diff --git a/packages/paperclip-runner/src/drivers/codex/codex-harness-session.ts b/packages/paperclip-runner/src/drivers/codex/codex-harness-session.ts
index fdde2d14d90..6bbbe3e7585 100644
--- a/packages/paperclip-runner/src/drivers/codex/codex-harness-session.ts
+++ b/packages/paperclip-runner/src/drivers/codex/codex-harness-session.ts
@@ -886,9 +886,11 @@ export class CodexHarnessSession
     };
   }
 
-  async close(input?: { reason: string }): Promise<void> {
+  // `force` is part of the contract. Codex closes the same way with or without it.
+  async close(input: { reason: string; force?: boolean }): Promise<void> {
     this.cancelPendingRequests("session_closed");
     this.eventQueue.close();
+    // Untyped callers (for example generated scripts) may still omit the input.
     await this.transport.close(input?.reason);
   }
 
diff --git a/packages/paperclip-runner/src/live/runnerd-codex-transport.ts b/packages/paperclip-runner/src/live/runnerd-codex-transport.ts
index 2f19d3aa461..503ef9cf878 100644
--- a/packages/paperclip-runner/src/live/runnerd-codex-transport.ts
+++ b/packages/paperclip-runner/src/live/runnerd-codex-transport.ts
@@ -2104,7 +2104,8 @@ export function readRunnerdArtifactBinding(runnerBinaryPath: string): {
 export interface RetainedRunnerdCleanupProof {
   readonly binding: Readonly<NativeRunIdentity>;
   readonly identity: Readonly<DurableRecoveryIdentity>;
-  readonly backend: Readonly<{ kind: string; name: string }>;
+  /** Must match the quarantine domain, which every production backend builds with kind "runner". */
+  readonly backend: Readonly<{ kind: "runner"; name: string }>;
   readonly providerSessionId: string;
   readonly activationDirectory: string;
   readonly sourceFingerprint: string;
@@ -2400,7 +2401,7 @@ export interface RetainedRunnerdMaintenanceInput {
   requestId: string;
   binding: NativeRunIdentity;
   identity: DurableRecoveryIdentity;
-  backend: { kind: string; name: string };
+  backend: { kind: "runner"; name: string };
   stateDirectory: string;
   activationDirectory: string;
   sourceFingerprint: string;
diff --git a/packages/paperclip-runner/src/live/runnerd-contract-types.test.ts b/packages/paperclip-runner/src/live/runnerd-contract-types.test.ts
new file mode 100644
index 00000000000..09f52dacba0
--- /dev/null
+++ b/packages/paperclip-runner/src/live/runnerd-contract-types.test.ts
@@ -0,0 +1,22 @@
+import { describe, expectTypeOf, it } from "vitest";
+import type { HarnessSession } from "../contracts/harness-driver.js";
+import type { CodexHarnessSession } from "../drivers/codex/codex-harness-session.js";
+import type { RetainedRunnerdCleanupProof, RetainedRunnerdMaintenanceInput } from "./runnerd-codex-transport.js";
+
+// The Runner test typecheck checks these assertions. Vitest only runs them.
+describe("Runner contract types", () => {
+  it("accepts only the runner backend kind for retained cleanup", () => {
+    // The quarantine domain uses the descriptor kind, which is "runner" for every
+    // production backend. Another kind would release nothing.
+    expectTypeOf<RetainedRunnerdMaintenanceInput["backend"]["kind"]>().toEqualTypeOf<"runner">();
+    expectTypeOf<RetainedRunnerdCleanupProof["backend"]["kind"]>().toEqualTypeOf<"runner">();
+
+    // @ts-expect-error: a backend kind other than "runner" is not allowed.
+    const remote: RetainedRunnerdMaintenanceInput["backend"] = { kind: "remote", name: "codex_app_server" };
+    void remote;
+  });
+
+  it("matches the HarnessSession close contract in the Codex session", () => {
+    expectTypeOf<Parameters<CodexHarnessSession["close"]>>().toEqualTypeOf<Parameters<HarnessSession["close"]>>();
+  });
+});