diff --git a/src/capability-language/file-lock.ts b/src/capability-language/file-lock.ts
index a1419ba..2a27ece 100644
--- a/src/capability-language/file-lock.ts
+++ b/src/capability-language/file-lock.ts
@@ -1,14 +1,19 @@
 import { spawn } from "node:child_process";
 
+export class FileLockBusyError extends Error {}
+
 /** Kernel-owned lock: a crashed coordinator cannot leave a stale ownership file.
  * The persistent file is just an inode; EOF releases the helper's lock. */
-export async function withFileLock<T>(filename: string, work: () => Promise<T>): Promise<T> {
+export async function withFileLock<T>(
+  filename: string,
+  work: () => Promise<T>,
+  { nonblocking = false }: { nonblocking?: boolean } = {},
+): Promise<T> {
   const child = spawn(
     "flock",
     [
       "--exclusive",
-      "--timeout",
-      "120",
+      ...(nonblocking ? ["--nonblock"] : ["--timeout", "120"]),
       "--conflict-exit-code",
       "75",
       filename,
@@ -32,15 +37,17 @@ export async function withFileLock<T>(filename: string, work: () => Promise<T>):
     await new Promise<void>((resolve, reject) => {
       let output = "";
       child.once("error", reject);
-      child.once("exit", (code) =>
+      child.once("exit", (code) => {
         reject(
-          new Error(
-            code === 75
-              ? "Timed out after 120 seconds waiting for another authoring command; inspect that command before retrying"
-              : `Cannot acquire authoring lock: ${diagnostics}`,
-          ),
-        ),
-      );
+          code === 75
+            ? new FileLockBusyError(
+                nonblocking
+                  ? "Another process owns this lock"
+                  : "Timed out after 120 seconds waiting for another authoring command; inspect that command before retrying",
+              )
+            : new Error(`Cannot acquire authoring lock: ${diagnostics}`),
+        );
+      });
       child.stdout.on("data", (chunk) => {
         output += chunk;
         if (output.includes("locked\n")) resolve();