From c5a5535e6ebc2e74ab0e3f1b249d7d989aba26a2 Mon Sep 17 00:00:00 2001
From: Sergey Sklyarov <sergey.sklyarov@gmail.com>
Date: Thu, 3 Sep 2026 22:37:50 +0200
Subject: [PATCH] common/json-schema : fix GBNF grammar generation for empty
 object schemas (#28279)

---
 common/json-schema-to-grammar.cpp     | 4 ++++
 examples/json_schema_to_grammar.py    | 3 +++
 tests/test-json-schema-to-grammar.cpp | 2 +-
 3 files changed, 8 insertions(+), 1 deletion(-)

diff --git a/common/json-schema-to-grammar.cpp b/common/json-schema-to-grammar.cpp
index 0aee51b26..a7a18857d 100644
--- a/common/json-schema-to-grammar.cpp
+++ b/common/json-schema-to-grammar.cpp
@@ -748,6 +748,10 @@ private:
             optional_props.push_back("*");
         }
 
+        if (required_props.empty() && optional_props.empty()) {
+            return "\"{\" space \"}\"";
+        }
+
         std::string rule = "\"{\" space ";
         for (size_t i = 0; i < required_props.size(); i++) {
             if (i > 0) {
diff --git a/examples/json_schema_to_grammar.py b/examples/json_schema_to_grammar.py
index 83abd259d..02b7ef15a 100755
--- a/examples/json_schema_to_grammar.py
+++ b/examples/json_schema_to_grammar.py
@@ -734,6 +734,9 @@ class SchemaConverter:
             )
             optional_props.append("*")
 
+        if not required_props and not optional_props:
+            return '"{" space "}"'
+
         rule = '"{" space '
         rule += ' "," space '.join(prop_kv_rule_names[k] for k in required_props)
 
diff --git a/tests/test-json-schema-to-grammar.cpp b/tests/test-json-schema-to-grammar.cpp
index 214dbe199..2a1b6348c 100755
--- a/tests/test-json-schema-to-grammar.cpp
+++ b/tests/test-json-schema-to-grammar.cpp
@@ -994,7 +994,7 @@ static void test_all(const std::string & lang, std::function<void(const TestCase
             "additionalProperties": false
         })""",
         R"""(
-            root ::= "{" space  space "}"
+            root ::= "{" space "}"
             space ::= | " " | "\n"{1,2} [ \t]{0,20}
         )"""
     });