diff --git a/src/resources/extensions/gsd/tests/worker-model-override.test.ts b/src/resources/extensions/gsd/tests/worker-model-override.test.ts
new file mode 100644
index 000000000..0b1e49edf
--- /dev/null
+++ b/src/resources/extensions/gsd/tests/worker-model-override.test.ts
@@ -0,0 +1,48 @@
+/**
+ * Worker model override — tests for parallel.worker_model preference.
+ *
+ * Verifies validation, resolveParallelConfig pass-through, and type definitions.
+ */
+
+import test from "node:test";
+import assert from "node:assert/strict";
+import { readFileSync } from "node:fs";
+import { join, dirname } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+
+const typesSrc = readFileSync(join(__dirname, "..", "types.ts"), "utf-8");
+const validationSrc = readFileSync(join(__dirname, "..", "preferences-validation.ts"), "utf-8");
+const preferencesSrc = readFileSync(join(__dirname, "..", "preferences.ts"), "utf-8");
+
+// ─── Type definition ──────────────────────────────────────────────────────
+
+test("ParallelConfig includes worker_model optional field", () => {
+  assert.ok(
+    typesSrc.includes("worker_model?: string"),
+    "ParallelConfig should have optional worker_model field",
+  );
+});
+
+// ─── Validation ───────────────────────────────────────────────────────────
+
+test("validatePreferences accepts valid worker_model string", () => {
+  assert.ok(
+    validationSrc.includes("p.worker_model"),
+    "validation should check parallel.worker_model",
+  );
+  assert.ok(
+    validationSrc.includes('parallel.worker_model must be a non-empty string'),
+    "validation should reject invalid worker_model",
+  );
+});
+
+// ─── resolveParallelConfig ────────────────────────────────────────────────
+
+test("resolveParallelConfig passes through worker_model", () => {
+  assert.ok(
+    preferencesSrc.includes("worker_model: prefs?.parallel?.worker_model"),
+    "resolveParallelConfig should pass through worker_model",
+  );
+});