diff --git a/tests/e2e/claude_code/_driver_unit_tests/test_request_determinism.py b/tests/e2e/claude_code/_driver_unit_tests/test_request_determinism.py
index 5046f35c73b..b7d330b7da6 100644
--- a/tests/e2e/claude_code/_driver_unit_tests/test_request_determinism.py
+++ b/tests/e2e/claude_code/_driver_unit_tests/test_request_determinism.py
@@ -36,6 +36,8 @@ import pytest
 from claude_code.cli_driver import _FIXED_CLI_USER_ID, _seed_cli_identity, _stable_cli_state, run_claude
 from claude_code.rate_limiter import RateLimiter
 
+pytestmark = pytest.mark.cli_determinism
+
 _STUB_REPLY = {
     "id": "msg_stub",
     "type": "message",
diff --git a/tests/e2e/conftest.py b/tests/e2e/conftest.py
index 430e16525d5..ac4cfb71407 100644
--- a/tests/e2e/conftest.py
+++ b/tests/e2e/conftest.py
@@ -23,6 +23,7 @@ from typing import Final
 import pytest
 import requests
 from e2e_config import (
+    CLI_DETERMINISM_OPT_IN_ENV,
     CONTROL_PLANE_BASE_URL,
     FIXTURE_DIR,
     FIXTURE_MODE_RAW,
@@ -53,6 +54,7 @@ OPT_IN_MARKERS: Final = MappingProxyType(
         "managed_files": MANAGED_FILES_OPT_IN_ENV,
         "prompt_caching_stack": PROMPT_CACHING_OPT_IN_ENV,
         "redis_chaos": REDIS_CHAOS_OPT_IN_ENV,
+        "cli_determinism": CLI_DETERMINISM_OPT_IN_ENV,
     }
 )
 
@@ -120,6 +122,11 @@ def pytest_configure(config: pytest.Config) -> None:
         "prompt_caching_stack: needs a proxy running with router_settings.optional_pre_call_checks including "
         "prompt_caching; deselected unless E2E_PROMPT_CACHING_STACK is set",
     )
+    config.addinivalue_line(
+        "markers",
+        "cli_determinism: drives the real claude CLI for several seconds, which widens the window in which "
+        "another test's in-flight upstream call is attributed to it; deselected unless E2E_CLI_DETERMINISM is set",
+    )
     config.addinivalue_line(
         "markers",
         "redis_chaos: load test that pauses the proxy's Redis outright mid-run; needs a proxy booted from "
diff --git a/tests/e2e/e2e_config.py b/tests/e2e/e2e_config.py
index 896cb3e7efe..82ddb09f7f5 100644
--- a/tests/e2e/e2e_config.py
+++ b/tests/e2e/e2e_config.py
@@ -145,6 +145,7 @@ WEEKLY_ANOMALY_OPT_IN_ENV = "E2E_WEEKLY_ANOMALY"
 MANAGED_FILES_OPT_IN_ENV = "E2E_MANAGED_FILES_STACK"
 PROMPT_CACHING_OPT_IN_ENV = "E2E_PROMPT_CACHING_STACK"
 REDIS_CHAOS_OPT_IN_ENV = "E2E_REDIS_CHAOS"
+CLI_DETERMINISM_OPT_IN_ENV = "E2E_CLI_DETERMINISM"
 ANOMALY_SESSIONS = int(os.environ.get("E2E_ANOMALY_SESSIONS", "6"))
 ANOMALY_TURNS_PER_SESSION = int(os.environ.get("E2E_ANOMALY_TURNS_PER_SESSION", "6"))
 ANOMALY_TURN_ATTEMPTS = int(os.environ.get("E2E_ANOMALY_TURN_ATTEMPTS", "3"))
diff --git a/tests/e2e/pytest.ini b/tests/e2e/pytest.ini
index 1fdd3bd28ad..f6d23a3ec12 100644
--- a/tests/e2e/pytest.ini
+++ b/tests/e2e/pytest.ini
@@ -10,4 +10,5 @@ markers =
     weekly: real-provider anomaly load test that spends real money; deselected unless E2E_WEEKLY_ANOMALY is set
     managed_files: needs a proxy running with require_managed_files enabled; deselected unless E2E_MANAGED_FILES_STACK is set
     prompt_caching_stack: needs a proxy running with router_settings.optional_pre_call_checks including prompt_caching; deselected unless E2E_PROMPT_CACHING_STACK is set
+    cli_determinism: drives the real claude CLI for several seconds, which widens the window in which another test's in-flight upstream call is attributed to it; deselected unless E2E_CLI_DETERMINISM is set
     redis_chaos: load test that pauses the proxy's Redis outright mid-run; needs a proxy booted from gateway/redis_chaos_ci_config.yml on the same host, and is deselected unless E2E_REDIS_CHAOS is set