feat(plugins): add thread-local tool whitelist to pre_tool_call gate

Adds set_thread_tool_whitelist / clear_thread_tool_whitelist to
hermes_cli/plugins.py. When set on the current thread, restricts which
tools can pass through get_pre_tool_call_block_message; non-whitelisted
tools are blocked with a configurable deny message.

Mirrors the per-thread approval-callback pattern already used by
set_approval_callback (tools/terminal_tool.py:190). Used by
_spawn_background_review to deny non-memory/non-skill tools at runtime
while inheriting the parent agent's full tools schema for prefix-cache
parity (see follow-up commit).

Tests cover allow / deny / clear / cross-thread isolation.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

hermes_cli/plugins.py

@@ -1339,6 +1339,21 @@ def invoke_hook(hook_name: str, **kwargs: Any) -> List[Any]:
 
 
 
+_thread_tool_whitelist = threading.local()
+
+
+def set_thread_tool_whitelist(
+    allowed: Optional[Set[str]],
+    deny_msg_fmt: str = "Tool '{tool_name}' denied: not in this thread's tool whitelist",
+) -> None:
+    _thread_tool_whitelist.allowed = allowed
+    _thread_tool_whitelist.fmt = deny_msg_fmt
+
+
+def clear_thread_tool_whitelist() -> None:
+    _thread_tool_whitelist.allowed = None
+
+
 def get_pre_tool_call_block_message(
     tool_name: str,
     args: Optional[Dict[str, Any]],
@@ -1357,6 +1372,11 @@ def get_pre_tool_call_block_message(
     directive wins.  Invalid or irrelevant hook return values are
     silently ignored so existing observer-only hooks are unaffected.
     """
+    allowed = getattr(_thread_tool_whitelist, "allowed", None)
+    if allowed is not None and tool_name not in allowed:
+        fmt = getattr(_thread_tool_whitelist, "fmt", "Tool '{tool_name}' denied")
+        return fmt.format(tool_name=tool_name)
+
     hook_results = invoke_hook(
         "pre_tool_call",
         tool_name=tool_name,

tests/hermes_cli/test_plugins.py

@@ -538,6 +538,95 @@ class TestPreToolCallBlocking:
         assert get_pre_tool_call_block_message("terminal", {}) == "first blocker"
 
 
+class TestThreadToolWhitelist:
+    """Tests for the thread-local tool whitelist used by background review forks."""
+
+    def test_allowed_tool_passes_through_to_hooks(self, monkeypatch):
+        from hermes_cli.plugins import (
+            set_thread_tool_whitelist,
+            clear_thread_tool_whitelist,
+        )
+
+        monkeypatch.setattr(
+            "hermes_cli.plugins.invoke_hook",
+            lambda hook_name, **kwargs: [],
+        )
+        set_thread_tool_whitelist({"memory", "skill_manage"})
+        try:
+            assert get_pre_tool_call_block_message("memory", {}) is None
+        finally:
+            clear_thread_tool_whitelist()
+
+    def test_disallowed_tool_blocked_with_message(self, monkeypatch):
+        from hermes_cli.plugins import (
+            set_thread_tool_whitelist,
+            clear_thread_tool_whitelist,
+        )
+
+        monkeypatch.setattr(
+            "hermes_cli.plugins.invoke_hook",
+            lambda hook_name, **kwargs: [],
+        )
+        set_thread_tool_whitelist(
+            {"memory"}, deny_msg_fmt="denied: {tool_name}"
+        )
+        try:
+            msg = get_pre_tool_call_block_message("terminal", {})
+            assert msg == "denied: terminal"
+        finally:
+            clear_thread_tool_whitelist()
+
+    def test_clear_restores_unrestricted_behavior(self, monkeypatch):
+        from hermes_cli.plugins import (
+            set_thread_tool_whitelist,
+            clear_thread_tool_whitelist,
+        )
+
+        monkeypatch.setattr(
+            "hermes_cli.plugins.invoke_hook",
+            lambda hook_name, **kwargs: [],
+        )
+        set_thread_tool_whitelist({"memory"})
+        clear_thread_tool_whitelist()
+        # After clearing, any tool should pass through to plugin hooks (which
+        # return [] here, so result is None).
+        assert get_pre_tool_call_block_message("terminal", {}) is None
+
+    def test_whitelist_is_thread_local(self, monkeypatch):
+        """Setting a whitelist in one thread must NOT leak into another."""
+        import threading
+
+        from hermes_cli.plugins import (
+            set_thread_tool_whitelist,
+            clear_thread_tool_whitelist,
+        )
+
+        monkeypatch.setattr(
+            "hermes_cli.plugins.invoke_hook",
+            lambda hook_name, **kwargs: [],
+        )
+
+        # Main thread: install a restrictive whitelist.
+        set_thread_tool_whitelist({"memory"})
+        try:
+            assert get_pre_tool_call_block_message("terminal", {}) is not None
+
+            # Worker thread: should NOT inherit main thread's whitelist.
+            result = {}
+
+            def worker():
+                result["msg"] = get_pre_tool_call_block_message("terminal", {})
+
+            t = threading.Thread(target=worker)
+            t.start()
+            t.join()
+            assert result["msg"] is None, (
+                "thread-local whitelist leaked across threads"
+            )
+        finally:
+            clear_thread_tool_whitelist()
+
+
 # ── TestPluginContext ──────────────────────────────────────────────────────