feat: surface MCP server status in System Status page

Add per-tool in-memory call counters to the MCP module and a
GET /api/v1/status/mcp endpoint that joins them with the live tool
registry and endpoint config, then render it as a new card on the
System Status page with a one-click client-config copy button.

- app/mcp/stats.py: lock-guarded MCPStatsTracker (the mcp SDK runs sync
  tool bodies via anyio.to_thread.run_sync, so this is genuinely
  multi-threaded, unlike the async REST routes)
- app/mcp/server.py: instrument search_regulations, add get_mcp_status()
- app/config/settings.py: optional MCP_PUBLIC_URL override, required
  because the Vite proxy and reverse proxies rewrite the Host header
- StatusPage.tsx: MCP Server card, joins the existing parallel fetch

Counters are process-local by design; token usage is already persisted
by ModelUsageTracker since MCP calls route through ask().

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/mcp/server.py

@@ -9,6 +9,7 @@ dict. No new retrieval, ranking, or LLM orchestration logic lives here.
 from __future__ import annotations
 
 import logging
+import time
 from typing import Annotated
 
 from mcp.server import MCPServer
@@ -18,6 +19,7 @@ from starlette.responses import PlainTextResponse
 from starlette.types import ASGIApp, Receive, Scope, Send
 
 from app.config.settings import settings
+from app.mcp.stats import get_mcp_stats_tracker
 from app.shared.bootstrap import get_agent_conversation_service, get_jwt_handler
 
 logger = logging.getLogger(__name__)
@@ -50,7 +52,24 @@ def search_regulations(
     # No session_id is passed: this keeps each call stateless (no
     # ConversationStore reads/writes), matching "search" semantics rather
     # than multi-turn chat semantics.
-    _, result = get_agent_conversation_service().ask(query=query, top_k=top_k)
+    started = time.perf_counter()
+    try:
+        _, result = get_agent_conversation_service().ask(query=query, top_k=top_k)
+    except Exception:
+        # Record the failure, then re-raise unchanged so the MCP SDK still
+        # converts it into a protocol-level error for the client. Swallowing
+        # it here would report success to the caller.
+        get_mcp_stats_tracker().record(
+            tool="search_regulations",
+            duration_ms=(time.perf_counter() - started) * 1000,
+            success=False,
+        )
+        raise
+    get_mcp_stats_tracker().record(
+        tool="search_regulations",
+        duration_ms=(time.perf_counter() - started) * 1000,
+        success=True,
+    )
     return {
         "answer": result.answer,
         "sources": [source.__dict__ for source in result.sources],
@@ -99,6 +118,13 @@ class MCPAuthMiddleware:
         await self.app(scope, receive, send)
 
 
+def _parse_allowed_hosts() -> list[str]:
+    """Split the configured MCP host allow-list into individual entries."""
+    # Shared by the transport-security builder and the status endpoint so the
+    # panel can never display an allow-list different from the enforced one.
+    return [h.strip() for h in settings.mcp_allowed_hosts.split(",") if h.strip()]
+
+
 def _build_transport_security() -> TransportSecuritySettings:
     """Translate the configured MCP host allow-list into SDK transport settings.
 
@@ -106,7 +132,7 @@ def _build_transport_security() -> TransportSecuritySettings:
     defaults to 127.0.0.1 and therefore rejects every remote client with HTTP
     421 — fatal for a remotely deployed backend.
     """
-    allowed = [h.strip() for h in settings.mcp_allowed_hosts.split(",") if h.strip()]
+    allowed = _parse_allowed_hosts()
     if "*" in allowed:
         # Explicit, logged opt-out. Kept as an escape hatch for environments
         # behind a proxy that rewrites Host unpredictably, but never the default.
@@ -140,3 +166,36 @@ def build_mcp_asgi_app() -> ASGIApp:
     )
     asgi_app.add_middleware(MCPAuthMiddleware)
     return asgi_app
+
+
+async def get_mcp_status(public_url: str) -> dict:
+    """Assemble the MCP status payload shown on the System Status page.
+
+    Owned by this module rather than the status route so that MCP internals
+    (the tool registry, the allow-list format, the stats tracker) stay behind
+    one boundary; the route only supplies public_url, which is the one value
+    only the HTTP layer can know.
+    """
+    stats = get_mcp_stats_tracker().snapshot()
+    # list_tools() reads the in-memory registry populated by @mcp.tool() at
+    # import time, so the panel always reflects what is actually advertised
+    # rather than a hand-maintained duplicate list.
+    tools = await mcp.list_tools()
+    return {
+        "endpoint_url": public_url,
+        "auth_required": settings.auth_enabled,
+        "allowed_hosts": _parse_allowed_hosts(),
+        "tools": [
+            {
+                "name": tool.name,
+                "description": (tool.description or "").strip().split("\n")[0],
+                "calls": entry.calls if entry else 0,
+                "errors": entry.errors if entry else 0,
+                "avg_duration_ms": entry.avg_duration_ms if entry else None,
+                "last_called_at": (
+                    entry.last_called_at.isoformat() if entry and entry.last_called_at else None
+                ),
+            }
+            for tool, entry in ((tool, stats.get(tool.name)) for tool in tools)
+        ],
+    }

backend/app/mcp/stats.py

@@ -0,0 +1,91 @@
+"""In-memory per-tool call counters for the MCP server.
+
+Lives in `app/mcp/` rather than `app/shared/` because these counters are
+meaningful only for the MCP transport: they answer "is anything actually
+calling our MCP endpoint, and does it work?" for the System Status page.
+Token consumption is deliberately not tracked here — MCP tool calls route
+through AgentConversationService.ask() like every other caller, so the
+existing ModelUsageTracker already accounts for it.
+
+Counters are process-local and reset on restart. That is an accepted
+tradeoff, recorded in the design spec: nothing billable depends on them.
+"""
+
+from __future__ import annotations
+
+import threading
+from dataclasses import dataclass
+from datetime import datetime, timezone
+from functools import lru_cache
+
+from loguru import logger
+
+
+@dataclass
+class MCPToolStats:
+    """Accumulated call outcomes for a single MCP tool."""
+
+    calls: int = 0
+    errors: int = 0
+    total_duration_ms: float = 0.0
+    last_called_at: datetime | None = None
+
+    @property
+    def avg_duration_ms(self) -> float | None:
+        """Mean call duration, or None when the tool has never been called.
+
+        Returning None rather than 0.0 keeps "never called" distinguishable
+        from "called, but instantaneous" in the status UI.
+        """
+        if self.calls == 0:
+            return None
+        return self.total_duration_ms / self.calls
+
+
+class MCPStatsTracker:
+    """Thread-safe registry of per-tool MCP call statistics.
+
+    The lock is load-bearing, not defensive habit: the mcp SDK dispatches
+    synchronous tool functions through anyio.to_thread.run_sync, so tool
+    bodies genuinely run on multiple worker threads at once — unlike the
+    async REST routes, which are serialized by the event loop.
+    """
+
+    def __init__(self) -> None:
+        """Initialize an empty registry guarded by a single lock."""
+        self._tools: dict[str, MCPToolStats] = {}
+        # One coarse lock is enough: record() runs once per MCP tool call and
+        # snapshot() is only read by the low-traffic status endpoint.
+        self._lock = threading.Lock()
+
+    def record(self, *, tool: str, duration_ms: float, success: bool) -> None:
+        """Record the outcome of one MCP tool invocation.
+
+        Never raises: a defect in observability code must not turn a working
+        tool call into a protocol error for the client.
+        """
+        try:
+            # Coerce outside the lock so a bad argument cannot abort mid-update
+            # and leave calls incremented but duration unaccounted for.
+            duration = float(duration_ms)
+            now = datetime.now(timezone.utc)
+            with self._lock:
+                stats = self._tools.setdefault(tool, MCPToolStats())
+                stats.calls += 1
+                if not success:
+                    stats.errors += 1
+                stats.total_duration_ms += duration
+                stats.last_called_at = now
+        except Exception as exc:  # noqa: BLE001 - tracking must never break a real call
+            logger.warning("MCPStatsTracker.record failed for tool {} - {}", tool, exc)
+
+    def snapshot(self) -> dict[str, MCPToolStats]:
+        """Return a shallow copy of all tracked tools, safe to read outside the lock."""
+        with self._lock:
+            return dict(self._tools)
+
+
+@lru_cache
+def get_mcp_stats_tracker() -> MCPStatsTracker:
+    """Return the process-wide singleton tracker (mirrors get_model_usage_tracker())."""
+    return MCPStatsTracker()