diff --git a/plugins/voyage/tests/synthetic/plan-determinism.test.mjs b/plugins/voyage/tests/synthetic/plan-determinism.test.mjs
index 009d579..15938e5 100644
--- a/plugins/voyage/tests/synthetic/plan-determinism.test.mjs
+++ b/plugins/voyage/tests/synthetic/plan-determinism.test.mjs
@@ -61,3 +61,65 @@ test('plan determinism — no duplicate step titles within run', () => {
     );
   }
 });
+
+// --- v4.1 forward-compat block (SC #10) ---
+//
+// Adding the optional frontmatter key `profile_used` (Step 3 OPTIONAL_STRING_KEYS)
+// must not break parsing of EITHER:
+//   - Existing plans WITHOUT profile_used (plan-run-A.md, plan-run-B.md)
+//   - New plans WITH profile_used (profile-plan-run-{economy,premium}-*.md)
+//
+// This is the forward-compat assertion required by Step 19. Extend-in-place
+// keeps the determinism + forward-compat checks colocated.
+
+test('plan determinism — forward-compat: legacy fixtures (no profile_used) parse cleanly', () => {
+  for (const rel of ['tests/synthetic/plan-run-A.md', 'tests/synthetic/plan-run-B.md']) {
+    const text = readFileSync(join(ROOT, rel), 'utf-8');
+    const doc = parseDocument(text);
+    assert.ok(doc.valid, `${rel}: frontmatter parse failed: ${(doc.errors || []).map((e) => e.message).join(', ')}`);
+    assert.equal(
+      doc.parsed.frontmatter.profile_used,
+      undefined,
+      `${rel}: legacy fixture must NOT have profile_used set`,
+    );
+    assert.ok(
+      Array.isArray(doc.parsed.frontmatter.steps),
+      `${rel}: steps array still loads after parser extension`,
+    );
+  }
+});
+
+test('plan determinism — forward-compat: new fixtures with profile_used parse cleanly', () => {
+  const cases = [
+    { rel: 'tests/synthetic/profile-plan-run-economy-1.md', profile: 'economy' },
+    { rel: 'tests/synthetic/profile-plan-run-economy-2.md', profile: 'economy' },
+    { rel: 'tests/synthetic/profile-plan-run-premium-1.md', profile: 'premium' },
+    { rel: 'tests/synthetic/profile-plan-run-premium-2.md', profile: 'premium' },
+  ];
+  for (const { rel, profile } of cases) {
+    const text = readFileSync(join(ROOT, rel), 'utf-8');
+    const doc = parseDocument(text);
+    assert.ok(doc.valid, `${rel}: frontmatter parse failed: ${(doc.errors || []).map((e) => e.message).join(', ')}`);
+    assert.equal(
+      doc.parsed.frontmatter.profile_used,
+      profile,
+      `${rel}: profile_used must be ${profile}`,
+    );
+    assert.ok(
+      Array.isArray(doc.parsed.frontmatter.steps) && doc.parsed.frontmatter.steps.length >= 10,
+      `${rel}: steps array must be non-empty`,
+    );
+  }
+});
+
+test('plan determinism — forward-compat: real v4.1 plan validates with --strict (no PLAN_VERSION_MISMATCH)', async () => {
+  // Sanity check that adding profile_used to manifest-yaml schema doesn't
+  // regress full plan-validator strict-mode behaviour on a real plan that
+  // ships profile_used in step manifests.
+  const realPlan = '.claude/projects/2026-05-08-voyage-v4.1-modellprofiler/plan.md';
+  const { validatePlan } = await import('../../lib/validators/plan-validator.mjs');
+  const result = await validatePlan(join(ROOT, realPlan), { strict: true });
+  assert.equal(result.valid, true, `real plan must validate strict: ${JSON.stringify(result.errors)}`);
+  const versionMismatch = (result.warnings || []).find((w) => w.code === 'PLAN_VERSION_MISMATCH');
+  assert.equal(versionMismatch, undefined, 'real plan must NOT emit PLAN_VERSION_MISMATCH warning');
+});