diff --git a/src/google/adk/cli/cli_eval.py b/src/google/adk/cli/cli_eval.py
index f191f5efba..88ca680ba3 100644
--- a/src/google/adk/cli/cli_eval.py
+++ b/src/google/adk/cli/cli_eval.py
@@ -53,7 +53,12 @@
 # This is always optional unless explicitly specified.
 RESPONSE_EVALUATION_SCORE_KEY = "response_evaluation_score"
 
-EVAL_SESSION_ID_PREFIX = "___eval___session___"
+# Must stay in sync with google.adk.evaluation.local_eval_service. The prefix
+# is only allowed to contain lowercase letters, digits and hyphens so the
+# generated eval session IDs satisfy the custom-session-ID constraints of
+# remote backends such as Vertex AI Agent Engine. See
+# https://github.com/google/adk-python/issues/6683
+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 16dcc514c8..065d7a7942 100644
--- a/src/google/adk/evaluation/local_eval_service.py
+++ b/src/google/adk/evaluation/local_eval_service.py
@@ -62,7 +62,13 @@
 
 logger = logging.getLogger("google_adk." + __name__)
 
-EVAL_SESSION_ID_PREFIX = "___eval___session___"
+# Note: must only contain lowercase letters, digits and hyphens, and start
+# and end with an alphanumeric character once the UUID is appended. Custom
+# session IDs are passed to the configured SessionService (e.g. Vertex
+# AiSessionService), which forwards them to remote backends such as Agent
+# Engine that enforce this constraint. See
+# https://github.com/google/adk-python/issues/6683
+EVAL_SESSION_ID_PREFIX = "eval-session-"
 
 
 def _get_session_id() -> str:
diff --git a/tests/unittests/evaluation/test_eval_session_id.py b/tests/unittests/evaluation/test_eval_session_id.py
new file mode 100644
index 0000000000..f544a208f8
--- /dev/null
+++ b/tests/unittests/evaluation/test_eval_session_id.py
@@ -0,0 +1,41 @@
+# Copyright 2026 Google LLC
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from __future__ import annotations
+
+import re
+
+from google.adk.evaluation.local_eval_service import _get_session_id
+
+# Vertex AI Agent Engine Sessions only accept custom session IDs that match
+# `[a-z0-9-]`, with a letter or digit as the first and last character.
+# See https://github.com/google/adk-python/issues/6683
+_AGENT_ENGINE_SESSION_ID_PATTERN = re.compile(r'^[a-z0-9-]+$')
+
+
+def test_eval_session_id_matches_agent_engine_constraints():
+  # Regression test for https://github.com/google/adk-python/issues/6683
+  # The generated eval session ID is passed straight to the configured
+  # SessionService (e.g. VertexAiSessionService), which forwards it to remote
+  # backends such as Agent Engine as a custom session ID. IDs containing
+  # underscores or uppercase letters are rejected by the remote API, so the
+  # generated ID must match `[a-z0-9-]` and start/end with an alphanumeric
+  # character.
+  session_id = _get_session_id()
+  assert _AGENT_ENGINE_SESSION_ID_PATTERN.fullmatch(session_id), (
+      f'Generated eval session id {session_id!r} must match'
+      f' {_AGENT_ENGINE_SESSION_ID_PATTERN.pattern}.'
+  )
+  assert session_id[0].isalnum()
+  assert session_id[-1].isalnum()