diff --git a/haystack/components/generators/utils.py b/haystack/components/generators/utils.py
index 4504a05708..25bf764c15 100644
--- a/haystack/components/generators/utils.py
+++ b/haystack/components/generators/utils.py
@@ -160,7 +160,13 @@ def _convert_streaming_chunks_to_chat_message(chunks: list[StreamingChunk]) -> C
 
 
 def _serialize_object(obj: Any) -> Any:
-    """Convert an object to a serializable dict recursively"""
+    """
+    Convert an object to a serializable dict recursively.
+
+    Used to serialize `logprobs` and `usage` from OpenAI SDK response objects, so it skips any
+    attribute starting with "_" (SDK-internal fields). `base_serialization._serialize_value_with_schema`
+    doesn't skip those, so don't swap this out for it.
+    """
     if hasattr(obj, "model_dump"):
         return obj.model_dump()
     if hasattr(obj, "__dict__"):
diff --git a/haystack/core/serialization.py b/haystack/core/serialization.py
index 85f7d60940..344a6bac95 100644
--- a/haystack/core/serialization.py
+++ b/haystack/core/serialization.py
@@ -193,6 +193,11 @@ def default_to_dict(obj: Any, **init_parameters: Any) -> dict[str, Any]:
     Objects in `init_parameters` that have a `to_dict()` method are automatically
     serialized by calling that method.
 
+    This is the format used for saved pipeline files (`Pipeline.dump`/`Pipeline.load`). Don't merge
+    it with `base_serialization._serialize_value_with_schema` — that one uses a different envelope
+    for a different job (arbitrary runtime values, not Components) and changing either would break
+    saved files.
+
     An example usage:
 
     ```python
@@ -257,7 +262,10 @@ def default_from_dict(cls: type[T], data: dict[str, Any]) -> T:
     """
     Utility function to deserialize a dictionary to an object.
 
-    This is mostly necessary for components but can be used by any object.
+    This is mostly necessary for components but can be used by any object. Reverses the
+    `{"type": ..., "init_parameters": ...}` envelope produced by `default_to_dict` — see that
+    function's docstring for why this envelope is not interchangeable with
+    `haystack.utils.base_serialization._serialize_value_with_schema`'s.
 
     The function will raise a `DeserializationError` if the `type` field in `data` is
     missing or it doesn't match the type of `cls`.
diff --git a/haystack/tracing/utils.py b/haystack/tracing/utils.py
index 0f3b18ff50..10ff4ce43a 100644
--- a/haystack/tracing/utils.py
+++ b/haystack/tracing/utils.py
@@ -43,6 +43,10 @@ def _serializable_value(value: Any, use_placeholders: bool = True) -> Any:
     """
     Serializes a value into a format suitable for tracing.
 
+    One-way only: this is never deserialized, so it's allowed to lose data (e.g. replace large
+    objects with a placeholder). Don't swap it for `base_serialization`'s schema serializer, which
+    is built to round-trip exactly.
+
     :param value:
         The value to serialize.
     :param use_placeholders:
diff --git a/haystack/utils/base_serialization.py b/haystack/utils/base_serialization.py
index fe1929e1cd..64e61d250d 100644
--- a/haystack/utils/base_serialization.py
+++ b/haystack/utils/base_serialization.py
@@ -77,6 +77,10 @@ def _serialize_value_with_schema(payload: Any) -> dict[str, Any]:  # noqa: PLR09
     - Lists, tuples, and sets. Lists with mixed types are not supported.
     - Primitive types (str, int, float, bool, None)
 
+    This is for runtime values (Agent/State data, pipeline inputs/outputs at a breakpoint), not
+    Component definitions — see `default_to_dict` in `core/serialization.py` for that other format.
+    Don't merge the two; they're not interchangeable.
+
     :param payload: The value to serialize (can be any type)
     :returns: The serialized dict representation of the given value. Contains two keys:
         - "serialization_schema": Contains type information for each field.