diff --git a/src/google/adk/cli/cli_eval.py b/src/google/adk/cli/cli_eval.py
index f191f5efbad..26645d575f1 100644
--- a/src/google/adk/cli/cli_eval.py
+++ b/src/google/adk/cli/cli_eval.py
@@ -53,7 +53,7 @@
 # This is always optional unless explicitly specified.
 RESPONSE_EVALUATION_SCORE_KEY = "response_evaluation_score"
 
-EVAL_SESSION_ID_PREFIX = "___eval___session___"
+EVAL_SESSION_ID_PREFIX = "eval-session-"
 DEFAULT_CRITERIA = {
     TOOL_TRAJECTORY_SCORE_KEY: 1.0,  # 1-point scale; 1.0 is perfect.
     RESPONSE_MATCH_SCORE_KEY: 0.8,
diff --git a/src/google/adk/evaluation/local_eval_service.py b/src/google/adk/evaluation/local_eval_service.py
index 16dcc514c88..5b7c28d5375 100644
--- a/src/google/adk/evaluation/local_eval_service.py
+++ b/src/google/adk/evaluation/local_eval_service.py
@@ -62,10 +62,14 @@
 
 logger = logging.getLogger("google_adk." + __name__)
 
-EVAL_SESSION_ID_PREFIX = "___eval___session___"
+EVAL_SESSION_ID_PREFIX = "eval-session-"
 
 
 def _get_session_id() -> str:
+  # uuid4's default str form is lowercase hex with hyphens, so together
+  # with the prefix this always matches Vertex AI Agent Engine's session
+  # id constraint ([a-z0-9-], alnum first/last char) as well as every
+  # other session service's id format.
   return f"{EVAL_SESSION_ID_PREFIX}{str(uuid.uuid4())}"