diff --git a/tests/acp/test_server.py b/tests/acp/test_server.py
index c1ff1bf4e63..6dce8d8702b 100644
--- a/tests/acp/test_server.py
+++ b/tests/acp/test_server.py
@@ -971,6 +971,18 @@ class TestSessionConfiguration:
             "hermes_cli.runtime_provider.resolve_runtime_provider",
             fake_resolve_runtime_provider,
         )
+        # Pin the parser so this test doesn't depend on live
+        # ``_KNOWN_PROVIDER_NAMES`` / ``_PROVIDER_ALIASES`` module state
+        # (sibling of the same hardening on
+        # ``test_model_switch_uses_requested_provider``).
+        monkeypatch.setattr(
+            "hermes_cli.models.parse_model_input",
+            lambda raw, current: ("anthropic", "claude-sonnet-4-6"),
+        )
+        monkeypatch.setattr(
+            "hermes_cli.models.detect_provider_for_model",
+            lambda model, current: None,
+        )
         manager = SessionManager(db=SessionDB(tmp_path / "state.db"))
 
         with patch("run_agent.AIAgent", side_effect=fake_agent):
@@ -1543,6 +1555,20 @@ class TestSlashCommands:
             "hermes_cli.runtime_provider.resolve_runtime_provider",
             fake_resolve_runtime_provider,
         )
+        # Pin the model-string parser independently of the live
+        # ``_KNOWN_PROVIDER_NAMES`` / ``_PROVIDER_ALIASES`` module state.
+        # Otherwise any test in the same xdist worker that mutates those
+        # globals (e.g. registers a custom provider that shadows
+        # ``anthropic``) flakes this one — observed once in CI as
+        # ``'custom' == 'anthropic'``.
+        monkeypatch.setattr(
+            "hermes_cli.models.parse_model_input",
+            lambda raw, current: ("anthropic", "claude-sonnet-4-6"),
+        )
+        monkeypatch.setattr(
+            "hermes_cli.models.detect_provider_for_model",
+            lambda model, current: None,
+        )
         manager = SessionManager(db=SessionDB(tmp_path / "state.db"))
 
         with patch("run_agent.AIAgent", side_effect=fake_agent):