diff --git a/client/src/lib/api.ts b/client/src/lib/api.ts
index 592ab231..ac93133d 100644
--- a/client/src/lib/api.ts
+++ b/client/src/lib/api.ts
@@ -1703,27 +1703,31 @@ export const api = {
      * Coerces the CLI's JSON output (which may be an object) to an array.
      * @returns `{ mcp }` — {@link CodexMcpServer} items.
      */
-    mcp: async () => {
+    mcp: async (): Promise<{ mcp: CodexMcpServer[] }> => {
       const res = await request<{ mcp: unknown }>("/codex-config/mcp");
-      return { mcp: Array.isArray(res.mcp) ? res.mcp : [] };
+      return { mcp: Array.isArray(res.mcp) ? (res.mcp as CodexMcpServer[]) : [] };
     },
     /**
      * GET /api/codex-config/plugins - installed plugins from the codex CLI.
      * Coerces the CLI's JSON output (which may be an object) to an array.
      * @returns `{ plugins }` — {@link CodexPlugin} items.
      */
-    plugins: async () => {
+    plugins: async (): Promise<{ plugins: CodexPlugin[] }> => {
       const res = await request<{ plugins: unknown }>("/codex-config/plugins");
-      return { plugins: Array.isArray(res.plugins) ? res.plugins : [] };
+      return { plugins: Array.isArray(res.plugins) ? (res.plugins as CodexPlugin[]) : [] };
     },
     /**
      * GET /api/codex-config/marketplaces - registered marketplaces from the codex CLI.
      * Coerces the CLI's JSON output (which may be an object) to an array.
      * @returns `{ marketplaces }` — {@link CodexMarketplace} items.
      */
-    marketplaces: async () => {
+    marketplaces: async (): Promise<{ marketplaces: CodexMarketplace[] }> => {
       const res = await request<{ marketplaces: unknown }>("/codex-config/marketplaces");
-      return { marketplaces: Array.isArray(res.marketplaces) ? res.marketplaces : [] };
+      return {
+        marketplaces: Array.isArray(res.marketplaces)
+          ? (res.marketplaces as CodexMarketplace[])
+          : [],
+      };
     },
     /**
      * POST /api/codex-config/mcp/add - add an MCP server via the `codex` CLI.
@@ -3006,15 +3010,15 @@ export interface CodexMarketplace {
   [key: string]: unknown;
 }
 
-/** Arguments for POST /api/codex-config/mcp/add. */
-export interface CodexMcpAddArgs {
-  name: string;
-  url?: string;
-  bearerTokenEnvVar?: string;
-  command?: string;
-  args?: string[];
-  env?: Record<string, string>;
-}
+/**
+ * Arguments for POST /api/codex-config/mcp/add. A union, not one optional-heavy
+ * shape: `codex mcp add` takes either a URL or a command and rejects both or
+ * neither, and `env` is stdio-only. Modelling that here turns three of the
+ * server's `EBADARG` responses into compile errors.
+ */
+export type CodexMcpAddArgs =
+  | { name: string; url: string; bearerTokenEnvVar?: string }
+  | { name: string; command: string; args?: string[]; env?: Record<string, string> };
 
 /** Arguments for POST /api/codex-config/plugins/action. */
 export interface CodexPluginActionArgs {