From 102946aa3ba6d0dc73cb3508d1856e753e02c639 Mon Sep 17 00:00:00 2001
From: Pratik Dhanave <i.pratikdhanave@gmail.com>
Date: Sun, 2 Aug 2026 09:33:08 +0530
Subject: [PATCH] mcp: prevent MemoryEventStore.After panic on index past the
 end
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After guarded only the lower bound (dl.first > start → ErrEventsPurged), so
an index greater than the highest stored event made the final
slices.Clone(dl.data[start-dl.first:]) panic with slice-bounds-out-of-range.
The boundary start == dl.first+len(dl.data) (resume from the last event seen)
is a valid empty replay, so only strictly-greater indices faulted.

This is reachable from untrusted input: parseEventID accepts any
non-negative index from the client-supplied Last-Event-ID header, and
serveGET passes it to eventStore.After.

Return an empty replay for an index at or beyond the latest stored event —
there is nothing after it — matching the already-valid index == last case.
Add regression coverage for indices past the end.

Fixes #1131
---
 mcp/event.go      | 9 +++++++++
 mcp/event_test.go | 6 ++++++
 2 files changed, 15 insertions(+)

diff --git a/mcp/event.go b/mcp/event.go
index bbb3e6c8..1d86b703 100644
--- a/mcp/event.go
+++ b/mcp/event.go
@@ -342,6 +342,15 @@ func (s *MemoryEventStore) After(_ context.Context, sessionID, streamID string,
 			return nil, fmt.Errorf("MemoryEventStore.After: index %d, stream ID %v, session %q: %w",
 				index, streamID, sessionID, ErrEventsPurged)
 		}
+		// An index at or beyond the latest stored event has nothing after it, so
+		// there is nothing to replay. The boundary start == dl.first+len(dl.data)
+		// (resume from the last event seen) already yields an empty slice; guard
+		// the strictly-greater case too, otherwise dl.data[start-dl.first:]
+		// panics with a slice-bounds-out-of-range on an index past the end (for
+		// example a client-supplied Last-Event-ID beyond any event ever sent).
+		if start-dl.first > len(dl.data) {
+			return nil, nil
+		}
 		return slices.Clone(dl.data[start-dl.first:]), nil
 	}
 
diff --git a/mcp/event_test.go b/mcp/event_test.go
index 91ddb9d7..9a902dab 100644
--- a/mcp/event_test.go
+++ b/mcp/event_test.go
@@ -299,7 +299,13 @@ func TestMemoryEventStoreAfter(t *testing.T) {
 		{"S1", "1", 0, []string{"d2", "d3"}, ""},
 		{"S1", "1", 1, []string{"d3"}, ""},
 		{"S1", "1", 2, nil, ""},
+		// An index past the latest stored event (highest here is 2) must yield an
+		// empty replay, not panic with a slice-bounds-out-of-range. This is
+		// reachable from a client-supplied Last-Event-ID beyond any event sent.
+		{"S1", "1", 3, nil, ""},
+		{"S1", "1", 100, nil, ""},
 		{"S1", "2", 0, nil, ""},
+		{"S1", "2", 5, nil, ""}, // past the end of stream "2" (highest is 0)
 		{"S1", "3", 0, nil, "unknown stream ID"},
 		{"S2", "0", 0, nil, "unknown session ID"},
 	} {