fix(openai): folded system messages keep their JSON blocks

crates/tinyinference-llm/src/providers/openai/mod_tests.rs

@@ -2003,6 +2003,28 @@ fn merge_system_preserves_user_image_blocks() {
     }
 }
 
+#[test]
+fn folded_system_messages_keep_their_json_blocks_as_sent_unfolded() {
+    // A system message's JSON block is serialized on the pass-through path;
+    // hoisting or merging it into the user turn must not drop it, and its
+    // blocks join exactly as the pass-through path joins them.
+    use crate::message::SystemMessage;
+    let structured = Message::System(SystemMessage {
+        content: vec![
+            ContentBlock::Text("Context: ".to_string()),
+            ContentBlock::Json(serde_json::json!({"tenant": "acme"})),
+        ],
+        ..Default::default()
+    });
+    let sent_as_is = r#"Context: {"tenant":"acme"}"#;
+
+    let hoisted = single_system_message(&[Message::user("hi"), structured.clone()]);
+    assert_eq!(hoisted[0].text(), sent_as_is);
+
+    let merged = merge_system_into_user(&[structured, Message::user("hi")]);
+    assert_eq!(merged[0].text(), format!("{sent_as_is}\n\nhi"));
+}
+
 #[test]
 fn merge_system_is_noop_without_system_messages() {
     let input = vec![Message::user("just user")];

crates/tinyinference-llm/src/providers/openai/transport.rs

@@ -186,17 +186,20 @@ pub(super) fn auth_headers(auth: &AuthStyle, api_key: &str) -> Vec<(String, Stri
     }
 }
 
-/// Concatenates the text of a message's content blocks (non-text blocks — images,
-/// json, thinking — are ignored). Used to gather system-prompt text for merging.
+/// A system message's content as the pass-through path sends it
+/// (`translate_text_content`): text verbatim, JSON serialized, reasoning
+/// dropped, blocks joined with nothing between them. Folding with anything
+/// else would make a hoisted or merged prompt differ from the same prompt sent
+/// as-is, and would silently drop JSON blocks.
 fn content_text(content: &[ContentBlock]) -> String {
     content
         .iter()
         .filter_map(|block| match block {
-            ContentBlock::Text(text) => Some(text.as_str()),
+            ContentBlock::Text(text) => Some(text.clone()),
+            ContentBlock::Json(value) => Some(value.to_string()),
             _ => None,
         })
-        .collect::<Vec<_>>()
-        .join("\n")
+        .collect()
 }
 
 /// Folds all `system` messages into the first `user` message and drops the