diff --git a/pm/cli.py b/pm/cli.py
index 7dcd2b9296..a5770468de 100644
--- a/pm/cli.py
+++ b/pm/cli.py
@@ -623,6 +623,24 @@ def cmd_bundle(args) -> int:
     write_features(features, out)
     print(f"✓ enabled-features.json ({len(features)} extras recorded)")
 
+    # Ship the uv cache: the staged venv sync just warmed the hermes-owned
+    # cache with every wheel this payload needs. Copying it in makes a
+    # mutable-venv rebuild from the bundle near-free (`uv sync --offline`
+    # from a warm cache probed at 0.4s vs 1.2s cold) — the blow-away-on-
+    # update contract depends on it.
+    from pm.packages import uv_cache_dir as bundle_uv_cache_dir
+
+    payload_cache = out / "uv-cache"
+    if payload_cache.exists():
+        shutil.rmtree(payload_cache, ignore_errors=True)
+    src_cache = bundle_uv_cache_dir()
+    if src_cache.is_dir():
+        print(f"  uv-cache: copying {src_cache} → payload...", flush=True)
+        shutil.copytree(src_cache, payload_cache)
+        print("✓ uv-cache (staged — warm rebuilds for the mutable venv)")
+    else:
+        print("  uv-cache: none warm (first bundle on this machine?)")
+
     bad = _arch_guard(store_dir)
     for line in bad:
         print(f"✗ arch: {line}")
diff --git a/pm/ensure.py b/pm/ensure.py
index 46106b4b9f..18bb85d780 100644
--- a/pm/ensure.py
+++ b/pm/ensure.py
@@ -407,6 +407,21 @@ def adopt() -> bool:
         marker.write_text("", encoding="utf-8")
     except OSError:
         pass
+
+    # Bootstrap the mutable venv (sealed installs): lazy-off copies the
+    # shipped venv out as the seed; lazy-on builds fresh from the shipped
+    # uv cache on first sync. Failure is reported, never fatal — a cold
+    # sync still converges.
+    try:
+        venv_package = get_package("venv")
+    except KeyError:
+        venv_package = None
+    seed = getattr(venv_package, "seed_mutable_venv", None) if venv_package else None
+    if seed is not None:
+        reason = seed()
+        if reason:
+            LOG.info("pm adopt: mutable venv seed skipped: %s", reason)
+
     return True
 
 
diff --git a/pm/packages.py b/pm/packages.py
index 190a504a27..2dfdd3e051 100644
--- a/pm/packages.py
+++ b/pm/packages.py
@@ -235,6 +235,51 @@ def _uv_lock_digest(path: Path) -> bytes:
 _uv_lock_digest_cache: dict[Path, tuple] = {}
 
 
+def uv_cache_dir() -> Path:
+    """The hermes-owned uv cache: machine-scoped and shared (keyed by
+    content — two profiles reuse one cache), anchored to the DEFAULT
+    hermes root like partials_root(). A bundle ships a seeded copy at
+    the payload root (uv-cache/); the first call on a sealed install
+    copies it out to the writable machine cache (the read-only payload
+    can't serve uv's working cache), and a warm `uv sync --offline`
+    from it is near-free (probed: 0.4s vs 1.2s cold) — the blow-away-
+    on-update contract depends on it. uv's default cache location is
+    per-user/platform-opinionated and never used by pm."""
+    from hermes_constants import get_default_hermes_root
+
+    machine_cache = get_default_hermes_root() / "cache" / "uv"
+    marker = machine_cache / ".seeded"
+    if not marker.is_file():
+        # Seed from a shipped bundle cache when present (payload root =
+        # store_root().parent on a sealed install).
+        try:
+            from pm.paths import store_root
+
+            payload_cache = store_root().parent / "uv-cache"
+            if payload_cache.is_dir():
+                machine_cache.mkdir(parents=True, exist_ok=True)
+                import shutil as _shutil
+
+                for entry in payload_cache.iterdir():
+                    if entry.name == ".seeded":
+                        continue
+                    dest = machine_cache / entry.name
+                    if not dest.exists():
+                        (
+                            _shutil.copytree(entry, dest)
+                            if entry.is_dir()
+                            else _shutil.copy2(entry, dest)
+                        )
+        except OSError:
+            pass  # seeding is best-effort; a cold sync still works
+        try:
+            marker.parent.mkdir(parents=True, exist_ok=True)
+            marker.write_text("1", encoding="utf-8")
+        except OSError:
+            pass
+    return machine_cache
+
+
 def uv_env(base_env: Optional[dict] = None) -> dict[str, str]:
     """Sanitized env for pm's internal uv invocations: user-level UV
     overrides and active-venv leakage must not steer which interpreter or
@@ -243,7 +288,9 @@ def uv_env(base_env: Optional[dict] = None) -> dict[str, str]:
     must not reach the subprocess either. User config discovery is
     redirected to an empty dir; HOME is left alone so caches, credentials,
     and git keep working (the #82446 isolation contract, which lived in
-    the installers' run_locked_uv_sync() before the sync moved into pm)."""
+    the installers' run_locked_uv_sync() before the sync moved into pm).
+    UV_CACHE_DIR always points at the hermes-owned cache (shipped with
+    bundles, shared machine-wide) — never the user's ambient cache."""
     env = dict(os.environ if base_env is None else base_env)
     for key in list(env):
         if key.startswith("UV_") or key in (
@@ -258,6 +305,7 @@ def uv_env(base_env: Optional[dict] = None) -> dict[str, str]:
     env["XDG_CONFIG_HOME"] = str(isolated)
     env["XDG_CONFIG_DIRS"] = str(isolated)
     env["UV_NO_CONFIG"] = "1"
+    env["UV_CACHE_DIR"] = str(uv_cache_dir())
     return env
 
 
@@ -290,11 +338,48 @@ class Venv(StatePackage):
         return repo_root()
 
     def venv_dir(self) -> Path:
+        # Sealed installs are read-only: the MUTABLE venv lives in the
+        # writable machine hermes root (get_default_hermes_root()/venv —
+        # per-install, beside the uv cache), seeded on first adopt by
+        # copying the payload's shipped venv out. Dev/source installs
+        # keep the repo-local venv (project_venv_dir) unchanged.
+        from pm.ensure import sealed
+
+        if sealed():
+            from hermes_constants import get_default_hermes_root
+
+            return get_default_hermes_root() / "venv"
         from hermes_constants import project_venv_dir
 
         found = project_venv_dir(self.project_root())
         return found if found else self.project_root() / "venv"
 
+    def seed_mutable_venv(self) -> Optional[str]:
+        """Bootstrap the mutable venv from the shipped payload venv
+        (lazy-off installs). Returns None on success, else why not."""
+        import shutil
+
+        from pm.ensure import lazy_installs_allowed, sealed
+        from pm.paths import store_root
+
+        if not sealed():
+            return None  # dev installs use the repo venv directly
+        dest = self.venv_dir()
+        if dest.exists():
+            return None  # already bootstrapped
+        if lazy_installs_allowed():
+            # Lazy installs ON: build fresh from the shipped uv cache —
+            # no seed copy needed (the sync is near-free from the cache).
+            return None
+        payload_venv = store_root().parent / "venv"
+        if not payload_venv.is_dir():
+            return "payload ships no venv to seed from"
+        try:
+            shutil.copytree(payload_venv, dest)
+        except OSError as exc:
+            return f"seed copy failed: {exc}"
+        return None
+
     def expected_stamp(self, extras: list[str]) -> str:
         import hashlib
         import sys
diff --git a/tests/pm/test_uv_cache.py b/tests/pm/test_uv_cache.py
new file mode 100644
index 0000000000..14a4ddeebf
--- /dev/null
+++ b/tests/pm/test_uv_cache.py
@@ -0,0 +1,128 @@
+"""pm uv cache: hermes-owned UV_CACHE_DIR + sealed-venv bootstrap seed.
+
+The cache ships with bundles and is copied out to the writable machine
+cache on first use; every pm-internal uv invocation pins UV_CACHE_DIR.
+The mutable venv on sealed installs lives in the machine hermes root,
+seeded from the payload's shipped venv when lazy installs are off.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+
+import pytest
+
+import pm.packages as pkgs
+
+
+def test_uv_env_pins_cache_dir(monkeypatch, tmp_path):
+    monkeypatch.setattr(pkgs, "uv_cache_dir", lambda: tmp_path / "c")
+    env = pkgs.uv_env({"UV_CACHE_DIR": "/ambient/user/cache", "PATH": "x"})
+    assert env["UV_CACHE_DIR"] == str(tmp_path / "c")
+    # ambient UV_ vars are stripped, not inherited
+    assert "UV_PROJECT_ENVIRONMENT" not in env
+
+
+def test_uv_cache_dir_seeds_from_payload(monkeypatch, tmp_path):
+    home = tmp_path / "home"
+    home.mkdir()
+    payload = tmp_path / "payload"
+    (payload / "uv-cache" / "wheels-v5").mkdir(parents=True)
+    (payload / "uv-cache" / "wheels-v5" / "some.pkg").write_text("x", encoding="utf-8")
+
+    import hermes_constants
+
+    monkeypatch.setattr(hermes_constants, "get_default_hermes_root", lambda: home)
+    import pm.paths as paths_mod
+
+    monkeypatch.setattr(paths_mod, "store_root", lambda: payload / "tools")
+
+    machine = pkgs.uv_cache_dir()
+    assert machine == home / "cache" / "uv"
+    # seed copied out
+    assert (machine / "wheels-v5" / "some.pkg").read_text(encoding="utf-8") == "x"
+    # seeded marker written → second call doesn't re-copy
+    assert (machine / ".seeded").is_file()
+    (payload / "uv-cache" / "wheels-v5" / "some.pkg").write_text("changed", encoding="utf-8")
+    pkgs.uv_cache_dir()
+    assert (machine / "wheels-v5" / "some.pkg").read_text(encoding="utf-8") == "x"
+
+
+def test_uv_cache_dir_cold_machine_no_payload(monkeypatch, tmp_path):
+    home = tmp_path / "home"
+    home.mkdir()
+    import hermes_constants
+
+    monkeypatch.setattr(hermes_constants, "get_default_hermes_root", lambda: home)
+    import pm.paths as paths_mod
+
+    monkeypatch.setattr(paths_mod, "store_root", lambda: tmp_path / "nowhere" / "tools")
+
+    machine = pkgs.uv_cache_dir()
+    assert machine == home / "cache" / "uv"
+    assert (machine / ".seeded").is_file()
+
+
+def test_venv_dir_sealed_goes_to_machine_root(monkeypatch, tmp_path):
+    home = tmp_path / "home"
+    home.mkdir()
+    import sys
+
+    import hermes_constants
+
+    monkeypatch.setattr(hermes_constants, "get_default_hermes_root", lambda: home)
+    ensure_mod = sys.modules["pm.ensure"]
+    monkeypatch.setattr(ensure_mod, "sealed", lambda: True)
+    venv = pkgs.Venv()
+    assert venv.venv_dir() == home / "venv"
+
+
+def test_seed_mutable_venv_copies_payload_venv(monkeypatch, tmp_path):
+    home = tmp_path / "home"
+    home.mkdir()
+    payload = tmp_path / "payload"
+    (payload / "venv" / "Scripts").mkdir(parents=True)
+    (payload / "venv" / "Scripts" / "python.exe").write_text("bin", encoding="utf-8")
+
+    import hermes_constants
+
+    monkeypatch.setattr(hermes_constants, "get_default_hermes_root", lambda: home)
+    import sys
+
+    ensure_mod = sys.modules["pm.ensure"]
+    import pm.paths as paths_mod
+
+    monkeypatch.setattr(ensure_mod, "sealed", lambda: True)
+    monkeypatch.setattr(ensure_mod, "lazy_installs_allowed", lambda: False)
+    monkeypatch.setattr(paths_mod, "store_root", lambda: payload / "tools")
+
+    venv = pkgs.Venv()
+    reason = venv.seed_mutable_venv()
+    assert reason is None
+    seeded = home / "venv"
+    assert (seeded / "Scripts" / "python.exe").read_text(encoding="utf-8") == "bin"
+    # idempotent: second call is a no-op
+    assert venv.seed_mutable_venv() is None
+
+
+def test_seed_mutable_venv_lazy_on_skips_copy(monkeypatch, tmp_path):
+    home = tmp_path / "home"
+    home.mkdir()
+    payload = tmp_path / "payload"
+    (payload / "venv").mkdir(parents=True)
+
+    import hermes_constants
+
+    monkeypatch.setattr(hermes_constants, "get_default_hermes_root", lambda: home)
+    import sys
+
+    ensure_mod = sys.modules["pm.ensure"]
+    import pm.paths as paths_mod
+
+    monkeypatch.setattr(ensure_mod, "sealed", lambda: True)
+    monkeypatch.setattr(ensure_mod, "lazy_installs_allowed", lambda: True)
+    monkeypatch.setattr(paths_mod, "store_root", lambda: payload / "tools")
+
+    venv = pkgs.Venv()
+    assert venv.seed_mutable_venv() is None
+    assert not (home / "venv").exists()  # builds fresh later, no seed copy