diff --git a/packages/engine/src/__tests__/project-engine-merge-lane-fixture-drift.test.ts b/packages/engine/src/__tests__/project-engine-merge-lane-fixture-drift.test.ts
new file mode 100644
index 0000000000..5b176d8aa4
--- /dev/null
+++ b/packages/engine/src/__tests__/project-engine-merge-lane-fixture-drift.test.ts
@@ -0,0 +1,55 @@
+import { readFileSync } from "node:fs";
+import { describe, expect, it } from "vitest";
+import { seedMergeLaneState } from "./_project-engine-merge-lane-fixture.js";
+
+/**
+ * FNXC:MergeQueue 2026-08-09-08:06:
+ * Object.create(ProjectEngine.prototype) runs no class field initializers, so every merge-lane
+ * field a prototype fake needs must be registered in seedMergeLaneState. FN-8871
+ * (capacityDeferredMergeReasons) and FN-8882 (prMergeRetryTimers) each reached production as an
+ * operator-reported TypeError before this ratchet made an unregistered field a fast test failure.
+ */
+const AUTO_MERGE_STATE_MARKER = "// ── Auto-merge state ──";
+const WORKSPACE_BUSY_REENQUEUE_MARKER = "private scheduleWorkspaceBusyReenqueue(";
+
+/** Merge-lane fields deliberately omitted because the prototype merge drain never reads them. */
+const NOT_SEEDED_BY_FIXTURE: Record<string, string> = {
+  unregisterMergeAdmissionProvider: "registration cleanup is used only during engine lifecycle management",
+  autostashSweepTimer: "autostash maintenance is not part of the prototype merge drain",
+  mergeActiveReconcileTimer: "reconciliation maintenance is not part of the prototype merge drain",
+};
+
+function getMergeLaneFields(): string[] {
+  const source = readFileSync(new URL("../project-engine.ts", import.meta.url), "utf8");
+  const start = source.indexOf(AUTO_MERGE_STATE_MARKER);
+  const end = source.indexOf(WORKSPACE_BUSY_REENQUEUE_MARKER, start);
+
+  expect(start, `Missing ${AUTO_MERGE_STATE_MARKER} marker in project-engine.ts`).toBeGreaterThanOrEqual(0);
+  expect(end, `Missing ${WORKSPACE_BUSY_REENQUEUE_MARKER} marker in project-engine.ts`).toBeGreaterThan(start);
+
+  const fields = [...source.slice(start, end).matchAll(/^\s*private\s+(?:readonly\s+)?(\w+)(?=\??!?\s*(?:=|:))/gm)]
+    .map((match) => match[1]);
+
+  expect(fields, "Expected the auto-merge state block to declare instance fields").not.toHaveLength(0);
+  return fields;
+}
+
+describe("ProjectEngine merge-lane fixture drift", () => {
+  it("requires prototype merge fakes to seed every auto-merge state field they can exercise", () => {
+    const mergeLaneFields = getMergeLaneFields();
+    const fixtureKeys = new Set(Object.keys(seedMergeLaneState({} as object)));
+    const allowlistedFields = Object.keys(NOT_SEEDED_BY_FIXTURE);
+
+    for (const field of mergeLaneFields) {
+      expect(
+        fixtureKeys.has(field) || field in NOT_SEEDED_BY_FIXTURE,
+        `ProjectEngine auto-merge field "${field}" is not seeded by _project-engine-merge-lane-fixture.ts. Add its production-equivalent default to seedMergeLaneState or allowlist it with a drain-path reason.`,
+      ).toBe(true);
+    }
+
+    for (const field of allowlistedFields) {
+      expect(mergeLaneFields, `Allowlisted field "${field}" is no longer in the auto-merge state block`).toContain(field);
+      expect(fixtureKeys, `Allowlisted field "${field}" is now seeded; remove it from NOT_SEEDED_BY_FIXTURE`).not.toContain(field);
+    }
+  });
+});