diff --git a/src/container/ca.ts b/src/container/ca.ts
index 0d526b9..52a0eba 100644
--- a/src/container/ca.ts
+++ b/src/container/ca.ts
@@ -9,12 +9,17 @@ const SYSTEM_CA_PATHS = [
   "/etc/pki/tls/certs/ca-bundle.crt", // RHEL / CentOS / Fedora
 ];
 
+/** Per-user cache dir. XDG_RUNTIME_DIR is per-user on Linux; tmpdir() fallback covers macOS/Windows. */
+function caCacheDir(): string {
+  return process.env.XDG_RUNTIME_DIR ?? tmpdir();
+}
+
 /**
- * Write the proxy CA certificate PEM to a temp file on the host.
+ * Write the proxy CA certificate PEM to a per-user file on the host.
  * Returns the path to the written file.
  */
 export function writeCaCertificate(caCertificate: string): string {
-  const outPath = join(tmpdir(), "onecli-proxy-ca.pem");
+  const outPath = join(caCacheDir(), "onecli-proxy-ca.pem");
   writeFileSync(outPath, caCertificate);
   return outPath;
 }
@@ -28,7 +33,7 @@ export function buildCombinedCaBundle(caCertificate: string): string | null {
     try {
       const sysCa = readFileSync(sysPath, "utf8");
       const combined = sysCa.trimEnd() + "\n" + caCertificate.trimEnd() + "\n";
-      const outPath = join(tmpdir(), "onecli-combined-ca.pem");
+      const outPath = join(caCacheDir(), "onecli-combined-ca.pem");
       writeFileSync(outPath, combined);
       return outPath;
     } catch {
diff --git a/test/container/ca.test.ts b/test/container/ca.test.ts
index 6e5c8b5..8ef8c54 100644
--- a/test/container/ca.test.ts
+++ b/test/container/ca.test.ts
@@ -16,6 +16,18 @@ vi.mock("os", async (importOriginal) => {
   return { ...original, tmpdir: () => isolatedTmp };
 });
 
+// The per-user-isolation fix prefers XDG_RUNTIME_DIR over tmpdir(). Point it at
+// the isolated dir so cert writes are deterministic on hosts where it's set.
+let savedXdgRuntimeDir: string | undefined;
+beforeAll(() => {
+  savedXdgRuntimeDir = process.env.XDG_RUNTIME_DIR;
+  process.env.XDG_RUNTIME_DIR = isolatedTmp;
+});
+afterAll(() => {
+  if (savedXdgRuntimeDir === undefined) delete process.env.XDG_RUNTIME_DIR;
+  else process.env.XDG_RUNTIME_DIR = savedXdgRuntimeDir;
+});
+
 describe("writeCaCertificate", () => {
   it("writes PEM to tmpdir and returns path", () => {
     const path = writeCaCertificate(PROXY_CA);
diff --git a/test/container/client.test.ts b/test/container/client.test.ts
index b65f8c0..7799b87 100644
--- a/test/container/client.test.ts
+++ b/test/container/client.test.ts
@@ -255,7 +255,7 @@ describe("ContainerClient", () => {
         addHostMapping: false,
       });
 
-      const expectedHostPath = join(tmpdir(), "onecli-proxy-ca.pem");
+      const expectedHostPath = join(process.env.XDG_RUNTIME_DIR ?? tmpdir(), "onecli-proxy-ca.pem");
       const mountArg = args.find((a) => a.includes(":") && a.includes("onecli-proxy-ca.pem") && a.endsWith(":ro"));
       expect(mountArg).toBe(
         `${expectedHostPath}:${MOCK_CONFIG.caCertificateContainerPath}:ro`,