summary"] == {"via": "ep"}
 
     async def test_builtin_kind_does_not_load_colliding_entry_point(self, isolated_sink_plugins, capsys):
         ep = MagicMock()
@@ -350,7 +350,7 @@ async def test_missing_plugin_sink_does_not_break_fanout(self, isolated_sink_plu
         assert any("unknown sink kind 'demo_ndjson'" in r.getMessage() for r in caplog.records)
         run_id = uuid4()
         await fanout.emit_final(run_id, {"ok": True}, attempt=1)
-        assert json.loads(path.read_text().strip())["summary"] == {"ok": True}
+        assert json.loads(path.read_text(encoding="utf-8").strip())["summary"] == {"ok": True}
 
 
 def _demo_example_sink_installed() -> bool:
@@ -372,7 +372,7 @@ async def test_path_dot_writes_default_ndjson(self, tmp_path, monkeypatch):
         sink = create_demo_sink({"path": "."})
         await sink.emit_final(uuid4(), {"x": 1}, attempt=1)
         out = tmp_path / "agentevals-demo-sink.ndjson"
-        assert json.loads(out.read_text().strip())["summary"] == {"x": 1}
+        assert json.loads(out.read_text(encoding="utf-8").strip())["summary"] == {"x": 1}
 
     async def test_existing_directory_appends_default_filename(self, tmp_path):
         d = tmp_path / "outdir"
@@ -390,7 +390,7 @@ async def test_directory_with_custom_filename(self, tmp_path):
 
         sink = create_demo_sink({"path": str(d), "filename": "runs.jsonl"})
         await sink.emit_final(uuid4(), {"n": 2}, attempt=1)
-        assert json.loads((d / "runs.jsonl").read_text().strip())["summary"] == {"n": 2}
+        assert json.loads((d / "runs.jsonl").read_text(encoding="utf-8").strip())["summary"] == {"n": 2}
 
 
 class TestSinkFanoutErrorIsolation:
diff --git a/tests/storage/test_config.py b/tests/storage/test_config.py
index 910b96e..4ae3f27 100644
--- a/tests/storage/test_config.py
+++ b/tests/storage/test_config.py
@@ -76,7 +76,7 @@ def test_from_env_reads_postgres(self, monkeypatch):
 
     def test_from_env_url_file_takes_precedence(self, tmp_path, monkeypatch):
         dsn_file = tmp_path / "dsn"
-        dsn_file.write_text("postgresql://from-file/db\n")
+        dsn_file.write_text("postgresql://from-file/db\n", encoding="utf-8")
         monkeypatch.setenv("AGENTEVALS_STORAGE_BACKEND", "postgres")
         monkeypatch.setenv("AGENTEVALS_DATABASE_URL", "postgresql://from-env/db")
         monkeypatch.setenv("AGENTEVALS_DATABASE_URL_FILE", str(dsn_file))
diff --git a/tests/test_api.py b/tests/test_api.py
index 627707d..1d5af56 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -1330,7 +1330,9 @@ def test_download_missing(self):
         assert resp.status_code == 404
 
     def test_download_success(self):
-        with tempfile.NamedTemporaryFile(mode="w", suffix=".json", delete=False, dir=tempfile.gettempdir()) as f:
+        with tempfile.NamedTemporaryFile(
+            mode="w", suffix=".json", delete=False, dir=tempfile.gettempdir(), encoding="utf-8"
+        ) as f:
             f.write('{"test": true}')
             fname = os.path.basename(f.name)
 
diff --git a/tests/test_cli.py b/tests/test_cli.py
index 58b72f9..d0a27ec 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -93,7 +93,7 @@ def fake_server_factory(config):
 
 def test_run_preserves_config_file_settings_when_flags_omitted(monkeypatch, tmp_path):
     trace_file = tmp_path / "trace.json"
-    trace_file.write_text("{}")
+    trace_file.write_text("{}", encoding="utf-8")
     config_file = tmp_path / "eval.yaml"
     config_file.write_text(
         textwrap.dedent(
@@ -105,7 +105,8 @@ def test_run_preserves_config_file_settings_when_flags_omitted(monkeypatch, tmp_
             trace_format: otlp-json
             output: json
             """
-        )
+        ),
+        encoding="utf-8",
     )
 
     captured = {}
@@ -129,7 +130,7 @@ async def fake_run_evaluation(config):
 
 def test_run_merges_explicit_metrics_with_config_file(monkeypatch, tmp_path):
     trace_file = tmp_path / "trace.json"
-    trace_file.write_text("{}")
+    trace_file.write_text("{}", encoding="utf-8")
     config_file = tmp_path / "eval.yaml"
     config_file.write_text(
         textwrap.dedent(
@@ -141,7 +142,8 @@ def test_run_merges_explicit_metrics_with_config_file(monkeypatch, tmp_path):
                 type: code
                 path: ./examples/custom_evaluators/tool_call_checker.py
             """
-        )
+        ),
+        encoding="utf-8",
     )
 
     captured = {}
diff --git a/tests/test_eval_config_loader.py b/tests/test_eval_config_loader.py
index ad89323..d2c578a 100644
--- a/tests/test_eval_config_loader.py
+++ b/tests/test_eval_config_loader.py
@@ -26,7 +26,8 @@ def test_load_eval_config_rejects_legacy_keys(tmp_path):
             trace_format: otlp-json
             output: json
             """
-        )
+        ),
+        encoding="utf-8",
     )
 
     with pytest.raises(ValueError, match="Legacy eval config keys are no longer supported"):
diff --git a/tests/test_jaeger_loader.py b/tests/test_jaeger_loader.py
index bfdc9b7..7d05a8f 100644
--- a/tests/test_jaeger_loader.py
+++ b/tests/test_jaeger_loader.py
@@ -51,7 +51,7 @@ def test_format_name(self, loader):
 
     def test_load_minimal(self, loader, minimal_jaeger_json, tmp_path):
         path = tmp_path / "test.json"
-        path.write_text(json.dumps(minimal_jaeger_json))
+        path.write_text(json.dumps(minimal_jaeger_json), encoding="utf-8")
 
         traces = loader.load(str(path))
         assert len(traces) == 1
@@ -63,7 +63,7 @@ def test_load_minimal(self, loader, minimal_jaeger_json, tmp_path):
 
     def test_span_tree_structure(self, loader, minimal_jaeger_json, tmp_path):
         path = tmp_path / "test.json"
-        path.write_text(json.dumps(minimal_jaeger_json))
+        path.write_text(json.dumps(minimal_jaeger_json), encoding="utf-8")
 
         trace = loader.load(str(path))[0]
 
@@ -79,7 +79,7 @@ def test_span_tree_structure(self, loader, minimal_jaeger_json, tmp_path):
 
     def test_tags_parsed(self, loader, minimal_jaeger_json, tmp_path):
         path = tmp_path / "test.json"
-        path.write_text(json.dumps(minimal_jaeger_json))
+        path.write_text(json.dumps(minimal_jaeger_json), encoding="utf-8")
 
         trace = loader.load(str(path))[0]
         root = trace.root_spans[0]
@@ -89,7 +89,7 @@ def test_tags_parsed(self, loader, minimal_jaeger_json, tmp_path):
 
     def test_invalid_format_raises(self, loader, tmp_path):
         path = tmp_path / "bad.json"
-        path.write_text(json.dumps({"not_data": []}))
+        path.write_text(json.dumps({"not_data": []}), encoding="utf-8")
 
         with pytest.raises(ValueError, match="Invalid Jaeger JSON format"):
             loader.load(str(path))
@@ -97,14 +97,14 @@ def test_invalid_format_raises(self, loader, tmp_path):
     def test_empty_spans_skipped(self, loader, tmp_path):
         data = {"data": [{"traceID": "empty", "spans": []}]}
         path = tmp_path / "empty.json"
-        path.write_text(json.dumps(data))
+        path.write_text(json.dumps(data), encoding="utf-8")
 
         traces = loader.load(str(path))
         assert len(traces) == 0
 
     def test_find_spans_by_operation(self, loader, minimal_jaeger_json, tmp_path):
         path = tmp_path / "test.json"
-        path.write_text(json.dumps(minimal_jaeger_json))
+        path.write_text(json.dumps(minimal_jaeger_json), encoding="utf-8")
 
         trace = loader.load(str(path))[0]
         found = trace.find_spans_by_operation("child")
@@ -113,7 +113,7 @@ def test_find_spans_by_operation(self, loader, minimal_jaeger_json, tmp_path):
 
     def test_find_spans_by_tag(self, loader, minimal_jaeger_json, tmp_path):
         path = tmp_path / "test.json"
-        path.write_text(json.dumps(minimal_jaeger_json))
+        path.write_text(json.dumps(minimal_jaeger_json), encoding="utf-8")
 
         trace = loader.load(str(path))[0]
         found = trace.find_spans_by_tag("key1", "val1")
diff --git a/tests/test_loader_auto.py b/tests/test_loader_auto.py
index 484e7a0..bcdacd9 100644
--- a/tests/test_loader_auto.py
+++ b/tests/test_loader_auto.py
@@ -15,7 +15,7 @@
 
 
 def _write_tmp(content: str, suffix: str = ".json") -> str:
-    f = tempfile.NamedTemporaryFile(mode="w", suffix=suffix, delete=False)
+    f = tempfile.NamedTemporaryFile(mode="w", suffix=suffix, delete=False, encoding="utf-8")
     f.write(content)
     f.close()
     return f.name
diff --git a/tests/test_openai_eval_backend.py b/tests/test_openai_eval_backend.py
index c58f0bd..7ea0f2a 100644
--- a/tests/test_openai_eval_backend.py
+++ b/tests/test_openai_eval_backend.py
@@ -1,6 +1,7 @@
-import pytest
 from unittest.mock import MagicMock
 
+import pytest
+
 from agentevals.config import OpenAIEvalDef
 from agentevals.openai_eval_backend import (
     _build_jsonl_items,
diff --git a/tests/test_otlp_loader.py b/tests/test_otlp_loader.py
index 9e496e3..c3a3428 100644
--- a/tests/test_otlp_loader.py
+++ b/tests/test_otlp_loader.py
@@ -43,7 +43,7 @@ def test_otlp_loader_jsonl_format(sample_otlp_span):
     """Test loading JSONL format (one span per line)."""
     loader = OtlpJsonLoader()
 
-    with tempfile.NamedTemporaryFile(mode="w", suffix=".jsonl", delete=False) as f:
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".jsonl", delete=False, encoding="utf-8") as f:
         f.write(json.dumps(sample_otlp_span) + "\n")
         temp_path = f.name
 
@@ -110,7 +110,7 @@ def test_otlp_loader_full_export():
         ]
     }
 
-    with tempfile.NamedTemporaryFile(mode="w", suffix=".json", delete=False) as f:
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".json", delete=False, encoding="utf-8") as f:
         json.dump(otlp_export, f)
         temp_path = f.name
 
@@ -169,7 +169,7 @@ def test_otlp_loader_parent_child_relationships():
         },
     ]
 
-    with tempfile.NamedTemporaryFile(mode="w", suffix=".jsonl", delete=False) as f:
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".jsonl", delete=False, encoding="utf-8") as f:
         for span in spans:
             f.write(json.dumps(span) + "\n")
         temp_path = f.name
@@ -200,7 +200,7 @@ def test_otlp_loader_empty_file():
     """Test loading an empty file."""
     loader = OtlpJsonLoader()
 
-    with tempfile.NamedTemporaryFile(mode="w", suffix=".json", delete=False) as f:
+    with tempfile.NamedTemporaryFile(mode="w", suffix=".json", delete=False, encoding="utf-8") as f:
         temp_path = f.name
 
     try:
diff --git a/tests/test_runner.py b/tests/test_runner.py
index e537d18..ae56d8a 100644
--- a/tests/test_runner.py
+++ b/tests/test_runner.py
@@ -273,7 +273,9 @@ def _run(self, match_type, tmp_path):
         conv_result = convert_traces([_make_tool_trace(["helm_get_release", "helm_list_releases"])])[0]
 
         eval_set_path = tmp_path / "eval_set.json"
-        eval_set_path.write_text(json.dumps(_make_eval_set_json(["helm_list_releases", "helm_get_release"])))
+        eval_set_path.write_text(
+            json.dumps(_make_eval_set_json(["helm_list_releases", "helm_get_release"])), encoding="utf-8"
+        )
         eval_set = load_eval_set(str(eval_set_path))
 
         return asyncio.run(
@@ -312,7 +314,9 @@ def test_builtin_custom_evaluator_uses_per_evaluator_match_type(self, tmp_path):
         conv_result = convert_traces([_make_tool_trace(["helm_get_release", "helm_list_releases"])])[0]
 
         eval_set_path = tmp_path / "eval_set.json"
-        eval_set_path.write_text(json.dumps(_make_eval_set_json(["helm_list_releases", "helm_get_release"])))
+        eval_set_path.write_text(
+            json.dumps(_make_eval_set_json(["helm_list_releases", "helm_get_release"])), encoding="utf-8"
+        )
         eval_set = load_eval_set(str(eval_set_path))
 
         trace_result = asyncio.run(