1 files changed, 14 insertions, 2 deletions
diff --git a/libpanto/src/anthropic_messages_json.zig b/libpanto/src/anthropic_messages_json.zig
index d382de8..c54db86 100644
--- a/libpanto/src/anthropic_messages_json.zig
+++ b/libpanto/src/anthropic_messages_json.zig
@@ -79,10 +79,12 @@ pub fn serializeRequest(
         try s.endArray();
     }
 
-    // Emit messages (everything that isn't .system).
+    // Emit messages (everything that isn't .system). If the conversation
+    // has been compacted, only the latest compaction summary and the
+    // messages after it are active; the superseded prefix is dropped.
     try s.objectField("messages");
     try s.beginArray();
-    for (conv.messages.items) |msg| {
+    for (conversation.activeMessageWindow(conv.messages.items)) |msg| {
         if (msg.role == .system) continue;
         try writeMessage(&s, msg, allocator);
     }
@@ -239,6 +241,16 @@ fn writeBlock(s: *std.json.Stringify, block: conversation.ContentBlock) !void {
         // system text is hoisted into the top-level `system` string by
         // `collectSystemPrompt`. Present only to keep the switch exhaustive.
         .System => {},
+        // A compaction summary is the synthetic seed text standing in for
+        // a compacted prefix; the model reads it as ordinary user text.
+        .CompactionSummary => |cs| {
+            try s.beginObject();
+            try s.objectField("type");
+            try s.write("text");
+            try s.objectField("text");
+            try s.write(cs.text.items);
+            try s.endObject();
+        },
     }
 }