diff --git a/hermes_cli/plugin_packs.py b/hermes_cli/plugin_packs.py
index 5487f37f68..d88c93507a 100644
--- a/hermes_cli/plugin_packs.py
+++ b/hermes_cli/plugin_packs.py
@@ -317,6 +317,7 @@ def install_pack_plugins(
         _get_disabled_set,
         _get_enabled_set,
         _install_plugin_core,
+        _install_python_dependencies,
         _prompt_plugin_env_vars,
         _run_capability_consent,
         _save_disabled_set,
@@ -350,6 +351,7 @@ def install_pack_plugins(
             _prompt_plugin_env_vars(manifest, console)
         except Exception:
             logger.debug("requires_env prompt failed for %s", installed_name, exc_info=True)
+        _install_python_dependencies(target, console)
 
         enabled = _get_enabled_set()
         disabled = _get_disabled_set()
diff --git a/hermes_cli/plugin_python_deps.py b/hermes_cli/plugin_python_deps.py
new file mode 100644
index 0000000000..9cbfb152f4
--- /dev/null
+++ b/hermes_cli/plugin_python_deps.py
@@ -0,0 +1,440 @@
+"""Python dependencies declared by user plugins: read, resolve against Hermes' own ranges, install,
+and re-apply after ``hermes update`` rebuilds the venv.
+
+A plugin declares deps in its ``pyproject.toml`` (``[project].dependencies``) or, without one, in
+``plugin.yaml`` ``python_dependencies`` (``pip_dependencies`` is an accepted alias). ``python_runtime:
+external`` in the manifest opts a plugin out: it manages its own interpreter (sidecar venv) and its tree
+is never handed to the resolver.
+
+Contract (agreed with the Mnemosyne team, Sep 2026): the union of every enabled plugin's declarations
+is resolved together with Hermes' declared ranges; a candidate that has no solution is refused without
+touching the live venv or disabling anything already installed. After an update, the union is
+re-applied; if core moved and the union no longer resolves, non-memory plugins are dropped first and
+disabled with a loud warning, because a Hermes that boots without memory reads as data loss.
+
+Lifted in shape from ethernet8023's ``pm/plugin_declarations.py`` / ``pm/workspace.py`` (#102765).
+"""
+
+from __future__ import annotations
+
+import logging
+import re
+import tomllib
+from dataclasses import dataclass
+from pathlib import Path
+from typing import Callable, Iterable, Optional
+
+import yaml
+from hermes_constants import get_hermes_home
+from packaging.markers import InvalidMarker, UndefinedEnvironmentName
+from packaging.requirements import InvalidRequirement, Requirement
+from packaging.utils import canonicalize_name
+
+logger = logging.getLogger(__name__)
+
+_MANIFEST_FILES = ("plugin.yaml", "plugin.yml")
+_DEP_KEYS = ("python_dependencies", "pip_dependencies")
+EXTERNAL_RUNTIME_KEY = "python_runtime"
+_CORE_DIST = "hermes-agent"
+# Substrings uv/pip print only when the resolver itself proved there is no solution. Deliberately
+# narrow: a fetch timeout or index outage must never be misread as a conflict.
+_CONFLICT_MARKERS = (
+    "no solution found",
+    "conflicting requirements",
+    "resolutionimpossible",
+    "because only the following versions",
+)
+
+
+class DependencyConflict(Exception):
+    """The resolver proved core + enabled plugins + candidate has no valid solution."""
+
+
+class DependencyInstallError(Exception):
+    """The install failed for a reason other than a proven conflict (network, build, gate)."""
+
+
+@dataclass(frozen=True)
+class PythonDeclaration:
+    plugin_dir: Path
+    name: str
+    specs: tuple[str, ...]
+    source: str            # "pyproject" | "manifest" | ""
+    external: bool = False
+    memory_provider: bool = False
+
+    @property
+    def wants_install(self) -> bool:
+        return bool(self.specs) and not self.external
+
+
+# ── reading ───────────────────────────────────────────────────────────────────
+
+def _read_manifest(plugin_dir: Path) -> dict:
+    for candidate in _MANIFEST_FILES:
+        path = plugin_dir / candidate
+        if path.is_file():
+            data = yaml.safe_load(path.read_text(encoding="utf-8-sig")) or {}
+            return data if isinstance(data, dict) else {}
+    return {}
+
+
+def _pyproject_specs(plugin_dir: Path) -> Optional[list[str]]:
+    """``[project].dependencies`` from the plugin's pyproject, or ``None`` when there is none."""
+    path = plugin_dir / "pyproject.toml"
+    if not path.is_file():
+        return None
+    document = tomllib.loads(path.read_text(encoding="utf-8-sig"))
+    specs = document.get("project", {}).get("dependencies", [])
+    if not isinstance(specs, list) or any(not isinstance(s, str) for s in specs):
+        raise ValueError(f"{path}: [project].dependencies must be a list of strings")
+    return specs
+
+
+def _manifest_specs(manifest: dict) -> list[str]:
+    specs: list[str] = []
+    for key in _DEP_KEYS:
+        values = manifest.get(key) or []
+        if not isinstance(values, list) or any(not isinstance(v, str) for v in values):
+            raise ValueError(f"{key} must be a list of requirement strings")
+        specs.extend(v.strip() for v in values if v.strip())
+    return list(dict.fromkeys(specs))
+
+
+def _is_memory_provider(plugin_dir: Path, manifest: dict) -> bool:
+    if str(manifest.get("kind") or "").strip().lower() == "exclusive":
+        return True
+    init = plugin_dir / "__init__.py"
+    if not init.is_file():
+        return False
+    head = init.read_text(encoding="utf-8", errors="replace")[:8192]
+    return "register_memory_provider" in head or "MemoryProvider" in head
+
+
+def read_declaration(plugin_dir: Path) -> PythonDeclaration:
+    """One effective dependency surface for a plugin directory. A real pyproject owns packaging; the
+    manifest list bridges plugins without one. Raises ``ValueError`` on a malformed declaration."""
+    plugin_dir = Path(plugin_dir)
+    manifest = _read_manifest(plugin_dir)
+    name = str(manifest.get("name") or plugin_dir.name)
+    external = str(manifest.get(EXTERNAL_RUNTIME_KEY) or "").strip().lower() == "external"
+    pyproject = _pyproject_specs(plugin_dir)
+    specs, source = (pyproject, "pyproject") if pyproject is not None else (_manifest_specs(manifest), "manifest")
+    return PythonDeclaration(
+        plugin_dir=plugin_dir, name=name, specs=tuple(specs), source=source if specs else "",
+        external=external, memory_provider=_is_memory_provider(plugin_dir, manifest))
+
+
+# ── requirement hygiene ───────────────────────────────────────────────────────
+
+def _parse_requirement(spec: str) -> Requirement:
+    try:
+        return Requirement(spec)
+    except InvalidRequirement as exc:
+        raise ValueError(f"invalid requirement {spec!r}: {exc}") from exc
+
+
+def _marker_applies(req: Requirement) -> bool:
+    try:
+        return req.marker is None or req.marker.evaluate()
+    except (InvalidMarker, UndefinedEnvironmentName) as exc:
+        raise ValueError(f"invalid marker in {req!s}: {exc}") from exc
+
+
+def applicable_requirement(spec: str) -> Optional[str]:
+    """``spec`` reduced to ``name[extras]specifier`` when Hermes should install it here; ``None`` when
+    its environment marker excludes this platform, when it names Hermes itself (always satisfied by the
+    running checkout — installing ``hermes-agent`` from an index would clobber it), or when it points
+    at a URL/path (see :func:`unsupported_specs`; the resolver must only see index packages)."""
+    req = _parse_requirement(spec)
+    if req.url or canonicalize_name(req.name) == _CORE_DIST or not _marker_applies(req):
+        return None
+    extras = f"[{','.join(sorted(req.extras))}]" if req.extras else ""
+    return f"{req.name}{extras}{req.specifier}"
+
+
+def applicable_specs(specs: Iterable[str]) -> list[str]:
+    return [r for r in (applicable_requirement(s) for s in specs) if r]
+
+
+def unsupported_specs(specs: Iterable[str]) -> list[str]:
+    """Direct URL/path requirements: never installed by Hermes (arbitrary git/tarball sources are a
+    supply-chain surface the reviewed pin does not cover); surfaced so the user can install them."""
+    return [s for s in specs if _parse_requirement(s).url]
+
+
+def core_constraints(project_root: Path) -> list[str]:
+    """Hermes' own declared ranges (``[project].dependencies`` + every extra), as constraint lines.
+    Plugins resolve inside these, so they can move transitives but never a core package out of range."""
+    document = tomllib.loads((Path(project_root) / "pyproject.toml").read_text(encoding="utf-8"))
+    project = document.get("project", {})
+    declared: list[str] = list(project.get("dependencies", []))
+    for group in project.get("optional-dependencies", {}).values():
+        declared.extend(group)
+    lines = []
+    for spec in declared:
+        try:
+            reduced = applicable_requirement(spec)
+        except ValueError:
+            continue
+        if reduced and not reduced.lower().startswith(_CORE_DIST):
+            lines.append(_strip_extras(reduced))
+    return sorted(set(lines))
+
+
+def _strip_extras(spec: str) -> str:
+    """Constraint files may not carry extras."""
+    return re.sub(r"\[[^\]]*\]", "", spec, count=1)
+
+
+# ── enumeration across homes ──────────────────────────────────────────────────
+
+def _name_set(config: dict, key: str) -> set[str]:
+    values = (config.get("plugins") or {}).get(key) if isinstance(config.get("plugins"), dict) else None
+    return set(values) if isinstance(values, list) else set()
+
+
+def _plugin_enabled(decl: PythonDeclaration, enabled: set[str], disabled: set[str]) -> bool:
+    keys = {decl.name, decl.plugin_dir.name}
+    return bool(keys & enabled) and not (keys & disabled)
+
+
+def _read_home_config(home: Path) -> dict:
+    path = home / "config.yaml"
+    if not path.is_file():
+        return {}
+    data = yaml.safe_load(path.read_text(encoding="utf-8")) or {}
+    return data if isinstance(data, dict) else {}
+
+
+def enabled_declarations(home: Path) -> list[PythonDeclaration]:
+    """Declarations of every enabled, non-external user plugin under ``<home>/plugins`` that declares
+    something. Malformed declarations are skipped with a warning (one bad plugin must not stop the
+    union for everyone else)."""
+    plugins_dir = Path(home) / "plugins"
+    if not plugins_dir.is_dir():
+        return []
+    config = _read_home_config(Path(home))
+    enabled, disabled = _name_set(config, "enabled"), _name_set(config, "disabled")
+    found: list[PythonDeclaration] = []
+    for plugin_dir in sorted(p for p in plugins_dir.iterdir() if p.is_dir() and not p.name.startswith(".")):
+        try:
+            decl = read_declaration(plugin_dir)
+        except (ValueError, OSError, tomllib.TOMLDecodeError, yaml.YAMLError) as exc:
+            logger.warning("Plugin %s: dependency declaration skipped: %s", plugin_dir.name, exc)
+            continue
+        if decl.wants_install and _plugin_enabled(decl, enabled, disabled):
+            found.append(decl)
+    return found
+
+
+def dependency_homes() -> list[Path]:
+    """Every home whose plugins share this venv: the default home plus live named profiles."""
+    from hermes_cli.profiles import profiles_to_serve
+    return [home for _name, home in profiles_to_serve(multiplex=True)]
+
+
+def union_specs(declarations: Iterable[PythonDeclaration]) -> list[str]:
+    seen: dict[str, None] = {}
+    for decl in declarations:
+        for spec in applicable_specs(decl.specs):
+            seen.setdefault(spec, None)
+    return list(seen)
+
+
+# ── resolve / install ─────────────────────────────────────────────────────────
+
+def _classify(result) -> Optional[Exception]:
+    if result.ok:
+        return None
+    text = (result.stderr + result.stdout).lower()
+    if result.blocked:
+        return DependencyInstallError(result.reason)
+    if any(marker in text for marker in _CONFLICT_MARKERS):
+        return DependencyConflict(_tail(result.stderr or result.stdout))
+    return DependencyInstallError(_tail(result.stderr or result.stdout) or "installer failed")
+
+
+def _tail(text: str, limit: int = 600) -> str:
+    return text.strip()[-limit:]
+
+
+def resolve(specs: list[str], constraints: list[str], *, dry_run: bool, timeout: int = 600):
+    """Run the shared installer ladder on *specs* under *constraints*. Raises ``DependencyConflict`` or
+    ``DependencyInstallError``; returns the installer result on success."""
+    from tools.lazy_deps import install_specs
+    result = install_specs(specs, timeout=timeout, constraints=constraints, dry_run=dry_run)
+    error = _classify(result)
+    if error is not None:
+        raise error
+    return result
+
+
+def check_candidate(candidate: PythonDeclaration, *, home: Path, project_root: Path) -> list[str]:
+    """Prove core + every enabled plugin + *candidate* resolves, without installing. Returns the
+    candidate's applicable specs (empty when it declares nothing for this platform)."""
+    own = applicable_specs(candidate.specs)
+    if not own or candidate.external:
+        return []
+    resolve(union_specs([candidate, *_peers(candidate, home)]), core_constraints(project_root), dry_run=True)
+    return own
+
+
+def _satisfied(spec: str) -> bool:
+    """Installed and inside the specifier (extras are not checked; a missing extra surfaces at import)."""
+    from importlib.metadata import PackageNotFoundError, version
+    req = Requirement(spec)
+    try:
+        installed = version(req.name)
+    except PackageNotFoundError:
+        return False
+    return req.specifier.contains(installed, prereleases=True)
+
+
+def _peers(decl: PythonDeclaration, home: Path) -> list[PythonDeclaration]:
+    """Enabled plugins other than *decl* itself (matched by directory, so a re-enable is not its own peer)."""
+    return [d for d in enabled_declarations(home) if d.plugin_dir.resolve() != decl.plugin_dir.resolve()]
+
+
+def install_declaration(decl: PythonDeclaration, *, home: Path, project_root: Path,
+                        timeout: int = 900) -> list[str]:
+    """Install one plugin's applicable specs into the venv, resolved TOGETHER with every enabled
+    peer's specs under core constraints, so installing A can never downgrade what enabled B needs
+    (uv refuses instead). No-op when every spec is already satisfied. Returns what applies."""
+    specs = applicable_specs(decl.specs)
+    if specs and not decl.external and not all(_satisfied(s) for s in specs):
+        resolve(union_specs([decl, *_peers(decl, home)]), core_constraints(project_root),
+                dry_run=False, timeout=timeout)
+    return specs
+
+
+# ── install-time glue shared by CLI, dashboard and pack installs ──────────────
+
+@dataclass(frozen=True)
+class DepsOutcome:
+    """What happened to a freshly installed plugin's Python dependencies. ``status`` is one of
+    ``none`` (nothing declared / external runtime), ``installed``, ``failed`` (network, build, gate;
+    the plugin stays installed and the loader warns at import), ``invalid`` (malformed declaration)."""
+    status: str
+    specs: tuple[str, ...] = ()
+    detail: str = ""
+    skipped: tuple[str, ...] = ()  # direct-URL requirements Hermes never installs
+
+    @property
+    def message(self) -> str:
+        text = _OUTCOME_MESSAGES[self.status](self)
+        if self.skipped:
+            text += (f"\nNot installed (direct URL requirements are left to you): "
+                     f"uv pip install {' '.join(repr(s) for s in self.skipped)}")
+        return text.strip()
+
+
+_OUTCOME_MESSAGES: dict[str, Callable[[DepsOutcome], str]] = {
+    "none": lambda o: "",
+    "installed": lambda o: f"Installed Python dependencies: {', '.join(o.specs)}",
+    "failed": lambda o: (f"Python dependencies not installed ({o.detail}). Run manually: "
+                         f"uv pip install {' '.join(o.specs)}"),
+    "invalid": lambda o: f"Python dependency declaration ignored: {o.detail}",
+}
+
+
+def project_root() -> Path:
+    return Path(__file__).resolve().parent.parent
+
+
+def refuse_conflicting_candidate(plugin_dir: Path, *, home: Path) -> None:
+    """Install-time gate, run BEFORE a plugin tree is moved into place: a candidate whose deps cannot
+    resolve with core + the enabled plugins is refused; a malformed declaration is refused too. A
+    network failure during the dry run is not a verdict and lets the install continue."""
+    try:
+        check_candidate(read_declaration(plugin_dir), home=home, project_root=project_root())
+    except DependencyConflict as exc:
+        raise DependencyConflict(
+            f"its Python dependencies conflict with Hermes or an enabled plugin:\n{exc}") from exc
+    except DependencyInstallError as exc:
+        logger.warning("Dependency pre-check skipped (%s); the install will retry for real", exc)
+    except (ValueError, tomllib.TOMLDecodeError, yaml.YAMLError) as exc:
+        raise ValueError(f"invalid Python dependency declaration: {exc}") from exc
+
+
+def install_for_plugin_dir(plugin_dir: Path) -> DepsOutcome:
+    """Install an installed plugin's declared deps into the venv. Never raises."""
+    try:
+        decl = read_declaration(plugin_dir)
+    except (ValueError, tomllib.TOMLDecodeError, yaml.YAMLError) as exc:
+        return DepsOutcome("invalid", detail=str(exc))
+    skipped = () if decl.external else tuple(unsupported_specs(decl.specs))
+    try:
+        specs = install_declaration(decl, home=get_hermes_home(), project_root=project_root())
+    except (DependencyConflict, DependencyInstallError) as exc:
+        return DepsOutcome("failed", tuple(applicable_specs(decl.specs)), detail=_tail(str(exc), 300),
+                           skipped=skipped)
+    return DepsOutcome("installed" if specs else "none", tuple(specs), skipped=skipped)
+
+
+# ── post-update re-apply ──────────────────────────────────────────────────────
+
+@dataclass
+class ReapplyReport:
+    installed: list[str]
+    dropped: list[tuple[str, str]]   # (plugin name, reason)
+    failed: str = ""
+
+    @property
+    def ok(self) -> bool:
+        return not self.failed
+
+
+def _drop_order(declarations: list[PythonDeclaration]) -> list[PythonDeclaration]:
+    """Plugins in the order they may be sacrificed on conflict: non-memory first, memory last."""
+    return sorted(declarations, key=lambda d: (d.memory_provider, d.name))
+
+
+def _resolves(declarations: list[PythonDeclaration], constraints: list[str]) -> bool:
+    try:
+        resolve(union_specs(declarations), constraints, dry_run=True)
+    except DependencyConflict:
+        return False
+    return True
+
+
+def _first_resolving_subset(declarations: list[PythonDeclaration], constraints: list[str]
+                            ) -> tuple[list[PythonDeclaration], list[PythonDeclaration]]:
+    """Largest subset of *declarations* that resolves. Fast path: the whole union. Otherwise every
+    plugin that cannot resolve on its own against core is a culprit and is dropped (never an innocent
+    neighbour); what remains is peeled non-memory-first only for plugin-vs-plugin conflicts."""
+    if _resolves(declarations, constraints):
+        return list(declarations), []
+    kept = [d for d in declarations if _resolves([d], constraints)]
+    dropped = [d for d in declarations if d not in kept]
+    order = _drop_order(kept)
+    while kept and not _resolves(kept, constraints):
+        victim = order.pop(0)
+        kept.remove(victim)
+        dropped.append(victim)
+    return kept, dropped
+
+
+def reapply_all(*, project_root: Path, disable: Callable[[Path, str], None]) -> ReapplyReport:
+    """Re-install the union of every enabled plugin's deps after a venv rebuild. Conflicts drop
+    non-memory plugins first and disable them through *disable(home, name)*; anything else (network,
+    gate) is reported without changing plugin state."""
+    per_home = [(home, enabled_declarations(home)) for home in dependency_homes()]
+    everything = [d for _home, decls in per_home for d in decls]
+    if not everything:
+        return ReapplyReport(installed=[], dropped=[])
+    constraints = core_constraints(project_root)
+    try:
+        kept, dropped = _first_resolving_subset(everything, constraints)
+        specs = union_specs(kept)
+        if specs:
+            resolve(specs, constraints, dry_run=False)
+    except DependencyInstallError as exc:
+        return ReapplyReport(installed=[], dropped=[], failed=str(exc))
+    report = ReapplyReport(installed=specs, dropped=[])
+    for victim in dropped:
+        home = next(h for h, decls in per_home if victim in decls)
+        reason = "its Python dependencies no longer resolve against this Hermes"
+        disable(home, victim.name)
+        report.dropped.append((victim.name, reason))
+    return report
diff --git a/hermes_cli/plugins_cmd.py b/hermes_cli/plugins_cmd.py
index 59e8715c92..6c9880c831 100644
--- a/hermes_cli/plugins_cmd.py
+++ b/hermes_cli/plugins_cmd.py
@@ -342,25 +342,46 @@ def _missing_env_specs(manifest: dict) -> list[dict]:
     return [s for s in env_specs if not get_env_value(s["name"])]
 
 
-def _print_python_dependencies(manifest: dict, console) -> None:
-    """Print declared ``python_dependencies`` with an install hint — Hermes never auto-installs
-    plugin pip dependencies.
+def _refuse_conflicting_python_deps(tmp_target: Path, plugin_name: str) -> None:
+    """Dependency pre-check on the staged tree: a conflict with core or an enabled plugin refuses the
+    install before anything is moved into place (nothing installed, nothing disabled)."""
+    from hermes_cli.plugin_python_deps import DependencyConflict, refuse_conflicting_candidate
+    try:
+        refuse_conflicting_candidate(tmp_target, home=get_hermes_home())
+    except DependencyConflict as exc:
+        raise PluginOperationError(
+            f"Plugin '{plugin_name}' was not installed: {exc}\n"
+            "The plugin author should relax that requirement. To install the plugin anyway and manage "
+            "its Python packages yourself: hermes plugins install <identifier> --no-deps") from exc
+    except ValueError as exc:
+        raise PluginOperationError(f"Plugin '{plugin_name}' was not installed: {exc}") from exc
 
-    See #64165.
-    See #15220, #64165.
-    """
-    deps = manifest.get("python_dependencies") or []
-    if not isinstance(deps, list):
+
+def _install_python_dependencies_quietly(target: Path, warnings: list[str]) -> list[str]:
+    """Dashboard variant: install, append a failure to *warnings*, return the applicable specs."""
+    from hermes_cli.plugin_python_deps import install_for_plugin_dir
+    outcome = install_for_plugin_dir(target)
+    if outcome.status in ("failed", "invalid"):
+        warnings.append(outcome.message)
+    return list(outcome.specs)
+
+
+def _install_python_dependencies_for_key(key: str, console) -> None:
+    """``plugins enable`` variant: a user plugin enabled after a bare install gets its deps now."""
+    entry = _find_plugin_entry(key)
+    if entry is not None and entry[4]:
+        _install_python_dependencies(Path(entry[4]), console)
+
+
+def _install_python_dependencies(target: Path, console, *, skip: bool = False) -> None:
+    """Install the plugin's declared Python deps (pyproject ``[project].dependencies`` or manifest
+    ``python_dependencies``) into the venv and report; the plugin stays installed on failure."""
+    from hermes_cli.plugin_python_deps import install_for_plugin_dir
+    outcome = install_for_plugin_dir(target) if not skip else None
+    if outcome is None or outcome.status == "none":
         return
-    deps = [d.strip() for d in deps if isinstance(d, str) and d.strip()]
-    if not deps:
-        return
-    plugin_name = manifest.get("name", "this plugin")
-    console.print(f"\n[bold]{plugin_name}[/bold] declares Python dependencies (not installed automatically):")
-    for dep in deps:
-        console.print(f"  - {dep}")
-    console.print(
-        f"[dim]Install them yourself if needed: pip install {' '.join(repr(d) for d in deps)}[/dim]\n")
+    style = {"installed": "green", "failed": "yellow", "invalid": "yellow"}[outcome.status]
+    console.print(f"[{style}]{'✓' if outcome.status == 'installed' else '⚠'}[/{style}] {outcome.message}")
 
 
 def _prompt_plugin_env_vars(manifest: dict, console) -> None:
@@ -683,11 +704,13 @@ def _install_plugin_core(
     ref: Optional[str] = None,
     scan_decision_cb=None,
     reviewed_pin: Optional[str] = None,
+    python_deps: bool = True,
 ) -> tuple[Path, dict, str]:
     """Clone a Git plugin and atomically record its source and exact revision.
 
     *reviewed_pin* is the curated-catalog sha for this install; the scan trusts the tree
-    only when the checked-out revision is exactly that sha."""
+    only when the checked-out revision is exactly that sha. *python_deps* False skips the
+    dependency conflict gate (``--no-deps``: the user installs them by hand)."""
     requested_revision = _normalize_exact_revision(ref) if ref is not None else None
     try:
         git_url, subdir = _resolve_git_url(identifier)
@@ -721,6 +744,8 @@ def _install_plugin_core(
         # Scan BEFORE anything is moved into place; raises PluginScanBlocked when blocked.
         _scan_plugin_tree(tmp_target, identifier, force=force, scan_decision_cb=scan_decision_cb,
                           reviewed_pin=bool(reviewed_pin) and installed_revision == reviewed_pin)
+        if python_deps:
+            _refuse_conflicting_python_deps(tmp_target, plugin_name)
 
         if target.exists() and not force:
             raise PluginOperationError(
@@ -751,6 +776,7 @@ def cmd_install(
     enable: Optional[bool] = None,
     ref: Optional[str] = None,
     allow_removed: bool = False,
+    no_deps: bool = False,
 ) -> None:
     """Install a plugin from the curated catalog (bare name), a Git URL, or owner/repo shorthand.
 
@@ -798,10 +824,12 @@ def cmd_install(
     try:
         if entry is not None:
             target, installed_manifest, installed_name = catalog.install_catalog_entry(
-                entry, force=force, ref=ref, allow_removed=True, scan_decision_cb=_interactive_scan_decision)
+                entry, force=force, ref=ref, allow_removed=True, scan_decision_cb=_interactive_scan_decision,
+                python_deps=not no_deps)
         else:
             target, installed_manifest, installed_name = _install_plugin_core(
-                identifier, force=force, ref=ref, scan_decision_cb=_interactive_scan_decision)
+                identifier, force=force, ref=ref, scan_decision_cb=_interactive_scan_decision,
+                python_deps=not no_deps)
     except PluginOperationError as e:
         _fail(console, f"[red]{'Blocked' if isinstance(e, PluginScanBlocked) else 'Error'}:[/red] {e}")
     if not _looks_like_plugin_dir(target):
@@ -809,7 +837,7 @@ def cmd_install(
             f"[yellow]Warning:[/yellow] {installed_name} doesn't contain plugin.yaml, "
             f"plugin.json, or __init__.py. It may not be a valid Hermes plugin.")
     _prompt_plugin_env_vars(installed_manifest, console)
-    _print_python_dependencies(installed_manifest, console)
+    _install_python_dependencies(target, console, skip=no_deps)
     _display_after_install(target, identifier)
 
     if enable is None:
@@ -921,11 +949,13 @@ def _rescan_after_update(target: Path, name: str, console) -> None:
 
 
 def _post_pull_housekeeping(target: Path, console) -> None:
-    """After ``git pull``: drop stale ``__pycache__`` and copy any new ``.example`` files."""
+    """After ``git pull``: drop stale ``__pycache__``, copy any new ``.example`` files, and install
+    dependencies the new revision declares (a version bump commonly adds or moves a package)."""
     # Same stale-bytecode class as the main checkout (#6207/#60242): the pull just changed .py files under
     # this plugin dir, so drop any __pycache__ compiled from the previous revision.
     _clear_plugin_bytecode(target)
     _copy_example_files(target, console)
+    _install_python_dependencies(target, console)
 
 
 def _remove_plugin_core(target: Path) -> None:
@@ -1094,6 +1124,7 @@ def cmd_enable(name: str, allow_tool_override: Optional[bool] = None) -> None:
     # Built-in tool override is a privileged grant; bundled plugins are trusted.
     if source == "bundled":
         return
+    _install_python_dependencies_for_key(key, console)
     # When the manifest declares capabilities the consent screen is the canonical grant path
     # (it covers tools.override too); the legacy prompt then only runs on an explicit flag.
     # See #64228.
@@ -1818,9 +1849,11 @@ def dashboard_install_plugin(
 
     if enable:
         _set_plugin_enabled(installed_name, enable=True)
+    deps = _install_python_dependencies_quietly(target, warnings)
     ap = target / "after-install.md"
     return {
         "ok": True, "plugin_name": installed_name, "warnings": warnings,
+        "python_dependencies": deps,
         "missing_env": [s["name"] for s in _missing_env_specs(installed_manifest)],
         "after_install_path": str(ap) if ap.exists() else None, "enabled": enable,
     }
@@ -1916,7 +1949,10 @@ def dashboard_update_user_plugin(name: str) -> dict[str, Any]:
     try:
         if sidecar:
             sha, changed = catalog.repin_catalog_plugin(target, sidecar)
-            return {"ok": True, "name": name, "sha": sha, "unchanged": not changed}
+            warnings: list[str] = []
+            deps = _install_python_dependencies_quietly(target, warnings) if changed else []
+            return {"ok": True, "name": name, "sha": sha, "unchanged": not changed,
+                    "python_dependencies": deps, "warnings": warnings}
         msg = _pull_plugin_update(
             target,
             lambda rec: (
@@ -2134,7 +2170,8 @@ _PLUGIN_ACTIONS = {
         force=getattr(args, "force", False),
         enable=_tri_state_flag(args, "enable", "no_enable"),
         ref=getattr(args, "ref", None),
-        allow_removed=getattr(args, "allow_removed", False)),
+        allow_removed=getattr(args, "allow_removed", False),
+        no_deps=getattr(args, "no_deps", False)),
     "search": lambda args: _catalog().cmd_search(
         getattr(args, "term", "") or "", json_output=getattr(args, "json", False)),
     "browse": lambda args: _catalog().cmd_search(""),
diff --git a/hermes_cli/plugins_cmd_catalog.py b/hermes_cli/plugins_cmd_catalog.py
index 10d86373ca..213a1839f3 100644
--- a/hermes_cli/plugins_cmd_catalog.py
+++ b/hermes_cli/plugins_cmd_catalog.py
@@ -106,7 +106,7 @@ def removed_annotation(name: str, dir_path) -> Optional[str]:
 # ── Catalog-aware install / update ───────────────────────────────────────────
 
 def install_catalog_entry(entry: PluginCatalogEntry, *, force: bool, ref: Optional[str] = None,
-                          allow_removed: bool = False, scan_decision_cb=None) -> tuple:
+                          allow_removed: bool = False, scan_decision_cb=None, python_deps: bool = True) -> tuple:
     """``_install_plugin_core`` at the catalog pin (an explicit *ref* wins) + provenance sidecar.
     Returns the core's ``(target, manifest, installed_name)``."""
     from hermes_cli.plugins_cmd import _install_plugin_core
@@ -114,7 +114,7 @@ def install_catalog_entry(entry: PluginCatalogEntry, *, force: bool, ref: Option
         raise_if_removed(entry.name, entry.repo)
     target, manifest, installed_name = _install_plugin_core(
         entry.install_identifier, force=force, ref=ref or entry.sha, scan_decision_cb=scan_decision_cb,
-        reviewed_pin=entry.sha)
+        reviewed_pin=entry.sha, python_deps=python_deps)
     write_catalog_sidecar(target, entry)
     return target, manifest, installed_name
 
@@ -147,6 +147,9 @@ def cmd_update_catalog(name: str, target: Path, sidecar: dict, console) -> None:
         raise SystemExit(1)
     verb = "updated to" if changed else "is already at catalog pin"
     console.print(f"[green]✓[/green] Plugin [bold]{name}[/bold] {verb} {sha[:8]}.")
+    if changed:
+        from hermes_cli.plugins_cmd import _install_python_dependencies
+        _install_python_dependencies(target, console)
 
 
 # ── search / browse / info / validate ────────────────────────────────────────
diff --git a/hermes_cli/plugins_loader.py b/hermes_cli/plugins_loader.py
index 788aeb744c..3af0120ebf 100644
--- a/hermes_cli/plugins_loader.py
+++ b/hermes_cli/plugins_loader.py
@@ -217,13 +217,9 @@ class PluginLoaderMixin:
             )
 
     def _warn_python_dependencies(self, manifest: PluginManifest) -> None:
-        """Warn about missing declared pip dependencies with an install hint — NEVER auto-install.
-
-        See #64165.
-        python_dependencies is a declaration seam ONLY: Hermes validates and prints the requirements with an
-        install hint but NEVER auto-installs them. The isolation design (constraints installs vs. vendored
-        dirs vs. conflict-detection-and-refusal) is an explicitly deferred follow-up — see the round-2
-        review on #64165 and #15220.
+        """Warn about declared pip dependencies missing at load time. Installing happens at
+        ``hermes plugins install``/``enable`` and after ``hermes update`` (``hermes_cli.plugin_python_deps``)
+        under core constraints; the loader itself never installs — import time is not a consent point.
         """
         deps = manifest.python_dependencies
         if not deps:
@@ -233,9 +229,9 @@ class PluginLoaderMixin:
         if missing:
             logger.warning(
                 "Plugin %s declares Python dependencies that are not "
-                "installed: %s. Hermes does not install plugin dependencies "
-                "automatically; install them yourself, e.g.: pip install %s",
-                key, ", ".join(missing), " ".join(f"'{m}'" for m in missing),
+                "installed: %s. Run `hermes plugins enable %s` to install them, "
+                "or install them yourself: pip install %s",
+                key, ", ".join(missing), key, " ".join(f"'{m}'" for m in missing),
             )
         else:
             logger.debug("Plugin %s python_dependencies satisfied: %s", key, ", ".join(deps))
diff --git a/hermes_cli/plugins_manifest.py b/hermes_cli/plugins_manifest.py
index ce5c34e40a..92b36904b3 100644
--- a/hermes_cli/plugins_manifest.py
+++ b/hermes_cli/plugins_manifest.py
@@ -36,7 +36,7 @@ _KNOWN_MANIFEST_FIELDS: Set[str] = {
     "pip_dependencies", "provides_browser_providers", "provides_web_providers",
     "manifest_version", "api_version", "requires_plugins", "python_dependencies", "config_schema",
     "license", "homepage", "tags", "capabilities", "emits", "listens", "hermes", "depends",
-    "requires_hermes",
+    "requires_hermes", "python_runtime",
 }
 
 # Highest manifest schema version this Hermes understands.
diff --git a/hermes_cli/subcommands/plugins.py b/hermes_cli/subcommands/plugins.py
index e67e63fc82..c977e35e68 100644
--- a/hermes_cli/subcommands/plugins.py
+++ b/hermes_cli/subcommands/plugins.py
@@ -29,6 +29,10 @@ def build_plugins_parser(subparsers, *, cmd_plugins: Callable) -> None:
     plugins_install.add_argument(
         "--allow-removed", action="store_true",
         help="DANGEROUS: bypass the catalog removed-plugin blocklist check")
+    plugins_install.add_argument(
+        "--no-deps", action="store_true",
+        help="Skip the plugin's declared Python dependencies (no conflict check, nothing installed); "
+             "you manage them yourself")
     _install_enable_group = plugins_install.add_mutually_exclusive_group()
     _install_enable_group.add_argument(
         "--enable", action="store_true",
diff --git a/hermes_cli/update_cmd.py b/hermes_cli/update_cmd.py
index 816f59a85a..a40fa522fd 100644
--- a/hermes_cli/update_cmd.py
+++ b/hermes_cli/update_cmd.py
@@ -80,6 +80,7 @@ from hermes_cli.update_cmd_deps import (  # noqa: F401
     _ensure_venv_pip, _install_psutil_android_compat, _is_android_python, _npm_bin_exists,
     _npm_lockfile_changed, _npm_manifest_paths, _npm_manifests_digest, _path_uid,
     _rebuild_desktop_after_update, _record_npm_lockfile_hash, _refresh_active_lazy_features,
+    _reapply_plugin_python_dependencies,
     _refresh_active_memory_provider_dependencies, _refuse_update_if_venv_foreign_owned,
     _repair_node_deps_on_current_checkout, _restore_active_tool_dependencies,
     _sync_python_dependencies_after_pull, _update_node_dependencies,
@@ -620,6 +621,7 @@ def _repair_venv_on_current_checkout(
     _m()._install_python_dependencies_with_optional_fallback(repair_prefix, env=repair_env, group="all")
     _m()._refresh_active_lazy_features(repair_prefix, env=repair_env, features=active_lazy_features)
     _m()._restore_active_tool_dependencies(active_tool_dependencies, repair_prefix, env=repair_env)
+    _m()._reapply_plugin_python_dependencies()
     # Core ``.[all]`` install finished. Clear the generic core breadcrumb before the lazy-refresh phase —
     # that phase uses its own marker so a later lazy failure cannot be "healed" by clearing the core marker
     # based on a narrow 7-package import probe (#58004 review).
@@ -710,6 +712,7 @@ def _repair_current_checkout(
                       "to finish import-based venv repair.")
             _m()._restore_active_tool_dependencies(
                 active_tool_dependencies, repair_prefix, env=repair_env)
+            _m()._reapply_plugin_python_dependencies()
         current_checkout_complete = _repair_node_deps_on_current_checkout(
             _print_verified_update_completion, assume_yes=assume_yes, gateway_mode=gateway_mode,
             pre_update_snapshot_id=pre_update_snapshot_id,
diff --git a/hermes_cli/update_cmd_deps.py b/hermes_cli/update_cmd_deps.py
index 4198a8b735..0452d0f5b7 100644
--- a/hermes_cli/update_cmd_deps.py
+++ b/hermes_cli/update_cmd_deps.py
@@ -388,6 +388,41 @@ def _refresh_active_memory_provider_dependencies() -> None:
         print(f"  ⚠ {provider} dependencies failed to refresh: {exc}")
 
 
+def _reapply_plugin_python_dependencies() -> None:
+    """Re-install every enabled user plugin's declared Python deps after the venv was rebuilt (a
+    ``uv sync``/reinstall strips anything Hermes' own lock does not know). Non-memory plugins whose
+    deps no longer resolve are disabled loudly, memory providers last. Never raises."""
+    from hermes_cli.plugin_python_deps import reapply_all
+    from hermes_cli.update_cmd import _m
+
+    def _disable(home, name: str) -> None:
+        # Write through the real config writer (comments/defaults preserved), scoped to *home*.
+        from hermes_cli.config import load_config, save_config
+        from hermes_constants import reset_hermes_home_override, set_hermes_home_override
+        token = set_hermes_home_override(home)
+        try:
+            config = load_config()
+            plugins = config.setdefault("plugins", {})
+            plugins["enabled"] = sorted(set(plugins.get("enabled") or []) - {name})
+            plugins["disabled"] = sorted(set(plugins.get("disabled") or []) | {name})
+            save_config(config, merge_existing=True)
+        finally:
+            reset_hermes_home_override(token)
+
+    try:
+        report = reapply_all(project_root=_m().PROJECT_ROOT, disable=_disable)
+    except Exception as exc:  # the update must finish even if the plugin step blows up
+        print(f"  ⚠ Plugin Python dependencies not re-applied: {exc}")
+        return
+    if report.installed:
+        print(f"  ✓ Plugin Python dependencies re-applied: {', '.join(report.installed)}")
+    for name, reason in report.dropped:
+        print(f"  ✗ Plugin '{name}' DISABLED: {reason}. Fix the plugin's declared dependencies, "
+              f"then `hermes plugins enable {name}`.")
+    if report.failed:
+        print(f"  ⚠ Plugin Python dependencies not re-applied: {report.failed}")
+
+
 def _is_android_python() -> bool:
     from hermes_cli.update_cmd import _m
     return _m().sys.platform == "android"
@@ -1046,6 +1081,7 @@ def _sync_python_dependencies_after_pull(
 
     # Heal memory-provider bridge packages last — the steps above may have stripped them.
     _m()._refresh_active_memory_provider_dependencies()
+    _m()._reapply_plugin_python_dependencies()
 
     # Remaining import failures are real breakage. Warn only — never roll back: `cannot import
     # name X` is also the stale-bytecode signature, which self-heals next launch.
diff --git a/hermes_cli/update_cmd_zip.py b/hermes_cli/update_cmd_zip.py
index 86cf6ef5b4..5a981cc0e2 100644
--- a/hermes_cli/update_cmd_zip.py
+++ b/hermes_cli/update_cmd_zip.py
@@ -354,6 +354,7 @@ def _reinstall_python_deps_after_zip(active_tool_dependencies) -> None:
     _m()._restore_active_tool_dependencies(active_tool_dependencies, install_prefix, env=install_env)
     # Parity with git-pull path: heal the active memory provider's bridge packages after the reinstall.
     _m()._refresh_active_memory_provider_dependencies()
+    _m()._reapply_plugin_python_dependencies()
 
 
 def _update_via_zip(args, *, had_desktop_app_before_update: bool = False, _windows_gateway_resume=None) -> bool:
diff --git a/website/docs/developer-guide/plugins/index.md b/website/docs/developer-guide/plugins/index.md
index 850070ad1d..88dbbc325f 100644
--- a/website/docs/developer-guide/plugins/index.md
+++ b/website/docs/developer-guide/plugins/index.md
@@ -300,7 +300,8 @@ this Hermes understands still loads with a warning.
 | `manifest_version` | int | Manifest **file-format** version. Absent = `1`. Current max: `2`. Independent from `api_version`. |
 | `api_version` | int | Runtime **plugin API generation** the plugin targets (ctx surface / hook signatures). Deliberately a separate axis from `manifest_version` — an `api_version: 1` plugin can use a v2 manifest. |
 | `requires_plugins` | list | Inter-plugin dependencies: `- id: other-plugin` with optional `version_range: ">=1.0,<2"`. **Advisory**: a missing dependency logs a clear warning but the plugin still loads — probe at runtime with `ctx.has_plugin("other-plugin")`. Load **order** honors these edges: when A requires B, B's `register()` runs before A's (topological sort, alphabetical tiebreak; cycles warn and fall back to alphabetical order). |
-| `python_dependencies` | list of str | Declared pip requirements (e.g. `"requests>=2.0,<3"`). **Declaration seam only** — Hermes validates them, and `hermes plugins install` / `hermes plugins doctor` surface missing ones with a `pip install` hint, but Hermes **never auto-installs** them. Pin upper bounds. |
+| `python_dependencies` | list of str | PEP 508 requirements (e.g. `"requests>=2.0,<3"`). Installed into Hermes' venv on `hermes plugins install` / `enable` and **re-applied after every `hermes update`** (see [Python dependencies](#python-dependencies)). A `pyproject.toml` beside `plugin.yaml` with `[project].dependencies` is the equivalent, preferred form. |
+| `python_runtime` | str | `external` — the plugin manages its own interpreter/venv (sidecar pattern); Hermes installs nothing and leaves any `pyproject.toml` alone. |
 | `config_schema` | mapping | JSON-schema-ish description of keys under `plugins.entries.<id>.settings`: `api_url: {type: str, default: "", description: "...", required: false}`. Validated at load; mismatches log actionable warnings naming the key and expected type — never load failures. Types: `str`, `int`, `float`, `bool`, `list`, `dict` (plus JSON-schema aliases). |
 | `license` | str | SPDX-style license id (e.g. `MIT`). |
 | `homepage` | str | Project URL. |
@@ -319,21 +320,57 @@ requires_plugins:
   - id: other-plugin
     version_range: ">=1.0,<2"
 python_dependencies:
-  - "somepkg>=1.0,<2"     # surfaced, never auto-installed
+  - "somepkg>=1.0,<2"     # installed on install/enable, re-applied after hermes update
 config_schema:
   api_url: {type: str, default: "", description: "Service endpoint"}
 ```
 
-:::note pip-dependency isolation is deferred
-`python_dependencies` is intentionally declare-and-surface only. Installing
-arbitrary packages into Hermes' shared venv is a conflict and supply-chain
-surface, so the install seam's isolation design (constraints-file installs
-against the host lock vs. per-plugin vendored dirs vs. conflict detection
-with refusal) is an explicitly deferred follow-up — see the round-2 review on
-[#64165](https://github.com/NousResearch/hermes-agent/issues/64165) and
-[#15220](https://github.com/NousResearch/hermes-agent/issues/15220). Plugin
-packs (#64166) build on these v2 fields.
-:::
+### Python dependencies
+
+A directory plugin can bring its own PyPI packages. Declare them either in the manifest
+(`python_dependencies`, above) or, preferably, in a `pyproject.toml` next to `plugin.yaml`:
+
+```toml
+[project]
+name = "my-plugin"
+version = "1.0.0"
+requires-python = ">=3.11"
+dependencies = [
+    "somepkg>=1.0,<2",
+    "other[extra]>=3.11",
+]
+```
+
+When both exist the `pyproject.toml` wins. What Hermes does with them:
+
+- **Install / enable** — the declared packages are installed into Hermes' venv with
+  `uv pip install` (pip fallback) under a **constraints file built from Hermes' own pinned
+  dependencies**, so a plugin can never move a core package (httpx, pydantic, …) off the version
+  Hermes was tested with. Environment markers (`; sys_platform == "win32"`) are honoured.
+- **Conflict = refusal, never a silent drop** — before the plugin tree is moved into place, its
+  dependencies are dry-run resolved together with every already-enabled plugin's. A candidate that
+  cannot resolve is *not installed* and the error names the conflict; existing plugins are untouched.
+- **`hermes update` re-applies them** — the update's `uv sync` rebuilds the venv from Hermes' lock
+  and strips anything else. Afterwards Hermes walks every profile's enabled plugins and reinstalls
+  their declared dependencies. If the union no longer resolves (a core pin moved), non-memory
+  plugins are dropped one at a time until it does; each dropped plugin is **disabled with a loud
+  message** naming it, and memory providers are kept over everything else, because a Hermes that
+  boots without memory looks like data loss.
+- **`hermes plugins update`** re-runs the install for whatever the new revision declares.
+- **`--no-deps`** on `hermes plugins install` skips all of this for one plugin (no conflict gate,
+  nothing installed) when you would rather manage its packages yourself.
+- **Opt out with `python_runtime: external`** — plugins that keep a heavy runtime (torch, native
+  extensions) in their own sidecar venv and talk to it over a subprocess declare this in
+  `plugin.yaml`; Hermes then installs nothing and the plugin never joins the shared resolution.
+- **Nothing to load is an error** — `hermes plugins validate` (and the catalog CI) fail a
+  `plugin.yaml` with no `__init__.py`, `desktop/plugin.js` or `plugin.json` beside it. A pip-layout
+  package whose code sits under `src/` behind an entry point needs a thin directory-plugin wrapper
+  whose `pyproject.toml` depends on the package.
+- `security.allow_lazy_installs: false` disables all of this; the plugin installs, its dependencies
+  do not, and the loader warns at import.
+
+`HERMES_HOME/plugins/` survives `hermes update` and Desktop updates: the updater only rebuilds the
+venv and the checkout, never the home directory.
 
 ## Step 3: Write the tool schemas