diff --git a/mcp_clickhouse/mcp_server.py b/mcp_clickhouse/mcp_server.py
index c8a12243..0ae309fe 100644
--- a/mcp_clickhouse/mcp_server.py
+++ b/mcp_clickhouse/mcp_server.py
@@ -188,8 +188,29 @@ def result_to_column(query_columns, result) -> List[Column]:
     return [Column(**dict(zip(query_columns, row))) for row in result]
 
 
+# Integers outside JavaScript's safe range lose precision once a JSON consumer
+# (browsers, Node-based MCP clients, ...) parses them as IEEE-754 doubles, e.g.
+# UInt64 1875924584784080993 -> 1875924584784081000. Emit such values as strings
+# so the exact value survives; smaller ints stay numeric.
+JS_MAX_SAFE_INTEGER = (1 << 53) - 1
+
+
+def _json_safe(obj: Any) -> Any:
+    """Recursively stringify integers that exceed JavaScript's safe integer range."""
+    # bool is a subclass of int but is always JSON-safe, so leave it numeric.
+    if isinstance(obj, bool):
+        return obj
+    if isinstance(obj, int):
+        return str(obj) if abs(obj) > JS_MAX_SAFE_INTEGER else obj
+    if isinstance(obj, dict):
+        return {key: _json_safe(value) for key, value in obj.items()}
+    if isinstance(obj, (list, tuple)):
+        return [_json_safe(item) for item in obj]
+    return obj
+
+
 def _serialize_tool_result(obj: Any) -> str:
-    return json.dumps(obj, default=str)
+    return json.dumps(_json_safe(obj), default=str)
 
 
 def list_databases() -> str:
diff --git a/tests/test_serialization.py b/tests/test_serialization.py
new file mode 100644
index 00000000..cc544a4a
--- /dev/null
+++ b/tests/test_serialization.py
@@ -0,0 +1,62 @@
+"""Unit tests for JSON serialization of tool results.
+
+These are pure-function tests and do not require a ClickHouse connection.
+"""
+
+import json
+
+from mcp_clickhouse.mcp_server import (
+    JS_MAX_SAFE_INTEGER,
+    _json_safe,
+    _serialize_tool_result,
+)
+
+
+def test_large_uint64_is_stringified():
+    # Value from the bug report: it would otherwise round to ...081000.
+    value = 1875924584784080993
+    result = json.loads(_serialize_tool_result({"columns": ["id"], "rows": [[value]]}))
+    assert result["rows"][0][0] == str(value)
+
+
+def test_large_negative_int_is_stringified():
+    value = -(1 << 63)
+    assert _json_safe(value) == str(value)
+
+
+def test_boundary_values_stay_numeric():
+    # The largest safe integer and everything below it must remain a JSON number.
+    assert _json_safe(JS_MAX_SAFE_INTEGER) == JS_MAX_SAFE_INTEGER
+    assert _json_safe(-JS_MAX_SAFE_INTEGER) == -JS_MAX_SAFE_INTEGER
+    assert _json_safe(0) == 0
+    assert _json_safe(42) == 42
+    # One past the boundary flips to a string.
+    assert _json_safe(JS_MAX_SAFE_INTEGER + 1) == str(JS_MAX_SAFE_INTEGER + 1)
+
+
+def test_booleans_stay_boolean():
+    # bool is an int subclass; it must never be stringified.
+    assert _json_safe(True) is True
+    assert _json_safe(False) is False
+
+
+def test_nested_structures_are_walked():
+    payload = {
+        "rows": [
+            (1875924584784080993, "web-01", [9999999999999999999, 7]),
+        ],
+        "meta": {"count": 2, "huge": 12345678901234567890},
+    }
+    result = json.loads(_serialize_tool_result(payload))
+    assert result == {
+        "rows": [["1875924584784080993", "web-01", ["9999999999999999999", 7]]],
+        "meta": {"count": 2, "huge": "12345678901234567890"},
+    }
+
+
+def test_non_int_types_still_serialize_via_default():
+    # Types json can't encode natively fall through to json.dumps(default=str).
+    from decimal import Decimal
+
+    result = json.loads(_serialize_tool_result({"price": Decimal("1.5")}))
+    assert result["price"] == "1.5"