diff --git a/ddtrace/llmobs/_writer.py b/ddtrace/llmobs/_writer.py
index b80675bcc47..e7c05b03da3 100644
--- a/ddtrace/llmobs/_writer.py
+++ b/ddtrace/llmobs/_writer.py
@@ -111,7 +111,7 @@ def __init__(
     ) -> None:
         super(BaseLLMObsWriter, self).__init__(interval=interval)
         self._lock = forksafe.RLock()
-        self._buffer: List[Union[LLMObsSpanEvent, LLMObsEvaluationMetricEvent]] = []
+        self._buffer: List[str] = []
         self._buffer_size: int = 0
         self._timeout: float = timeout
         self._api_key: str = _api_key or config._dd_api_key
@@ -148,7 +148,7 @@ def stop(self, timeout=None):
     def on_shutdown(self):
         self.periodic()
 
-    def _enqueue(self, event: Union[LLMObsSpanEvent, LLMObsEvaluationMetricEvent], event_size: int) -> None:
+    def _enqueue(self, encoded_event: str, event_size: int) -> None:
         """Internal shared logic of enqueuing events to be submitted to LLM Observability."""
         with self._lock:
             if len(self._buffer) >= self.BUFFER_LIMIT:
@@ -160,7 +160,7 @@ def _enqueue(self, event: Union[LLMObsSpanEvent, LLMObsEvaluationMetricEvent], e
             if self._buffer_size + event_size > EVP_PAYLOAD_SIZE_LIMIT:
                 logger.debug("manually flushing buffer because queueing next event will exceed EVP payload limit")
                 self.periodic()
-            self._buffer.append(event)
+            self._buffer.append(encoded_event)
             self._buffer_size += event_size
 
     def _encode(self, payload, num_events):
@@ -177,7 +177,7 @@ def periodic(self) -> None:
         with self._lock:
             if not self._buffer:
                 return
-            events = self._buffer
+            enc_events = self._buffer
             self._buffer = []
             self._buffer_size = 0
 
@@ -188,16 +188,17 @@ def periodic(self) -> None:
                 "`LLMObs.enable(api_key=...)` before running your application."
             )
             return
-        data = self._data(events)
-        enc_llm_events = self._encode(data, len(events))
-        if not enc_llm_events:
-            return
+        payload = self._data(enc_events)
         try:
-            self._send_payload_with_retry(enc_llm_events, len(events))
+            self._send_payload_with_retry(payload, len(enc_events))
         except Exception:
-            telemetry.record_dropped_payload(len(events), event_type=self.EVENT_TYPE, error="connection_error")
+            telemetry.record_dropped_payload(len(enc_events), event_type=self.EVENT_TYPE, error="connection_error")
             logger.error(
-                "failed to send %d LLMObs %s events to %s", len(events), self.EVENT_TYPE, self._intake, exc_info=True
+                "failed to send %d LLMObs %s events to %s",
+                len(enc_events),
+                self.EVENT_TYPE,
+                self._intake,
+                exc_info=True,
             )
 
     def _send_payload(self, payload: bytes, num_events: int):
@@ -263,11 +264,16 @@ class LLMObsEvalMetricWriter(BaseLLMObsWriter):
     ENDPOINT = EVAL_ENDPOINT
 
     def enqueue(self, event: LLMObsEvaluationMetricEvent) -> None:
-        event_size = len(safe_json(event))
-        self._enqueue(event, event_size)
+        encoded_event = self._encode(event, 1)
+        if not encoded_event:
+            return
+        event_size = len(encoded_event)
+        self._enqueue(encoded_event, event_size)
 
-    def _data(self, events: List[LLMObsEvaluationMetricEvent]) -> Dict[str, Any]:
-        return {"data": {"type": "evaluation_metric", "attributes": {"metrics": events}}}
+    def _data(self, events: List[str]) -> str:
+        metrics = ",".join(events)
+        enc_payload = f'{{"data": {{"type": "evaluation_metric", "attributes": {{"metrics": [{metrics}]}}}}}}'
+        return enc_payload
 
 
 class LLMObsSpanWriter(BaseLLMObsWriter):
@@ -279,7 +285,10 @@ class LLMObsSpanWriter(BaseLLMObsWriter):
     ENDPOINT = SPAN_ENDPOINT
 
     def enqueue(self, event: LLMObsSpanEvent) -> None:
-        raw_event_size = len(safe_json(event))
+        encoded_event = self._encode(event, 1)
+        if not encoded_event:
+            return
+        raw_event_size = len(encoded_event)
         truncated_event_size = None
         should_truncate = raw_event_size >= EVP_EVENT_SIZE_LIMIT
         if should_truncate:
@@ -288,16 +297,20 @@ def enqueue(self, event: LLMObsSpanEvent) -> None:
                 raw_event_size,
             )
             event = _truncate_span_event(event)
-            truncated_event_size = len(safe_json(event))
+            encoded_event = self._encode(event, 1)
+            truncated_event_size = len(encoded_event)
         telemetry.record_span_event_raw_size(event, raw_event_size)
         telemetry.record_span_event_size(event, truncated_event_size or raw_event_size)
-        self._enqueue(event, truncated_event_size or raw_event_size)
-
-    def _data(self, events: List[LLMObsSpanEvent]) -> List[Dict[str, Any]]:
-        return [
-            {"_dd.stage": "raw", "_dd.tracer_version": ddtrace.__version__, "event_type": "span", "spans": [event]}
-            for event in events
-        ]
+        self._enqueue(encoded_event, truncated_event_size or raw_event_size)
+
+    def _data(self, events: List[str]) -> str:
+        payload = []
+        tracer_version = ddtrace.__version__
+        for event in events:
+            payload.append(
+                f'{{"_dd.stage": "raw", "_dd.tracer_version": "{tracer_version}", "event_type": "span", "spans": [{event}]}}'
+            )
+        return "[{}]".format(",".join(payload))
 
 
 def _truncate_span_event(event: LLMObsSpanEvent) -> LLMObsSpanEvent: