fix: address coordinator/idle-sweep quality issues from review

- CRITICAL: idle sweep now calls mark_stopped() after successful HTTP stop,
  preventing repeated stop POSTs on every 3rd tick for the same instance
- CRITICAL: active_allocations() now filters by gpu_id to avoid marking wrong
  instance idle on multi-GPU nodes when an allocation is released
- CRITICAL: VRAM pre-flight guard in ensure_service was dead code — added the
  actual HTTPException(503) before the candidate loop
- IMPORTANT: register() now updates agent_url on re-registration if it changed,
  so relocated agents are tracked correctly
- IMPORTANT: updated test_service_registry.py callers of active_allocations()
  to pass the now-required gpu_id argument

circuitforge_core/resources/coordinator/agent_supervisor.py

@@ -45,6 +45,10 @@ class AgentSupervisor:
         if node_id not in self._agents:
             self._agents[node_id] = AgentRecord(node_id=node_id, agent_url=agent_url)
             logger.info("Registered agent node: %s @ %s", node_id, agent_url)
+        else:
+            if self._agents[node_id].agent_url != agent_url:
+                self._agents[node_id].agent_url = agent_url
+                logger.info("Updated agent URL for %s → %s", node_id, agent_url)
 
     def get_node_info(self, node_id: str) -> NodeInfo | None:
         record = self._agents.get(node_id)
@@ -156,7 +160,11 @@ class AgentSupervisor:
                 "Idle sweep: stopping %s on %s gpu%s (idle timeout)",
                 instance.service, instance.node_id, instance.gpu_id,
             )
-            await self._http_post(stop_url)
+            success = await self._http_post(stop_url)
+            if success:
+                self._service_registry.mark_stopped(
+                    instance.service, instance.node_id, instance.gpu_id
+                )
 
     async def run_heartbeat_loop(self) -> None:
         self._running = True

circuitforge_core/resources/coordinator/app.py

@@ -227,6 +227,14 @@ def create_coordinator_app(
                 service_max_mb = svc.max_mb
                 break
 
+        # Filter candidates by VRAM headroom — skip models where free VRAM
+        # is less than half of the service's max_mb ceiling.
+        if service_max_mb > 0 and free_mb < service_max_mb // 2:
+            raise HTTPException(
+                503,
+                detail=f"Insufficient VRAM on gpu {req.gpu_id}: {free_mb}MB free, need at least {service_max_mb // 2}MB",
+            )
+
         last_error: str = ""
         async with httpx.AsyncClient(timeout=120.0) as client:
             for model in candidates:

circuitforge_core/resources/coordinator/service_registry.py

@@ -84,17 +84,17 @@ class ServiceRegistry:
             return False
         # If no active allocations remain for this instance, mark it idle
         key = f"{alloc.service}:{alloc.node_id}:{alloc.gpu_id}"
-        if self.active_allocations(alloc.service, alloc.node_id) == 0:
+        if self.active_allocations(alloc.service, alloc.node_id, alloc.gpu_id) == 0:
             if key in self._instances:
                 self._instances[key] = dataclasses.replace(
                     self._instances[key], state="idle", idle_since=time.time()
                 )
         return True
 
-    def active_allocations(self, service: str, node_id: str) -> int:
+    def active_allocations(self, service: str, node_id: str, gpu_id: int) -> int:
         return sum(
             1 for a in self._allocations.values()
-            if a.service == service and a.node_id == node_id
+            if a.service == service and a.node_id == node_id and a.gpu_id == gpu_id
         )
 
     # ── instance API ─────────────────────────────────────────────────
@@ -127,6 +127,14 @@ class ServiceRegistry:
     def all_instances(self) -> list[ServiceInstance]:
         return list(self._instances.values())
 
+    def mark_stopped(self, service: str, node_id: str, gpu_id: int) -> None:
+        """Transition an instance to 'stopped' state and clear idle_since."""
+        key = f"{service}:{node_id}:{gpu_id}"
+        if key in self._instances:
+            self._instances[key] = dataclasses.replace(
+                self._instances[key], state="stopped", idle_since=None
+            )
+
     def idle_past_timeout(self, idle_stop_config: dict[str, int]) -> list[ServiceInstance]:
         """
         Return instances in 'idle' state whose idle time exceeds their configured timeout.

tests/test_resources/test_service_registry.py

@@ -25,13 +25,13 @@ def test_allocate_creates_allocation(registry):
 def test_active_allocations_count(registry):
     registry.allocate("vllm", "heimdall", 0, "M", "http://h:8000", "a", 300.0)
     registry.allocate("vllm", "heimdall", 0, "M", "http://h:8000", "b", 300.0)
-    assert registry.active_allocations("vllm", "heimdall") == 2
+    assert registry.active_allocations("vllm", "heimdall", 0) == 2
 
 
 def test_release_decrements_count(registry):
     alloc = registry.allocate("vllm", "heimdall", 0, "M", "http://h:8000", "a", 300.0)
     registry.release(alloc.allocation_id)
-    assert registry.active_allocations("vllm", "heimdall") == 0
+    assert registry.active_allocations("vllm", "heimdall", 0) == 0
 
 
 def test_release_nonexistent_returns_false(registry):