diff --git a/agent/secret_scope.py b/agent/secret_scope.py
index 2dd0728ebe..cc414f9c3f 100644
--- a/agent/secret_scope.py
+++ b/agent/secret_scope.py
@@ -29,9 +29,18 @@ _MULTIPLEX_ACTIVE: bool = False
 
 
 def set_multiplex_active(active: bool) -> None:
-    """Mark whether the process is a profile multiplexer (get_secret fails closed)."""
+    """Mark whether the process is a profile multiplexer (get_secret fails closed).
+
+    Activation also freezes the launch home (``hermes_constants.pin_process_hermes_home``): from
+    here on "is this task routed" compares the override against the home the process was launched
+    with, not against whatever a host later mirrors into ``os.environ["HERMES_HOME"]``."""
     global _MULTIPLEX_ACTIVE
+    from hermes_constants import pin_process_hermes_home, unpin_process_hermes_home
     _MULTIPLEX_ACTIVE = bool(active)
+    if _MULTIPLEX_ACTIVE:
+        pin_process_hermes_home()
+    else:
+        unpin_process_hermes_home()
 
 
 def is_multiplex_active() -> bool:
diff --git a/hermes_constants.py b/hermes_constants.py
index bec5d17d96..40b0bdba5a 100644
--- a/hermes_constants.py
+++ b/hermes_constants.py
@@ -159,12 +159,39 @@ def get_process_hermes_home() -> Path:
     """Hermes home of the running process, ignoring task overrides.
 
     For process-level assets (theme YAML, dashboard plugin manifests) that must stay visible while a
-    request is scoped to another profile (e.g. embedded ``/chat`` under ``--open-profile``).
+    request is scoped to another profile (e.g. embedded ``/chat`` under ``--open-profile``), and the
+    reference every "does this task serve a ROUTED home" decision compares the override against.
+    Once pinned (``pin_process_hermes_home``) the answer is frozen: a host that mirrors the served
+    profile into ``os.environ["HERMES_HOME"]`` per turn would otherwise re-label the launch home on
+    every turn and every served profile would look like the launch one (#119242).
     """
+    if _PINNED_PROCESS_HOME is not None:
+        return _PINNED_PROCESS_HOME
     val = os.environ.get("HERMES_HOME", "").strip()
     return _expand_hermes_home(val) if val else _get_platform_default_hermes_home()
 
 
+# The launch home, frozen the moment this process starts serving a second profile
+# (``agent.secret_scope.set_multiplex_active(True)``) or when an embedding host pins it explicitly.
+_PINNED_PROCESS_HOME: Path | None = None
+
+
+def pin_process_hermes_home(path: "str | Path | None" = None) -> Path:
+    """Freeze the launch home; the first pin wins. ``None`` pins the home the process env names NOW,
+    so it must run before any per-turn mirror of ``HERMES_HOME`` — the same moment
+    ``tui_gateway.launch_profile_policy.capture_launch_env`` freezes the env."""
+    global _PINNED_PROCESS_HOME
+    if _PINNED_PROCESS_HOME is None:
+        _PINNED_PROCESS_HOME = _expand_hermes_home(str(path)) if path else get_process_hermes_home()
+    return _PINNED_PROCESS_HOME
+
+
+def unpin_process_hermes_home() -> None:
+    """Follow ``HERMES_HOME`` live again (single-profile mode; tests that stand hosts up and down)."""
+    global _PINNED_PROCESS_HOME
+    _PINNED_PROCESS_HOME = None
+
+
 # Hermes-managed runtime downloads at the root of a home (GGUF models, llama.cpp runtimes,
 # managed Node): re-downloadable on demand and routinely tens to hundreds of GB. Shared by
 # ``hermes backup`` (excludes them) and ``profile create --clone-all`` (skips them from the
diff --git a/tests/agent/test_secret_scope_pinned_launch_home.py b/tests/agent/test_secret_scope_pinned_launch_home.py
new file mode 100644
index 0000000000..32f580ee51
--- /dev/null
+++ b/tests/agent/test_secret_scope_pinned_launch_home.py
@@ -0,0 +1,66 @@
+"""The launch home is frozen once the process serves several profiles (#119242).
+
+Every "does this task serve a ROUTED home" decision (``agent.secret_scope.serves_routed_profile``,
+``_is_process_home``, ``tools.environments.local._is_routed_home``,
+``hermes_cli.env_loader._process_hermes_home``) compares the task's home override with
+``get_process_hermes_home()``. That used to read ``os.environ["HERMES_HOME"]`` live, so a host that
+mirrors the served profile into the env on every turn (Hermes WebUI does) made every served
+profile look like the launch one: MCP connections fell back to bare cross-profile names, the launch
+residue survived ``strip_launch_profile_env``, the launch profile's bridged grants seeded the
+served scope. ``set_multiplex_active(True)`` now pins the launch home; a later env mutation cannot
+re-label it. Standalone ``hermes -p x gateway run`` (multiplex inactive) keeps following the env.
+"""
+from __future__ import annotations
+
+import pytest
+
+import hermes_constants
+from agent.secret_scope import _is_process_home, serves_routed_profile, set_multiplex_active
+from hermes_cli.env_loader import _process_hermes_home
+from hermes_constants import get_process_hermes_home, reset_hermes_home_override, set_hermes_home_override
+from tools.environments.local import _is_routed_home
+
+
+@pytest.fixture
+def homes(tmp_path, monkeypatch):
+    launch = tmp_path / "launch"
+    served = tmp_path / "profiles" / "served"
+    launch.mkdir()
+    served.mkdir(parents=True)
+    monkeypatch.setenv("HERMES_HOME", str(launch))
+    monkeypatch.setattr(hermes_constants, "_PINNED_PROCESS_HOME", None, raising=False)
+    return launch, served
+
+
+def test_a_per_turn_env_mirror_cannot_relabel_the_launch_home_under_multiplex(homes, monkeypatch):
+    launch, served = homes
+    set_multiplex_active(True)
+    monkeypatch.setenv("HERMES_HOME", str(served))  # the host's per-turn mirror
+    token = set_hermes_home_override(served)
+    try:
+        assert get_process_hermes_home() == launch
+        assert _process_hermes_home() == launch
+        assert _is_routed_home(served) and not _is_routed_home(launch)
+        assert not _is_process_home(served) and _is_process_home(launch)
+        assert serves_routed_profile()
+    finally:
+        reset_hermes_home_override(token)
+    set_multiplex_active(False)
+    # Pin released with the mode: the env is authoritative again.
+    assert get_process_hermes_home() == served
+
+
+def test_a_standalone_profile_process_keeps_following_its_env(homes, monkeypatch):
+    """T1 (``hermes -p x gateway run``): multiplex inactive, the env IS the profile — an override
+    naming that same home is not routed, and a later env change is followed."""
+    launch, served = homes
+    monkeypatch.setenv("HERMES_HOME", str(served))
+    token = set_hermes_home_override(served)
+    try:
+        assert get_process_hermes_home() == served
+        assert not _is_routed_home(served)
+        assert not serves_routed_profile()
+    finally:
+        reset_hermes_home_override(token)
+    monkeypatch.setenv("HERMES_HOME", str(launch))
+    assert get_process_hermes_home() == launch
diff --git a/tests/conftest.py b/tests/conftest.py
index 40a65085bb..c3db9a4265 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -565,6 +565,9 @@ def _hermetic_environment(tmp_path, monkeypatch):
     secret_scope_mod = sys.modules.get("agent.secret_scope")
     if secret_scope_mod is not None and hasattr(secret_scope_mod, "_MULTIPLEX_ACTIVE"):
         monkeypatch.setattr(secret_scope_mod, "_MULTIPLEX_ACTIVE", False)
+    hermes_constants_mod = sys.modules.get("hermes_constants")
+    if hermes_constants_mod is not None and hasattr(hermes_constants_mod, "_PINNED_PROCESS_HOME"):
+        monkeypatch.setattr(hermes_constants_mod, "_PINNED_PROCESS_HOME", None)
     launch_policy_mod = sys.modules.get("tui_gateway.launch_profile_policy")
     if launch_policy_mod is not None and hasattr(launch_policy_mod, "_snapshot"):
         monkeypatch.setattr(launch_policy_mod, "_snapshot", None)