diff --git a/pm/client.py b/pm/client.py
index fb5fbf957d..eff4735da5 100644
--- a/pm/client.py
+++ b/pm/client.py
@@ -5,6 +5,7 @@ from collections.abc import Mapping, Sequence
 import json
 from pathlib import Path
 import subprocess
+import sys
 import threading
 import uuid
 
@@ -183,6 +184,20 @@ def ensure(name, *, base_env=None, explicit=False, progress=None, pause_event=No
 
 def sync_venv(extras=None, *, explicit=False, plugin_dirs=None, extra_plugin_dirs=(), selection=None, staged_plugin=None, repair=False,
               project_root: Path | None = None) -> None:
+    from pm.environments import running_from_selected_environment
+
+    if extras and not explicit and not repair and not running_from_selected_environment(
+            paths.repo_root() if project_root is None else Path(project_root)):
+        # A lazy extra may only extend the environment this process runs from. From any other
+        # interpreter (a build_environment test venv, a developer venv, a Nix Python) the sync would
+        # commit a selection this process never activates while every process booted afterwards
+        # swaps onto it — a generation without whatever the foreign interpreter carried.
+        from pm.install import _refuse_lazy
+        raise _refuse_lazy(
+            "venv",
+            f"{list(extras)}: this process is not running from the install's dependency environment "
+            f"({sys.prefix}); only an explicit install may change what later processes boot into",
+        )
     if selection is not None and "expected_config" not in selection:
         from hermes_cli.runtime_state import _digest
         selection = {**selection, "expected_config": _digest(Path(selection["home"]) / "config.yaml") or "missing"}
diff --git a/pm/environments.py b/pm/environments.py
index 892f2c3dc7..8cc5510020 100644
--- a/pm/environments.py
+++ b/pm/environments.py
@@ -169,6 +169,27 @@ def site_packages(venv: Path) -> Path:
     return venv / f"lib/python{version[0]}.{version[1]}/site-packages"
 
 
+def running_from_selected_environment(project_root: Path) -> bool:
+    """Does this process run on the environment PM selected for the install (base venv or committed
+    generation)?
+
+    A lazy sync from any other interpreter — a build_environment test venv, a developer's own venv,
+    a Nix store Python — must not commit the install's selection: activation is a boot decision, so
+    this process keeps running unchanged while every process booted afterwards swaps onto a
+    generation that lacks whatever the foreign interpreter carried.
+
+    activate_dependencies puts the selection's site-packages on sys.path without changing
+    sys.prefix, so sys.path is the signal (the same one ensure_import reads after a sync).
+    """
+    import sys
+
+    try:
+        selected = site_packages(selected_venv(project_root)).resolve()
+    except (OSError, RuntimeError, ValueError):
+        return False
+    return any(Path(entry).resolve() == selected for entry in sys.path if entry)
+
+
 def activate_dependencies(project_root: Path) -> None:
     """Select the committed tree at process boot, before third-party imports.
 
diff --git a/tests/pm/test_features.py b/tests/pm/test_features.py
index e790c13494..d6cfaed603 100644
--- a/tests/pm/test_features.py
+++ b/tests/pm/test_features.py
@@ -88,3 +88,38 @@ def test_sync_venv_allows_frozen_extras_when_lazy_off(rooted, monkeypatch):
     from pm.package import InstallError
     with pytest.raises(InstallError, match="lazy installs are disabled"):
         ensure_mod.sync_venv(["web"])
+
+
+def test_lazy_sync_never_creates_the_first_selection_for_a_foreign_interpreter(rooted, monkeypatch):
+    """A process running from an environment PM did not select (a build_environment test venv,
+    a dev venv, nix) imports an adapter whose extra is missing. Letting that lazy sync commit
+    the install's FIRST selection strands every later process: they boot into a generation
+    that lacks whatever the foreign interpreter carried (the CI "anthropic/aiohttp vanished"
+    class). Only an explicit install may create it. Exercised at the client seam every
+    ensure_import caller goes through, in the in-process (is_runtime) shape."""
+    import pm.client as client
+    import pm.install as ensure_mod
+    from pm import paths
+    from pm.package import InstallError
+    from pm.environments import runtime_facts_path
+
+    repo = rooted / "repo"
+    repo.mkdir()
+    (repo / "venv").mkdir()  # the install's own base venv, which this pytest process is NOT running from
+    monkeypatch.setattr(paths, "repo_root", lambda: repo)
+    monkeypatch.setattr(client, "is_runtime", lambda: True)
+    monkeypatch.setattr(ensure_mod, "lazy_installs_allowed", lambda: True)
+    venv_pkg = ensure_mod.get_package("venv")
+    monkeypatch.setattr(venv_pkg, "expected_stamp", lambda extras, **kwargs: "stamp")
+    monkeypatch.setattr(
+        venv_pkg, "apply", lambda *args, **kwargs: pytest.fail("lazy sync built a generation for a foreign interpreter"))
+
+    assert not runtime_facts_path(repo).exists()
+    with pytest.raises(InstallError, match="not running from the install's dependency environment"):
+        client.sync_venv(["bedrock"])
+    assert not runtime_facts_path(repo).exists(), "a refused sync must not commit a selection"
+
+    # The remedy the refusal names still works: an explicit install creates the selection.
+    monkeypatch.setattr(venv_pkg, "apply", lambda *args, **kwargs: {})
+    client.sync_venv(["bedrock"], explicit=True)
+    assert runtime_facts_path(repo).is_file()