diff --git a/server/src/__tests__/documents-routes.test.ts b/server/src/__tests__/documents-routes.test.ts
index ad7601c5c89..c40c4efefc6 100644
--- a/server/src/__tests__/documents-routes.test.ts
+++ b/server/src/__tests__/documents-routes.test.ts
@@ -158,4 +158,28 @@ describeEmbeddedPostgres("document routes", () => {
       }),
     ]));
   });
+
+  it("returns 400 for a malformed document id instead of a 500 (PAP-10582)", async () => {
+    const companyA = await seedCompany("A");
+    const app = createApp();
+
+    const res = await request(app)
+      .get(`/api/companies/${companyA}/documents/not-a-uuid`)
+      .expect(400);
+    expect(res.body).toEqual({ error: "Invalid document id format" });
+
+    // Other :documentId routes are covered by the same param guard.
+    await request(app)
+      .get(`/api/companies/${companyA}/documents/not-a-uuid/backlinks`)
+      .expect(400);
+    await request(app)
+      .patch(`/api/companies/${companyA}/documents/not-a-uuid`)
+      .send({ status: "archived" })
+      .expect(400);
+
+    // A well-formed but non-existent id still resolves to a 404, not a 400.
+    await request(app)
+      .get(`/api/companies/${companyA}/documents/${randomUUID()}`)
+      .expect(404);
+  });
 });
diff --git a/server/src/routes/documents.ts b/server/src/routes/documents.ts
index 5a40079fcb8..20f439ce534 100644
--- a/server/src/routes/documents.ts
+++ b/server/src/routes/documents.ts
@@ -3,6 +3,7 @@ import type { Db } from "@paperclipai/db";
 import {
   companyDocumentListQuerySchema,
   createDocumentLinkSchema,
+  isUuidLike,
   updateDocumentMetadataSchema,
 } from "@paperclipai/shared";
 import { validate } from "../middleware/validate.js";
@@ -15,6 +16,17 @@ export function documentRoutes(db: Db) {
   const projects = projectService(db);
   const access = accessService(db);
 
+  // Document ids are UUID-keyed. Reject malformed ids early so they surface as a
+  // structured 400 instead of bubbling up as a Postgres "invalid input syntax for
+  // type uuid" error (which would become an opaque 500). See PAP-10582.
+  router.param("documentId", (req, res, next, value) => {
+    if (!isUuidLike(value)) {
+      res.status(400).json({ error: "Invalid document id format" });
+      return;
+    }
+    next();
+  });
+
   router.get("/companies/:companyId/documents", async (req, res) => {
     const companyId = req.params.companyId as string;
     assertCompanyAccess(req, companyId);