diff --git a/server/src/__tests__/environment-custom-images-service.test.ts b/server/src/__tests__/environment-custom-images-service.test.ts
index 883e4c7f32d..ae0e10dd8bf 100644
--- a/server/src/__tests__/environment-custom-images-service.test.ts
+++ b/server/src/__tests__/environment-custom-images-service.test.ts
@@ -419,6 +419,46 @@ describeEmbeddedPostgres("environmentCustomImageService", () => {
     })).rejects.toThrow("Setup template must be the active template");
   });
 
+  it("turns missing provider template setup failures into conflicts", async () => {
+    const { environmentId } = await seed();
+    const workerManager = createWorkerManager();
+    workerManager.call.mockImplementation(async (_pluginId, method) => {
+      if (method === "environmentStartInteractiveSetup") {
+        throw new Error("Snapshot not found");
+      }
+      throw new Error(`Unexpected plugin call: ${method}`);
+    });
+    const service = environmentCustomImageService(db, { pluginWorkerManager: workerManager });
+    const [template] = await db.insert(environmentCustomImageTemplates).values({
+      environmentId,
+      provider: "fake-plugin",
+      templateKind: "snapshot",
+      templateRef: "snapshot-missing",
+      status: "active",
+    }).returning();
+
+    await expect(service.startSetupSession({
+      environmentId,
+      templateId: template!.id,
+      actor: { userId: "user-1" },
+    })).rejects.toMatchObject({
+      status: 409,
+      message: expect.stringContaining("customImage template no longer exists"),
+    });
+
+    const [session] = await db
+      .select({
+        status: environmentCustomImageSetupSessions.status,
+        failureReason: environmentCustomImageSetupSessions.failureReason,
+      })
+      .from(environmentCustomImageSetupSessions)
+      .where(eq(environmentCustomImageSetupSessions.templateId, template!.id));
+    expect(session).toEqual({
+      status: "failed",
+      failureReason: "Selected provider template is missing.",
+    });
+  });
+
   it("applies the active template regardless of base-config changes and falls back when none exists", async () => {
     const { companyId, environmentId } = await seed();
     const environment = await db.select().from(environments).where(eq(environments.id, environmentId)).then((rows) => rows[0]!);
diff --git a/server/src/services/environment-custom-images.ts b/server/src/services/environment-custom-images.ts
index 1cef716cf5f..f27bf8a1ffb 100644
--- a/server/src/services/environment-custom-images.ts
+++ b/server/src/services/environment-custom-images.ts
@@ -185,6 +185,24 @@ function isActiveSetupStatus(status: EnvironmentCustomImageSetupSessionStatus):
   return (ACTIVE_SETUP_STATUSES as readonly string[]).includes(status);
 }
 
+function providerErrorMessage(error: unknown): string {
+  return error instanceof Error ? error.message : String(error);
+}
+
+function isProviderMissingTemplateError(error: unknown): boolean {
+  const message = providerErrorMessage(error).toLowerCase();
+  return (
+    (message.includes("snapshot") || message.includes("template") || message.includes("image"))
+    && (message.includes("not found") || message.includes("missing"))
+  );
+}
+
+function missingProviderTemplateConflict() {
+  return conflict(
+    "The selected environment customImage template no longer exists in the sandbox provider. Disable this custom image or roll back to a previous template, then start image setup again.",
+  );
+}
+
 function templateConfigBindingFromDriver(input: {
   templateRefKind?: string | null | undefined;
   templateConfigBinding?: unknown;
@@ -700,11 +718,17 @@ export function environmentCustomImageService(
           connectionPayload: providerSession.connectionPayload ?? null,
         };
       } catch (error) {
+        const failureReason = selectedTemplate && isProviderMissingTemplateError(error)
+          ? "Selected provider template is missing."
+          : providerErrorMessage(error);
         await markSessionStatus({
           sessionId,
           status: "failed",
-          failureReason: error instanceof Error ? error.message : String(error),
+          failureReason,
         });
+        if (selectedTemplate && isProviderMissingTemplateError(error)) {
+          throw missingProviderTemplateConflict();
+        }
         throw error;
       }
     },
diff --git a/ui/src/api/environments.ts b/ui/src/api/environments.ts
index ab3cf09aaa5..c8f620e03c7 100644
--- a/ui/src/api/environments.ts
+++ b/ui/src/api/environments.ts
@@ -65,7 +65,10 @@ export const environmentsApi = {
     config?: Record<string, unknown>;
     metadata?: Record<string, unknown> | null;
   }) => api.patch<Environment>(`/environments/${environmentId}`, body),
-  probe: (environmentId: string) => api.post<EnvironmentProbeResult>(`/environments/${environmentId}/probe`, {}),
+  probe: (environmentId: string, companyId?: string | null) => {
+    const query = companyId ? `?companyId=${encodeURIComponent(companyId)}` : "";
+    return api.post<EnvironmentProbeResult>(`/environments/${environmentId}/probe${query}`, {});
+  },
   probeConfig: (companyId: string, body: {
     name?: string;
     driver: "local" | "ssh" | "sandbox" | "plugin";
diff --git a/ui/src/pages/CompanyEnvironments.test.tsx b/ui/src/pages/CompanyEnvironments.test.tsx
index c787bea861d..4b576c9c522 100644
--- a/ui/src/pages/CompanyEnvironments.test.tsx
+++ b/ui/src/pages/CompanyEnvironments.test.tsx
@@ -485,7 +485,50 @@ describe("CompanyEnvironments — test provider button", () => {
     expect(buttonsAfter[0].disabled).toBe(true);
     expect(buttonsAfter[1].textContent?.trim()).toBe("Test provider");
     expect(buttonsAfter[1].disabled).toBe(false);
-    expect(mockEnvironmentsApi.probe).toHaveBeenCalledExactlyOnceWith("env-1");
+    expect(mockEnvironmentsApi.probe).toHaveBeenCalledExactlyOnceWith("env-1", "company-1");
+  });
+
+  it("explains that successful sandbox provider tests use a temporary sandbox", async () => {
+    root = createRoot(container);
+    const queryClient = new QueryClient({ defaultOptions: { queries: { retry: false } } });
+    mockEnvironmentsApi.list.mockResolvedValue([
+      { id: "env-1", name: "Daytona", driver: "sandbox", description: null, config: { provider: "daytona" } },
+    ]);
+    mockEnvironmentsApi.capabilities.mockResolvedValue(supportedDaytonaCapabilities());
+    mockEnvironmentsApi.probe.mockResolvedValue({
+      ok: true,
+      driver: "sandbox",
+      summary: "Connected to Daytona sandbox paperclip-probe.",
+      details: {
+        provider: "daytona",
+        diagnostics: [],
+        metadata: {
+          provider: "daytona",
+          sandboxId: "473167E9",
+          sandboxName: "paperclip-probe",
+        },
+      },
+    });
+
+    await act(async () => {
+      root!.render(
+        <QueryClientProvider client={queryClient}>
+          <TooltipProvider>
+            <CompanyEnvironments />
+          </TooltipProvider>
+        </QueryClientProvider>,
+      );
+    });
+    await flushReact();
+
+    await act(async () => {
+      testProviderButtons(container)[0].dispatchEvent(new MouseEvent("click", { bubbles: true }));
+    });
+    await flushReact();
+
+    expect(mockEnvironmentsApi.probe).toHaveBeenCalledExactlyOnceWith("env-1", "company-1");
+    expect(container.textContent).toContain("Verified temporary daytona sandbox paperclip-probe (473167E9).");
+    expect(container.textContent).toContain("Test probes clean up the validation sandbox after the check");
   });
 
   it("keeps the second environment's testing state when an earlier probe settles", async () => {
diff --git a/ui/src/pages/CompanyEnvironments.tsx b/ui/src/pages/CompanyEnvironments.tsx
index 8b0d3c68806..6229d9b2820 100644
--- a/ui/src/pages/CompanyEnvironments.tsx
+++ b/ui/src/pages/CompanyEnvironments.tsx
@@ -199,6 +199,36 @@ function readEnvironmentSandboxProvider(environment: Environment): string | null
     : null;
 }
 
+function readRecord(value: unknown): Record<string, unknown> | null {
+  return value && typeof value === "object" && !Array.isArray(value)
+    ? value as Record<string, unknown>
+    : null;
+}
+
+function readProbeMetadata(probe: EnvironmentProbeResult): Record<string, unknown> {
+  const details = readRecord(probe.details);
+  return readRecord(details?.metadata) ?? {};
+}
+
+function readStringValue(value: unknown): string | null {
+  return typeof value === "string" && value.trim().length > 0 ? value.trim() : null;
+}
+
+function sandboxProbeLifecycleNote(probe: EnvironmentProbeResult): string | null {
+  if (probe.driver !== "sandbox") return null;
+  const metadata = readProbeMetadata(probe);
+  const provider = readStringValue(metadata.provider) ?? readStringValue(readRecord(probe.details)?.provider) ?? "sandbox";
+  const sandboxName = readStringValue(metadata.sandboxName);
+  const sandboxId = readStringValue(metadata.sandboxId);
+  const sandboxLabel = sandboxName && sandboxId
+    ? `${sandboxName} (${sandboxId})`
+    : sandboxName ?? sandboxId;
+  const base = sandboxLabel
+    ? `Verified temporary ${provider} sandbox ${sandboxLabel}.`
+    : `Verified a temporary ${provider} sandbox.`;
+  return `${base} Test probes clean up the validation sandbox after the check, so it may not remain visible in the provider dashboard.`;
+}
+
 function formatDateTime(value: string | Date | null | undefined): string | null {
   if (!value) return null;
   const date = value instanceof Date ? value : new Date(value);
@@ -1150,7 +1180,7 @@ export function CompanyEnvironments() {
   });
 
   const environmentProbeMutation = useMutation({
-    mutationFn: async (environmentId: string) => await environmentsApi.probe(environmentId),
+    mutationFn: async (environmentId: string) => await environmentsApi.probe(environmentId, selectedCompanyId),
     onMutate: (environmentId) => {
       setTestingEnvironmentId(environmentId);
     },
@@ -1409,6 +1439,7 @@ export function CompanyEnvironments() {
           </div>
           {savedEnvironments.map((environment) => {
             const probe = probeResults[environment.id] ?? null;
+            const sandboxLifecycleNote = probe ? sandboxProbeLifecycleNote(probe) : null;
             const isEditing = editingEnvironmentId === environment.id;
             const sandboxProvider = readEnvironmentSandboxProvider(environment);
             const sandboxProviderCapability = sandboxProvider
@@ -1478,6 +1509,9 @@ export function CompanyEnvironments() {
                     }
                   >
                     <div className="font-medium">{probe.summary}</div>
+                    {sandboxLifecycleNote ? (
+                      <div className="mt-1 text-muted-foreground">{sandboxLifecycleNote}</div>
+                    ) : null}
                     {probe.details?.error && typeof probe.details.error === "string" ? (
                       <div className="mt-1 font-mono text-(length:--text-micro)">{probe.details.error}</div>
                     ) : null}