diff --git a/test/credential-file.test.mjs b/test/credential-file.test.mjs
new file mode 100644
index 0000000..91dfc2e
--- /dev/null
+++ b/test/credential-file.test.mjs
@@ -0,0 +1,30 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+import fs from 'node:fs/promises';
+import os from 'node:os';
+import path from 'node:path';
+import crypto from 'node:crypto';
+import {spawnSync} from 'node:child_process';
+
+test('supervised SDK reads a credential file and proves instance identity without an environment token', async t => {
+  const directory = await fs.mkdtemp(path.join(os.tmpdir(), 'qx-sdk-credential-'));
+  t.after(() => fs.rm(directory, {recursive:true, force:true}));
+  const token = crypto.randomBytes(32).toString('hex');
+  const filename = path.join(directory, 'process-token');
+  await fs.writeFile(filename, token, {mode:0o600});
+  const env = {...process.env, CAMINO_RUNTIME_AUTH_TOKEN_FILE: filename,
+    CAMINO_RUNTIME_AUTH_REQUIRED:'1', QUIXOS_RUNTIME_INSTANCE_ID:'instance:test'};
+  delete env.CAMINO_RUNTIME_AUTH_TOKEN;
+  const result = spawnSync(process.execPath, ['--input-type=module', '-e', `
+    import {createPackageRuntimeRoutes} from ${JSON.stringify(new URL('../dist/index.js', import.meta.url).href)};
+    createPackageRuntimeRoutes({packageRevisionId:'package:test',exports:{}})({
+      service(_type, implementation) { console.log(JSON.stringify(implementation.handshake({nonce:'challenge'}))); }
+    });
+  `], {env, encoding:'utf8'});
+  assert.equal(result.status, 0, result.stderr);
+  const handshake = JSON.parse(result.stdout);
+  assert.equal(handshake.authenticationProof, crypto.createHmac('sha256', token)
+    .update(JSON.stringify(['challenge','instance:test','package:test'])).digest('hex'));
+  assert.ok(handshake.capabilities.includes('epoch-grants-v1'));
+  assert.ok(!result.stdout.includes(token));
+});