diff --git a/app/api/cases/start/route.js b/app/api/cases/start/route.js
index 4ea1ebc..ce5d220 100644
--- a/app/api/cases/start/route.js
+++ b/app/api/cases/start/route.js
@@ -10,6 +10,17 @@ async function post(request) {
       return Response.json(result, { status: 200 });
     }
 
+    if (result.code === "PROVIDER_UNAVAILABLE") {
+      console.error("[api/cases/start] provider unavailable", {
+        error: result.error ?? "Start case failed",
+        providerApiPath: result.providerApiPath,
+      });
+      return Response.json(
+        { success: false, error: "Reasoning service is temporarily unavailable." },
+        { status: 503 },
+      );
+    }
+
     const status =
       result.statusCode === 400
         ? 400
diff --git a/tests/app/api/cases-start-route.test.js b/tests/app/api/cases-start-route.test.js
index 4bac8c8..d36afca 100644
--- a/tests/app/api/cases-start-route.test.js
+++ b/tests/app/api/cases-start-route.test.js
@@ -8,6 +8,10 @@ vi.mock("@/lib/graph/orchestrator.js", () => ({
   startCase: (...args) => mockStartCase(...args),
 }));
 
+vi.mock("@/lib/supabase/api-auth.js", () => ({
+  withAuthenticatedApi: (handler) => handler,
+}));
+
 describe("app/api/cases/start route", () => {
   beforeEach(() => {
     vi.resetModules();
@@ -123,6 +127,33 @@ describe("app/api/cases/start route", () => {
     expect(errorSpy).not.toHaveBeenCalled();
   });
 
+  it("returns a sanitized 503 when the provider is unavailable", async () => {
+    mockStartCase.mockResolvedValue({
+      success: false,
+      code: "PROVIDER_UNAVAILABLE",
+      error: "Ollama /api/generate request timed out after 5 minutes",
+      providerApiPath: "/api/generate",
+      providerExecution: { generateRequestAttempted: true },
+    });
+
+    const { POST } = await import("@/app/api/cases/start/route.js");
+    const response = await POST(
+      new Request("http://localhost/api/cases/start", {
+        method: "POST",
+        body: JSON.stringify({ scenario: "Scenario text" }),
+        headers: { "content-type": "application/json" },
+      }),
+    );
+
+    expect(response.status).toBe(503);
+    const body = await response.json();
+    expect(body).toEqual({
+      success: false,
+      error: "Reasoning service is temporarily unavailable.",
+    });
+    expect(JSON.stringify(body)).not.toMatch(/ollama|generate|timed out/i);
+  });
+
   it("returns provider/internal failures as 5xx without stack traces", async () => {
     const rawResponse = `{"reconstruction":{"observedStates":[{"id":"obs-1"${"x".repeat(2500)}}]}}`;
     mockStartCase.mockResolvedValue({