diff --git a/server/src/audit.ts b/server/src/audit.ts
index 2b5357c41..d6965cf85 100644
--- a/server/src/audit.ts
+++ b/server/src/audit.ts
@@ -501,6 +501,13 @@ function encodeCursor(cursor: AuditCursor) {
   return Buffer.from(JSON.stringify(cursor)).toString("base64url");
 }
 
+export class AuditQueryError extends Error {
+  constructor(message: string) {
+    super(message);
+    this.name = "AuditQueryError";
+  }
+}
+
 function decodeCursor(cursor: string): AuditCursor {
   try {
     const parsed = JSON.parse(
@@ -508,11 +515,12 @@ function decodeCursor(cursor: string): AuditCursor {
     ) as AuditCursor;
 
     if (!parsed.id || Number.isNaN(Date.parse(parsed.createdAt))) {
-      throw new Error("invalid cursor");
+      throw new AuditQueryError("cursor must be a valid audit page cursor");
     }
     return parsed;
-  } catch {
-    throw new Error("cursor must be a valid audit page cursor");
+  } catch (error) {
+    if (error instanceof AuditQueryError) throw error;
+    throw new AuditQueryError("cursor must be a valid audit page cursor");
   }
 }
 
@@ -583,13 +591,6 @@ export function createAuditReader(database: Database): AuditReader {
   };
 }
 
-export class AuditQueryError extends Error {
-  constructor(message: string) {
-    super(message);
-    this.name = "AuditQueryError";
-  }
-}
-
 export function auditQueryFromUrl(url: URL): AuditEventQuery {
   const rawLimit = url.searchParams.get("limit") ?? "50";
   const trimmedLimit = rawLimit.trim();
@@ -610,8 +611,19 @@ export function auditQueryFromUrl(url: URL): AuditEventQuery {
     throw new AuditQueryError('Query parameter "to" must be a valid date.');
   }
 
+  /*
+   * Fail fast on a stale or hand-edited bookmark. Without this the raw string travels into
+   * `createAuditReader.list`, where `decodeCursor` threw a generic `Error` that escaped the
+   * route's `AuditQueryError` catch as a 500. A corrupt cursor is a caller error, not a server
+   * failure, and answers 400 like a bad `from`/`to` already does.
+   */
+  const cursor = optional("cursor");
+  if (cursor !== undefined) {
+    decodeCursor(cursor);
+  }
+
   return {
-    cursor: optional("cursor"),
+    cursor,
     limit,
     eventType: optional("eventType"),
     actorUserId: optional("actorUserId"),
diff --git a/server/tests/audit-cursor.test.ts b/server/tests/audit-cursor.test.ts
new file mode 100644
index 000000000..ef58ccf9a
--- /dev/null
+++ b/server/tests/audit-cursor.test.ts
@@ -0,0 +1,93 @@
+import { describe, expect, test } from "bun:test";
+import { createApp } from "../src/app";
+import { AuditQueryError, auditQueryFromUrl } from "../src/audit";
+import { loadConfig } from "../src/config";
+import { testEnvironment } from "./support/environment";
+
+const config = loadConfig({ ...testEnvironment() });
+
+const adminAuth = {
+  handler: () => new Response(null, { status: 204 }),
+  api: {
+    getSession: async () => ({
+      user: { id: "admin", email: "admin@openbot.test" },
+    }),
+  },
+};
+
+function validCursor(): string {
+  return Buffer.from(
+    JSON.stringify({ id: "event-1", createdAt: "2026-08-13T12:00:00.000Z" }),
+  ).toString("base64url");
+}
+
+describe("audit cursor validation", () => {
+  test("a corrupt cursor is a query error, not a server failure", () => {
+    expect(() =>
+      auditQueryFromUrl(
+        new URL("http://openbot.local/api/admin/audit-events?cursor=!!bogus!!"),
+      ),
+    ).toThrow(AuditQueryError);
+    expect(() =>
+      auditQueryFromUrl(
+        new URL(
+          "http://openbot.local/api/admin/audit-events?cursor=bm90LWpzb24=",
+        ),
+      ),
+    ).toThrow(/cursor must be a valid audit page cursor/);
+  });
+
+  test("a well-formed cursor still parses", () => {
+    const query = auditQueryFromUrl(
+      new URL(
+        `http://openbot.local/api/admin/audit-events?cursor=${validCursor()}&limit=10`,
+      ),
+    );
+    expect(query.cursor).toBe(validCursor());
+    expect(query.limit).toBe(10);
+  });
+
+  test("the admin route answers 400 for a corrupt cursor instead of 500", async () => {
+    const app = createApp(
+      config,
+      adminAuth,
+      { rolesForUser: async () => ["admin"] },
+      {
+        list: async () => {
+          throw new Error("must not reach the store with a bad cursor");
+        },
+      },
+    );
+
+    const response = await app.request(
+      "http://openbot.local/api/admin/audit-events?cursor=!!bogus!!",
+    );
+
+    expect(response.status).toBe(400);
+    await expect(response.json()).resolves.toEqual({
+      error: "cursor must be a valid audit page cursor",
+    });
+  });
+
+  test("the admin route still pages with a valid cursor", async () => {
+    const queries: unknown[] = [];
+    const app = createApp(
+      config,
+      adminAuth,
+      { rolesForUser: async () => ["admin"] },
+      {
+        list: async (query) => {
+          queries.push(query);
+          return { events: [], nextCursor: undefined };
+        },
+      },
+    );
+
+    const response = await app.request(
+      `http://openbot.local/api/admin/audit-events?cursor=${validCursor()}`,
+    );
+
+    expect(response.status).toBe(200);
+    expect(queries).toHaveLength(1);
+  });
+});