dashboard: a success rate never rounds up to 100.0% while failures stand

admin/src/features/dashboard/UpstreamHealthTable.test.tsx

@@ -149,3 +149,32 @@ test("an empty pool says so instead of drawing a table", () => {
 	expect(screen.getByText("No upstreams configured.")).toBeTruthy();
 	expect(screen.queryByRole("table")).toBeNull();
 });
+
+test("a rate a hair under perfect never rounds up to 100.0% while failures stand", () => {
+	// The real row that produced this: 12,698 attempts, 2 failures, 99.984%.
+	renderTable([
+		entry({
+			period: period({ attempts: 12_698, successes: 12_696, failures: 2, success_rate: 12_696 / 12_698 }),
+		}),
+	]);
+
+	expect(screen.queryByText("100.0%")).toBeNull();
+	expect(screen.getByText("99.9%")).toBeTruthy();
+});
+
+test("a rate a hair above nothing never rounds down to 0.0% while successes stand", () => {
+	renderTable([
+		entry({
+			period: period({ attempts: 12_698, successes: 2, failures: 12_696, success_rate: 2 / 12_698 }),
+		}),
+	]);
+
+	expect(screen.queryByText("0.0%")).toBeNull();
+	expect(screen.getByText("0.1%")).toBeTruthy();
+});
+
+test("a window with no failures at all still reads 100.0%", () => {
+	renderTable([entry({ period: period({ attempts: 500, successes: 500, failures: 0, success_rate: 1 }) })]);
+
+	expect(screen.getByText("100.0%")).toBeTruthy();
+});

admin/src/features/dashboard/UpstreamHealthTable.tsx

@@ -122,9 +122,18 @@ function statusNow(upstream: UpstreamHealthEntry): "Available" | "Backing off" |
 /**
  * `success_rate` is null exactly when the window holds no attempt, and that must
  * not read as perfect reliability — hence the em-dash rather than `100.0%`.
+ *
+ * One decimal place cannot hold 12,696 of 12,698: it rounds to `100.0%`, and the
+ * row then claims perfection beside a failure count of 2. Neither endpoint may
+ * be reached by rounding — only by actually having no failure, or no success.
  */
 function successRate(period: UpstreamPeriodStats): string {
-	return period.success_rate === null ? "—" : `${(period.success_rate * 100).toFixed(1)}%`;
+	if (period.success_rate === null) return "—";
+
+	const rounded = period.success_rate * 100;
+	if (rounded > 99.9 && period.failures > 0) return "99.9%";
+	if (rounded < 0.1 && period.successes > 0) return "0.1%";
+	return `${rounded.toFixed(1)}%`;
 }
 
 /**