diff --git a/tests/tools/test_budget_config.py b/tests/tools/test_budget_config.py
new file mode 100644
index 000000000..aeacc6219
--- /dev/null
+++ b/tests/tools/test_budget_config.py
@@ -0,0 +1,176 @@
+"""Unit tests for tools/budget_config.py.
+
+Covers default values, resolve_threshold() priority chain
+(pinned > tool_overrides > registry > default), immutability,
+and the PINNED_THRESHOLDS escape-hatch for read_file.
+"""
+
+import dataclasses
+import math
+from unittest.mock import patch
+
+import pytest
+
+from tools.budget_config import (
+    DEFAULT_BUDGET,
+    DEFAULT_PREVIEW_SIZE_CHARS,
+    DEFAULT_RESULT_SIZE_CHARS,
+    DEFAULT_TURN_BUDGET_CHARS,
+    PINNED_THRESHOLDS,
+    BudgetConfig,
+)
+
+
+# ---------------------------------------------------------------------------
+# Module-level constants
+# ---------------------------------------------------------------------------
+
+
+class TestModuleConstants:
+    """Verify documented default values haven't drifted."""
+
+    def test_default_result_size(self):
+        assert DEFAULT_RESULT_SIZE_CHARS == 100_000
+
+    def test_default_turn_budget(self):
+        assert DEFAULT_TURN_BUDGET_CHARS == 200_000
+
+    def test_default_preview_size(self):
+        assert DEFAULT_PREVIEW_SIZE_CHARS == 1_500
+
+
+class TestPinnedThresholds:
+    """PINNED_THRESHOLDS – tools whose values must never be overridden."""
+
+    def test_read_file_is_inf(self):
+        assert PINNED_THRESHOLDS["read_file"] == float("inf")
+        assert math.isinf(PINNED_THRESHOLDS["read_file"])
+
+    def test_pinned_is_not_empty(self):
+        assert len(PINNED_THRESHOLDS) >= 1
+
+
+# ---------------------------------------------------------------------------
+# BudgetConfig defaults
+# ---------------------------------------------------------------------------
+
+
+class TestBudgetConfigDefaults:
+    """BudgetConfig() should match the module-level defaults exactly."""
+
+    def test_default_result_size(self):
+        cfg = BudgetConfig()
+        assert cfg.default_result_size == DEFAULT_RESULT_SIZE_CHARS
+
+    def test_default_turn_budget(self):
+        cfg = BudgetConfig()
+        assert cfg.turn_budget == DEFAULT_TURN_BUDGET_CHARS
+
+    def test_default_preview_size(self):
+        cfg = BudgetConfig()
+        assert cfg.preview_size == DEFAULT_PREVIEW_SIZE_CHARS
+
+    def test_default_tool_overrides_empty(self):
+        cfg = BudgetConfig()
+        assert cfg.tool_overrides == {}
+
+    def test_default_budget_singleton_matches(self):
+        """DEFAULT_BUDGET should equal a freshly constructed BudgetConfig."""
+        assert DEFAULT_BUDGET == BudgetConfig()
+
+
+# ---------------------------------------------------------------------------
+# Immutability (frozen=True)
+# ---------------------------------------------------------------------------
+
+
+class TestBudgetConfigFrozen:
+    """Frozen dataclass must reject attribute mutation."""
+
+    def test_cannot_set_default_result_size(self):
+        cfg = BudgetConfig()
+        with pytest.raises(dataclasses.FrozenInstanceError):
+            cfg.default_result_size = 999
+
+    def test_cannot_set_turn_budget(self):
+        cfg = BudgetConfig()
+        with pytest.raises(dataclasses.FrozenInstanceError):
+            cfg.turn_budget = 999
+
+    def test_cannot_set_preview_size(self):
+        cfg = BudgetConfig()
+        with pytest.raises(dataclasses.FrozenInstanceError):
+            cfg.preview_size = 999
+
+    def test_cannot_set_tool_overrides(self):
+        cfg = BudgetConfig()
+        with pytest.raises(dataclasses.FrozenInstanceError):
+            cfg.tool_overrides = {"foo": 1}
+
+
+# ---------------------------------------------------------------------------
+# Custom construction
+# ---------------------------------------------------------------------------
+
+
+class TestBudgetConfigCustom:
+    """BudgetConfig can be created with non-default values."""
+
+    def test_custom_values(self):
+        cfg = BudgetConfig(
+            default_result_size=50_000,
+            turn_budget=100_000,
+            preview_size=500,
+            tool_overrides={"my_tool": 42},
+        )
+        assert cfg.default_result_size == 50_000
+        assert cfg.turn_budget == 100_000
+        assert cfg.preview_size == 500
+        assert cfg.tool_overrides == {"my_tool": 42}
+
+
+# ---------------------------------------------------------------------------
+# resolve_threshold() priority chain
+# ---------------------------------------------------------------------------
+
+
+class TestResolveThreshold:
+    """Priority: pinned > tool_overrides > registry > default."""
+
+    def test_pinned_wins_over_override(self):
+        """Even if tool_overrides contains read_file, pinned value wins."""
+        cfg = BudgetConfig(tool_overrides={"read_file": 1})
+        result = cfg.resolve_threshold("read_file")
+        assert result == float("inf")
+
+    def test_tool_override_wins_over_default(self):
+        """tool_overrides should be returned before falling back to registry."""
+        cfg = BudgetConfig(tool_overrides={"my_tool": 42})
+        result = cfg.resolve_threshold("my_tool")
+        assert result == 42
+
+    @patch("tools.registry.registry")
+    def test_falls_back_to_registry(self, mock_registry):
+        """When not pinned and not in overrides, delegate to registry."""
+        mock_registry.get_max_result_size.return_value = 77_777
+        cfg = BudgetConfig()
+        result = cfg.resolve_threshold("some_tool")
+        mock_registry.get_max_result_size.assert_called_once_with(
+            "some_tool", default=DEFAULT_RESULT_SIZE_CHARS
+        )
+        assert result == 77_777
+
+    @patch("tools.registry.registry")
+    def test_registry_receives_custom_default(self, mock_registry):
+        """Custom default_result_size flows through to registry call."""
+        mock_registry.get_max_result_size.return_value = 50_000
+        cfg = BudgetConfig(default_result_size=50_000)
+        cfg.resolve_threshold("unknown_tool")
+        mock_registry.get_max_result_size.assert_called_once_with(
+            "unknown_tool", default=50_000
+        )
+
+    def test_pinned_read_file_returns_inf(self):
+        """Canonical case: read_file must always return inf."""
+        cfg = BudgetConfig()
+        assert cfg.resolve_threshold("read_file") == float("inf")