diff --git a/packages/opencode/src/config/mcp.ts b/packages/opencode/src/config/mcp.ts
index ae558a76b2..2a49745dd8 100644
--- a/packages/opencode/src/config/mcp.ts
+++ b/packages/opencode/src/config/mcp.ts
@@ -26,7 +26,7 @@ export const OAuth = Schema.Struct({
     description: "OAuth client secret (if required by the authorization server)",
   }),
   scope: Schema.optional(Schema.String).annotate({ description: "OAuth scopes to request during authorization" }),
-  callbackPort: Schema.optional(PositiveInt).annotate({
+  callbackPort: Schema.optional(Schema.Int.check(Schema.isBetween({ minimum: 1, maximum: 65535 }))).annotate({
     description:
       "Port for the local OAuth callback server (default: 19876). Shorthand for redirectUri when only the port needs changing. Ignored if redirectUri is set.",
   }),