diff --git a/packages/engine/src/__tests__/_project-engine-merge-lane-fixture.ts b/packages/engine/src/__tests__/_project-engine-merge-lane-fixture.ts
new file mode 100644
index 0000000000..9a1577df88
--- /dev/null
+++ b/packages/engine/src/__tests__/_project-engine-merge-lane-fixture.ts
@@ -0,0 +1,62 @@
+type MergeLaneState = {
+  mergeQueue: string[];
+  mergeActive: Set<string>;
+  capacityDeferredMergeTaskIds: Set<string>;
+  capacityDeferredMergeReasons: Map<string, string>;
+  capacityDeferredMerges: Map<string, unknown>;
+  coordinatorAdmittedMergeTaskIds: Set<string>;
+  pausedReviewTaskIds: Set<string>;
+  mergeRunning: boolean;
+  mergeRunningSince: number;
+  activeMergeSession: { dispose(): void } | null;
+  activeMergeTaskId: string | null;
+  activeMergeStartedAtMs: number | null;
+  mergeBodyInFlight: Promise<unknown> | null;
+  mergeAbortController: AbortController | null;
+  mergeRetryTimer: ReturnType<typeof setTimeout> | null;
+  prMergeRetryTimers: Map<string, ReturnType<typeof setTimeout>>;
+  workspaceBusyReenqueues: Map<string, number>;
+  workspaceBusyReenqueueTimers: Set<ReturnType<typeof setTimeout>>;
+  manualMergeResolvers: Map<string, unknown[]>;
+  shuttingDown: boolean;
+  startupGeneration: number;
+  started: boolean;
+};
+
+/**
+ * FNXC:MergeQueue 2026-08-09-06:22:
+ * Object.create(ProjectEngine.prototype) runs no class field initializers, so a prototype-only
+ * merge fake starts with every merge-lane field undefined. FN-8871 requires this fixture to include
+ * capacity and PR-retry merge state, preventing production drain additions from drifting test fakes.
+ */
+export function seedMergeLaneState<T extends object>(
+  engine: T,
+  overrides: Partial<MergeLaneState> = {},
+): T & MergeLaneState {
+  const defaults: MergeLaneState = {
+    mergeQueue: [],
+    mergeActive: new Set(),
+    capacityDeferredMergeTaskIds: new Set(),
+    capacityDeferredMergeReasons: new Map(),
+    capacityDeferredMerges: new Map(),
+    coordinatorAdmittedMergeTaskIds: new Set(),
+    pausedReviewTaskIds: new Set(),
+    mergeRunning: false,
+    mergeRunningSince: 0,
+    activeMergeSession: null,
+    activeMergeTaskId: null,
+    activeMergeStartedAtMs: null,
+    mergeBodyInFlight: null,
+    mergeAbortController: null,
+    mergeRetryTimer: null,
+    prMergeRetryTimers: new Map(),
+    workspaceBusyReenqueues: new Map(),
+    workspaceBusyReenqueueTimers: new Set(),
+    manualMergeResolvers: new Map(),
+    shuttingDown: false,
+    startupGeneration: 0,
+    started: true,
+  };
+
+  return Object.assign(engine, defaults, overrides);
+}
diff --git a/packages/engine/src/__tests__/group-merge-coordinator.test.ts b/packages/engine/src/__tests__/group-merge-coordinator.test.ts
index e4828e8c75..b0fca6872f 100644
--- a/packages/engine/src/__tests__/group-merge-coordinator.test.ts
+++ b/packages/engine/src/__tests__/group-merge-coordinator.test.ts
@@ -26,6 +26,7 @@ import {
 } from "../merge/group-merge-coordinator.js";
 import { ProjectEngine } from "../project-engine.js";
 import { runAiMerge } from "../merge/merger-ai.js";
+import { seedMergeLaneState } from "./_project-engine-merge-lane-fixture.js";
 
 const dirs: string[] = [];
 
@@ -1490,14 +1491,12 @@ describe("resolveBranchGroupMergeRouting", () => {
         getSessionStats: vi.fn(() => ({ tokens: { input: 1, output: 1 } })),
       },
     }));
-    engine.manualMergeResolvers = new Map();
-    engine.mergeActive = new Set();
-    engine.mergeQueue = [];
-    engine.capacityDeferredMergeTaskIds = new Set();
-    engine.capacityDeferredMerges = new Map();
-    engine.coordinatorAdmittedMergeTaskIds = new Set();
-    engine.started = true;
-    engine.shuttingDown = false;
+    /*
+    FNXC:SharedBranchMemberHold 2026-08-09-05:22:
+    FN-8811's release half runs the production drain, so this prototype fake must carry complete
+    merge-lane state rather than only the fields the test happened to seed when it was written.
+    */
+    seedMergeLaneState(engine);
 
     const released = await ProjectEngine.prototype.onMerge.call(engine, "FN-3324");
     createResolvedAgentSessionMock.mockReset();