From 1ae20b96290fa5b4fd3cfffc18ce3bfc32444c7e Mon Sep 17 00:00:00 2001
From: Jonathan Amsterdam <jba@google.com>
Date: Fri, 27 Mar 2026 10:04:36 -0400
Subject: [PATCH] mcp: purge makes a change if it removes any chunk

Don't require the chunk to be non-empty.

Fixes #860.
---
 mcp/event.go      |  9 +++++----
 mcp/event_test.go | 23 +++++++++++++++++++++++
 2 files changed, 28 insertions(+), 4 deletions(-)

diff --git a/mcp/event.go b/mcp/event.go
index 62dd2ad2..80b42f51 100644
--- a/mcp/event.go
+++ b/mcp/event.go
@@ -376,10 +376,11 @@ func (s *MemoryEventStore) purge() {
 			for _, dl := range sm {
 				if dl.size > 0 {
 					r := dl.removeFirst()
-					if r > 0 {
-						changed = true
-						s.nBytes -= r
-					}
+					// Even if we remove an empty chunk, that's
+					// still progress. There may be non-empty
+					// chunks after it.
+					changed = true
+					s.nBytes -= r
 				}
 			}
 		}
diff --git a/mcp/event_test.go b/mcp/event_test.go
index 685e6486..91ddb9d7 100644
--- a/mcp/event_test.go
+++ b/mcp/event_test.go
@@ -327,6 +327,29 @@ func TestMemoryEventStoreAfter(t *testing.T) {
 	}
 }
 
+func TestMemoryEventStorePurgeEmpty(t *testing.T) {
+	ctx := context.Background()
+	s := NewMemoryEventStore(nil)
+	s.SetMaxBytes(100)
+
+	// Append an empty chunk first.
+	if err := s.Append(ctx, "S1", "1", []byte("")); err != nil {
+		t.Fatal(err)
+	}
+	// Append a non-empty chunk.
+	if err := s.Append(ctx, "S1", "1", []byte("1234567890")); err != nil {
+		t.Fatal(err)
+	}
+	// Now nBytes = 10, but the first chunk is empty.
+
+	// This should not panic.
+	s.SetMaxBytes(5)
+
+	if s.nBytes > 5 {
+		t.Errorf("got nBytes %d, want <= 5", s.nBytes)
+	}
+}
+
 func BenchmarkMemoryEventStore(b *testing.B) {
 	// Benchmark with various settings for event store size, number of session,
 	// and payload size.