feat(server): support json_object response format (#2265)

Author: EricLBuehler

docs/openapi.json

@@ -4689,6 +4689,21 @@
               }
             }
           },
+          {
+            "type": "object",
+            "description": "Structured response as any JSON object",
+            "required": [
+              "type"
+            ],
+            "properties": {
+              "type": {
+                "type": "string",
+                "enum": [
+                  "json_object"
+                ]
+              }
+            }
+          },
           {
             "type": "object",
             "description": "Structured response following a JSON schema",

mistralrs-server-core/src/chat_completion.rs

@@ -1008,6 +1008,7 @@ pub async fn parse_request(
             Some(ResponseFormat::JsonSchema {
                 json_schema: JsonSchemaResponseFormat { name: _, schema },
             }) => Constraint::JsonSchema(schema),
+            Some(ResponseFormat::JsonObject) => Constraint::JsonSchema(json!({"type": "object"})),
             Some(ResponseFormat::Text) => Constraint::None,
             None => Constraint::None,
         },

mistralrs-server-core/src/openai.rs

@@ -561,6 +561,9 @@ pub enum ResponseFormat {
     /// Free-form text response
     #[serde(rename = "text")]
     Text,
+    /// Structured response as any JSON object
+    #[serde(rename = "json_object")]
+    JsonObject,
     /// Structured response following a JSON schema
     #[serde(rename = "json_schema")]
     JsonSchema {
@@ -1848,6 +1851,16 @@ mod tests {
         tool
     }
 
+    #[test]
+    fn chat_response_format_accepts_json_object() {
+        let format: ResponseFormat = serde_json::from_value(json!({
+            "type": "json_object"
+        }))
+        .unwrap();
+
+        assert!(matches!(format, ResponseFormat::JsonObject));
+    }
+
     #[test]
     fn openai_tool_deserializes_chat_function_tool() {
         let tool = assert_tool_roundtrips(json!({

mistralrs-server-core/src/responses.rs

@@ -1666,15 +1666,7 @@ async fn parse_openresponses_request(
                     schema: schema.unwrap_or(serde_json::Value::Object(Default::default())),
                 },
             },
-            TextFormat::JsonObject => {
-                // JsonObject is treated as a schema with empty object
-                crate::openai::ResponseFormat::JsonSchema {
-                    json_schema: crate::openai::JsonSchemaResponseFormat {
-                        name: "json_object".to_string(),
-                        schema: serde_json::json!({"type": "object"}),
-                    },
-                }
-            }
+            TextFormat::JsonObject => crate::openai::ResponseFormat::JsonObject,
         })
     } else {
         oairequest.response_format