diff --git a/langfuse/_utils/serializer.py b/langfuse/_utils/serializer.py
index 27294bf80..6dcf2a6ce 100644
--- a/langfuse/_utils/serializer.py
+++ b/langfuse/_utils/serializer.py
@@ -1,6 +1,7 @@
 """@private"""
 
 import datetime as dt
+import decimal
 import enum
 import math
 from asyncio import Queue
@@ -72,6 +73,14 @@ def _default_inner(self, obj: Any) -> Any:
             if np is not None and isinstance(obj, np.ndarray):
                 return obj.tolist()
 
+            # Serialize Decimal as its exact string form rather than via float():
+            # float() would silently round high-precision values (and overflow on
+            # very large ones), and JSON numbers are parsed as doubles downstream
+            # anyway. str() preserves the exact value; NaN/Infinity render as
+            # "NaN"/"Infinity"/"-Infinity", matching the float handling below.
+            if isinstance(obj, decimal.Decimal):
+                return str(obj)
+
             if isinstance(obj, float) and math.isnan(obj):
                 return "NaN"
 
@@ -140,7 +149,19 @@ def _default_inner(self, obj: Any) -> Any:
                 return list(obj)
 
             if isinstance(obj, dict):
-                return {self.default(k): self.default(v) for k, v in obj.items()}
+                result = {}
+                for k, v in obj.items():
+                    serialized_key = self.default(k)
+                    # JSON object keys must be scalars. If a key serializes to a
+                    # container/object (e.g. tuple, set, or custom object), fall back
+                    # to its string form so a single non-primitive key does not
+                    # discard the whole dict.
+                    if not isinstance(
+                        serialized_key, (str, int, float, bool, type(None))
+                    ):
+                        serialized_key = str(k)
+                    result[serialized_key] = self.default(v)
+                return result
 
             if isinstance(obj, list):
                 return [self.default(item) for item in obj]
diff --git a/tests/unit/test_serializer.py b/tests/unit/test_serializer.py
index f4c8dde86..baf3a529a 100644
--- a/tests/unit/test_serializer.py
+++ b/tests/unit/test_serializer.py
@@ -2,6 +2,7 @@
 import threading
 from dataclasses import dataclass
 from datetime import date, datetime, timezone
+from decimal import Decimal
 from enum import Enum
 from pathlib import Path
 from uuid import UUID
@@ -304,3 +305,43 @@ def test_dict_with_non_string_keys_is_serialized(input_obj, expected):
     result = json.loads(EventSerializer().encode(input_obj))
 
     assert result == expected
+
+
+def test_decimal_is_preserved_exactly():
+    # Serialized to its exact string form (never the "<Decimal>" fallback)
+    assert json.loads(EventSerializer().encode(Decimal("19.99"))) == "19.99"
+    assert json.loads(EventSerializer().encode({"price": Decimal("19.99")})) == {
+        "price": "19.99"
+    }
+    # High-precision values are preserved exactly (a float() conversion would
+    # silently round these).
+    assert (
+        json.loads(EventSerializer().encode(Decimal("1.0000000000000001")))
+        == "1.0000000000000001"
+    )
+    assert (
+        json.loads(EventSerializer().encode(Decimal("123456789012345678")))
+        == "123456789012345678"
+    )
+
+
+def test_decimal_special_values():
+    assert EventSerializer().encode(Decimal("NaN")) == '"NaN"'
+    assert EventSerializer().encode(Decimal("Infinity")) == '"Infinity"'
+    assert EventSerializer().encode(Decimal("-Infinity")) == '"-Infinity"'
+
+
+def test_dict_with_non_primitive_keys_preserves_values():
+    # A tuple key must not discard the entire dict (previously the whole dict
+    # serialized to a "<not serializable ...>" string).
+    result = json.loads(EventSerializer().encode({(1, 2): "v", "other": "data"}))
+    assert result == {"(1, 2)": "v", "other": "data"}
+
+
+def test_dict_with_custom_object_key_uses_str():
+    class _Key:
+        def __str__(self) -> str:
+            return "custom-key"
+
+    result = json.loads(EventSerializer().encode({_Key(): "val", "k2": "x"}))
+    assert result == {"custom-key": "val", "k2": "x"}