diff --git a/desktop/src/features/onboarding/ui/RuntimeErrorTooltip.tsx b/desktop/src/features/onboarding/ui/RuntimeErrorTooltip.tsx
new file mode 100644
index 0000000000..15708d3087
--- /dev/null
+++ b/desktop/src/features/onboarding/ui/RuntimeErrorTooltip.tsx
@@ -0,0 +1,49 @@
+import { AlertTriangle } from "lucide-react";
+
+import { Tooltip, TooltipContent, TooltipTrigger } from "@/shared/ui/tooltip";
+
+type RuntimeErrorTooltipProps = {
+  className?: string;
+  detail: string;
+  label: string;
+  showIcon?: boolean;
+  testId?: string;
+};
+
+export function RuntimeErrorTooltip({
+  className,
+  detail,
+  label,
+  showIcon = false,
+  testId,
+}: RuntimeErrorTooltipProps) {
+  return (
+    <Tooltip>
+      <TooltipTrigger asChild>
+        <span
+          aria-label={`${label}. ${detail}`}
+          className={className}
+          data-testid={testId}
+          role="status"
+          // biome-ignore lint/a11y/noNoninteractiveTabindex: Focus exposes the error tooltip to keyboard users without adding a nested card action
+          tabIndex={0}
+        >
+          {showIcon ? (
+            <AlertTriangle
+              aria-hidden="true"
+              className="h-3.5 w-3.5 shrink-0"
+            />
+          ) : null}
+          <span className="min-w-0 truncate">{label}</span>
+        </span>
+      </TooltipTrigger>
+      <TooltipContent
+        className="max-w-80 text-left"
+        side="bottom"
+        sideOffset={12}
+      >
+        <span className="leading-4">{detail}</span>
+      </TooltipContent>
+    </Tooltip>
+  );
+}
diff --git a/desktop/src/features/onboarding/ui/SetupStep.tsx b/desktop/src/features/onboarding/ui/SetupStep.tsx
index 9056e6486d..27f4f4bfce 100644
--- a/desktop/src/features/onboarding/ui/SetupStep.tsx
+++ b/desktop/src/features/onboarding/ui/SetupStep.tsx
@@ -23,6 +23,7 @@ import {
   runtimeCanBeSelected,
 } from "./onboardingRuntimeSelection";
 import { ONBOARDING_PRIMARY_CTA_CLASS } from "./OnboardingChrome";
+import { RuntimeErrorTooltip } from "./RuntimeErrorTooltip";
 import { OnboardingFooter } from "./OnboardingFooter";
 import { getRuntimeDisplayLabel, RuntimeIcon } from "./RuntimeIcon";
 import {
@@ -153,6 +154,7 @@ function RuntimeStatus({
       runtime.authStatus.status === "logged_out",
   });
   const connectMutation = useConnectAcpRuntimeMutation();
+  const runtimesQuery = useAcpRuntimesQuery();
   const authMethods = getOnboardingAuthMethods(
     runtime,
     methodsQuery.data?.methods ?? [],
@@ -197,14 +199,18 @@ function RuntimeStatus({
           SET UP
         </Button>
         {methodsQuery.error instanceof Error ? (
-          <span className="text-2xs text-destructive">
-            Couldn’t load sign-in options.
-          </span>
+          <RuntimeErrorTooltip
+            className="absolute inset-x-3 bottom-2 truncate text-xs leading-4 text-destructive"
+            detail="Couldn’t load sign-in options."
+            label="Sign-in unavailable"
+          />
         ) : null}
         {connectMutation.error instanceof Error ? (
-          <span className="text-2xs text-destructive">
-            {connectMutation.error.message}
-          </span>
+          <RuntimeErrorTooltip
+            className="absolute inset-x-3 bottom-2 truncate text-xs leading-4 text-destructive"
+            detail="Couldn’t start sign-in. Try again."
+            label="Sign-in failed"
+          />
         ) : null}
       </div>
     );
@@ -223,11 +229,44 @@ function RuntimeStatus({
     );
   }
 
-  if (installError) {
+  if (
+    runtime.availability === "available" &&
+    runtime.authStatus.status === "unknown"
+  ) {
     return (
-      <div className="flex h-8 w-8 items-center justify-center">
-        <AlertTriangle className="h-4 w-4 text-destructive" />
-      </div>
+      <Button
+        aria-label={`Check ${runtime.label} again`}
+        className="buzz-onboarding-runtime-setup h-5 rounded-full bg-[var(--buzz-welcome-chartreuse)]/30 px-2.5 font-mono !text-badge font-normal uppercase text-foreground hover:bg-[var(--buzz-welcome-chartreuse)]/40"
+        disabled={runtimesQuery.isFetching}
+        onClick={(event) => {
+          event.stopPropagation();
+          void runtimesQuery.refetch();
+        }}
+        type="button"
+        variant="ghost"
+      >
+        {runtimesQuery.isFetching ? "CHECKING…" : "CHECK AGAIN"}
+      </Button>
+    );
+  }
+
+  if (installError && runtime.canAutoInstall) {
+    return (
+      <Button
+        aria-label={`Retry ${runtime.label} setup`}
+        className="buzz-onboarding-runtime-setup h-5 rounded-full bg-[var(--buzz-welcome-chartreuse)]/30 px-2.5 font-mono !text-badge font-normal uppercase text-foreground hover:bg-[var(--buzz-welcome-chartreuse)]/40"
+        data-setup-flash={isSetupFlashing ? "true" : undefined}
+        data-testid={`onboarding-runtime-install-${runtime.id}`}
+        onClick={(event) => {
+          event.stopPropagation();
+          onSelect();
+          onInstall();
+        }}
+        type="button"
+        variant="ghost"
+      >
+        SET UP
+      </Button>
     );
   }
 
@@ -402,7 +441,7 @@ function runtimeDetailText(runtime: AcpRuntimeCatalogEntry): string {
   if (runtime.availability === "cli_missing") {
     return "ACP adapter detected; CLI missing.";
   }
-  return "Not installed yet.";
+  return "";
 }
 
 function isSupportedOnboardingAuthMethod(
@@ -452,36 +491,26 @@ function getOnboardingAuthMethods(
   return supported;
 }
 
-function RuntimeAuthActions({ runtime }: { runtime: AcpRuntimeCatalogEntry }) {
-  const runtimesQuery = useAcpRuntimesQuery();
-
+function RuntimeAuthError({ runtime }: { runtime: AcpRuntimeCatalogEntry }) {
   if (runtime.authStatus.status === "config_invalid") {
     return (
-      <p className="mt-2 text-2xs leading-4 text-destructive">
-        {runtime.authStatus.diagnostic}
-      </p>
+      <RuntimeErrorTooltip
+        className="absolute inset-x-3 bottom-2 truncate text-xs leading-4 text-destructive"
+        detail="Check this runtime’s configuration and try again."
+        label="Configuration invalid"
+      />
     );
   }
-  if (runtime.authStatus.status === "unknown") {
+  if (
+    runtime.availability === "available" &&
+    runtime.authStatus.status === "unknown"
+  ) {
     return (
-      <div className="mt-2 flex flex-col items-center gap-1.5">
-        <span className="text-2xs text-muted-foreground">
-          Couldn’t verify authentication.
-        </span>
-        <Button
-          disabled={runtimesQuery.isFetching}
-          className="h-6 rounded-full px-2 text-2xs"
-          onClick={(event) => {
-            event.stopPropagation();
-            void runtimesQuery.refetch();
-          }}
-          size="sm"
-          type="button"
-          variant="ghost"
-        >
-          {runtimesQuery.isFetching ? "Checking…" : "Check again"}
-        </Button>
-      </div>
+      <RuntimeErrorTooltip
+        className="absolute inset-x-3 bottom-2 truncate text-xs leading-4 text-destructive"
+        detail="Couldn’t verify authentication."
+        label="Status unavailable"
+      />
     );
   }
   return null;
@@ -552,18 +581,29 @@ function RuntimeCard({
           runtime={runtime}
           setupFlashToken={setupFlashToken}
         />
-        {!isAvailable && !installError ? (
-          <p className="max-w-[13rem] text-2xs leading-4 text-muted-foreground">
+        {!isAvailable && runtimeDetailText(runtime) ? (
+          <p
+            aria-hidden={installError ? "true" : undefined}
+            className={cn(
+              "max-w-[13rem] text-2xs leading-4 text-muted-foreground",
+              installError && "invisible",
+            )}
+          >
             {runtimeDetailText(runtime)}
           </p>
         ) : null}
-        {installError ? (
-          <p className="max-w-[13rem] text-2xs leading-4 text-destructive">
-            {installError}
-          </p>
-        ) : null}
-        <RuntimeAuthActions runtime={runtime} />
       </div>
+      {installError ? (
+        <RuntimeErrorTooltip
+          className="absolute inset-x-3 bottom-2 flex min-w-0 items-center justify-center gap-1.5 overflow-hidden whitespace-nowrap text-xs leading-4 text-destructive"
+          detail="Setup couldn’t be completed. Try again."
+          label="Setup failed"
+          showIcon
+          testId={`onboarding-runtime-error-${runtime.id}`}
+        />
+      ) : (
+        <RuntimeAuthError runtime={runtime} />
+      )}
     </div>
   );
 }
diff --git a/desktop/src/testing/e2eBridge.ts b/desktop/src/testing/e2eBridge.ts
index de9190f284..83c84a40c7 100644
--- a/desktop/src/testing/e2eBridge.ts
+++ b/desktop/src/testing/e2eBridge.ts
@@ -128,6 +128,7 @@ type E2eConfig = {
     acpRuntimesCatalog?: RawAcpRuntimeCatalogEntry[];
     acpRuntimesDelayMs?: number;
     acpAuthMethods?: Record<string, RawAcpAuthMethodsResult>;
+    acpAuthMethodsError?: string;
     connectAcpRuntimeResult?: RawConnectAcpRuntimeResult;
     connectAcpRuntimeDelayMs?: number;
     connectAcpRuntimeError?: string;
@@ -6699,6 +6700,10 @@ async function handleDiscoverAcpAuthMethods(
   args: { runtimeId?: string },
   config: E2eConfig | undefined,
 ): Promise<RawAcpAuthMethodsResult> {
+  const error = config?.mock?.acpAuthMethodsError;
+  if (error) {
+    throw new Error(error);
+  }
   const runtimeId = args.runtimeId ?? "";
   const configured = config?.mock?.acpAuthMethods?.[runtimeId];
   if (configured) {
diff --git a/desktop/tests/e2e/onboarding-agent-defaults.spec.ts b/desktop/tests/e2e/onboarding-agent-defaults.spec.ts
index 7c2f3aabf9..6159fd64f3 100644
--- a/desktop/tests/e2e/onboarding-agent-defaults.spec.ts
+++ b/desktop/tests/e2e/onboarding-agent-defaults.spec.ts
@@ -636,13 +636,35 @@ for (const authStatus of [
       await setupButton.click();
       await expect(card).toHaveAttribute("aria-checked", "true");
     } else if (authStatus.status === "unknown") {
+      const error = card.getByRole("status", {
+        name: /Status unavailable/,
+      });
+      await expect(error).toBeVisible();
+      await expect(error).toHaveCSS("font-size", "12px");
+      await expect(error).toHaveCSS("position", "absolute");
+      await error.hover();
+      await expect(page.getByRole("tooltip")).toHaveText(
+        "Couldn’t verify authentication.",
+      );
       await expect(
-        card.getByText("Couldn’t verify authentication"),
-      ).toBeVisible();
+        card.getByRole("button", { name: "Check Claude again" }),
+      ).toHaveText("CHECK AGAIN");
       await card.click();
       await expect(card).toHaveAttribute("aria-checked", "true");
     } else {
-      await expect(card.getByText("Fix Claude config")).toBeVisible();
+      const error = card.getByRole("status", {
+        name: /Configuration invalid/,
+      });
+      await expect(error).toBeVisible();
+      await expect(error).toHaveCSS("font-size", "12px");
+      await expect(error).toHaveCSS("position", "absolute");
+      await error.hover();
+      await expect(page.getByRole("tooltip")).toHaveText(
+        "Check this runtime’s configuration and try again.",
+      );
+      await expect(page.getByRole("tooltip")).not.toContainText(
+        "Fix Claude config",
+      );
       await card.click();
       await expect(card).toHaveAttribute("aria-checked", "true");
     }
@@ -678,6 +700,199 @@ test("setup cards only show checks after user selection", async ({ page }) => {
   ).toHaveText("INSTALLED");
 });
 
+test("unavailable sign-in options use the compact error and tooltip pattern", async ({
+  page,
+}) => {
+  const discoveryError = "Auth discovery failed with sensitive details";
+  await installMockBridge(
+    page,
+    {
+      acpRuntimesCatalog: [
+        availableRuntime("claude", { status: "logged_out" }),
+      ],
+      acpAuthMethodsError: discoveryError,
+    },
+    { skipCommunitySeed: true, skipOnboardingSeed: true },
+  );
+  await page.goto("/");
+  await navigateToSetupPage(page);
+
+  const card = page.getByTestId("onboarding-runtime-claude");
+  const setupButton = card.getByTestId(
+    "onboarding-runtime-instructions-claude",
+  );
+  const error = card.getByRole("status", { name: /Sign-in unavailable/ });
+  await expect(error).toBeVisible();
+  await expect(error).toHaveCSS("font-size", "12px");
+  await expect(error).toHaveCSS("position", "absolute");
+  await expect(error).toHaveCSS("white-space", "nowrap");
+  await expect(error).not.toHaveAttribute("title");
+  await error.hover();
+  await expect(page.getByRole("tooltip")).toHaveText(
+    "Couldn’t load sign-in options.",
+  );
+  await expect(page.getByRole("tooltip")).not.toContainText(discoveryError);
+  await expect(setupButton).toBeVisible();
+  await expect(setupButton).toHaveText("SET UP");
+});
+
+test("failed sign-in uses the compact error and tooltip pattern", async ({
+  page,
+}) => {
+  const connectionError = "Terminal launch failed with sensitive details";
+  await installMockBridge(
+    page,
+    {
+      acpRuntimesCatalog: [
+        availableRuntime("claude", { status: "logged_out" }),
+      ],
+      acpAuthMethods: {
+        claude: {
+          methods: [
+            {
+              id: "login",
+              name: "Sign in",
+              description: null,
+              type: "terminal",
+            },
+          ],
+        },
+      },
+      connectAcpRuntimeError: connectionError,
+    },
+    { skipCommunitySeed: true, skipOnboardingSeed: true },
+  );
+  await page.goto("/");
+  await navigateToSetupPage(page);
+
+  const card = page.getByTestId("onboarding-runtime-claude");
+  const setupButton = card.getByTestId(
+    "onboarding-runtime-instructions-claude",
+  );
+  const heading = card.getByRole("heading", { name: "Claude" });
+  const headingTopBefore = await heading.evaluate(
+    (element) => element.getBoundingClientRect().top,
+  );
+  const setupTopBefore = await setupButton.evaluate(
+    (element) => element.getBoundingClientRect().top,
+  );
+
+  await setupButton.click();
+
+  const error = card.getByRole("status", { name: /Sign-in failed/ });
+  await expect(error).toBeVisible();
+  await expect(error).toHaveCSS("font-size", "12px");
+  await expect(error).toHaveCSS("position", "absolute");
+  await expect(error).toHaveCSS("white-space", "nowrap");
+  await expect(error).not.toHaveAttribute("title");
+  await error.hover();
+  await expect(page.getByRole("tooltip")).toHaveText(
+    "Couldn’t start sign-in. Try again.",
+  );
+  await expect(page.getByRole("tooltip")).not.toContainText(connectionError);
+  await page.keyboard.press("Escape");
+  await expect(page.getByRole("tooltip")).toBeHidden();
+  await error.focus();
+  const tooltip = page.getByRole("tooltip");
+  await expect(tooltip).toHaveText("Couldn’t start sign-in. Try again.");
+  await expect(error).toHaveAccessibleName(
+    "Sign-in failed. Couldn’t start sign-in. Try again.",
+  );
+  const [cardBox, setupBox, tooltipBox] = await Promise.all([
+    card.boundingBox(),
+    setupButton.boundingBox(),
+    tooltip.boundingBox(),
+  ]);
+  expect(cardBox).not.toBeNull();
+  expect(setupBox).not.toBeNull();
+  expect(tooltipBox).not.toBeNull();
+  expect(tooltipBox?.y).toBeGreaterThanOrEqual(
+    (cardBox?.y ?? 0) + (cardBox?.height ?? 0),
+  );
+  expect(tooltipBox?.y).toBeGreaterThanOrEqual(
+    (setupBox?.y ?? 0) + (setupBox?.height ?? 0),
+  );
+  await expect(setupButton).toBeVisible();
+  expect(
+    await heading.evaluate((element) => element.getBoundingClientRect().top),
+  ).toBe(headingTopBefore);
+  expect(
+    await setupButton.evaluate(
+      (element) => element.getBoundingClientRect().top,
+    ),
+  ).toBe(setupTopBefore);
+});
+
+test("failed install pins a single-line 12px error without moving card content", async ({
+  page,
+}) => {
+  const installError = "Install already in progress with additional details";
+  await installMockBridge(
+    page,
+    {
+      installAcpRuntimeResult: {
+        success: false,
+        steps: [
+          {
+            step: "Install adapter",
+            command: "npm install adapter",
+            success: false,
+            stdout: "",
+            stderr: installError,
+            exit_code: 1,
+          },
+        ],
+      },
+    },
+    { skipCommunitySeed: true, skipOnboardingSeed: true },
+  );
+  await page.goto("/");
+  await navigateToSetupPage(page);
+
+  const card = page.getByTestId("onboarding-runtime-claude");
+  const setupButton = page.getByTestId("onboarding-runtime-install-claude");
+  const heading = card.getByRole("heading", { name: "Claude" });
+  const headingTopBefore = await heading.evaluate(
+    (element) => element.getBoundingClientRect().top,
+  );
+  const detail = card.getByText("CLI detected; ACP adapter missing.");
+  const detailTopBefore = await detail.evaluate(
+    (element) => element.getBoundingClientRect().top,
+  );
+  const setupTopBefore = await setupButton.evaluate(
+    (element) => element.getBoundingClientRect().top,
+  );
+
+  await setupButton.click();
+
+  const error = page.getByTestId("onboarding-runtime-error-claude");
+  await expect(error).toBeVisible();
+  await expect(error).toHaveText(/Setup failed/);
+  await expect(error).not.toHaveAttribute("title");
+  await expect(setupButton).toBeVisible();
+  await expect(setupButton).toHaveText("SET UP");
+  await expect(error).toHaveCSS("font-size", "12px");
+  await expect(error).toHaveCSS("position", "absolute");
+  await expect(error).toHaveCSS("white-space", "nowrap");
+  await expect(error.locator("span")).toHaveCSS("text-overflow", "ellipsis");
+  await error.hover();
+  await expect(page.getByRole("tooltip")).toHaveText(
+    "Setup couldn’t be completed. Try again.",
+  );
+  await expect(page.getByRole("tooltip")).not.toContainText(installError);
+  expect(
+    await heading.evaluate((element) => element.getBoundingClientRect().top),
+  ).toBe(headingTopBefore);
+  expect(
+    await detail.evaluate((element) => element.getBoundingClientRect().top),
+  ).toBe(detailTopBefore);
+  expect(
+    await setupButton.evaluate(
+      (element) => element.getBoundingClientRect().top,
+    ),
+  ).toBe(setupTopBefore);
+});
+
 test("successful install still waits for refreshed runtime readiness", async ({
   page,
 }) => {
diff --git a/desktop/tests/helpers/bridge.ts b/desktop/tests/helpers/bridge.ts
index 4c91b10ccc..f4fe926817 100644
--- a/desktop/tests/helpers/bridge.ts
+++ b/desktop/tests/helpers/bridge.ts
@@ -130,6 +130,7 @@ type MockBridgeOptions = {
   acpRuntimesCatalog?: Record<string, unknown>[];
   acpRuntimesDelayMs?: number;
   acpAuthMethods?: Record<string, { methods: Record<string, unknown>[] }>;
+  acpAuthMethodsError?: string;
   connectAcpRuntimeResult?: { launched: boolean };
   connectAcpRuntimeDelayMs?: number;
   connectAcpRuntimeError?: string;