diff --git a/gateway/code_skew.py b/gateway/code_skew.py
new file mode 100644
index 00000000000..f7bc4ef3cee
--- /dev/null
+++ b/gateway/code_skew.py
@@ -0,0 +1,64 @@
+"""Detect when the gateway is running stale code after a hot ``git pull``.
+
+The gateway is a single long-lived process; its ``sys.modules`` is frozen at
+boot. If the checkout is updated underneath it (a manual ``git pull``, or the
+window before ``hermes update``'s graceful restart fires), a first-time lazy
+import on a new code path can resolve a freshly-pulled consumer module against a
+stale cached dependency -> ImportError (see
+``tests/test_stale_utils_module_import.py`` for the exact failure).
+
+We snapshot the checkout revision at gateway startup and compare on demand, so
+risky callers (e.g. ``/model`` switching) can refuse with a clear "restart the
+gateway" message instead of crashing on a cryptic import error.
+
+If the revision can't be read (non-git install, IO error), the boot snapshot
+stays ``None`` and skew detection no-ops — it never produces a false positive.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+_PROJECT_ROOT = Path(__file__).resolve().parent.parent
+_boot_fingerprint: str | None = None
+
+
+def _fingerprint() -> str | None:
+    """Current checkout fingerprint, reusing the CLI's git-rev reader.
+
+    ``hermes_cli.main`` is always already imported in a gateway process (it's
+    the entry point), so this import is free and avoids duplicating the
+    worktree-aware ref resolution.
+    """
+    try:
+        from hermes_cli.main import _read_git_revision_fingerprint
+
+        return _read_git_revision_fingerprint(_PROJECT_ROOT)
+    except Exception:
+        return None
+
+
+def record_boot_fingerprint() -> None:
+    """Snapshot the checkout revision at gateway startup (idempotent)."""
+    global _boot_fingerprint
+    if _boot_fingerprint is None:
+        _boot_fingerprint = _fingerprint()
+
+
+def _short(fingerprint: str) -> str:
+    """Render a ``git:<ref>:<sha>`` fingerprint as a compact label."""
+    sha = fingerprint.rsplit(":", 1)[-1]
+    if sha and sha != "unresolved" and len(sha) > 10:
+        return sha[:10]
+    return sha or fingerprint
+
+
+def detect_code_skew() -> tuple[str, str] | None:
+    """Return ``(boot_rev, disk_rev)`` short labels if the checkout drifted
+    since boot, else ``None``."""
+    if _boot_fingerprint is None:
+        return None
+    current = _fingerprint()
+    if current is None or current == _boot_fingerprint:
+        return None
+    return _short(_boot_fingerprint), _short(current)
diff --git a/gateway/run.py b/gateway/run.py
index 5ec99eddcd2..bc7f42aa8e9 100644
--- a/gateway/run.py
+++ b/gateway/run.py
@@ -17369,6 +17369,13 @@ async def start_gateway(config: Optional[GatewayConfig] = None, replace: bool =
                  Useful for systemd services to avoid restart-loop deadlocks
                  when the previous process hasn't fully exited yet.
     """
+    # Snapshot the checkout revision now, while sys.modules still matches disk,
+    # so a later `git pull` under this long-lived process can be detected (and
+    # risky work like model switching refused) instead of crashing on a stale
+    # in-memory module.
+    from gateway.code_skew import record_boot_fingerprint
+    record_boot_fingerprint()
+
     # ── Duplicate-instance guard ──────────────────────────────────────
     # Prevent two gateways from running under the same HERMES_HOME.
     # The PID file is scoped to HERMES_HOME, so future multi-profile
diff --git a/gateway/slash_commands.py b/gateway/slash_commands.py
index ab9ea9759bd..c7420bc645e 100644
--- a/gateway/slash_commands.py
+++ b/gateway/slash_commands.py
@@ -45,6 +45,35 @@ from utils import (
 logger = logging.getLogger("gateway.run")
 
 
+def _model_switch_skew_guard() -> Optional[str]:
+    """Refuse a model switch when the gateway is running stale code.
+
+    A long-lived gateway holds its modules in memory from boot. If the checkout
+    changed underneath it (e.g. a manual ``git pull``), switching models can hit
+    a first-time lazy import on a new code path and crash on a stale cached
+    dependency — the cryptic ``cannot import name 'env_float' from 'utils'``.
+    Detect the drift and tell the user to restart instead.
+
+    Intentionally scoped to model switching — the known, highest-risk trigger.
+    Any first-time lazy import on a stale process is technically exposed; we
+    don't guard every import site, only this one.
+    """
+    from gateway.code_skew import detect_code_skew
+
+    skew = detect_code_skew()
+    if not skew:
+        return None
+    boot_rev, disk_rev = skew
+    return t(
+        "gateway.model.error_prefix",
+        error=(
+            f"This gateway is running code from {boot_rev} but the checkout on "
+            f"disk is now {disk_rev}. Switching models would risk a stale-module "
+            f"crash — restart the gateway to load the new code: hermes gateway restart"
+        ),
+    )
+
+
 class GatewaySlashCommandsMixin:
     """In-session slash-command handlers for GatewayRunner."""
 
@@ -1146,6 +1175,9 @@ class GatewaySlashCommandsMixin:
                         _chat_id: str, model_id: str, provider_slug: str
                     ) -> str:
                         """Perform the model switch and return confirmation text."""
+                        skew_error = _model_switch_skew_guard()
+                        if skew_error:
+                            return skew_error
                         result = _switch_model(
                             raw_input=model_id,
                             current_provider=_cur_provider,
@@ -1366,6 +1398,9 @@ class GatewaySlashCommandsMixin:
             return "\n".join(lines)
 
         # Perform the switch
+        skew_error = _model_switch_skew_guard()
+        if skew_error:
+            return skew_error
         result = _switch_model(
             raw_input=model_input,
             current_provider=current_provider,
diff --git a/tests/test_code_skew.py b/tests/test_code_skew.py
new file mode 100644
index 00000000000..0773fd6b8b4
--- /dev/null
+++ b/tests/test_code_skew.py
@@ -0,0 +1,79 @@
+"""Tests for gateway code-skew detection (stale-checkout guard).
+
+Companion to ``tests/test_stale_utils_module_import.py``: that test proves the
+crash; these prove the guard that turns it into a clear "restart the gateway"
+message before a model switch can hit it.
+"""
+
+import pytest
+
+from gateway import code_skew
+
+
+@pytest.fixture(autouse=True)
+def _reset_boot_fingerprint(monkeypatch):
+    """Each test starts with no recorded boot fingerprint."""
+    monkeypatch.setattr(code_skew, "_boot_fingerprint", None)
+
+
+class TestDetectCodeSkew:
+    def test_no_boot_fingerprint_means_no_skew(self, monkeypatch):
+        # Nothing recorded (e.g. non-git install) -> never a false positive.
+        monkeypatch.setattr(code_skew, "_fingerprint", lambda: "git:refs/heads/main:def456")
+        assert code_skew.detect_code_skew() is None
+
+    def test_unchanged_checkout_is_not_skew(self, monkeypatch):
+        monkeypatch.setattr(code_skew, "_fingerprint", lambda: "git:refs/heads/main:abc1234567890")
+        code_skew.record_boot_fingerprint()
+        assert code_skew.detect_code_skew() is None
+
+    def test_drift_is_detected_with_short_revs(self, monkeypatch):
+        monkeypatch.setattr(code_skew, "_fingerprint", lambda: "git:refs/heads/main:abc1234567890")
+        code_skew.record_boot_fingerprint()
+
+        monkeypatch.setattr(code_skew, "_fingerprint", lambda: "git:refs/heads/main:def4567890123")
+        skew = code_skew.detect_code_skew()
+        assert skew == ("abc1234567", "def4567890")
+
+    def test_unreadable_current_rev_does_not_false_positive(self, monkeypatch):
+        monkeypatch.setattr(code_skew, "_fingerprint", lambda: "git:refs/heads/main:abc1234567890")
+        code_skew.record_boot_fingerprint()
+
+        monkeypatch.setattr(code_skew, "_fingerprint", lambda: None)
+        assert code_skew.detect_code_skew() is None
+
+    def test_record_is_idempotent(self, monkeypatch):
+        monkeypatch.setattr(code_skew, "_fingerprint", lambda: "git:refs/heads/main:first")
+        code_skew.record_boot_fingerprint()
+        monkeypatch.setattr(code_skew, "_fingerprint", lambda: "git:refs/heads/main:second")
+        code_skew.record_boot_fingerprint()  # must not overwrite the boot snapshot
+        assert code_skew._boot_fingerprint == "git:refs/heads/main:first"
+
+
+class TestShort:
+    def test_shortens_long_sha(self):
+        assert code_skew._short("git:refs/heads/main:abcdef0123456789") == "abcdef0123"
+
+    def test_keeps_unresolved_marker(self):
+        assert code_skew._short("git:refs/heads/main:unresolved") == "unresolved"
+
+    def test_passes_short_sha_through_untruncated(self):
+        assert code_skew._short("git:HEAD:abc1234") == "abc1234"
+
+
+class TestModelSwitchSkewGuard:
+    def test_guard_returns_none_without_skew(self, monkeypatch):
+        from gateway import slash_commands
+
+        monkeypatch.setattr(code_skew, "detect_code_skew", lambda: None)
+        assert slash_commands._model_switch_skew_guard() is None
+
+    def test_guard_message_names_revs_and_restart(self, monkeypatch):
+        from gateway import slash_commands
+
+        monkeypatch.setattr(code_skew, "detect_code_skew", lambda: ("abc1234567", "def4567890"))
+        msg = slash_commands._model_switch_skew_guard()
+        assert msg is not None
+        assert "abc1234567" in msg
+        assert "def4567890" in msg
+        assert "hermes gateway restart" in msg
diff --git a/tests/test_stale_utils_module_import.py b/tests/test_stale_utils_module_import.py
new file mode 100644
index 00000000000..9514c447484
--- /dev/null
+++ b/tests/test_stale_utils_module_import.py
@@ -0,0 +1,90 @@
+"""Regression for the stale-``utils``-module ImportError after a hot ``git pull``.
+
+Real incident (gateway session 1518671026962174144)::
+
+    Sorry, I encountered an error (ImportError).
+    cannot import name 'env_float' from 'utils' (~/.hermes/hermes-agent/utils.py)
+
+Mechanism:
+
+1. A long-running gateway/agent process imported ``utils`` BEFORE ``env_float``
+   existed (added in 06ca1e99, 2026-06-20 14:00). The cached module object in
+   ``sys.modules`` therefore has no ``env_float`` attribute.
+2. ``hermes update`` ran ``git pull``, updating ``utils.py`` (now defining
+   ``env_float``) and ~22 consumer modules (now doing ``from utils import
+   env_float``) on disk -- WITHOUT restarting the process.
+3. Switching the live session's model (anthropic/opus -> opencode/glm) forced the
+   FIRST import of a consumer module on the new provider's code path. Its
+   top-level ``from utils import env_float`` resolved against the STALE cached
+   ``utils`` -> ImportError. The path in parentheses is the consumer-reported
+   ``utils.__file__`` on disk (which *does* define ``env_float``), which is why
+   the error is so confusing: the file on disk is fine, the in-memory module is not.
+
+``hermes_cli/main.py`` (the ``hermes update`` flow, ~line 9326) already
+acknowledges this exact hazard -- "source files on disk are newer than cached
+Python modules in this process" -- and reloads ``hermes_constants`` after the
+pull, but NOT ``utils``. Any ``utils`` consumer added in the same release stays
+exposed until the process restarts.
+
+The messaging client (Discord/Telegram/Feishu/...) is incidental: the trigger is
+a fresh import on a stale process, not the platform. We assert that below by
+reproducing the failure with the Discord adapter's exact import line.
+"""
+
+import sys
+import types
+
+import pytest
+
+
+def _import_fresh_consumer(name: str, source: str) -> types.ModuleType:
+    """Import a brand-new module whose body runs ``source`` -- mimicking a
+    consumer module being imported for the first time on the model-switch path."""
+    mod = types.ModuleType(name)
+    mod.__file__ = f"{name}.py"
+    sys.modules.pop(name, None)
+    exec(compile(source, mod.__file__, "exec"), mod.__dict__)
+    sys.modules[name] = mod
+    return mod
+
+
+class TestStaleUtilsModuleImport:
+    def test_fresh_consumer_import_fails_against_stale_utils(self, monkeypatch):
+        """The bug: stale in-memory ``utils`` + fresh ``from utils import env_float``."""
+        import utils
+
+        # Sanity: today's on-disk source is healthy.
+        assert hasattr(utils, "env_float")
+
+        # Simulate the pre-06-20 cached module (monkeypatch auto-restores after).
+        monkeypatch.delattr(utils, "env_float")
+
+        with pytest.raises(ImportError, match=r"cannot import name 'env_float' from 'utils'"):
+            _import_fresh_consumer("stale_switch_path_consumer", "from utils import env_float\n")
+
+    def test_client_is_incidental_discord_import_line_fails_identically(self, monkeypatch):
+        """Same failure via the Discord adapter's exact import line -- the client
+        does not determine the bug, the stale process does."""
+        import utils
+
+        monkeypatch.delattr(utils, "env_float")
+
+        # plugins/platforms/discord/adapter.py:106
+        with pytest.raises(ImportError, match=r"cannot import name 'env_float' from 'utils'"):
+            _import_fresh_consumer(
+                "stale_discord_consumer",
+                "from utils import atomic_json_write, env_float\n",
+            )
+
+    def test_healthy_process_imports_consumer_fine(self):
+        """Control: when the cached ``utils`` matches disk (env_float present),
+        the same consumer import succeeds -- proving the harness isolates the
+        staleness, not an unrelated import error."""
+        import utils
+
+        assert hasattr(utils, "env_float")
+        mod = _import_fresh_consumer(
+            "healthy_consumer",
+            "from utils import env_float\nVALUE = env_float('UNSET_FOR_TEST', 1.5)\n",
+        )
+        assert mod.VALUE == 1.5