diff --git a/core/tests/test_update.py b/core/tests/test_update.py
index 8bdf1ba4..d77055ec 100644
--- a/core/tests/test_update.py
+++ b/core/tests/test_update.py
@@ -229,7 +229,7 @@ def test_a_missing_requirement_is_installed_by_its_whole_line_so_a_pin_or_url_su
     monkeypatch.setattr(update, "_installed_version", lambda n: "1.0" if n == "numpy" else None)
     ran = []
     monkeypatch.setattr(update.subprocess, "run", lambda cmd, **k: ran.append(cmd) or subprocess.CompletedProcess(cmd, 0, "", ""))
-    monkeypatch.setattr(update, "_pip_freeze", lambda: {})
+    monkeypatch.setattr(update, "_pip_freeze", lambda: {"numpy": "1.26.4"})
     assert update.install_requirements()["ok"]
     assert [c[-1] for c in ran] == ["zz_absent_pkg>=3.8,<3.9", "zz_model @ https://example.com/zz_model.whl"]
 
@@ -244,7 +244,7 @@ def test_one_package_that_cannot_install_does_not_stop_the_others(repo, monkeypa
         return subprocess.CompletedProcess(cmd, 1 if "zz_bad_url" in cmd[-1] else 0, "", "404 not found")
 
     monkeypatch.setattr(update.subprocess, "run", fake_run)
-    monkeypatch.setattr(update, "_pip_freeze", lambda: {})
+    monkeypatch.setattr(update, "_pip_freeze", lambda: {"numpy": "1.26.4"})
     out = update.install_requirements()
     assert ran == ["zz_bad_url @ https://example.com/missing.whl", "zz_good_pkg"], "the second still installs"
     assert out["ok"] is False and "zz_bad_url" in out["detail"] and "404 not found" in out["detail"]
@@ -269,6 +269,32 @@ def test_a_spacy_model_that_does_not_load_is_downloaded_on_the_switch(repo, monk
     ran = []
     monkeypatch.setattr(update, "_spacy_model_loads", lambda: bool(ran) and ran[-1] == ["spacy", "download", "en_core_web_sm"])
     monkeypatch.setattr(update.subprocess, "run", lambda cmd, **k: ran.append(cmd[-3:]) or subprocess.CompletedProcess(cmd, 0, "", ""))
-    monkeypatch.setattr(update, "_pip_freeze", lambda: {})
+    monkeypatch.setattr(update, "_pip_freeze", lambda: {"numpy": "1.26.4"})
     assert update.install_requirements()["ok"]
     assert ran[-1] == ["spacy", "download", "en_core_web_sm"]
+
+
+def test_an_install_pins_every_installed_package_so_nothing_already_here_moves(repo, monkeypatch):
+    (repo / "requirements.txt").write_text("zz_new_pkg\n")
+    monkeypatch.setattr(update, "_installed_version", lambda n: None)
+    seen = {}
+
+    def fake_run(cmd, **k):
+        if "-c" in cmd:
+            seen["constraints"] = open(cmd[cmd.index("-c") + 1]).read()
+        return subprocess.CompletedProcess(cmd, 0, "", "")
+
+    monkeypatch.setattr(update.subprocess, "run", fake_run)
+    monkeypatch.setattr(update, "_pip_freeze", lambda: {"numpy": "1.26.4", "torch": "2.9.0"})
+    assert update.install_requirements()["ok"]
+    assert "numpy==1.26.4" in seen["constraints"] and "torch==2.9.0" in seen["constraints"]
+
+
+def test_an_unreadable_installed_list_installs_nothing(repo, monkeypatch):
+    (repo / "requirements.txt").write_text("zz_new_pkg\n")
+    monkeypatch.setattr(update, "_installed_version", lambda n: None)
+    ran = []
+    monkeypatch.setattr(update.subprocess, "run", lambda cmd, **k: ran.append(cmd) or subprocess.CompletedProcess(cmd, 0, "", ""))
+    monkeypatch.setattr(update, "_pip_freeze", lambda: {})
+    out = update.install_requirements()
+    assert out["ok"] is False and ran == []
diff --git a/core/update.py b/core/update.py
index e1a502b6..a8afffef 100644
--- a/core/update.py
+++ b/core/update.py
@@ -12,10 +12,12 @@ feature: no module can announce the thing that updates every module.
 """
 from __future__ import annotations
 
+import os
 import re
 import shutil
 import subprocess
 import sys
+import tempfile
 from pathlib import Path
 
 REPO_ROOT = Path(__file__).resolve().parents[1]
@@ -426,9 +428,25 @@ def install_requirements(timeout: int = 900) -> dict:
     # stop every other one from installing too. Dependencies are resolved normally: a new package
     # with nothing under it is not installed, it is broken. The freeze diff says whether anything
     # already present moved as a result.
+    if not before:
+        return {"ran": False, "ok": False,
+                "detail": "Could not read the installed packages, so nothing is installed (an install could move them). "
+                          f"Run it yourself: {sys.executable} -m pip install <the missing packages>"}
+    # Constraints: every installed package stays at its version, so a new package's dependencies can be added
+    # but never bump one that is already here. A dependency that needs a newer one fails, and says so.
+    constraints = tempfile.NamedTemporaryFile("w", suffix=".txt", delete=False, encoding="utf-8")
+    try:
+        constraints.write("\n".join(f"{n}=={v}" for n, v in before.items()) + "\n")
+        constraints.close()
+        return _install_each(status, constraints.name, before, floor_note, timeout, model_broken)
+    finally:
+        os.unlink(constraints.name)
+
+
+def _install_each(status, constraints_path, before, floor_note, timeout, model_broken):
     failed, done = [], []
     for line in status.get("install", []):
-        cmd = [sys.executable, "-m", "pip", "install", "--disable-pip-version-check", line]
+        cmd = [sys.executable, "-m", "pip", "install", "--disable-pip-version-check", "-c", constraints_path, line]
         try:
             proc = subprocess.run(cmd, cwd=REPO_ROOT, capture_output=True, text=True, timeout=timeout)
         except subprocess.TimeoutExpired:
@@ -442,7 +460,7 @@ def install_requirements(timeout: int = 900) -> dict:
         else:
             done.append(line)
     if model_broken:
-        cmd = [sys.executable, "-m", "spacy", "download", "en_core_web_sm"]
+        cmd = [sys.executable, "-m", "spacy", "download", "en_core_web_sm"]     # the model is data, not a dependency: no constraints
         try:
             proc = subprocess.run(cmd, cwd=REPO_ROOT, capture_output=True, text=True, timeout=timeout)
             if proc.returncode != 0 or not _spacy_model_loads():