diff --git a/src/agentevals/trace_attrs.py b/src/agentevals/trace_attrs.py
index 35da68a..e4cd16a 100644
--- a/src/agentevals/trace_attrs.py
+++ b/src/agentevals/trace_attrs.py
@@ -81,3 +81,6 @@
 GENAI_ATTRIBUTE_ALIASES: dict[str, list[str]] = {
     OTEL_GENAI_PROVIDER_NAME: [OTEL_GENAI_SYSTEM],
 }
+
+# agentevals custom attributes (repository-specific, outside OTel semconv)
+AGENTEVALS_SESSION_ID = "agentevals.session_id"
diff --git a/src/agentevals/utils/log_enrichment.py b/src/agentevals/utils/log_enrichment.py
index eecd3d4..c58f4fa 100644
--- a/src/agentevals/utils/log_enrichment.py
+++ b/src/agentevals/utils/log_enrichment.py
@@ -7,7 +7,7 @@
 from collections import defaultdict
 
 from ..trace_attrs import (
-    OTEL_GENAI_AGENT_NAME,
+    AGENTEVALS_SESSION_ID,
     OTEL_GENAI_INPUT_MESSAGES,
     OTEL_GENAI_OUTPUT_MESSAGES,
 )
@@ -28,7 +28,7 @@ def enrich_spans_with_logs(spans: list[dict], logs: list[dict], session_id: str
     Args:
         spans: List of OTLP span dictionaries
         logs: List of GenAI log event dictionaries
-        session_id: Optional session ID to add as agent.name attribute
+        session_id: Optional session ID to add as the agentevals.session_id attribute
 
     Returns:
         List of enriched span dictionaries with message attributes added
@@ -88,6 +88,22 @@ def _extract_messages_from_logs(
     return input_messages, output_messages
 
 
+def _append_session_id(attrs: list[dict], session_id: str | None) -> None:
+    """Append the session ID attribute to *attrs* in place, if present.
+
+    Written to the custom ``agentevals.session_id`` attribute rather than
+    ``gen_ai.agent.name`` so real agent names (from the SDK or OTLP) are
+    never overwritten.
+    """
+    if session_id:
+        attrs.append(
+            {
+                "key": AGENTEVALS_SESSION_ID,
+                "value": {"stringValue": session_id},
+            }
+        )
+
+
 def _inject_messages(
     span: dict,
     input_messages: list[dict],
@@ -113,13 +129,7 @@ def _inject_messages(
                 "value": {"stringValue": json.dumps(output_messages)},
             }
         )
-    if session_id:
-        attrs.append(
-            {
-                "key": OTEL_GENAI_AGENT_NAME,
-                "value": {"stringValue": session_id},
-            }
-        )
+    _append_session_id(attrs, session_id)
 
     return span_copy
 
@@ -148,12 +158,7 @@ def _enrich_per_span(
             span_copy = span.copy()
             if session_id:
                 attrs = list(span_copy.get("attributes", []))
-                attrs.append(
-                    {
-                        "key": OTEL_GENAI_AGENT_NAME,
-                        "value": {"stringValue": session_id},
-                    }
-                )
+                _append_session_id(attrs, session_id)
                 span_copy["attributes"] = attrs
             enriched.append(span_copy)
 
diff --git a/tests/test_log_enrichment.py b/tests/test_log_enrichment.py
index 42361a7..d5830a0 100644
--- a/tests/test_log_enrichment.py
+++ b/tests/test_log_enrichment.py
@@ -84,8 +84,8 @@ def test_session_id_injected(self):
         logs = [_make_log("gen_ai.user.message", {"content": "hi"})]
         result = enrich_spans_with_logs(spans, logs, session_id="my-session")
 
-        agent_name = _get_injected_attr(result[0], "gen_ai.agent.name", parse_json=False)
-        assert agent_name == "my-session"
+        session_attr = _get_injected_attr(result[0], "agentevals.session_id", parse_json=False)
+        assert session_attr == "my-session"
 
     def test_choice_extracts_nested_content(self):
         spans = [_make_span()]
@@ -139,6 +139,23 @@ def test_logs_matched_to_correct_span(self):
         msgs_s2 = _get_injected_attr(result[1], "gen_ai.input.messages")
         assert msgs_s2 == [{"role": "user", "content": "question 2"}]
 
+    def test_agent_name_preserved_with_session_id(self):
+        """Regression: real gen_ai.agent.name attr must survive enrichment,
+        session_id lands in its own attr, doesn't clobber it."""
+        span = _make_span(
+            "s1",
+            attrs=[{"key": "gen_ai.agent.name", "value": {"stringValue": "roll_die_agent"}}],
+        )
+        logs = [_make_log("gen_ai.user.message", {"content": "hi"}, span_id="s1")]
+        result = enrich_spans_with_logs([span], logs, session_id="sess-123")
+
+        agent_name_attrs = [a for a in result[0]["attributes"] if a["key"] == "gen_ai.agent.name"]
+        assert len(agent_name_attrs) == 1, f"expected exactly one gen_ai.agent.name attr, found {len(agent_name_attrs)}"
+        assert agent_name_attrs[0]["value"]["stringValue"] == "roll_die_agent"
+
+        session_attr = _get_injected_attr(result[0], "agentevals.session_id", parse_json=False)
+        assert session_attr == "sess-123"
+
     def test_span_without_logs_not_enriched(self):
         spans = [_make_span("s1"), _make_span("s2")]
         logs = [
@@ -156,8 +173,8 @@ def test_session_id_on_all_spans(self):
         ]
         result = enrich_spans_with_logs(spans, logs, session_id="test")
 
-        assert _get_injected_attr(result[0], "gen_ai.agent.name", parse_json=False) == "test"
-        assert _get_injected_attr(result[1], "gen_ai.agent.name", parse_json=False) == "test"
+        assert _get_injected_attr(result[0], "agentevals.session_id", parse_json=False) == "test"
+        assert _get_injected_attr(result[1], "agentevals.session_id", parse_json=False) == "test"
 
     def test_tool_calls_in_assistant_message(self):
         spans = [_make_span("s1")]