fix: keyword routing fallback — never return zero tools

Previously narrow_tools_by_keywords returned [] when no category matched,
silently disabling all tools for messages that didn't hit a keyword.

Now:
- Falls back to _FALLBACK_TOOLS (web_search, session_search, session_read,
  scratch_read) when no category matches — tools always available for casual use
- Detects explicit "use your tools / look that up / can you check" phrases
  and returns the full role tool list (None = all permitted)
- Return type updated to list[str] | None; orchestrator log handles both

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/tools/__init__.py

@@ -640,11 +640,23 @@ _KEYWORD_CATEGORY_MAP: dict[str, list[str]] = {
 }
 
 
-def classify_tool_categories(message: str) -> list[str]:
-    """Return category names whose keywords appear in message (case-insensitive).
+# Tools always available as a fallback when no keyword category matches.
+# Covers the most common casual needs: quick lookup, session context, scratchpad.
+_FALLBACK_TOOLS: list[str] = [
+    "web_search", "session_search", "session_read", "scratch_read",
+]
 
-    Empty return means no tool category matched — route as pure chat with zero tool overhead.
-    """
+# Phrases that signal the user explicitly wants the orchestrator to use tools.
+# When matched, the full role tool list is returned rather than a narrow subset.
+_EXPLICIT_TOOL_PHRASES: list[str] = [
+    "use your tools", "use tools", "with tools", "using tools",
+    "look that up", "can you check", "please check", "go check",
+    "use the tools", "tools to", "check on that",
+]
+
+
+def classify_tool_categories(message: str) -> list[str]:
+    """Return category names whose keywords appear in message (case-insensitive)."""
     low = message.lower()
     return [cat for cat, kws in _KEYWORD_CATEGORY_MAP.items() if any(kw in low for kw in kws)]
 
@@ -653,15 +665,15 @@ def narrow_tools_by_keywords(
     message: str,
     role_tools: list[str] | None,
     context_messages: list[dict] | None = None,
-) -> list[str]:
+) -> list[str] | None:
     """Narrow the active tool list to categories relevant to this message.
 
-    Also scans the last assistant message in context_messages — this catches follow-up
-    patterns like "yes, please do that" where the tool intent was expressed by the assistant
-    in the prior turn and the user is simply confirming.
+    Also scans the last assistant message in context_messages — catches follow-up
+    patterns like "yes, please do that" where tool intent was in the prior turn.
 
-    Returns [] if no keywords matched (zero tool overhead).
-    Returns keyword-matched tools, intersected with role_tools if role_tools is set.
+    Returns None  — explicit tool request detected; caller uses full role tool list.
+    Returns list  — keyword-matched tools (intersected with role_tools if set).
+                    Falls back to _FALLBACK_TOOLS rather than [] when no categories match.
     """
     scan_text = message
     if context_messages:
@@ -670,9 +682,20 @@ def narrow_tools_by_keywords(
                 scan_text = scan_text + " " + (m.get("content") or "")
                 break
 
+    low_scan = scan_text.lower()
+
+    # Explicit "use your tools" request → return full permitted list
+    if any(phrase in low_scan for phrase in _EXPLICIT_TOOL_PHRASES):
+        return role_tools  # None means all tools; list means role-scoped set
+
     matched = classify_tool_categories(scan_text)
     if not matched:
-        return []
+        # No category matched — return minimal fallback rather than empty
+        fallback = list(_FALLBACK_TOOLS)
+        if role_tools is not None:
+            role_set = set(role_tools)
+            fallback = [t for t in fallback if t in role_set]
+        return fallback
 
     seen: set[str] = set()
     dynamic: list[str] = []