diff --git a/changes/11273.test.md b/changes/11273.test.md
new file mode 100644
index 00000000000..cdc127cee83
--- /dev/null
+++ b/changes/11273.test.md
@@ -0,0 +1 @@
+Add unit tests for FixedQueryBuilder
diff --git a/tests/unit/common/clients/prometheus/test_fixed_query_builder.py b/tests/unit/common/clients/prometheus/test_fixed_query_builder.py
new file mode 100644
index 00000000000..a62b5589eec
--- /dev/null
+++ b/tests/unit/common/clients/prometheus/test_fixed_query_builder.py
@@ -0,0 +1,149 @@
+"""
+Tests for FixedQueryBuilder: query building, metric type classification,
+and live stat query construction.
+"""
+
+import re
+from uuid import UUID
+
+import pytest
+
+from ai.backend.common.clients.prometheus import (
+    FixedQueryBuilder,
+)
+from ai.backend.common.clients.prometheus.fixed_query_builder import _regex_union
+from ai.backend.common.clients.prometheus.metric_types import (
+    ContainerMetricOptionalLabel,
+    MetricType,
+)
+from ai.backend.common.clients.prometheus.preset import LabelMatcher, LabelOperator, MetricPreset
+from ai.backend.common.clients.prometheus.types import ValueType
+from ai.backend.common.types import KernelId
+
+
+class TestGetContainerMetricType:
+    @pytest.fixture
+    def builder(self) -> FixedQueryBuilder:
+        return FixedQueryBuilder("1m")
+
+    @pytest.mark.parametrize(
+        ("metric_name", "value_type", "expected"),
+        [
+            ("mem", ValueType.CURRENT, MetricType.GAUGE),
+            ("cpu_util", ValueType.CAPACITY, MetricType.GAUGE),
+            ("net_rx", ValueType.CURRENT, MetricType.RATE),
+            ("net_tx", ValueType.CURRENT, MetricType.RATE),
+            ("net_rx", ValueType.CAPACITY, MetricType.RATE),
+            ("cpu_util", ValueType.CURRENT, MetricType.DIFF),
+        ],
+        ids=[
+            "gauge-unknown-metric",
+            "gauge-capacity-overrides-diff",
+            "rate-net_rx",
+            "rate-net_tx",
+            "rate-precedence-over-value_type",
+            "diff-cpu_util-current",
+        ],
+    )
+    def test_metric_type_classification(
+        self,
+        builder: FixedQueryBuilder,
+        metric_name: str,
+        value_type: ValueType,
+        expected: MetricType,
+    ) -> None:
+        label = ContainerMetricOptionalLabel(value_type=value_type)
+        assert builder.get_container_metric_type(metric_name, label) == expected
+
+
+class TestGetContainerMetricQuery:
+    @pytest.fixture
+    def builder(self) -> FixedQueryBuilder:
+        return FixedQueryBuilder("5m")
+
+    def test_gauge_query_preset(self, builder: FixedQueryBuilder) -> None:
+        label = ContainerMetricOptionalLabel(value_type=ValueType.CURRENT)
+
+        result = builder.get_container_metric_query("mem", label)
+
+        assert isinstance(result, MetricPreset)
+        assert result.window == "5m"
+        assert result.labels["container_metric_name"] == LabelMatcher.exact("mem")
+        assert result.labels["value_type"] == LabelMatcher.exact("current")
+        assert "value_type" in result.group_by
+
+    @pytest.mark.parametrize("metric_name", ["net_rx", "cpu_util"])
+    def test_rate_based_query_uses_rate_function(
+        self, builder: FixedQueryBuilder, metric_name: str
+    ) -> None:
+        label = ContainerMetricOptionalLabel(value_type=ValueType.CURRENT)
+
+        rendered = builder.get_container_metric_query(metric_name, label).render()
+
+        assert "rate(" in rendered
+        assert "[5m]" in rendered
+
+    def test_query_with_optional_labels(self, builder: FixedQueryBuilder) -> None:
+        kid = UUID("12345678-1234-5678-1234-567812345678")
+        label = ContainerMetricOptionalLabel(
+            value_type=ValueType.CURRENT,
+            kernel_id=kid,
+        )
+
+        result = builder.get_container_metric_query("mem", label)
+
+        assert result.labels["kernel_id"] == LabelMatcher.exact(str(kid))
+        assert "kernel_id" in result.group_by
+
+
+class TestGetContainerLiveStatQueries:
+    def test_kernel_id_regex_filter(self) -> None:
+        builder = FixedQueryBuilder("1m")
+        kid1 = KernelId(UUID("aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa"))
+        kid2 = KernelId(UUID("bbbbbbbb-bbbb-bbbb-bbbb-bbbbbbbbbbbb"))
+
+        result = builder.get_container_live_stat_queries([kid1, kid2])
+
+        matcher = result.gauge.labels["kernel_id"]
+        assert matcher.operator == LabelOperator.REGEX
+        pattern = re.compile(matcher.value)
+        assert pattern.fullmatch(str(kid1))
+        assert pattern.fullmatch(str(kid2))
+        assert not pattern.fullmatch("cccccccc-cccc-cccc-cccc-cccccccccccc")
+
+    @pytest.mark.parametrize(
+        ("preset_attr", "expected_metrics"),
+        [
+            ("diff", ["cpu_util"]),
+            ("rate", ["net_rx", "net_tx"]),
+        ],
+    )
+    def test_preset_filters_by_metric_name_and_value_type(
+        self, preset_attr: str, expected_metrics: list[str]
+    ) -> None:
+        builder = FixedQueryBuilder("1m")
+        kid = KernelId(UUID("aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa"))
+
+        result = builder.get_container_live_stat_queries([kid])
+        labels = getattr(result, preset_attr).labels
+
+        assert labels["container_metric_name"].operator == LabelOperator.REGEX
+        for metric in expected_metrics:
+            assert metric in labels["container_metric_name"].value
+        assert labels["value_type"] == LabelMatcher.exact("current")
+
+    def test_gauge_has_no_metric_name_filter(self) -> None:
+        builder = FixedQueryBuilder("1m")
+        kid = KernelId(UUID("aaaaaaaa-aaaa-aaaa-aaaa-aaaaaaaaaaaa"))
+
+        result = builder.get_container_live_stat_queries([kid])
+
+        assert "container_metric_name" not in result.gauge.labels
+
+
+class TestRegexUnion:
+    def test_escapes_special_chars(self) -> None:
+        result = _regex_union(["a.b", "c+d"])
+        assert r"a\.b" in result
+        assert r"c\+d" in result
+        assert "|" in result