Respect batch size limits when flushing queue

langfuse-core/src/index.ts

@@ -695,7 +695,10 @@ abstract class LangfuseCoreStateless {
     const MAX_MSG_SIZE = 1_000_000;
     const BATCH_SIZE_LIMIT = 2_500_000;
 
-    this.processQueueItems(items, MAX_MSG_SIZE, BATCH_SIZE_LIMIT);
+    const { processedItems, remainingItems } = this.processQueueItems(items, MAX_MSG_SIZE, BATCH_SIZE_LIMIT);
+
+    // Add remaining items back to the start of the queue
+    queue.unshift(...remainingItems);
 
     const promiseUUID = generateUUID();
 
@@ -708,9 +711,9 @@ abstract class LangfuseCoreStateless {
     };
 
     const payload = JSON.stringify({
-      batch: items,
+      batch: processedItems,
       metadata: {
-        batch_size: items.length,
+        batch_size: processedItems.length,
         sdk_integration: this.sdkIntegration,
         sdk_version: this.getLibraryVersion(),
         sdk_variant: this.getLibraryId(),

langfuse-core/test/langfuse.flush.spec.ts

@@ -231,5 +231,37 @@ describe("Langfuse Core", () => {
       jest.advanceTimersByTime(300);
       expect(mocks.fetch).toHaveBeenCalledTimes(1);
     });
+
+    describe("when queue is completely full", () => {
+      const MAX_MSG_SIZE = 1_000_000;
+      const BATCH_SIZE_LIMIT = 2_500_000;
+      // Message is right under the message size limit
+      const MSG_SIZE = MAX_MSG_SIZE - 1000;
+      const BIG_STRING = "a".repeat(MSG_SIZE);
+
+      it("should flush remaining items on subsequent flush", () => {
+        const n = Math.floor(BATCH_SIZE_LIMIT / MSG_SIZE) + 1;
+
+        [langfuse, mocks] = createTestClient({
+          publicKey: "pk-lf-111",
+          secretKey: "sk-lf-111",
+          flushAt: n,
+          flushInterval: 200,
+        });
+
+        // Adds enough messages to exceed batch size limit
+        for (let i = 0; i < n; i++) {
+          langfuse.trace({ name: `test-trace-${i}`, input: { content: BIG_STRING } });
+        }
+
+        // First call flushes the messages that fit under the batch size limit
+        expect(mocks.fetch).toHaveBeenCalledTimes(1);
+
+        jest.advanceTimersByTime(300);
+
+        // Second call flushes the remaining messages
+        expect(mocks.fetch).toHaveBeenCalledTimes(2);
+      });
+    });
   });
 });