diff --git a/src/server/catalog-download.ts b/src/server/catalog-download.ts
index 984b22217c..4f8d64325c 100644
--- a/src/server/catalog-download.ts
+++ b/src/server/catalog-download.ts
@@ -11,6 +11,7 @@
  * copy is the one nobody looks at in the dashboard.
  */
 import { createHash } from "node:crypto";
+import { statSync } from "node:fs";
 
 /**
  * Upper bound for the REMOTE route only.
@@ -35,6 +36,29 @@ export interface SerializedCatalog {
   etag?: string;
   /** Byte length of `body`, present only when `body` is. */
   bytes?: number;
+  /** Remote-only preflight failure; management serialization never sets this. */
+  error?: "too_large";
+}
+
+interface CatalogFileIdentity {
+  key: string;
+  size: number;
+}
+
+let remoteCache: { path: string; identity: string; serialized: SerializedCatalog } | undefined;
+let remoteSerialization: { path: string; identity: string; result: Promise<SerializedCatalog> } | undefined;
+
+function catalogFileIdentity(path: string): CatalogFileIdentity | null {
+  try {
+    const stat = statSync(path, { bigint: true });
+    if (!stat.isFile()) return null;
+    return {
+      key: `${stat.dev}:${stat.ino}:${stat.size}:${stat.mtimeNs}:${stat.ctimeNs}`,
+      size: Number(stat.size),
+    };
+  } catch {
+    return null;
+  }
 }
 
 export function catalogEtag(body: string): string {
@@ -61,6 +85,46 @@ export async function serializePersistedCatalog(): Promise<SerializedCatalog> {
   return { body, etag: catalogEtag(body), bytes };
 }
 
+/**
+ * Serialize the catalog for the remotely reachable route.
+ *
+ * File size is checked before the synchronous reader can materialize it. A
+ * validated file identity caches the expensive parse/stringify/hash result,
+ * and the shared promise limits cache misses to one serialization at a time.
+ */
+export async function serializeRemotePersistedCatalog(): Promise<SerializedCatalog> {
+  const { readCodexCatalogPath } = await import("../codex/catalog");
+  const path = readCodexCatalogPath();
+  const identity = catalogFileIdentity(path);
+  if (!identity) return { body: null };
+  if (identity.size > MAX_REMOTE_CATALOG_BYTES) return { body: null, error: "too_large" };
+  if (remoteCache?.path === path && remoteCache.identity === identity.key) return remoteCache.serialized;
+  if (remoteSerialization) {
+    if (remoteSerialization.path === path && remoteSerialization.identity === identity.key) {
+      return remoteSerialization.result;
+    }
+    await remoteSerialization.result;
+    return serializeRemotePersistedCatalog();
+  }
+
+  const result = (async () => {
+    const serialized = await serializePersistedCatalog();
+    if (serialized.bytes !== undefined && serialized.bytes > MAX_REMOTE_CATALOG_BYTES) {
+      return { body: null, error: "too_large" } as SerializedCatalog;
+    }
+    const after = catalogFileIdentity(path);
+    if (after?.key !== identity.key) return { body: null };
+    if (serialized.body !== null) remoteCache = { path, identity: identity.key, serialized };
+    return serialized;
+  })();
+  remoteSerialization = { path, identity: identity.key, result };
+  try {
+    return await result;
+  } finally {
+    if (remoteSerialization?.result === result) remoteSerialization = undefined;
+  }
+}
+
 /**
  * The authoritative Codex version for a catalog response, or undefined.
  *
diff --git a/src/server/index.ts b/src/server/index.ts
index 6c7e53f062..9be8578239 100644
--- a/src/server/index.ts
+++ b/src/server/index.ts
@@ -1084,34 +1084,32 @@ export function startServer(port?: number, deps: StartServerDeps = {}): Server<W
         if (!isAllowedRequestOrigin(req, policy)) {
           return withCors(formatErrorResponse(403, "origin_rejected", "cross-origin data-plane request blocked"), req, policy);
         }
-        const { serializePersistedCatalog, persistedCodexVersion, MAX_REMOTE_CATALOG_BYTES } = await import("./catalog-download");
-        const serialized = await serializePersistedCatalog();
-        if (serialized.body === null) {
-          // Built directly rather than through formatErrorResponse: that helper derives
-          // `code` from the status and message via classifyError, and these two need stable,
-          // specific codes. `catalog_not_found` in particular is what lets a caller — and
-          // tests/api-key-attribution.test.ts — tell "this route exists and has no catalog"
-          // apart from "this route is gone", which is the difference between admission proof
-          // and a vacuous pass.
+        const { serializeRemotePersistedCatalog, persistedCodexVersion } = await import("./catalog-download");
+        const serialized = await serializeRemotePersistedCatalog();
+        if (serialized.error === "too_large") {
           return withCors(
             new Response(JSON.stringify({
-              error: { type: "invalid_request_error", code: "catalog_not_found", message: "no materialized catalog is available" },
+              error: { type: "server_error", code: "catalog_too_large", message: "catalog exceeds the maximum served size" },
             }), {
-              status: 404,
+              status: 507,
               headers: { "content-type": "application/json" },
             }),
             req,
             policy,
           );
         }
-        // Size policy belongs to this route, not the shared serializer: the management route
-        // must keep its existing behavior for a catalog of any supported size.
-        if (serialized.bytes !== undefined && serialized.bytes > MAX_REMOTE_CATALOG_BYTES) {
+        if (serialized.body === null) {
+          // Built directly rather than through formatErrorResponse: that helper derives
+          // `code` from the status and message via classifyError, and these two need stable,
+          // specific codes. `catalog_not_found` in particular is what lets a caller — and
+          // tests/api-key-attribution.test.ts — tell "this route exists and has no catalog"
+          // apart from "this route is gone", which is the difference between admission proof
+          // and a vacuous pass.
           return withCors(
             new Response(JSON.stringify({
-              error: { type: "server_error", code: "catalog_too_large", message: "catalog exceeds the maximum served size" },
+              error: { type: "invalid_request_error", code: "catalog_not_found", message: "no materialized catalog is available" },
             }), {
-              status: 507,
+              status: 404,
               headers: { "content-type": "application/json" },
             }),
             req,
diff --git a/tests/api-catalog-route.test.ts b/tests/api-catalog-route.test.ts
index 634ce3e7e1..84d875495c 100644
--- a/tests/api-catalog-route.test.ts
+++ b/tests/api-catalog-route.test.ts
@@ -230,6 +230,24 @@ describe("GET|HEAD /v1/catalog least-privilege data-plane route (#809)", () => {
     expect(mgmt?.status).toBe(200);
   });
 
+  test("caches remote serialization and preflights oversized files", async () => {
+    isolatedCodexHome = installIsolatedCodexHome("ocx-v1-catalog-cache-");
+    const path = join(isolatedCodexHome.path, "opencodex-catalog.json");
+    writeFileSync(path, JSON.stringify(catalogFixture));
+
+    const { serializeRemotePersistedCatalog, MAX_REMOTE_CATALOG_BYTES } = await import("../src/server/catalog-download");
+    const first = await serializeRemotePersistedCatalog();
+    const cached = await serializeRemotePersistedCatalog();
+    expect(cached).toBe(first);
+
+    // A sparse over-limit file proves the ceiling is enforced from metadata,
+    // before the synchronous JSON reader can allocate or parse its contents.
+    const { truncateSync } = await import("node:fs");
+    truncateSync(path, MAX_REMOTE_CATALOG_BYTES + 1);
+    const oversized = await serializeRemotePersistedCatalog();
+    expect(oversized).toEqual({ body: null, error: "too_large" });
+  });
+
   test("reports a distinguishable code when no catalog is materialized", async () => {
     isolatedCodexHome = installIsolatedCodexHome("ocx-v1-catalog-missing-");
     saveConfig(dataPlaneConfig());