diff --git a/python/packages/core/agent_framework/observability.py b/python/packages/core/agent_framework/observability.py
index 0fa017a2ed..362be2146e 100644
--- a/python/packages/core/agent_framework/observability.py
+++ b/python/packages/core/agent_framework/observability.py
@@ -2153,24 +2153,23 @@ def _capture_messages(
     finish_reason: FinishReason | None = None,
 ) -> None:
     """Log messages with extra information."""
-    from ._types import normalize_messages, prepend_instructions_to_messages
+    from ._types import normalize_messages
 
     normalized_messages = normalize_messages(messages)
-    # For logging: prepend framework instructions as messages so the log record
-    # mirrors the full message list as sent to the provider.
-    log_messages = prepend_instructions_to_messages(normalized_messages, system_instructions)
-    for index, message in enumerate(log_messages):
+    otel_messages: list[dict[str, Any]] = []
+    for index, message in enumerate(normalized_messages):
+        # Reuse the otel message representation for logging instead of calling to_dict()
+        # to avoid expensive Pydantic serialization overhead
+        otel_message = _to_otel_message(message)
         logger.info(
-            _to_otel_message(message),
+            otel_message,
             extra={
                 OtelAttr.EVENT_NAME: OtelAttr.CHOICE if output else ROLE_EVENT_MAP.get(message.role),
                 OtelAttr.PROVIDER_NAME: provider_name,
                 MessageListTimestampFilter.INDEX_KEY: index,
             },
         )
-    # For the span attribute, only include the original chat history so that
-    # framework/options instructions are not duplicated into gen_ai.input.messages.
-    otel_messages: list[dict[str, Any]] = [_to_otel_message(message) for message in normalized_messages]
+        otel_messages.append(otel_message)
     if finish_reason:
         otel_messages[-1]["finish_reason"] = FINISH_REASON_MAP[finish_reason]
     span.set_attribute(
diff --git a/python/packages/core/tests/core/test_observability.py b/python/packages/core/tests/core/test_observability.py
index 1966c7164b..d4403043af 100644
--- a/python/packages/core/tests/core/test_observability.py
+++ b/python/packages/core/tests/core/test_observability.py
@@ -3019,10 +3019,10 @@ async def test_system_instructions_preserves_non_ascii_characters(span_exporter:
     assert [msg.get("role") for msg in input_messages] == ["user"]
 
 
-def test_capture_messages_logs_prepended_instructions_excludes_them_from_span(
+def test_capture_messages_keeps_framework_instructions_out_of_logs_and_span_messages(
     span_exporter: InMemorySpanExporter,
 ):
-    """Test framework instructions are logged as prepended messages but excluded from the span attribute."""
+    """Test separate framework instructions do not appear in chat-history logs or span messages."""
     import json
 
     from opentelemetry import trace
@@ -3043,22 +3043,21 @@ def test_capture_messages_logs_prepended_instructions_excludes_them_from_span(
 
     spans = span_exporter.get_finished_spans()
     assert len(spans) == 1
-    # Span attribute must not contain the framework instruction.
     input_messages = json.loads(spans[0].attributes[OtelAttr.INPUT_MESSAGES])
     assert [msg.get("role") for msg in input_messages] == ["user"]
 
-    # Logging path must include the prepended framework instruction.
-    assert mock_logger_info.call_count == 2, f"Expected 2 log calls, got {mock_logger_info.call_count}"
-    logged_messages = [call.args[0] for call in mock_logger_info.call_args_list]
-    assert [msg["role"] for msg in logged_messages] == ["system", "user"]
-    assert logged_messages[0]["parts"][0]["content"] == "Framework system instruction"
-    assert logged_messages[1]["parts"][0]["content"] == "Test"
+    assert mock_logger_info.call_count == 1, f"Expected 1 log call, got {mock_logger_info.call_count}"
+    (first_call,) = mock_logger_info.call_args_list
+    assert first_call.args
+    logged_message = first_call.args[0]
+    assert logged_message["role"] == "user"
+    assert logged_message["parts"][0]["content"] == "Test"
 
 
-def test_capture_messages_logs_prepended_instructions_when_chat_history_has_system_message(
+def test_capture_messages_logs_only_chat_history_when_framework_instructions_are_separate(
     span_exporter: InMemorySpanExporter,
 ):
-    """Test framework instructions are prepended in logs; original system message stays in span attribute."""
+    """Test chat-history logging preserves original system messages without prepending framework instructions."""
     import json
 
     from opentelemetry import trace
@@ -3082,17 +3081,14 @@ def test_capture_messages_logs_prepended_instructions_when_chat_history_has_syst
 
     spans = span_exporter.get_finished_spans()
     assert len(spans) == 1
-    # Span attribute keeps the original chat history (including the original system message).
     input_messages = json.loads(spans[0].attributes[OtelAttr.INPUT_MESSAGES])
     assert [msg.get("role") for msg in input_messages] == ["system", "user"]
 
-    # Logging path includes the prepended framework instruction before the chat history.
-    assert mock_logger_info.call_count == 3, f"Expected 3 log calls, got {mock_logger_info.call_count}"
+    assert mock_logger_info.call_count == 2, f"Expected 2 log calls, got {mock_logger_info.call_count}"
     logged_messages = [call.args[0] for call in mock_logger_info.call_args_list]
-    assert [msg["role"] for msg in logged_messages] == ["system", "system", "user"]
-    assert logged_messages[0]["parts"][0]["content"] == "Framework system instruction"
-    assert logged_messages[1]["parts"][0]["content"] == "Original system message"
-    assert logged_messages[2]["parts"][0]["content"] == "Test"
+    assert [msg["role"] for msg in logged_messages] == ["system", "user"]
+    assert logged_messages[0]["parts"][0]["content"] == "Original system message"
+    assert logged_messages[1]["parts"][0]["content"] == "Test"
 
 
 @pytest.mark.parametrize("enable_sensitive_data", [True], indirect=True)