diff --git a/swift/mcap/MCAPWriter.swift b/swift/mcap/MCAPWriter.swift
index 8232b0203b..d120c67053 100644
--- a/swift/mcap/MCAPWriter.swift
+++ b/swift/mcap/MCAPWriter.swift
@@ -161,6 +161,9 @@ public final class MCAPWriter {
       }
     } else {
       message.serialize(to: &buffer)
+      if buffer.count >= options.chunkSize {
+        await _flush()
+      }
     }
   }
 
@@ -219,6 +222,7 @@ public final class MCAPWriter {
 
     let chunkStartOffset = _position()
     chunk.serialize(to: &buffer)
+    await _flush()
     let messageIndexStartOffset = _position()
 
     var messageIndexOffsets: [ChannelID: UInt64] = [:]