diff --git a/frontend/src/components/Flow/BrainNode.tsx b/frontend/src/components/Flow/BrainNode.tsx
index 7e32b1f..7de099d 100644
--- a/frontend/src/components/Flow/BrainNode.tsx
+++ b/frontend/src/components/Flow/BrainNode.tsx
@@ -13,13 +13,28 @@ export type BrainNodeData = {
   flows: string[]
   /** Circle diameter in pixels, from how many neurons this one is wired to. */
   size: number
+  /** Why this neuron cannot run, if validation found something. */
+  issue?: string | null
   [key: string]: unknown
 }
 
+/**
+ * Ring thickness as a share of the diameter, measured off the brand mark: a
+ * 1.4545 stroke on a 6.3 outer diameter in `fluksio-light.svg`.
+ *
+ * A share rather than a fixed width because the neurons run from 32px to 96px:
+ * the same ring in pixels would swallow the smallest and read as a hairline on
+ * the largest, and it is the proportion that makes a circle look like the mark.
+ */
+const RING = 0.23
+
 function BrainNodeComponent({ data }: NodeProps) {
-  const { label, kind, members, flows, size } = data as BrainNodeData
+  const { label, kind, members, flows, size, issue } = data as BrainNodeData
   const emits = useGroupEmits(members)
   const failed = useGroupError(members)
+  // One ring for both kinds of trouble: a run that broke and wiring that can
+  // never run are the same answer to "can I trust this neuron right now".
+  const problem = failed || !!issue
   // Brightest right after a publish, then left to decay. Only the peak needs
   // holding: the floor is the resting style, so letting go is the whole decay.
   const [firing, setFiring] = useState(false)
@@ -34,14 +49,16 @@ function BrainNodeComponent({ data }: NodeProps) {
   return (
     <span
       className={cn(
-        "brain-cell relative flex items-center justify-center rounded-full border",
+        "brain-cell relative flex items-center justify-center rounded-full",
         // Nothing seen yet this session is not the same as seen and gone quiet.
         emits === 0 && "brain-idle",
         firing && "brain-hot",
-        failed ? "border-destructive" : "border-border",
+        problem ? "border-destructive" : "border-foreground",
       )}
-      style={{ width: size, height: size }}
-      title={`${label} · ${kind} · ${members.join(", ")}`}
+      // Border-box, so the outer rim stays at `size` and the edges keep landing
+      // on it however thick the ring gets.
+      style={{ width: size, height: size, borderWidth: size * RING }}
+      title={`${label} · ${kind} · ${members.join(", ")}${issue ? ` · ${issue}` : ""}`}
     >
       {/* Remounting on each emit is what restarts the animation. */}
       {emits > 0 ? <span key={emits} className="node-pulse" /> : null}
@@ -63,15 +80,23 @@ function BrainNodeComponent({ data }: NodeProps) {
       </span>
       {/*
        * A name under every circle is what makes the graph unreadable, so only
-       * the neuron under the pointer or the keyboard says what it is — except a
-       * failing one, since colour is never the only carrier of a status.
+       * the neuron under the pointer or the keyboard says what it is — except
+       * one with something wrong, since colour is never the only carrier of a
+       * status. Two words rather than the reason itself: a cycle names every
+       * node in it, which is a sentence no label under a 32px circle can hold,
+       * so the phrase matches what Home says and the full text stays in the
+       * tooltip.
        */}
-      <span className={cn("brain-label", failed && "brain-label-on")}>
+      <span className={cn("brain-label", problem && "brain-label-on")}>
         <span className="max-w-[140px] truncate text-xs font-medium">
           {label}
         </span>
-        {failed ? (
-          <span className="text-xs font-medium text-destructive">failed</span>
+        {/* The label box is only as wide as the circle it hangs under, so two
+            words break onto two lines unless told not to. */}
+        {problem ? (
+          <span className="whitespace-nowrap text-xs font-medium text-destructive">
+            {failed ? "failed" : "cannot run"}
+          </span>
         ) : null}
       </span>
     </span>
diff --git a/frontend/src/components/Flow/BrainView.tsx b/frontend/src/components/Flow/BrainView.tsx
index 8658941..d256c56 100644
--- a/frontend/src/components/Flow/BrainView.tsx
+++ b/frontend/src/components/Flow/BrainView.tsx
@@ -129,6 +129,7 @@ function build(graph: BrainGraph): { nodes: Node[]; edges: Edge[] } {
           members: source.members ?? [],
           flows: source.flows ?? [],
           size: node.size,
+          issue: source.issue,
         } satisfies BrainNodeData,
       }
     }),
diff --git a/frontend/src/components/Flow/flow.css b/frontend/src/components/Flow/flow.css
index 48487ea..148bc59 100644
--- a/frontend/src/components/Flow/flow.css
+++ b/frontend/src/components/Flow/flow.css
@@ -140,41 +140,31 @@
 }
 
 /*
- * A neuron is a filled disc rather than an outline: a `--card` circle on
- * `--background` is a hairline in light and all but nothing in dark.
+ * A neuron is the node of the brand mark: a disc of the identity blue inside a
+ * thick ring, wired to its neighbours. The ring thickness is a share of the
+ * diameter and lives in `BrainNode`, which is where the diameter is known.
  *
- * The fill is neutral, and stays neutral — status still speaks only through the
- * border and the word under it. What its lightness carries is how recently this
- * page saw the neuron publish: brightest just after it fires, decaying to a
- * floor it never drops below, so a graph left open sorts itself into what is
- * busy and what is not. A fresh page has seen nothing either way, so everything
- * starts at `.brain-idle`, halfway, claiming no history it does not have.
+ * The fill is identity and carries no status — that is the ring's job, and the
+ * word under it. What the fill's lightness carries is how recently this page
+ * saw the neuron publish: brightest just after it fires, decaying to a floor it
+ * never drops below, so a graph left open sorts itself into what is busy and
+ * what is not. The floor stays blue enough to still read as the mark. A fresh
+ * page has seen nothing either way, so everything starts at `.brain-idle`,
+ * halfway, claiming no history it does not have.
  */
 .brain-cell {
   cursor: pointer;
-  background-color: color-mix(
-    in srgb,
-    var(--muted-foreground) 28%,
-    transparent
-  );
+  background-color: color-mix(in srgb, var(--primary) 50%, transparent);
   transition: border-color var(--duration-base) var(--ease-standard);
 }
 
 .brain-cell.brain-idle {
-  background-color: color-mix(
-    in srgb,
-    var(--muted-foreground) 42%,
-    transparent
-  );
+  background-color: color-mix(in srgb, var(--primary) 65%, transparent);
 }
 
 /* Snapped into, decayed out of — never the other way round. */
 .brain-cell.brain-hot {
-  background-color: color-mix(
-    in srgb,
-    var(--muted-foreground) 65%,
-    transparent
-  );
+  background-color: color-mix(in srgb, var(--primary) 90%, transparent);
   transition: none;
 }
 
@@ -186,9 +176,15 @@
   }
 }
 
+/*
+ * Outside the ring rather than in it: the ring is what says whether this neuron
+ * can run, and the disc is already `--primary`, so recolouring either to mark
+ * the pointer would either overwrite a status or vanish into the fill.
+ */
 .brain-cell:hover,
 .react-flow__node:focus-visible .brain-cell {
-  border-color: var(--primary);
+  outline: 2px solid var(--ring);
+  outline-offset: 2px;
 }
 
 /*
diff --git a/frontend/src/components/Health/HealthOverview.tsx b/frontend/src/components/Health/HealthOverview.tsx
index 1019414..2088225 100644
--- a/frontend/src/components/Health/HealthOverview.tsx
+++ b/frontend/src/components/Health/HealthOverview.tsx
@@ -117,12 +117,16 @@ export function HealthOverview({
           <Tile
             label="Flows running"
             value={`${summary?.flows.running ?? 0}/${summary?.flows.total ?? 0}`}
+            // Worst first: the engine gave up on a quarantined flow, a flow
+            // validation blocks never started, and a paused one is deliberate.
             note={
               summary?.flows.quarantined
                 ? `${summary.flows.quarantined} quarantined`
-                : summary?.flows.paused
-                  ? `${summary.flows.paused} paused`
-                  : "none paused"
+                : summary?.flows.invalid
+                  ? `${summary.flows.invalid} cannot run`
+                  : summary?.flows.paused
+                    ? `${summary.flows.paused} paused`
+                    : "none paused"
             }
           />
           <Tile