diff --git a/lib/firm-catalog.ts b/lib/firm-catalog.ts
index 5d74992..716a094 100644
--- a/lib/firm-catalog.ts
+++ b/lib/firm-catalog.ts
@@ -104,6 +104,10 @@ export function getCatalogMeta(): {
 export function getTradableFirms(): (CatalogFirm & { username: string; password: string })[] {
     const result: (CatalogFirm & { username: string; password: string })[] = [];
     for (const firm of getCachedCatalog()) {
+        // Only Tradovate has a client. A TopstepX firm in the catalog is
+        // config-only for now; handing it to the Tradovate login would just
+        // burn attempts against the wrong platform.
+        if (firm.platform !== 'tradovate') continue;
         const creds = getCredentials(firm.id);
         if (creds) result.push({ ...firm, ...creds });
     }
@@ -156,6 +160,10 @@ function validatePayload(raw: unknown): FirmCatalogPayload {
         return {
             id: firm.id,
             name: firm.name,
+            // Lenient on purpose: a master from before the platform field (or
+            // with a platform this build doesn't know) must not fail the whole
+            // sync, and 'tradovate' is what every firm was until then.
+            platform: firm.platform === 'topstepx' ? 'topstepx' : 'tradovate',
             bannedSymbols: Array.isArray(firm.bannedSymbols) ? (firm.bannedSymbols as string[]) : [],
             accountTypes,
             updatedAt: typeof firm.updatedAt === 'string' ? firm.updatedAt : new Date(0).toISOString(),
diff --git a/lib/master-protocol.ts b/lib/master-protocol.ts
index 07e6ce7..733e985 100644
--- a/lib/master-protocol.ts
+++ b/lib/master-protocol.ts
@@ -78,6 +78,10 @@ export interface CatalogAccountType {
 export interface CatalogFirm {
     id: string;
     name: string;
+    /** Which platform the firm's accounts trade on. Only 'tradovate' firms get
+     *  a client here; anything else is synced but sits idle until its trading
+     *  logic exists. Older masters omit the field — treated as 'tradovate'. */
+    platform: 'tradovate' | 'topstepx';
     bannedSymbols: string[];
     accountTypes: CatalogAccountType[];
     updatedAt: string;